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Water Production Rates and Activity of Interstellar Comet 2I/Borisov

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... ± 0.8) × 10 26 s −1 and T ≈ 23 K. However, the temperature is not well constrained by the observations, with a (1σ ) error range of ∼ 13-181 K. Using a water production rate Q(H 2 O) = (10.7 ± 1.2) × 10 26 s −1 on 01 December 2019 (see ref. 33 ), we derive a CO/H 2 O mixing ratio of (31 ± 8)%. ...
... Around perihelion, the water production rate is ∼ 30 kg s −1 (see ref. 33 ) and the CO production rate is ∼ 15-20 kg s −1 (this study; see also ref. 8 ). If we assume the CO 2 production rate is comparable to that of CO, similar to the case of 67P (ref. ...
... As discussed in refs. 7,33 , the release of water peaked when 2I/Borisov was closest to the Sun. After perihelion, the water production dropped sharply Reflectivity gradient, S' [%/100nm] . ...
Preprint
The interstellar traveler, 2I/Borisov, is the first clearly active extrasolar comet, ever detected in our Solar system. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA), and multi-color optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact "pebbles" of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass loss rate of >= 200 kg/s and a dust-to-gas ratio >=3. Our long term monitoring of 2I/Borisov with VLT indicates a steady dust mass loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide gas (CO) with ALMA and derived the gas production rate of Q(CO) (3.3+/-0.8)x10^{26} mole/s. We found that the CO/H$_2$O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov's home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO.
... Chemical abundances in the coma are often expressed relative to H 2 O. In Fig. 2, we compare our measurements of the CO production rates with H 2 O production rates [8][9][10] . Reported H 2 O production rates peak around perihelion on 8 December 2019, when 2I/Borisov came within 2.006 au of the Sun, and decreased rapidly afterwards 9 . ...
... In Fig. 2, we compare our measurements of the CO production rates with H 2 O production rates [8][9][10] . Reported H 2 O production rates peak around perihelion on 8 December 2019, when 2I/Borisov came within 2.006 au of the Sun, and decreased rapidly afterwards 9 . In contrast, we observed that the CO production rates remained constant during our observations, with a possible maximum around 30 December 2019. ...
... 2I/Borisov probably has a small nucleus with a radius between 0.2 and 0.5 km (ref. 21 ), and it has been estimated that the comet lost a layer of between 1.0 and 6.4 m thickness from its surface, depending on the size of the nucleus 9 . Results from the Rosetta mission suggest an orbital thermal skin depth of a few tens of centimetres to 1 m (ref. ...
Article
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Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our Solar System1. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles2, organic material and prebiotic chemicals around their host system3. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions4. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide5. Here we report that the coma of 2I/Borisov contains substantially more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System4. Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own. Hubble Space Telescope data show that interstellar comet 2I/Borisov has an unusually high CO/H2O ratio—higher than any other comet that has been seen in the inner regions of our Solar System. This allows us to constrain the nature and location of the circumstellar region from which 2I/Borisov originated.
... The presence of a gas and dust coma surrounding 2I/Borisov, on the other hand, has been confirmed by numerous observations, including the detection of cyanide (CN) and hydroxyl (OH) emission in the near-ultraviolet (UV) and optical [14][15][16] . The observations reported to date have revealed a CN/H 2 O ratio and optical dust colours consistent with typical Solar System comets, while the C 2 abundance may be somewhat depleted 17,18 . ...
... Assuming 2I/Borisov's water production rate on 15-16 December was within the range 4.9-10.7 × 10 26 s −1 as reported in ref. 16 (see Methods), we find that the HCN abundance (relative to H 2 O) was 0.06-0.16% and the CO abundance was 35-105%. ...
... The comet's H 2 O production rate was obtained from UV observations of OH using the SWIFT satellite 16 Table 1). ...
Article
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Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation1. To date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here, we present high-resolution interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15–16 December 2019. Our observations reveal emission from hydrogen cyanide (HCN) and carbon monoxide (CO) coincident with the expected position of 2I/Borisov’s nucleus, with production rates Q(HCN) = (7.0 ± 1.1) × 1023 s−1 and Q(CO) = (4.4 ± 0.7) × 1026 s−1. While the HCN abundance relative to water (0.06–0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35–105%) is among the highest observed in any comet within 2 au of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment—probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small icy bodies analogous to our Solar System’s own proto-Kuiper belt. Interstellar comet Borisov has thus far looked very much like a Solar System comet in terms of its volatile content, but with new data from ALMA that show robust detections of CO and HCN, it is clear that Borisov is rich in CO, helping to pinpoint its origin.
... Chemical abundances in the coma are often expressed relative to H2O. In Figure 2, we compare our measurements of the CO production rates with H2O production rates 8,9,10 . Reported water production rates peak around perihelion on Dec. 8, 2019, when 2I/Borisov came within 2.006 au of the Sun and decreased rapidly afterwards 9 . ...
... In Figure 2, we compare our measurements of the CO production rates with H2O production rates 8,9,10 . Reported water production rates peak around perihelion on Dec. 8, 2019, when 2I/Borisov came within 2.006 au of the Sun and decreased rapidly afterwards 9 . In contrast, we observed that the CO production rates remained constant during our observations, with a possible maximum around December 30, 2019. ...
... 2I/Borisov's orbit firmly places its origins outside our solar system. Most of its properties, including the colour of its dust, its brightness trend with respect to the distance to the Sun, and the presence of fragment species have been surprisingly similar to those of comets from our solar system 1,9,20,23,25 , which could partially be explained by the exposure to radiation during the time the comet spent in interstellar space. Two major differences between typical solar system comets and 2I/Borisov become apparent with our HST/COS observations. ...
Preprint
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Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our solar system[1]. Comets are condensed samples of the gas, ice, and dust that were in a star's protoplanetary disk during the formation of its planets and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration moves volatiles[2], organic material, and prebiotic chemicals in their host system[3]. In our solar system, hundreds of comets have been observed remotely, and a few have been studied up close by space missions[4]. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide[5]. Here we report that the coma of 2I/Borisov contains significantly more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) solar system[4]. Our ultraviolet observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own.
... The presence of a gas and dust coma surrounding 2I/Borisov, on the other hand, has been confirmed by numerous observations, including the detection of cyanide (CN) and hydroxyl (OH) emission in the near-ultraviolet (UV) and optical [14][15][16] . The observations reported to date have revealed a CN/H 2 O ratio and optical dust colours consistent with typical Solar System comets, while the C 2 abundance may be somewhat depleted 17,18 . ...
... Assuming 2I/Borisov's water production rate on 15-16 December was within the range 4.9-10.7 × 10 26 s −1 as reported in ref. 16 (see Methods), we find that the HCN abundance (relative to H 2 O) was 0.06-0.16% and the CO abundance was 35-105%. ...
... The comet's H 2 O production rate was obtained from UV observations of OH using the SWIFT satellite 16 . A linear interpolation of the observed values on 1 December ((10.7 ± 1.2) × 10 26 s −1 ) and 21 December ((4.9 ± 0.9) × 10 26 s −1 ) results in Q(H 2 O) = (6.5 ± 1.0) × 10 26 s −1 at the epoch of our ALMA observations. ...
Preprint
Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. To-date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here we present high-resolution, interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15th-16th December 2019. Our observations reveal emission from hydrogen cyanide (HCN), and carbon monoxide (CO), coincident with the expected position of 2I/Borisov's nucleus, with production rates Q(HCN)=$(7.0\pm1.1)\times10^{23}$ s$^{-1}$ and Q(CO)=$(4.4\pm0.7)\times10^{26}$ s$^{-1}$. While the HCN abundance relative to water (0.06-0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35-105%) is among the highest observed in any comet within 2~au of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment - probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small, icy bodies analogous to our Solar System's own proto-Kuiper Belt.
... Such a spatial flattening has not been reported yet in the case of 2I/Borisov. On the other hand, Xing et al. (2020) reports that the water production in 2I/Borisov had increased drastically from November to December, close to perihelion and then decreased rapidly by December 21 st . With the contribution from CHON grains being ruled out, only the activity in H 2 O ice can explain the increase in production rate of C 2 close to perihelion and hence the initial increase in Q(C 2 )/Q(CN). ...
... The available magnitudes were also used to compute A f ρ, a proxy to the amount of dust produced (A'Hearn et al. 1984b). The obtained values of A f ρ (see Table 3), for 30 th November and 22 nd December, in V band is found to be similar to the values reported by Xing et al. (2020), for the same wavelength band, during nearby epochs (1 st December and 21 st December respectively). The equation for average slope of the curve of reflectivity, as mentioned in A' Hearn et al. (1984b), when used with the observed magnitudes provides a slope S = (9.9 ...
... Inserting this value in the above defined relation for area, provides A = 0.4 Km 2 , which is equal to the surface area of a sphere of radius r = 0.18 Km. The computed nuclear radius is in good agreement with the lower limits reported in observations using the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope (Xing et al. 2020) and the Hubble Space Telescope (Jewitt et al. 2020a). ...
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Comet 2I/Borisov is the first true interstellar comet discovered. Here we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31/11/2019 and 22/12/2019, at distances of r$_H$ = 2.013 and 2.031 AU respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low $C_2$ and $C_3$ abundances. We find the production rate ratio, Q($C_2$)/Q(CN) = 0.54 $\pm$ 0.18, pre-perihelion and Q($C_2$)/Q(CN) = 0.34 $\pm$ 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. From imaging observations we infer a dust-to-gas ratio similar to carbon chain depleted comets of the Solar system. We also compute the nucleus size to be in the range $0.18\leq r \leq 3.1$ Km. Our observations show that 2I/Borisov's behaviour is analogous to that of the Solar system comets.
... However, the temperature is not well constrained by the observations, with a 1σ error range of ~13-181 K. Using a water production Around perihelion, the water production rate is ~30 kg s −1 (ref. 32 ) and the CO production rate is ~15-20 kg s −1 (this study; see also ref. 8 ). If we assume the CO 2 production rate is comparable to that of CO, similar to the case of 67P (ref. ...
... As discussed in refs. 7,32 , the release of water peaked when 2I/Borisov was closest to the Sun. After perihelion, the water production dropped sharply by a factor of 5 in 20 days 32 . ...
... 7,32 , the release of water peaked when 2I/Borisov was closest to the Sun. After perihelion, the water production dropped sharply by a factor of 5 in 20 days 32 . In contrast, the CO production rate increased steadily as the comet approached the Sun, and reached the peak about a month after perihelion. ...
Article
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The interstellar traveller, 2I/Borisov, is the first clearly active extrasolar comet ever detected in our Solar System. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA) and multi-colour optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact ‘pebbles’ of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass-loss rate of ≳200 kg s−1 and a dust-to-gas ratio ≳3. Our long-term monitoring of 2I/Borisov with the VLT indicates a steady dust mass-loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide (CO) gas with ALMA and derived the gas production rate of Q(CO) = (3.3 ± 0.8) × 1026 s−1. We found that the CO/H2O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov’s home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO. The dust in the dust coma of interstellar object 2I/Borisov is large (exceeding ~1 mm radius) and compact, differing from the fluffy aggregates typically found in Solar System comets. This compact dust is presumably a result of impacts in the comet’s home system, and suggests 2I/Borisov formed in a collapsing pebble cloud.
... This confirmed the existence of a galactic population of interstellar objects with spatial number densities of order n o ∼ 1 − 2 × 10 −1 au −3 (Trilling et al. 2017;Laughlin & Batygin 2017;Jewitt et al. 2017;Rafikov 2018b;Zwart et al. 2018;Do et al. 2018;). 2I exhibited a dusty coma (Jewitt & Luu 2019;Bolin et al. 2020b;Fitzsimmons et al. 2019;Ye et al. 2020;McKay et al. 2020;Guzik et al. 2020;Hui et al. 2020;Kim et al. 2020;Cremonese et al. 2020;Yang et al. 2021) with typical cometary carbon-and nitrogen-bearing species detected (Opitom et al. 2019;Kareta et al. 2020;Lin et al. 2020;Bannister et al. 2020;Xing et al. 2020;Aravind et al. 2021). 2I was enriched in CO relative to H 2 O Cordiner et al. 2020), indicating formation exterior to the CO snowline in its original protoplanetary disk (Price et al. 2021). ...
... Moreover, the extinction of 3100Å light is extremely sensitive to airmass, so the comet's sky position is as important as its brightness and H 2 O production rate. To circumvent this, the H 2 O production rate of Borisov was measured in the 0-0 band of OH emission with the Ultraviolet-Optical Telescope on the Swift Observatory (Xing et al. 2020). However, Swift is currently addressing a broken reaction wheel and not available for observations. ...
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Based on the occurrence rates implied by the discoveries of 1I/`Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect $\ge1$ interstellar comets every year (Hoover et al. 2021). We advocate for future measurements of the production rates of H$_2$O, CO$_2$ and CO in these comets to estimate their carbon to oxygen ratios, which traces formation locations within their original protoplanetary disks. We review similar measurements for Solar System comets, which indicate formation interior to the CO snowline. By quantifying the relative processing in the interstellar medium and Solar System, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO$_2$ relative to H$_2$O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of ${\rm Q}({\rm CO})/{\rm Q}({\rm H_2O})<.2$ and ${\rm Q}({\rm CO})/{\rm Q}({\rm H_2O})>1$ likely indicate formation interior and exterior to the CO snowline, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snowline. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snowline. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.
... Almost two years later on August 30th 2019 a second ISO, 2I/Borisov was discovered by Gennadiy Borisov using his MARGO observatory in Crimea. Unlike 1I/'Oumuamua, which remains somewhat of an enigma, 2I/Borisov was found to be a comet (Jewitt & Luu, 2019) similar to ones found in our solar system with an associated coma and tail as it approached and rounded the sun de León et al., 2019;Fitzsimmons et al., 2019;Guzik et al., 2020;Jewitt et al., 2020a;Jewitt & Luu, 2019;Manzini et al., 2020;Opitom et al., 2019;Xing et al., 2020;Ye et al., 2020). Its apparent fragmentation around February/March 2020 (perihelion was on 8th December 2019) was later established to be an ejection of material, around 0.1% of the total mass (Jewitt et al., 2020b). ...
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A potential second interstellar object C/2019 Q4 (Borisov) was discovered after the first known interstellar object 1I/'Oumuamua. Can we send a spacecraft to this object, using existing technologies? In this paper, we assess the technical feasibility of a mission to C/2019 Q4 (Borisov), using existing technologies. We apply the Optimum Interplanetary Trajectory Software (OITS) tool to generate trajectories to C/2019 Q4 (Borisov). As results, we get the minimal DeltaV trajectory with a launch date in July 2018. For this trajectory, a Falcon Heavy launcher could have hauled a 2 ton spacecraft to C/2019 Q4 (Borisov). For a later launch date, results for a combined powered Jupiter flyby with a Solar Oberth maneuver are presented. For a launch in 2030, we could reach C/2019 Q4 (Borisov) in 2045, using the Space Launch System, up-scaled Parker probe heatshield technology, and solid propulsion engines. A CubeSat-class spacecraft with a mass of 3 kg could be sent to C/2019 Q4 (Borisov). If C/2019 Q4 (Borisov) turns out to be indeed an interstellar object, its discovery shortly after the discovery of 1I/'Oumuamua implies that the next interstellar object might be discovered in the near future. The feasibility of a mission to both, 1I/'Oumuamua and C/2019 Q4 (Borisov) using existing technologies indicates that missions to further interstellar objects are likely to be feasible as well.
... Remarkably, 1I and 2I appear quite different. Whereas 1I appeared inert, 2I closely resembles a typical active solar system comet, with a prominent and persistent dust coma (Jewitt & Luu 2019;Guzik et al. 2020;Jewitt et al. 2020a;Kim et al. 2020;Ye et al. 2020) consisting of nonicy grains (Yang et al. 2020) and showing spectral lines indicative of ongoing sublimation (Fitzsimmons et al. 2019;Bodewits et al. 2020;McKay et al. 2020;Xing et al. 2020). Both 1I (Micheli et al. 2018) and 2I (Hui et al. 2020;Jewitt et al. 2020a) exhibit nongravitational accelerations, perhaps caused by recoil forces from anisotropic mass loss. ...
... When the second interstellar object, 2I/Borisov, was first discovered it already displayed a prominent tail, and subsequent archival searches in pre-discovery survey observations showed that it was active outside 5 au from the Sun (Ye et al. 2020). Borisov was brighter and observable for a much longer time than 'Oumuamua, and the emission of several fragment species common in solar system comets was observed, including CN, OH, C 2 , [O I], and NH 2 (e.g., Fitzsimmons et al. 2019;Bannister et al. 2020;Lin et al. 2020;McKay et al. 2020;Xing et al. 2020). This initially led to the conclusion that many of the properties of this object were surprisingly similar to those of comets from our solar system. ...
Article
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Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra solar systems. In the solar system, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between solar system comets and exocomets to allow for the development of new observational methods and techniques. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. The compositional properties of solar system comets and exocomets are summarized before providing an observational comparison between them. Exocomets likely vary in their composition depending on their formation environment like solar system comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of solar system comets. Observations of gas around main sequence stars, spectroscopic observations of “polluted” white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with solar system comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of solar system comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and solar system comets.
... Assuming our Solar System originated with a nominal amount of CO, there may be some selection bias that causes CO-poor comets to be observed more frequently. Fitzsimmons et al. (2019) and Xing et al. (2020) conclude that the extrasolar comet 2I/Borisov is in most ways -excluding its high CO/H 2 O ratio -similar to Solar System comets. Our results support the conclusion that the CO/H 2 O ice enhancement commonly occurs in the outer disk for solar-type stars, between 20 and 100 au. ...
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To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) -- have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H2O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H2O of unity, and one model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H2O ice. In light of this, a population of CO ice enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own Solar System and in extrasolar systems, than previously expected.
... The properties of the 2I dust substance (e.g. Xing et al. 2020) were compared with the corresponding quantities of comet 67P/Churyumov−Gerasimenko, which was studied at great length during the implementation of the Rosetta space mission (ESA). It should be noted that the 2I colour index E(R -I) = 0.49 is very close to that of 67P (0.47), but their E(V -R) colour indices differ, taking values of 0.47 and 0.53, respectively (Lamy & Toth 2009;Jewitt & Luu 2019). ...
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We present results from multiband photometry of the interstellar comet 2I/Borisov (C/2019 Q4). The observations were carried out in the two months before its perihelion passage. The UBVri photometric data obtained for comet 2I were converted to its reflectance by means of observations of neighbouring solar analogues, which allowed the reflectance of the comet with wavelength to be calculated. The registered prominent changes in the reflectance spectra of 2I while it was approaching the Sun from 2.40 to 2.01 au, and numerical simulations of the light scattering by aggregate particles provide insight into the chemical-mineralogical and structural properties of the cometary dust particles. The close agreement between the trends observed in the spectral characteristics of 2I and some models suggests that, during the observations, conglomerates of magnesium-ferrous (and, probably, organic) submicron particles prevailed in the scattering by its matter, H2O ice particles induced no noticeable spectral signals, and the gas contribution to the scattering by the coma was relatively small.
... With its high sensitivity, CUBES could also allow us to measure the D/H ratio for the first time in an interstellar object like 2I/Borisov, which would provide crucial information about the planetary formation process in its origin system. While photodissociation products of water were detected in the coma of 2I/Borisov [25][26][27], it was not active or bright enough for measurements of the D/H ratio with current facilities. ...
Article
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The proposed CUBES spectrograph for ESO’s Very Large Telescope will be an exceptionally powerful instrument for the study of comets. The gas coma of a comet contains a large number of emission features in the near-UV range covered by CUBES (305-400 nm), which are diagnostic of the composition of the ices in its nucleus and the chemistry in the coma. Production rates and relative ratios between different species reveal how much ice is present and inform models of the conditions in the early solar system. In particular, CUBES will lead to advances in detection of water from very faint comets, revealing how much ice may be hidden in the main asteroid belt, and in measuring isotopic and molecular composition ratios in a much wider range of comets than currently possible, provide constraints on their formation temperatures. CUBES will also be sensitive to emissions from gaseous metals (e.g., FeI and NiI), which have recently been identified in comets and offer an entirely new area of investigation to understand these enigmatic objects.
... In stark contrast to 'Oumuamua, the interstellar comet Borisov displayed physical characteristics that are broadly consistent with the census of cometary bodies in the Solar System. It had dusty cometary activity (Jewitt & Luu 2019;Bolin et al. 2020b;Fitzsimmons et al. 2019;Ye et al. 2020;McKay et al. 2020;Guzik et al. 2020;Hui et al. 2020;Kim et al. 2020;Cremonese et al. 2020;Yang et al. 2021), and early observations confirmed the presence of carbon and nitrogen based species in the outflow (Opitom et al. 2019;Kareta et al. 2020;Lin et al. 2020;Bannister et al. 2020;Xing et al. 2020;Aravind et al. 2021). It was particularly enriched in CO (Bodewits et al. 2020;Cordiner et al. 2020), which could be explained if it formed in the outer regions of a protoplanetary disk (Price et al. 2021). ...
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The recently discovered population of interstellar objects presents us with the opportunity to characterize material from extrasolar planetary and stellar systems up close. The forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) will provide an unprecedented increase in sensitivity to these objects compared to the capabilities of currently operational observational facilities. In this paper, we generate a synthetic population of interstellar objects drawn from their galactic kinematics, and identify the distribution of impact parameters, eccentricities, hyperbolic velocities and sky locations of objects detectable with the LSST. This population is characterized by a clustering of trajectories in the direction of the solar apex and anti-apex, centered at orbital inclinations of $\sim90^\circ$. We identify the ecliptic or solar apex as the optimal sky locations to search for future interstellar objects as a function of survey limiting magnitude. Moreover, we identify the trajectories of detectable objects that will be reachable for $\textit{in situ}$ rendezvous with a dedicated mission with the capabilities of the forthcoming $\textit{Comet Interceptor}$ or proposed $\textit{BRIDGE}$ concept. By scaling our fractional population statistics with the inferred spatial number density, we estimate that the LSST will detect of order $\sim50$ interstellar objects over the course of its $\sim10$ year observational campaign. Furthermore, we find that there should be of order $\sim10$ and $\sim0.05$ reachable targets for missions with propulsion capabilities comparable to $\textit{BRIDGE}$ and $\textit{Comet Interceptor}$, respectively. These number estimates will be readily updateable when the number density and size frequency distribution of interstellar objects is better constrained.
... Almost two years later on 2019 August 30 a second ISO, 2I/Borisov was discovered by Gennadiy Borisov using his MARGO observatory in Crimea. Unlike 1I/'Oumuamua, which remains somewhat of an enigma, 2I/Borisov was found to be a comet (Jewitt & Luu, 2019) similar to ones found in our solar system with an associated coma and tail as it approached and rounded the sun de León et al., 2019;Fitzsimmons et al., 2019;Guzik et al., 2020;Jewitt et al., 2020a;Jewitt & Luu, 2019;Manzini et al., 2020;Opitom et al., 2019;Xing et al., 2020;Ye et al., 2020). Its apparent fragmentation around 2020 February/March (perihelion was on 2019 December 8) was later established to be an ejection of material, around 0.1% of the total mass (Jewitt et al., 2020b). ...
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On 2019 August 30, a second interstellar object 2I/Borisov was discovered 2 years after the discovery of the first known interstellar object, 1I/’Oumuamua. Can we send a spacecraft to this object, using existing and near-term technologies? In this paper we assess the technical feasibility of a near-term mission to 2I/Borisov. We apply the Optimum Interplanetary Trajectory Software (OITS) tool to generate trajectories to 2I/Borisov. As results, we get the minimal ΔV trajectory with a launch date in 2018 July. For this trajectory, a Falcon Heavy launcher could have hauled an 8 ton spacecraft to 2I/Borisov. For a later launch date, results for a combined powered Jupiter flyby with a Solar Oberth maneuver are presented. For a launch in 2027, we could reach 2I/Borisov in 2052, using the Space Launch System (SLS), up-scaled Parker probe heat shield technology, and solid propulsion engines. Using a SLS a spacecraft with a mass of 765 kg could be sent to 2I/Borisov. A Falcon Heavy could deliver 202 kg to 2I/Borisov. Arrival times sooner than 2052 can potentially be achieved but with higher ΔV requirements and lower spacecraft payload masses. 2I/Borisov’s discovery shortly after the discovery of 1I/’Oumuamua implies that the next interstellar object might be discovered in the near future. The feasibility of a mission to both, 1I/’Oumuamua and 2I/Borisov using scaled versions of existing technologies indicates that missions to at least some future interstellar objects are feasible as well.
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To date, at least three comets—2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd)—have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H 2 O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H 2 O of unity, and one model reaches a CO/H 2 O ratio >10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H 2 O ice. In light of this, a population of CO-ice-enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own solar system and in extrasolar systems, than previously expected.
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Exocomets are small bodies releasing gas and dust which orbit stars other than the Sun. Their existence was first inferred from the detection of variable absorption features in stellar spectra in the late 1980s using spectroscopy. More recently, they have been detected through photometric transits from space, and through far-IR/mm gas emission within debris disks. As (exo)comets are considered to contain the most pristine material accessible in stellar systems, they hold the potential to give us information about early stage formation and evolution conditions of extra Solar Systems. In the Solar System, comets carry the physical and chemical memory of the protoplanetary disk environment where they formed, providing relevant information on processes in the primordial solar nebula. The aim of this paper is to compare essential compositional properties between Solar System comets and exocomets. The paper aims to highlight commonalities and to discuss differences which may aid the communication between the involved research communities and perhaps also avoid misconceptions. Exocomets likely vary in their composition depending on their formation environment like Solar System comets do, and since exocomets are not resolved spatially, they pose a challenge when comparing them to high fidelity observations of Solar System comets. Observations of gas around main sequence stars, spectroscopic observations of "polluted" white dwarf atmospheres and spectroscopic observations of transiting exocomets suggest that exocomets may show compositional similarities with Solar System comets. The recent interstellar visitor 2I/Borisov showed gas, dust and nuclear properties similar to that of Solar System comets. This raises the tantalising prospect that observations of interstellar comets may help bridge the fields of exocomet and Solar System comets.
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The recently discovered population of interstellar objects presents us with the opportunity to characterize material from extrasolar planetary and stellar systems up close. The forthcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will provide an unprecedented increase in sensitivity to these objects compared to the capabilities of currently operational observational facilities. We generate a synthetic population of ‘Oumuamua-like objects drawn from their galactic kinematics and identify the distribution of impact parameters, eccentricities, hyperbolic velocities, and sky locations of objects detectable with the LSST, assuming no cometary activity. This population is characterized by a clustering of trajectories in the direction of the solar apex and antiapex, centered at orbital inclinations of ∼90°. We identify the ecliptic or solar apex as the optimal sky location to search for future interstellar objects as a function of survey limiting magnitude. Moreover, we identify the trajectories of detectable objects that will be reachable for in situ rendezvous with a dedicated mission with the capabilities of the forthcoming Comet Interceptor or proposed Bridge concept. By scaling our fractional population statistics with the inferred spatial number density, we estimate that the LSST will detect of order ∼15 interstellar objects over the course of its ∼10 yr observational campaign. Furthermore, we find that there should be ∼1–3 and ∼0.0007–0.001 reachable targets for missions with propulsion capabilities comparable to Bridge and Comet Interceptor, respectively. These numbers are lower limits and will be readily updateable when the number density and size–frequency distribution of interstellar objects are better constrained.
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Comet 2I/Borisov is the first interstellar comet observed in the solar system, providing a unique opportunity to understand the physical conditions that prevailed in a distant unknown planetary system. Observations of the comet show that the CO/H 2 O ratio is higher than that observed in solar system comets at a heliocentric distance r h < 2.5 au. We aim to study the gas-phase coma of comet 2I/Borisov using a multifluid chemical-hydrodynamical model. The gas-phase model includes a host of chemical reactions, with the neutrals, ions, and electrons treated as three separate fluids. Energy exchange between the three fluids due to elastic and inelastic scattering and radiative losses are also considered. Our model results show that in the region of the coma beyond ∼100 km of the nucleus, e ⁻ −CO inelastic collisions leading to vibrational excitation of CO causes a loss of energy from the electron fluid. We find a high abundance of CO ⁺ and HCO ⁺ ions, and we show how these two ions affect the creation/destruction rates of other ions such as H 2 O ⁺ , H 3 O ⁺ , N-bearing ions, and large organic ions. We find that the presence of CO leads to a higher abundance of large organic ions and neutrals such as CH 3 OH 2 + , CH 3 OCH 4 + , and CH 3 OCH 3 , as compared to a typical H 2 O-rich solar system comet. We conclude that the presence of a large amount of CO in the coma of comet 2I/Borisov, combined with a low production rate, affects the coma temperature profile and flux of major ionic species significantly.
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The aim of this paper is to report on the results of a multicolour observing campaign performed at the LBT on the interstellar comet 2I/Borisov during three epochs while target was approaching its perihelion: UBVRI images have been observed in October and December 2019, when the comet was at ∼2.3 and 2.0 au from the Sun, respectively. Comet Borisov presented a rather complex morphology, especially in the December observing epoch, when a bi-lobate structure on a global scale was visible, with a jet-like structure characterised by a disconnection event, probably due to the presence of an active area on the nucleus' surface combined with its rotational properties. U – B colour, measured for the first time for this comet, monotonically decreases as Borisov approaches perihelion, from 0.35 ± 0.05 in October 19th to −0.07 ± 0.05 in December 2nd; B – V colour shows a more chaotic behaviour, with values for October epochs quite similar to solar colours, and a higher value of 0.76 ± 0.04 in December 2nd, probably due to a combination of an increase of the B flux (CN emission) and of an increase in V flux (due to C2). V – R and R–I colours derived in both the October and December observing epochs depict a scenario of a coma slightly redder than the Sun, with a RI-reddening value of ∼11%/1000 Å and ∼5%/1000 Å, respectively. The R- Afρ at the reference aperture of r ∼10⁴ km from the optocentre slightly decreases from ∼100 cm (October) to ∼ 70 cm (December), showing that the Borisov's dust environment is quite similar to those of Short Period Comets of our Solar System. The application of a first-order photometric model, with the plausible scenario of grains with radius a = 100 μm and bulk density ρd = 1000 kg/m³ moving at vd in the range of 3–30 m/s, allows to derive a dust production rate Qd decrease from a range of 3–25 kg/s in October to a range of 2–18 kg/s in December. All these results confirm that the interstellar comet 2I/Borisov has a coma environment quite similar to that of the Short Period Comets (mostly Jupiter Family targets) orbiting in our inner Solar System.
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Context. The orientation of the spin axis of a comet is defined by the values of its equatorial obliquity and its cometocentric longitude of the Sun at perihelion. These parameters can be computed from the components of the nongravitational force caused by outgassing, if the cometary activity is well characterized. The trajectories of known interstellar bodies passing through the solar system show nongravitational accelerations. Aims. The spin-axis orientation of 1I/2017 U1 (`Oumuamua) remains to be determined; for 2I/Borisov, the already released results are mutually exclusive. In both cases, the values of the components of the nongravitational force are relatively well constrained. Here, we investigate --within the framework of the forced precession model of a nonspherical cometary nucleus-- the orientation of the spin axes of `Oumuamua and 2I/Borisov using public orbit determinations that consider outgassing. Methods. We applied a Monte Carlo simulation using the covariance matrix method together with Monte Carlo random search techniques to compute the distributions of equatorial obliquities and cometocentric longitudes of the Sun at perihelion of `Oumuamua and 2I/Borisov from the values of the nongravitational parameters. Results. We find that the equatorial obliquity of `Oumuamua could be about 93 degrees, if it has a very prolate (fusiform) shape, or close to 16 degrees, if it is very oblate (disk-like). Different orbit determinations of 2I/Borisov gave obliquity values of 59 degrees and 90 degrees. The distributions of cometocentric longitudes were in general multimodal. Conclusions. Our calculations suggest that the most probable spin-axis direction of `Oumuamua in equatorial coordinates is (280 degrees, +46 degrees) if very prolate or (312 degrees, -50 degrees) if very oblate. Our analysis favors a prolate shape. For the orbit determinations of 2I/Borisov used here, we find most probable poles pointing near (275 degrees, +65 degrees) and (231 degrees, +30 degrees), respectively. Although our analysis favors an oblate shape for 2I/Borisov, a prolate one cannot be ruled out.
Article
Aims. 2I/Borisov (hereafter 2I) is the first visibly active interstellar comet observed in the Solar System, allowing us for the first time to sample the composition of a building block from another system. We report on the monitoring of 2I with the Ultraviolet-Visual Echelle Spectrograph, the high-resolution optical spectrograph of the ESO Very Large Telescope at Paranal, for four months from November 15, 2019 to March 16, 2020. Our goal is to characterise the activity and composition of 2I with respect to Solar System comets. Methods. We collected high-resolution spectra at 12 different epochs from 2.1 au pre-perihelion to 2.6 au post-perihelion. Results. On December 24 and 26, 2019, close to perihelion, we detected several OH lines of the 309 nm (0–0) band and derived a water production rate of 2.2 ± 0.2 × 10 ²⁶ molecules s ⁻¹ . The three [OI] forbidden oxygen lines were detected at different epochs and we derived a green-to-red doublet intensity ratio (G/R) of 0.31 ± 0.05 close to perihelion. The NH 2 ortho and para lines from various bands were measured and allowed us to derive an ortho-to-para abundance ratio (OPR) of 3.21 ± 0.15, corresponding to an OPR and spin temperature of ammonia of 1.11 ± 0.08 and 31 −5 ⁺¹⁰ K, respectively. These values are consistent with the values usually measured for Solar System comets. Emission lines of the radicals NH (336 nm), CN (388 nm), CH (431 nm), and C 2 (517 nm) were also detected. Several FeI and NiI lines were identified and their intensities were measured to provide a ratio of log (NiI/FeI) = 0.21 ± 0.18, which is in agreement with the value recently found in Solar System comets. Conclusions. Our high spectral resolution observations of 2I/Borisov and the associated measurements of the NH 2 OPR and the Ni/Fe abundance ratio are remarkably similar to Solar System comets. Only the G/R ratio is unusually high, but it is consistent with the high abundance ratio of CO/H 2 O found by other investigators.
Article
We present high resolution, deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI: Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI: Jewitt) before its outburst and fragmentation of 2020 March, thus observing the comet in a relatively undisrupted state. We locate 1–2 arcsec long (2000–3000 km projected length) jet-like structures near the optocentre of 2I that appear to change position angles from epoch to epoch. With the assumption that the jet is located near the rotational pole supported by its stationary appearance on ∼10–100 h time frames in HST images, we determine that 2I’s pole points near α = 322 ± 10° and δ = 37 ± 10° (λ = 341° and β = 48°) and may be in a simple rotation state. Additionally, we find evidence for possible periodicity in the HST time-series light curve on the time-scale of ∼5.3 h with a small amplitude of ∼0.05 mag implying a lower limit on its b/a ratio of ∼1.5 unlike the large ∼2 mag light curve observed for 1I/‘Oumuamua. However, these small light-curve variations may not be the result of the rotation of 2I’s nucleus due to its dust-dominated light-scattering cross-section. Therefore, uniquely constraining the pre-Solar system encounter, pre-outburst rotation state of 2I may not be possible even with the resolution and sensitivity provided by HST observations.
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The paper presents an analysis of the actual change in brightness of comet 2I/Borisov. The calculations take into account total scattering cross-sections coming from ice, dust particles, and their combinations. It was assumed that dust particles are so-called silicate agglomerates which consist of single monomers. We analyze the actual outburst of the interstellar comet 2I/Borisov in terms of mass loss. This parameter (mass ejected) is one of the main factors determining the amplitude of the comet’s brightness jump. A numerical model was developed and used to analyze the loss of mass ejected during the cometary outburst 2I/Borisov. The calculations presented in this paper are related to the actual outburst of comet 2I/Borisov which was observed in March 2020 with a change in brightness Δm = −0.7 magnitude. The ratio of the mass of thrown dust to the mass of ejected ice, which determines the observed change in brightness, was also determined. In the case of water ice controlled sublimation, the ratio is between 9.51 and 8.16. However, in the case of the sublimation of carbon monoxide, this parameter takes values from 7.47 to 3.07. The values of these mass ratios are significantly related to the η(t1) parameter.
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Life-bearing meteors, asteroids, comets and frozen bodies of water which had been ejected from Mars or other planets via bolide impact may have caused the Cambrian Explosion of life on Earth 540 million years ago. Reviewed in support of this theory are historical and worldwide reports of blood, gore, flesh and a variety of organisms raining from clear skies on warm days along with freezing rains and ice and sometimes embedded in ice and which a 2008 report in the International Journal of Astrobiology linked to comets and celestial events. Numerous reports have documented, within meteors, fossilized organisms resembling fungi, algae, and diatoms. In 1880 specimens resembling fossilized crinoids, corals and sponges were identified within an assemblage of meteorites that had fallen to Earth and investigators speculated that evolution may have occurred in a similar fashion on other planets. Russian scientists have reported that mosquito larvae, the majority of seeds from a variety of plants, and fish eggs and embryos from crustaceans develop and reproduce normally after 7 to 13 months exposure to space outside the ISS and could travel to and from Earth and Mars and survive. Investigators have identified specimens on Mars that resemble stromatolites, bacterial mats, algae, fungi, and lichens, and fossils resembling tube worms, Ediacarans, Metazoans and other organisms including those with eyes and multiple legs. McKay speculated that evolution may have taken place more rapidly on Mars and experienced a "Cambrian Explosion" in advance of Earth. Eight hundred million years ago an armada of asteroids, comets and meteors more numerous and several times more powerful than the Chicxulub impact, invaded the inner solar system and struck the Earth-Moon system. It is highly probable Mars was also struck and massive amounts of life-bearing debris was cast into space. Genetic studies indicate the first metazoans appeared on Earth 750 to 800 million years ago soon after this impacting event. Given the relatively sudden "explosive" appearance of complex life with bones, brains, and modern eyes, as well as those that were bizarre and quickly became extinct, and given there are no antecedent intermediate forms and that previous life forms consisted of only 11 cell types prior to the Cambrian Explosion, the evidence, in total, supports the theory that life on other planets and Mars may have been transported to Earth 800 million years ago and contributed to the Cambrian Explosion.
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Comet 2I/Borisov is the first true interstellar comet discovered. Here we present results from observational programs at two Indian observatories, 2 m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle (HCT) and 1.2 m telescope at the Mount Abu Infrared Observatory (MIRO). Two epochs of imaging and spectroscopy were carried out at the HCT and three epochs of imaging at MIRO. We found CN to be the dominant molecular emission on both epochs, 31/11/2019 and 22/12/2019, at distances of rH = 2.013 and 2.031 AU respectively. The comet was inferred to be relatively depleted in Carbon bearing molecules on the basis of low C2 and C3 abundances. We find the production rate ratio, Q(C2)/Q(CN) = 0.54 ± 0.18, pre-perihelion and Q(C2)/Q(CN) = 0.34 ± 0.12 post-perihelion. This classifies the comet as being moderately depleted in carbon chain molecules. Using the results from spectroscopic observations, we believe the comet to have a chemically heterogeneous surface having variation in abundance of carbon chain molecules. From imaging observations we infer a dust-to-gas ratio similar to carbon chain depleted comets of the Solar system. We also compute the nucleus size to be in the range 0.18 ≤ r ≤ 3.1 Km. Our observations show that 2I/Borisov’s behaviour is analogous to that of the Solar system comets.
Article
Based on the occurrence rates implied by the discoveries of 1I/‘Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect ≥one interstellar object every year. We advocate for future measurements of the production rates of H 2 O, CO 2 , and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. By quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO 2 relative to H 2 O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q (CO)/ Q (H 2 O) < 0.2 and Q (CO)/ Q (H 2 O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.
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According to Chandra Survey Observatory Near-Asteroid Belt Comets, the solar wind's contact with the comet produces a variety of spectral characteristics. The study of X-ray spectra produced by charge exchange is presented here. The spectrum of a comet can reveal a lot about its composition. This study has concentrated on the elemental abundance in six different comets, including 17P/Holmes, C/1999T1, C/2013A1, 9p/Temple1, and 103p/Hartley2 (NEAT). Numerous aspects of the comet's dynamics allow it to behave in a unique manner as it gets closer to the Near-Asteroid Belt. These characteristics are being examined, and some studies are still ongoing. The computations allow us to observe, for instance, how the composition of a comet's upper atmosphere affects how much gas it produces. For several comet morphologies, both linear and nonlinear, bow shock, contact surface, and stagnation point are investigated in relation to gas production rate. Our results shed light on the complex interactions between cometary ions and the solar wind. An increase in gas production rate was shown to be significantly correlated with sharp drops in average molecular weight.
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Interstellar comets penetrating through the Solar System had been anticipated for decades1,2. The discovery of asteroidal-looking ‘Oumuamua3,4 was thus a huge surprise and a puzzle. Furthermore, the physical properties of the ‘first scout’ turned out to be impossible to reconcile with Solar System objects4,5,6, challenging our view of interstellar minor bodies7,8. Here, we report the identification and early characterization of a new interstellar object, which has an evidently cometary appearance. The body was discovered by Gennady Borisov on 30 August 2019 ut and subsequently identified as hyperbolic by our data mining code in publicly available astrometric data. The initial orbital solution implies a very high hyperbolic excess speed of ~32 km s⁻¹, consistent with ‘Oumuamua⁹ and theoretical predictions2,7. Images taken on 10 and 13 September 2019 ut with the William Herschel Telescope and Gemini North Telescope show an extended coma and a faint, broad tail. We measure a slightly reddish colour with a g′–r′ colour index of 0.66 ± 0.01 mag, compatible with Solar System comets. The observed morphology is also unremarkable and best explained by dust with a power-law size-distribution index of –3.7 ± 1.8 and a low ejection speed (44 ± 14 m s⁻¹ for β = 1 particles, where β is the ratio of the solar gravitational attraction to the solar radiation pressure). The nucleus is probably ~1 km in radius, again a common value among Solar System comets, and has a negligible chance of experiencing rotational disruption. Based on these early characteristics, and putting its hyperbolic orbit aside, 2I/Borisov appears indistinguishable from the native Solar System comets.
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The TRAPPIST-1 system is unique in that it has a chain of seven terrestrial Earth-like planets located close to or in its habitable zone. In this paper, we study the effect of potential cometary impacts on the TRAPPIST-1 planets and how they would affect the primordial atmospheres of these planets. We consider both atmospheric mass loss and volatile delivery with a view to assessing whether any sort of life has a chance to develop. We ran N-body simulations to investigate the orbital evolution of potential impacting comets, to determine which planets are more likely to be impacted and the distributions of impact velocities. We consider three scenarios that could potentially throw comets into the inner region (i.e within 0.1au where the seven planets are located) from an (as yet undetected) outer belt similar to the Kuiper belt or an Oort cloud: Planet scattering, the Kozai-Lidov mechanism and Galactic tides. For the different scenarios, we quantify, for each planet, how much atmospheric mass is lost and what mass of volatiles can be delivered over the age of the system depending on the mass scattered out of the outer belt. We find that the resulting high velocity impacts can easily destroy the primordial atmospheres of all seven planets, even if the mass scattered from the outer belt is as low as that of the Kuiper belt. However, we find that the atmospheres of the outermost planets f, g and h can also easily be replenished with cometary volatiles (e.g. $\sim$ an Earth ocean mass of water could be delivered). These scenarios would thus imply that the atmospheres of these outermost planets could be more massive than those of the innermost planets, and have volatiles-enriched composition.
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Nine recently discovered long-period comets were observed by the Solar Wind Anisotropies (SWAN) Lyman-alpha all-sky camera on board the Solar and Heliosphere Observatory (SOHO) satellite during the period of 2013 to 2016. These were C/2012 K1 (PanSTARRS), C/2013 US10 (Catalina), C/2013 V5 (Oukaimeden), C/2013 R1 (Lovejoy), C/2014 E2 (Jacques), C/2014 Q2 (Lovejoy), C/2015 G2 (MASTER), C/2014 Q1 (PanSTARRS) and C/2013 X1 (PanSTARRS). Of these 9 comets 6 were long-period comets and 3 were possibly dynamically new. Water production rates were calculated from each of the 885 images using our standard time-resolved model that accounts for the whole water photodissociation chain, exothermic velocities and collisional escape of H atoms. For most of these comets there were enough observations over a broad enough range of heliocentric distances to calculate power-law fits to the variation of production rate with heliocentric distances for pre- and post-perihelion portions of the orbits. Comet C/2014 Q1 (PanSTARRS), with a perihelion distance of only ~0.3 AU, showed the most unusual variation of water production rate with heliocentric distance and the resulting active area variation, indicating that when the comet was within 0.7 AU its activity was dominated by the continuous release of icy grains and chunks, greatly increasing the active sublimation area by more than a factor of 10 beyond what it had at larger heliocentric distances. A possible interpretation suggests that a large fraction of the comet's mass was lost during the apparition.
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Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4$\sigma$ detection and determine that the ring's sky-projected rotation axis matches that of the star. The CO mass derived ($0.65-42 \times10^{-7}$ M$_{\oplus}$) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO$_2$ mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that $(49\pm 27)$% of the detected flux originates from a region near the eccentric belt's pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.
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The Swift mission, scheduled for launch in 2004, is a multiwavelength observatory for gamma-ray burst (GRB) astronomy. It is a first-of-its-kind autonomous rapid-slewing satellite for transient astronomy and pioneers the way for future rapid-reaction and multiwavelength missions. It will be far more powerful than any previous GRB mission, observing more than 100 bursts yr-1 and performing detailed X-ray and UV/optical afterglow observations spanning timescales from 1 minute to several days after the burst. The objectives are to (1) determine the origin of GRBs, (2) classify GRBs and search for new types, (3) study the interaction of the ultrarelativistic outflows of GRBs with their surrounding medium, and (4) use GRBs to study the early universe out to z > 10. The mission is being developed by a NASA-led international collaboration. It will carry three instruments: a new-generation wide-field gamma-ray (15-150 keV) detector that will detect bursts, calculate 1'-4' positions, and trigger autonomous spacecraft slews; a narrow-field X-ray telescope that will give 5'' positions and perform spectroscopy in the 0.2-10 keV band; and a narrow-field UV/optical telescope that will operate in the 170-600 nm band and provide 03 positions and optical finding charts. Redshift determinations will be made for most bursts. In addition to the primary GRB science, the mission will perform a hard X-ray survey to a sensitivity of ~1 mcrab (~2 × 10-11 ergs cm-2 s-1 in the 15-150 keV band), more than an order of magnitude better than HEAO 1 A-4. A flexible data and operations system will allow rapid follow-up observations of all types of high-energy transients, with rapid data downlink and uplink available through the NASA TDRSS system. Swift transient data will be rapidly distributed to the astronomical community, and all interested observers are encouraged to participate in follow-up measurements. A Guest Investigator program for the mission will provide funding for community involvement. Innovations from the Swift program applicable to the future include (1) a large-area gamma-ray detector using the new CdZnTe detectors, (2) an autonomous rapid-slewing spacecraft, (3) a multiwavelength payload combining optical, X-ray, and gamma-ray instruments, (4) an observing program coordinated with other ground-based and space-based observatories, and (5) immediate multiwavelength data flow to the community. The mission is currently funded for 2 yr of operations, and the spacecraft will have a lifetime to orbital decay of ~8 yr.
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Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency's Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10(-10) to 10(-7) kilograms, and 48 grains of mass 10(-5) to 10(-2) kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails. Copyright © 2015, American Association for the Advancement of Science.
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We describe recent results on the CO/CO{sub 2}/H{sub 2}O composition of comets together with a survey of older literature (primarily for CO/H{sub 2}O) and compare these with models of the protoplanetary disk. Even with the currently small sample, there is a wide dispersion in abundance ratios and little if any systematic difference between Jupiter-family comets (JFCs) and long-period and Halley-type comets (LPCs and HTCs). We argue that the cometary observations require reactions on grain surfaces to convert CO to CO{sub 2} and also require formation of all types of comets in largely, but not entirely, overlapping regions, probably between the CO and CO{sub 2} snow lines. Any difference in the regions of formation is in the opposite direction from the classical picture with the JFCs having formed closer to the Sun than the LPCs. In the classical picture, the LPCs formed in the region of the giant planets and the JFCs formed in the Kuiper Belt. However, these data suggest, consistent with suggestions on dynamical grounds, that the JFCs and LPCs formed in largely overlapping regions where the giant planets are today and with JFCs on average forming slightly closer to the Sun than did the LPCs. Presumably at least the JFCs passed through the scattered disk on their way to their present dynamical family.
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Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.
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The SWAN (Solar Wind ANisotropies) Lyman-alpha all-sky camera on the SOHO spacecraft observed the hydrogen coma of comet C/1999 S4 (LINEAR) from the end of May through mid-August 2000. A systematic set of water-production rates was obtained for this well-documented event of complete fragmentation of a cometary nucleus. The observations indicate that the lower limit for the sunlit surface area of the nucleus was about 1 square kilometer before the fragmentation and that the amount of water released throughout the observing period was 3.3 × 109 kilograms. Evidence suggests that the activity of the comet was dominated by successive fragmentation. There were four major outbursts, occurring about every 16 days. The 21 July event led to the complete fragmentation and sublimation of what remained of the nucleus, producing the last 3 × 108kilograms of water. A model where the fragment size distribution follows the power law N(R) ∼R − 2.7, where Nand R are the number and radius of fragments, reproduces the observed dissipation. This distribution possibly reflects the internal structure of the nucleus.
Article
2I/Borisov is the first-ever observed interstellar comet (and the second detected interstellar object (ISO)). It was discovered on 2019 August 30 and has a heliocentric orbital eccentricity of ~3.35, corresponding to a hyperbolic orbit that is unbound to the Sun. Given that it is an ISO, it is of interest to compare its properties—such as composition and activity—with the comets in our solar system. This study reports low-resolution optical spectra of 2I/Borisov. The spectra were obtained by the MDM Observatory Hiltner 2.4 m telescope/Ohio State Multi-Object Spectrograph (on 2019 October 31.5 and November 4.5, UT). The wavelength coverage spanned from 3700 to 9200 Å. The dust continuum reflectance spectra of 2I/Borisov show that the spectral slope is steeper in the blue end of the spectrum (compared to the red). The spectra of 2I/Borisov clearly show CN emission at 3880 Å, as well as C2 emission at both 4750 and 5150 Å. Using a Haser model to covert the observed fluxes into estimates for the molecular production rates, we find Q(CN) = 2.4 ± 0.2 × 10²⁴ s⁻¹, and Q(C₂) = (5.5 ± 0.4) × 10²³ s⁻¹ at the heliocentric distance of 2.145 au. Our Q(CN) estimate is consistent with contemporaneous observations, and the Q(C₂) estimate is generally below the upper limits of previous studies. We derived the ratio Q(C₂)/Q(CN) = 0.2 ± 0.1, which indicates that 2I/Borisov is depleted in carbon-chain species, but is not empty. This feature is not rare for the comets in our solar system, especially in the class of Jupiter-family comets.
Article
Aims. Interstellar objects passing through our Solar System offer a rare opportunity to probe the physical and chemical processes involved in solid body and planet formation in extrasolar systems. The main objective of our study is to search for diagnostic absorption features of water ice in the near-infrared (NIR) spectrum of the second interstellar object 2I/2019 Q4 (Borisov) and compare its ice features to those of the Solar System icy objects. Methods. We observed 2I in the NIR on three separate occasions. The first observation was made on 2019 September 19 UT using the SpeX spectrograph at the 3m IRTF and again on September 24 UT with the GNIRS spectrograph at the 8m GEMINI telescope; the last observation was made on October 09 UT with IRTF. Results. The spectra obtained from all three nights appear featureless. No absorption features associated with water ice are detected. Spectral modeling suggests that water grains, if present, comprise no more than 10% of the coma cross section. The comet consistently exhibits a red D-type like spectrum with a spectral slope of about 6% per 1000 Å, which is similar to that of 1I/’Oumuamua and is comparable to Solar System comets.
Article
Aims. The discovery of the first active interstellar object 2I/Borisov provides an unprecedented opportunity to study planetary formation processes in another planetary system. In particular, spectroscopic observations of 2I allow us to constrain the composition of its nuclear ices. Methods. We obtained optical spectra of 2I with the 4.2 m William Herschel and 2.5 m Isaac Newton telescopes between 2019 September 30 and October 13, when the comet was between 2.5 au and 2.4 au from the Sun. We also imaged the comet with broadband filters on 15 nights from September 11 to October 17, as well as with a CN narrow-band filter on October 18 and 20, with the TRAPPIST-North telescope. Results. Broadband imaging confirms that the dust coma colours (B-V=0.82�0.02, V-R=0.46�0.03, R-I=0.44�0.03, B-R=1.28�0.03) are the same as for Solar System comets. We detect CN emission in all spectra and in the TRAPPIST narrow-band images with production rates between 1:6 � 1024 and 2:1 � 1024 molec/s. No other species are detected. We determine three-sigma upper limits for C2, C3, and OH production rates of 6 � 1023 molec/s, 2 � 1023 molec/s and 2 � 1027 molec/s, respectively, on October 02. There is no significant increase of the CN production rate or A(0)f� during our observing period. Finally, we place a three-sigma upper limit on the Q(C2)/Q(CN) ratio of 0.3 (on October 13). From this, we conclude that 2I is highly depleted in C2, and may have a composition similar to Solar System carbon-chain depleted comets. Key words. comets: individual (2I/Borisov)— comets: general
Article
The radio science experiment RSI on-board Rosetta determined the mass of the nucleus of comet 67P/Churyumov–Gerasimenko at the start of the prime mission from 2014 August to November (GM = 666.2 ± 0.2 m³ s⁻² or 9982 ± 3 × 10¹² kg) and shortly before the end of the mission from 2016 July to September (GM = 665.5 ± 0.1 m³ s⁻² or 9971.5 ± 1.5 × 10¹² kg). The mass-loss is ΔM = 10.5 ± 3.4·10⁹ kg, about 0.1 per cent of the nucleus mass. Almost 50 per cent of the mass-loss occurred during the 32 d before and 62 d after perihelion. The nucleus mass combined with the new very precise nucleus volume of 18.56 ± 0.02 km³ yields a bulk density of 537.8 ± 0.6 kg m⁻³. This low bulk density suggests that the nucleus is highly porous. The porosity is constrained by the observed bulk density, the density of ices, mostly water ice, and the density of compacted nucleus dust material. For a range of compacted dust material density from 2000 to 3500 kg m⁻³, the porosity varies between 65–79 per cent when the dust-to-ice mass ratio Fnucleus for the nucleus body lies in the range 3 ≤ Fnucleus ≤ 7. The nucleus is thus a highly porous very dusty body with very little ice. The total mass-loss ΔM puts hard constraints on the models of interpretation of the observations from other instruments on Rosetta. The loss from gas, based on ROSINA and MIRO observations, suggests that Fcoma, the dust-to-gas mass ratio of cometary matter in the coma beyond the Hill sphere of the nucleus is of the order of Fcoma ≤ 0.5 and therefore at least six times, and perhaps as much as 14 times, smaller than Fnucleus. It seems that the lost gas mass was overestimated by the instruments. The lost gas mass shall be 5.25 × 10⁹ kg for Fcoma = 1. For any other gas mass values > 5.25 × 10⁹ kg Fcoma is < 1. Most of the lifted dust mass will not leave the cometary Hill sphere but falls back to the nucleus. For 3 ≤ Fnucleus ≤ 7 and Fcoma = 0.5, the fallback mass is between 1.8·ΔM and 4.3·ΔM.
Article
The Solar Wind Anisotropies (SWAN) instrument on the SOlar and Heliospheric Observatory (SOHO) satellite has observed 44 long period and new Oort cloud comets and 36 apparitions of 17 short period comets since its launch in December 1995. Water production rates have been determined from the over 3700 images producing a consistent set of activity variations over large parts of each comet's orbit. This has enabled the calculation of exponential power-law variations with heliocentric distance of these comets both before and after perihelion, as well as the absolute values of the water production rates. These various measures of overall water activity including pre- and post-perihelion exponents, absolute water production rates at 1 AU, active surface areas and their variations have been compared with a number of dynamical quantities for each comet including dynamical class, original semi-major axis, nucleus radius (when available), and compositional taxonomic class. Evidence for evolution of cometary nuclei is seen in both long-period and short-period comets.
Article
We present Infrared Telescope Facility/SpeX and NEOWISE observations of the dynamically new comet C/2013 US10 (Catalina), hereafter US10, from 5.8 au inbound, to near perihelion at 1.3 au, and back to 5.0 au outbound. We detect water ice in the coma of US10, assess and monitor the physical properties of the ice as insolation varies with heliocentric distance, and investigate the relationship between water ice and CO2. This set of measurements is unique in orbital coverage and can be used to infer both the physical evolution of the ice, and, potentially, the nucleus composition. We report (1) nearly identical near-infrared spectroscopic measurements of the coma at -5.8 au, -5.0 au, +3.9 au (where <0 au indicates pre-perihelion epochs), all presenting evidence of water-ice grains, (2) a dust-dominated coma at 1.3 and 2.3 au and, (3) an increasing CO2/Afρ ratio from -4.9 to 1.8 au. We propose that sublimation of the hyper-volatile CO2 is responsible for dragging water-ice grains into the coma throughout the orbit. Once in the coma, the observability of the water-ice grains is controlled by the ice grain sublimation lifetime, which seems to require some small dust contaminant (i.e., non-pure ice grains). At |Rh| ≥ 3.9, the ice grains are long-lived and may be unchanged since leaving the comet nucleus. We find that the nucleus of comet US10 is made of, among other components, ∼1 μm water-ice grains containing up to 1% refractory materials. © 2018. The American Astronomical Society. All rights reserved..
Article
We collect observational evidence that supports the scheme of mass transfer on the nucleus of comet 67P/Churyumov-Gerasimenko. The obliquity of the rotation axis of 67P causes strong seasonal variations. During perihelion the southern hemisphere is four times more active than the north. Northern territories are widely covered by granular material that indicates back fall originating from the active south. Decimetre sized chunks contain water ice and their trajectories are influenced by an anti-solar force instigated by sublimation. OSIRIS observations suggest that up to 20 % of the particles directly return to the nucleus surface taking several hours of travel time. The back fall covered northern areas are active if illuminated but produce mainly water vapour. The decimetre chunks from the nucleus surface are too small to contain more volatile compounds such as CO 2 or CO. This causes a north-south dichotomy of the composition measurements in the coma. Active particles are trapped in the gravitational minimum of Hapi during northern winter. They are "shock frozen" and only reactivated when the comet approaches the sun after its aphelion passage. The insolation of the big cavity is enhanced by self-heating, i. e. reflection and IR radiation from the walls. This, together with the pristinity of the active back fall, explains the early observed activity of the Hapi region. Sobek may be a role model for the consolidated bottom of Hapi. Mass transfer in the case of 67P strongly influences the evolution of the nucleus and the interpretation of coma measurements.
Article
The Wide Angle Camera of the OSIRIS instrument on board the Rosetta spacecraft is equipped with several narrowband filters that are centered on the emission lines and bands of various fragment species. These are used to determine the evolution of the production and spatial distribution of the gas in the inner coma of comet 67P with time and heliocentric distance, here between 2.6 - 1.3 AU pre-perihelion. Our observations indicate that the emission observed in the OH, OI, CN, NH, and NH2 filters is mostly produced by dissociative electron impact excitation of different parent species. We conclude that CO2 rather than H2O is a significant source of the [OI] 630 nm emission. A strong plume-like feature observed in the in CN and [OI] filters is present throughout our observations. This plume is not present in OH emission and indicates a local enhancement of the CO2/H2O ratio by as much as a factor of 3. We observed a sudden decrease in intensity levels after March 2015, which we attribute to decreased electron temperatures in the first kilometers above the nucleus surface.
Article
This paper presents a self-consistent model for the evolution of gas produced in the debris disc of $\beta$ Pictoris. Our model proposes that atomic carbon and oxygen are created from the photodissociation of CO, which is itself released from volatile-rich bodies in the debris disc due to grain-grain collisions or photodesorption. While the CO lasts less than one orbit, the atomic gas evolves by viscous spreading resulting in an accretion disc inside the parent belt and a decretion disc outside. The temperature, ionisation fraction and population levels of carbon and oxygen are followed with the photodissociation region model Cloudy, which is coupled to a dynamical viscous $\alpha$ model. We present new gas observations of $\beta$ Pic, of C I observed with APEX and O I observed with Herschel, and show that these along with published C II and CO observations can all be explained with this new model. Our model requires a viscosity $\alpha$ > 0.1, similar to that found in sufficiently ionised discs of other astronomical objects; we propose that the magnetorotational instability is at play in this highly ionised and dilute medium. This new model can be tested from its predictions for high resolution ALMA observations of C I. We also constrain the water content of the planetesimals in $\beta$ Pic. The scenario proposed here might be at play in all debris discs and this model could be used more generally on all discs with C, O or CO detections.
Article
A systematic analysis of the mixing ratios with respect to H2O for eight species (CH3OH, HCN, NH3, H2CO, C2H2, C2H6, CH4, and CO) measured with high-resolution infrared spectroscopy in thirty comets between 1997 and 2013 is presented. Some trends are beginning to emerge when mixing ratios in individual comets are compared to average mixing ratios obtained for all species within the population. The variation in mixing ratios for all measured species is at least an order of magnitude. Overall, Jupiter-family comets are depleted in volatile species with respect to H2O compared to long-period Oort cloud comets, with the most volatile species showing the greatest relative depletion. There is a high positive correlation between the mixing ratios of HCN, C2H6, and CH4, whereas NH3, H2CO, and C2H2 are moderately correlated with each other but generally uncorrelated or show only weak correlation with other species. CO is generally uncorrelated with the other measured species possibly because it has the highest volatility and is therefore more susceptible to thermal evolutionary effects. Most of these correlations appear to be independent of dynamical class with a few possible exceptions. Molecular mixing ratios for CH3OH, HCN, C2H6, and CH4 show an expected behavior with heliocentric distance suggesting a dominant ice source, whereas there is emerging evidence that the mixing ratios of NH3, H2CO, C2H2, NH2, and CN may increase at small heliocentric distances, suggesting the possibility of additional sources related to the thermal decomposition of organic dust. Although this provides information on the composition of the most volatile grains in comets, it presents an additional difficulty in classifying comet chemistry because most comets within this dataset were only observed over a limited range of heliocentric distance. Although there is remarkable compositional diversity resulting in a unique chemical fingerprint for each comet, a hierarchical tree cluster analysis is used to determine a taxonomic classification system containing four groups and eleven subgroups. Optical and infrared comparisons indicate that mixing ratios of daughter species and potential parents from cometary ices are sometimes but not always consistent with one another. This suggests that in many comets there are significant sources of C2 and/or CN from grains, and that the importance of these sources is variable within the comet population.
Article
Unlabelled: Thousands of exoplanets are known to orbit nearby stars. Plans for the next generation of space-based and ground-based telescopes are fueling the anticipation that a precious few habitable planets can be identified in the coming decade. Even more highly anticipated is the chance to find signs of life on these habitable planets by way of biosignature gases. But which gases should we search for? Although a few biosignature gases are prominent in Earth's atmospheric spectrum (O2, CH4, N2O), others have been considered as being produced at or able to accumulate to higher levels on exo-Earths (e.g., dimethyl sulfide and CH3Cl). Life on Earth produces thousands of different gases (although most in very small quantities). Some might be produced and/or accumulate in an exo-Earth atmosphere to high levels, depending on the exo-Earth ecology and surface and atmospheric chemistry. To maximize our chances of recognizing biosignature gases, we promote the concept that all stable and potentially volatile molecules should initially be considered as viable biosignature gases. We present a new approach to the subject of biosignature gases by systematically constructing lists of volatile molecules in different categories. An exhaustive list up to six non-H atoms is presented, totaling about 14,000 molecules. About 2500 of these are CNOPSH compounds. An approach for extending the list to larger molecules is described. We further show that about one-fourth of CNOPSH molecules (again, up to N = 6 non-H atoms) are known to be produced by life on Earth. The list can be used to study classes of chemicals that might be potential biosignature gases, considering their accumulation and possible false positives on exoplanets with atmospheres and surface environments different from Earth's. The list can also be used for terrestrial biochemistry applications, some examples of which are provided. We provide an online community usage database to serve as a registry for volatile molecules including biogenic compounds. Key words: Astrobiology-Atmospheric gases-Biosignatures-Exoplanets. Astrobiology 16, 465-485.
Article
We used the UltraViolet-Optical Telescope on board Swift to systematically follow the dynamically new comet C/2013 A1 (Siding Spring) on its approach to the Sun. The comet was observed from a heliocentric distance of 4.5 AU pre-perihelion to its perihelion at 1.4 AU. From our observations, we estimate that the water production rate during closest approach to Mars was 1.5 +/- 0.3 x 1E28 molecules/s, that peak gas delivery rates were between 4.5-8.8 kg/s, and that in total between 3.1-5.4 x 1E4 kg cometary gas was delivered to the planet. Seasonal and evolutionary effects on the nucleus govern the pre-perihelion activity of comet Siding Spring. The sudden increase of its water production between 2.46-2.06 AU suggests the onset of the sublimation of icy grains in the coma, likely driven by CO2. As the comet got closer to the Sun, the relative contribution of the nucleus' water production increased, while CO2 production rates decreased. The changes in the comet's activity can be explained by a depletion of CO2, but the comet's high mass loss rate suggests they may also reflect primordial heterogeneities in the nucleus.
Article
We used the UltraViolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km$^2$, estimated to be more than 20 percent of the surface. Based on our measurements, the comet lost $4x10^{11}$ kg of ice and dust during this apparition, corresponding to at most a few meters of its surface.Even though this was likely not Garradd's first passage through the inner solar system, the activity of the comet was complex and changed significantly during the time it was observed.
Article
A summary is presented of our spectroscopic survey of comets extending for roughly 19 years from 1985 to 2004 comprising data for 92 comets of which 50 showed good emissions. All data were re-analyzed using consistent reduction techniques. Our observations of comets over several apparitions and comets observed over an extended period indicate no major changes in compositional classification. To our regret, no major unidentified cometary features were found in our surveyed spectral region of 5200–10400 Å. Absolute production rates for the dominant parent molecule H2O and the daughter species C2, NH2 and CN are determined within the limits of the Haser model as are values for the dust continuum, Afρ. From these data, production rate ratios are calculated for C2/H2O, NH2/H2O, CN/H2O and Afρ/H2O. Excluding the odd Comets Yanaka (1988r), 43P/Wolf–Harrington and 19P/Borrelly, with unusual spectra, our set of comets exhibited relatively uniform composition. Detailed analyses of our data resulted in four taxonomic classes:
Article
We used a carefully selected set of observations of five comets along with a vectorial model and the Haser model to explore the lifetimes against photodissociation of H2O and OH in cometary comae. The photometric and long-slit CCD data described in this paper were obtained specifically for this project, while the IUE data are archival. The underlying parameter space was examined and our knowledge of such factors as the outflow and ejection velocities is discussed. We conclude that it is difficult to constrain the lifetime of OH but that the H2O lifetimes must be dependent on changes in the solar UV flux. Our data are consistent with the expectation that as the solar activity, and thus UV flux, increases, the H2O lifetimes decrease. From our data, we show that the combined theoretical lifetimes for H2O and OH do not produce good fits to the data; thus, at least one set of theoretical lifetimes must be in error. Our data require shorter H2O lifetimes and/or shorter OH lifetimes. We present some useful relations for determining which lifetimes and velocities to use for future applications. Additionally, we have derived improved lifetimes and scale lengths for use computing production rates with simple models. The new parent lifetimes and scale lengths are about 15-45% shorter than commonly used values which ignore effects due to solar activity. Finally, we discuss what data types are needed in the future to enable better modeling of cometary data.
Article
The Swift MIDEX mission is the first-of-its-kind observatory for multi-wavelength transient astronomy. The goal of the mission is to ascertain the origin of gamma-ray bursts and to utilize these bursts to probe the early universe. The Ultra- Violet/Optical Telescope (UVOT) is one of three telescopes flying aboard Swift. The UVOT is a working 'copy' of the Optical Monitor on the X-ray Multi-mirror Mission (XMM- Newton). It is a Ritchey-Chretien telescope with microchannel plate intensified charged-coupled devices (MICs) that provide sub-arcsecond imaging. These MICs are photon counting devices, capable of detecting very low signal levels. When flown above the atmosphere, the UVOT will have the equivalent sensitivity of a 4 m telescope on the ground, reaching a limiting magnitude of 24 for a 1000 second observation in the white light filter. A rotating filter wheel contains sensitive photometric broadband UV and visual filters for determining photometric redshifts. The filter wheel also contains UV and visual grisms for performing low-resolution spectroscopy.
Article
An absolute flux calibrated reference spectrum of the Sun covering the 0.12-2.5 μm wavelength range is presented. The ultraviolet and optical spectrum is based on absolute flux measurements from satellites and from the ground. The near-infrared spectrum is based on measurements using the NASA CV-990 aircraft and on a model spectrum. The synthetic optical and near-infrared magnitudes of the absolute calibrated solar reference spectrum agree with published values to 0.01-0.03 mag, i.e., within the uncertainties of the measurements. The absolute flux of the reference spectrum over the optical and near-infrared 0.4-2.5 μm range is known with an uncertainty of 5%, or better. In the blue and ultraviolet, especially for wavelengths in the 0.12-0.2 μm interval, the uncertainty increases up to about 20% due to the variability of the solar energy output at these wavelengths. The absolute flux spectrum of the Sun presented here will help to establish the absolute calibration of NICMOS, the HST near-infrared camera and multi-object spectrograph.
Article
Narrowband filter photometry of Comet 96P/Machholz 1 was obtained at Lowell Observatory during the comet's 2007 apparition. Production rates of OH, CN, C 2 , C 3 , and NH were derived from these data sets, and the quantity A()f — a proxy measure of the dust production — was also calculated. Relative abundances, expressed as ratios of production rates with respect to OH (a measure of the water abundance), were compared to those measured in other comets. Comet Machholz 1 is shown to be depleted in CN by about a factor of 72 from average, while C 2 and C 3 are also low but "only" by factors of 8 and 19 respectively from "typical" composition (based on an update to the classifications by A'Hearn et al., 1995, Icarus 118, 223). In contrast, NH is near the mid-to-upper end of its normal range. This extremely low CN-to-OH ratio for Machholz 1 indicates that it is either compositionally associated with Comet Yanaka (1988r; 1988 Y1) which was strongly depleted in CN and C 2 but not NH 2 (Fink, 1992, Science 257, 1926), or represents a new compositional class of comets, since Yanaka had a much greater depletion of C 2 (>100) than does Machholz 1 (8). Evidence is strongly suggestive that the extremely anomalous compositions of these two comets is primordial rather than from recent thermal processing. It remains unclear if these comets formed at a location in our solar system with unusual conditions and a low probability of being gravitationally perturbed into the inner solar system, or if one or both objects are interstellar interlopers.
Article
We have examined the effects of vaporization from the nucleus of a comet and show that a latitude dependence of vaporization can, in some cases, explain asymmetries in cometary light curves. We also find that a non-uniform distribution of solar radiation over a comet can considerably shorten the vaporization lifetime compared to the results normally obtained by assuming that the nuclear surface is isothermal. Independent of any latitude effects, comets with CO2-dominated nuclei and with perihelion distances less than 0.5 AU have vaporization lifetimes less than or comparable to their dynamical ejection times. This may explain the observed deficit of comets with small perihelion distances. Similarly comets with CO2-dominated nuclei and perihelia near Jupiter's orbit have vaporization lifetimes that are shorter than the time for capture into short-period orbits. We suggest, therefore, that at least some new comets are composed in large part of CO2, while only H2O-dominated comets, with lower vaporization rates, can survive to be captured into short-period orbits.
Article
We present the results of narrowband photometry of 85 comets observed over a period of 17 years. The data have been reduced homogeneously to molecular production rates and a proxy for the dust production rate. We confirm previous investigations, both our own and those of others, showing that there is no differentiation with depth in the cometary nucleus, that most comets are very similar to each other in chemical composition, and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is little variation of relative abundances with heliocentric distance, implying that for the species we observe the role of density-dependent processes in the coma is small. There is also little variation from one apparition to the next for most short-period comets.
Article
Oort cloud comets are currently believed to have formed in the Sun's protoplanetary disk and to have been ejected to large heliocentric orbits by the giant planets. Detailed models of this process fail to reproduce all of the available observational constraints, however. In particular, the Oort cloud appears to be substantially more populous than the models predict. Here we present numerical simulations that show that the Sun captured comets from other stars while it was in its birth cluster. Our results imply that a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary disks of other stars.
Article
Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.
Article
A new solar spectral atlas, improved oscillator strengths, and additional transitions have been incorporated in calculations of the fluorescent equilibrium of the OH molecule in comets as a function of heliocentric distance and radial velocity. Comparisons of detailed, ultraviolet band profiles have been made with the observed, high-resolution spectra of eight comets. While a purely fluorescent equilibrium calculation is sufficient to reproduce the major variations seen in the band structure from one comet to another, some collisional quenching of the inversion of the X2II3/2 Lambda-doublet ground state is required to match the detailed features. The relative population inversion of the upper and lower levels of the ground state is also calculated as a function of both distance and velocity, with the results being consistent with 18 cm radio observations. Predictions of the infrared OH vibrational bands are made; the 1-0 band should be observable if a sufficiently high spectral resolution is employed.
Article
We present the photometric calibration of the Swift UltraViolet/Optical Telescope (UVOT) which includes: optimum photometric and background apertures, effective area curves, colour transformations, conversion factors for count rates to flux, and the photometric zero points (which are accurate to better than 4 per cent) for each of the seven UVOT broadband filters. The calibration was performed with observations of standard stars and standard star fields that represent a wide range of spectral star types. The calibration results include the position dependent uniformity, and instrument response over the 1600-8000A operational range. Because the UVOT is a photon counting instrument, we also discuss the effect of coincidence loss on the calibration results. We provide practical guidelines for using the calibration in UVOT data analysis. The results presented here supersede previous calibration results. Comment: Minor improvements after referees report. Accepted for publication in MNRAS
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