Publications (45)72.29 Total impact
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Article: VLT-CRIRES Survey of Rovibrational CO Emission from Protoplanetary Disks
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ABSTRACT: We present a large, comprehensive survey of rovibrational CO line emission at 4.7 micron from 69 protoplanetary disks, obtained with CRIRES on the ESO Very Large Telescope at the highest available spectral resolving power (R=95,000, v=3.2 km/s). The CO fundamental band (Delta v=1) is a well-known tracer of warm gas in the inner, planet-forming regions of gas-rich disks around young stars, with the lines formed in the super-heated surfaces of the disks at radii of 0.1-10 AU. Our high spectral resolution data provide new insight into the kinematics of the inner disk gas. Pure double-peaked Keplerian profiles are surprisingly uncommon, beyond the frequency expected based on disk inclination. The majority of the profiles are consistent with emission from a disk plus a slow (few km/s) molecular disk wind. This is evidenced by analysis of different categories as well as an overall tendency for line profiles to have excess blue emission. Weak emission lines from isotopologues and vibrationally excited levels are readily detected. In general, 13CO lines trace cooler gas than the bulk 12CO emission and may arise from further out in the disk, as indicated by narrower line profiles. A high fraction of the sources show vibrationally excited emission (~50%) which is correlated with accretion luminosity, consistent with ultra-violet (UV) fluorescent excitation. Disks around early-type Herbig AeBe stars have narrower lines, on average, than their lower-mass late-type counterparts, due to their increased luminosity. Evolutionary changes in CO are also seen. Removal of the protostellar envelope between class I and II results in the disappearance of the strong absorption lines and CO ice feature characteristic of class I spectra. However, CO emission from class I and II objects is similar in detection frequency, excitation and line shape, indicating that inner disk characteristics are established early.04/2013; -
Article: The HCN-Water Ratio in the Planet Formation Region of Disks
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ABSTRACT: We find a trend between the mid-infrared HCN/H2O flux ratio and submillimeter disk mass among T Tauri stars in Taurus. While it may seem puzzling that the molecular emission properties of the inner disk (< few AU) are related to the properties of the outer disk (beyond ~20 AU) probed by the submillimeter continuum, an interesting possible interpretation is that the trend is a result of planetesimal and protoplanet formation. Because objects this large are decoupled from the accretion flow, when they form, they can lock up water (and oxygen) beyond the snow line, thereby enhancing the C/O ratio in the inner disk and altering the molecular abundances there. We discuss the assumptions that underlie this interpretation, a possible alternative explanation, and related open questions that motivate future work. Whatever its origin, understanding the meaning of the relation between the HCN/H2O ratio and disk mass is of interest as trends like this among T Tauri disk properties are relatively rare.03/2013; -
Article: Anomalous CO2 Ice Toward HOPS-68: A Tracer of Protostellar Feedback
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ABSTRACT: We report the detection of a unique CO2 ice band toward the deeply embedded, low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared Spectrograph onboard the Spitzer Space Telescope, reveals a 15.2 micron CO2 ice bending mode profile that cannot modeled with the same ice structure typically found toward other protostars. We develop a modified CO2 ice profile decomposition, including the addition of new high-quality laboratory spectra of pure, crystalline CO2 ice. Using this model, we find that 87-92% of the CO2 is sequestered as spherical, CO2-rich mantles, while typical interstellar ices show evidence of irregularly-shaped, hydrogen-rich mantles. We propose that (1) the nearly complete absence of unprocessed ices along the line-of-sight is due to the flattened envelope structure of HOPS-68, which lacks cold absorbing material in its outer envelope, and possesses an extreme concentration of material within its inner (10 AU) envelope region and (2) an energetic event led to the evaporation of inner envelope ices, followed by cooling and re-condensation, explaining the sequestration of spherical, CO2 ice mantles in a hydrogen-poor mixture. The mechanism responsible for the sublimation could be either a transient accretion event or shocks in the interaction region between the protostellar outflow and envelope. The proposed scenario is consistent with the rarity of the observed CO2 ice profile, the formation of nearly pure CO2 ice, and the production of spherical ice mantles. HOPS-68 may therefore provide a unique window into the protostellar feedback process, as outflows and heating shape the physical and chemical structure of protostellar envelopes and molecular clouds.02/2013; -
Article: The Physical Structure of Protoplanetary Disks: the Serpens Cluster Compared with Other Regions
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ABSTRACT: Spectral energy distributions are presented for 94 young stars surrounded by disks in the Serpens Molecular Cloud, based on photometry and Spitzer IRS spectra. Taking a distance to the cloud of 415 pc rather than 259 pc, the distribution of ages is shifted to lower values, in the 1-3 Myr range, with a tail up to 10 Myr. The mass distribution spans 0.2-1.2 Msun, with median mass of 0.7 Msun. The distribution of fractional disk luminosities in Serpens resembles that of the young Taurus Molecular Cloud, with most disks consistent with optically thick, passively irradiated disks in a variety of disk geometries (Ldisk/Lstar ~ 0.1). In contrast, the distributions for the older Upper Scorpius and Eta Chamaeleontis clusters are dominated by optically thin lower luminosity disks (Ldisk/Lstar ~ 0.02). This evolution in fractional disk luminosities is concurrent with that of disk fractions. The actively accreting and non-accreting stars (based on Ha data) in Serpens show very similar distributions in fractional disk luminosities, differing only in the brighter tail dominated by strongly accreting stars. In contrast with a sample of Herbig Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in fractional disk luminosities for different disk geometries: both flared and flat disks present wider, overlapping distributions. This result is consistent with previous suggestions of a faster evolution for disks around Herbig Ae/Be stars. Furthermore, the results for the mineralogy of the dust in the disk surface do not show any correlation to either stellar and disk characteristics or mean cluster age in the 1-10 Myr range probed here. A possible explanation for the lack of correlation is that the processes affecting the dust within disks have short timescales, happening repeatedly, making it difficult to distinguish long lasting evolutionary effects. [abridged]12/2012; -
Article: CO2 Ice toward Low-luminosity, Embedded Protostars: Evidence for Episodic Mass Accretion via Chemical History
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ABSTRACT: We present Spitzer IRS spectroscopy of CO2 ice bending mode spectra at 15.2 micrometer toward 19 young stellar objects with luminosity lower than 1 Lsun (3 with luminosity lower than 0.1 Lsun). Ice on dust grain surfaces can encode the history of heating because pure CO2 ice forms only at elevated temperature, T > 20 K, and thus around protostars of higher luminosity. Current internal luminosities of YSOs with L < 1 Lsun do not provide the conditions needed to produce pure CO2 ice at radii where typical envelopes begin. The presence of detectable amounts of pure CO2 ice would signify a higher past luminosity. Many of the spectra require a contribution from a pure, crystalline CO2 component, traced by the presence of a characteristic band splitting in the 15.2 micrometer bending mode. About half of the sources (9 out of 19) in the low luminosity sample have evidence for pure CO2 ice, and six of these have significant double-peaked features, which are very strong evidence of pure CO2 ice. The presence of the pure CO2 ice component indicates that the dust temperature, and hence luminosity of the central star/accretion disk system, must have been higher in the past. An episodic accretion scenario, in which mixed CO-CO2 ice is converted to pure CO2 ice during each high luminosity phase, explains the presence of pure CO2 ice, the total amount of CO2 ice, and the observed residual C18O gas.08/2012; -
Article: Water in star-forming regions with Herschel: highly excited molecular emission from the NGC 1333 IRAS 4B outflow
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ABSTRACT: During the embedded phase of pre-main sequence stellar evolution, a disk forms from the dense envelope while an accretion-driven outflow carves out a cavity within the envelope. Highly excited H2O emission in spatially unresolved Spitzer/IRS spectra of a low-mass Class 0 object, NGC 1333 IRAS 4B, has previously been attributed to the envelope-disk accretion shock but could instead be produced in an outflow. As part of the survey of low-mass sources in the Water in Star Forming Regions with Herschel (WISH-LM) program, we used Herschel/PACS to obtain a far-IR spectrum and several Nyquist-sampled spectral images with to determine the origin of excited H2O emission from NGC 1333 IRAS 4B. The spectrum has high signal-to-noise in a rich forest of H2O, CO, and OH lines, providing a near-complete census of far-IR molecular emission from a Class 0 protostar. The excitation diagrams for the three molecules all require fits with two excitation temperatures, indicating the presence of two physical components. The highly excited component of H2O emission is characterized by subthermal excitation of 1500 K gas with a density of 10^5 - 10^7 cm-3, conditions that also reproduce the mid-IR H2O emission detected by Spitzer. On the other hand, a high density, low temperature gas can reproduce the H2O spectrum observed by Spitzer but underpredicts the H2O lines seen by Herschel. Nyquist-sampled spectral maps of several lines show two spatial components of H2O emission, one centered at 1200 AU south of the central source at the position of the blueshifted outflow lobe and a second centered on-source. Both spatial components of the far-IR H2O emission are consistent with emission from the outflow. The gas cooling from the IRAS 4B envelope cavity walls is dominated by far-IR H2O emission, in contrast to stronger [O I] and CO cooling from more evolved protostars. [one sentence truncated]11/2011; -
Article: The Spitzer Ice Legacy: Ice Evolution from Cores to Protostars
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ABSTRACT: Ices regulate much of the chemistry during star formation and account for up to 80% of the available oxygen and carbon. In this paper, we use the Spitzer c2d Legacy ice survey, complimented with data sets on ices in cloud cores and high-mass protostars, to determine standard ice abundances and to present a coherent picture of the evolution of ices during low- and high-mass star formation. The median ice composition H2O:CO:CO2:CH3OH:NH3:CH4:XCN is 100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low- and high-mass protostars, respectively, and 100:31:38:4:-:-:- in cloud cores. In the low-mass sample, the ice abundances with respect to H2O of CH4, NH3, and the component of CO2 mixed with H2O typically vary by <25%, indicative of co-formation with H2O. In contrast, some CO and CO2 ice components, XCN, and CH3OH vary by factors 2-10 between the lower and upper quartile. The XCN band correlates with CO, consistent with its OCN– identification. The origin(s) of the different levels of ice abundance variations are constrained by comparing ice inventories toward different types of protostars and background stars, through ice mapping, analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on the analysis, the first ice formation phase is driven by hydrogenation of atoms, which results in an H2O-dominated ice. At later prestellar times, CO freezes out and variations in CO freezeout levels and the subsequent CO-based chemistry can explain most of the observed ice abundance variations. The last important ice evolution stage is thermal and UV processing around protostars, resulting in CO desorption, ice segregation, and the formation of complex organic molecules. The distribution of cometary ice abundances is consistent with the idea that most cometary ices have a protostellar origin.The Astrophysical Journal 10/2011; 740(2):109. · 6.02 Impact Factor -
Article: Ices in starless and starforming cores
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ABSTRACT: Icy grain mantles are commonly observed through infrared spectroscopy toward dense clouds, cloud cores, protostellar envelopes and protoplanetary disks. Up to 80% of the available oxygen, carbon and nitrogen are found in such ices; the most common ice constituents - H2O, CO2 and CO - are second in abundance only to H2 in many star forming regions. In addition to being a molecular reservoir, ice chemistry is responsible for much of the chemical evolution from H2O to complex, prebiotic molecules. Combining the existing ISO, Spitzer, VLT and Keck ice data results in a large sample of ice sources (\sime80) that span all stages of star formation and a large range of protostellar luminosities (<0.1-105 L\odot). Here we summarize the different techniques that have been applied to mine this ice data set on information on typical ice compositions in different environments and what this implies about how ices form and evolve during star and planet formation. The focus is on how to maximize the use of empirical constraints from ice observations, followed by the application of information from experiments and models. This strategy is used to identify ice bands and to constrain which ices form early during cloud formation, which form later in the prestellar core and which require protostellar heat and/or UV radiation to form. The utility of statistical tests, survival analysis and ice maps is highlighted; the latter directly reveals that the prestellar ice formation takes place in two phases, associated with H2O and CO ice formation, respectively, and that most protostellar ice variation can be explained by differences in the prestellar CO ice formation stage. Finally, special attention is paid to the difficulty of observing complex ices directly and how gas observations, experiments and models help in constraining this ice chemistry stage.07/2011; -
Article: Disks and Outflows in CO Rovibrational Emission from Embedded, Low-Mass Young Stellar Objects
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ABSTRACT: Young circumstellar disks that are still embedded in dense molecular envelopes may differ from their older counterparts, but are historically difficult to study because emission from a disk can be confused with envelope or outflow emission. CO fundamental emission is a potentially powerful probe of the disk/wind structure within a few AU of young protostars. In this paper, we present high spectral (R=90,000) and spatial (0.3") resolution VLT/CRIRES M-band spectra of 18 low-mass young stellar objects (YSOs) with dense envelopes in nearby star-froming regions to explore the utility of CO fundamental 4.6 micron emission as a probe of very young disks. CO fundamental emission is detected from 14 of the YSOs in our sample. The emission line profiles show a range of strengths and shapes, but can generally be classified into a broad, warm component and a narrow, cool component. The broad CO emission is detected more frequently from YSOs with bolometric luminosities of <15 Lsun than those with >15 Lsun, and as with CO emission from CTTSs is attributed to the warm (~1000 K) inner AU of the disk. The CO emission from objects with high bolometric luminosity is produced in cooler (~320 K), narrow lines in 12CO and in rarer isotopologues. From some objects, the narrow lines are blueshifted by up to ~10 km/s, indicating a slow wind origin. For other sources the lines are located at the systemic velocity of the star and likely arise in the disk. For a few YSOs, spatially-extended CO and H2 S(9) emission is detected up to 2" from the central source and is attributed to interactions between the wind and surrounding molecular material. Warm CO absorption is detected in the wind of six objects with velocities up to 100 km/s, often in discrete velocity components. That the wind is partially molecular where it is launched favors ejection in a disk wind rather than a coronal or chromospheric wind.06/2011; -
Article: On the Evolution of Dust Mineralogy, from Protoplanetary Disks to Planetary Systems
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ABSTRACT: Mineralogical studies of silicate features emitted by dust grains in protoplanetary disks and solar system bodies can shed light on the progress of planet formation. The significant fraction of crystalline material in comets, chondritic meteorites, and interplanetary dust particles indicates a modification of the almost completely amorphous interstellar medium dust from which they formed. The production of crystalline silicates, thus, must happen in protoplanetary disks, where dust evolves to build planets and planetesimals. Different scenarios have been proposed, but it is still unclear how and when this happens. This paper presents dust grain mineralogy (composition, crystallinity, and grain size distribution) of a complete sample of protoplanetary disks in the young Serpens cluster. These results are compared to those in the young Taurus region and to sources that have retained their protoplanetary disks in the older Upper Scorpius and η Chamaeleontis stellar clusters, using the same analysis technique for all samples. This comparison allows an investigation of the grain mineralogy evolution with time for a total sample of 139 disks. The mean cluster age and disk fraction are used as indicators of the evolutionary stage of the different populations. Our results show that the disks in the different regions have similar distributions of mean grain sizes and crystallinity fractions (~10%-20%) despite the spread in mean ages. Furthermore, there is no evidence of preferential grain sizes for any given disk geometry nor for the mean cluster crystallinity fraction to increase with mean age in the 1-8 Myr range. The main implication is that a modest level of crystallinity is established in the disk surface early on (≤1 Myr), reaching an equilibrium that is independent of what may be happening in the disk midplane. These results are discussed in the context of planet formation, in comparison with mineralogical results from small bodies in our own solar system.The Astrophysical Journal 05/2011; 734(1):51. · 6.02 Impact Factor -
Article: The structure and dynamics of molecular gas in planet-forming zones: A CRIRES spectro-astrometric survey
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ABSTRACT: We present a spectro-astrometric survey of molecular gas in the inner regions of 16 protoplanetary disks using CRIRES, the high resolution infrared imaging spectrometer on the Very Large Telescope. Spectro-astrometry with CRIRES measures the spatial extent of line emission to sub-milliarcsecond precision, or <0.2 AU at the distance of the observed targets. The sample consists of gas-rich disks surrounding stars with spectral types ranging from K to A. The properties of the spectro-astrometric signals divide the sources into two distinct phenomenological classes: one that shows clear Keplerian astrometric spectra, and one in which the astrometric signatures are dominated by gas with strong non-Keplerian (radial) motions. Similarly to the near-infrared continuum emission, as determined by interferometry, we find that the size of the CO line emitting region in the Keplerian sources obeys a size-luminosity relation as $R_CO L_*^0.5. The non-Keplerian spectro-astrometric signatures are likely indicative of the presence of wide-angle disk winds. The central feature of the winds is a strong sub-Keplerian velocity field due to conservation of angular momentum as the wind pressure drives the gas outwards. We construct a parametrized 2-dimensional disk+wind model that reproduces the observed characteristics the observed CO spectra and astrometry. The modeled winds indicate mass-loss rates of >10^-10 to 10^-8 Msol/yr. We suggest a unifying model in which all disks have slow molecular winds, but where the magnitude of the mass-loss rate determines the degree to which the mid-infrared molecular lines are dominated by the wind relative to the Keplerian disk surface.03/2011; -
Article: Observational Constraints on Submillimeter Dust Opacity
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ABSTRACT: Infrared extinction maps and submillimeter dust continuum maps are powerful probes of the density structure in the envelope of star-forming cores. We make a direct comparison between infrared and submillimeter dust continuum observations of the low-mass Class 0 core, B335, to constrain the ratio of submillimeter to infrared opacity (κsmm/κir) and the submillimeter opacity power-law index (κ λ–β). Using the average value of theoretical dust opacity models at 2.2 μm, we constrain the dust opacity at 850 and 450 μm. Using new dust continuum models based upon the broken power-law density structure derived from interferometric observations of B335 and the infall model derived from molecular line observations of B335, we find that the opacity ratios are and with a submillimeter opacity power-law index of βsmm = (2.18-2.58)+0.30 –0.30. The range of quoted values is determined from the uncertainty in the physical model for B335. For an average 2.2 μm opacity of 3800 ± 700 cm2 g–1, we find a dust opacity at 850 and 450 μm of κ850 = (1.18-1.77)+0.36 –0.24 and κ450 = (4.72-9.13)+1.9 –0.98 cm2 g–1 of dust. These opacities are from (0.65-0.97)κOH5 850 of the widely used theoretical opacities of Ossenkopf and Henning for coagulated ice grains with thin mantles at 850 μm.The Astrophysical Journal 01/2011; 728(2):143. · 6.02 Impact Factor -
Article: Astronomical Oxygen Isotopic Evidence for Supernova Enrichment of the Solar System Birth Environment by Propagating Star Formation
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ABSTRACT: New infrared absorption measurements of oxygen isotope ratios in CO gas from individual young stellar objects confirm that the solar system is anomalously high in its 18O/17O ratio compared with extra-solar oxygen in the Galaxy. We show that this difference in oxygen isotope ratios is best explained by 1 per cent enrichment of the proto-solar molecular cloud by ejecta from type II supernovae from a cluster having of order a few hundred stars that predated the Sun by at least 10 to 20 Myr. The likely source of exogenous oxygen was the explosion of one or more B stars during a process of propagating star formation.12/2010; -
Article: VLT/X-shooter Spectroscopy of a Dusty Planetary Nebula Discovered with Spitzer/IRS
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ABSTRACT: As part of a mid-infrared spectroscopic survey of young stars with the Spitzer Space Telescope, an unclassified red emission line object was discovered. Based on its high ionization state indicated by the Spitzer spectrum, this object could either be a dusty Supernova Remnant (SNR) or a Planetary Nebula (PN). In this research note, the object is classified and the available spectroscopic data are presented to the community for further analysis. UV/optical/NIR spectra were obtained during the science verification run of the VLT/X-shooter. A large number of emission lines are identified allowing the determination of the nature of this object. The presence of strong, narrow (Delta_v ~ 8 - 74 km/s) emission lines, combined with very low line ratios of, e.g., [N II]/Halpha and [S II]/Halpha show that the object is a Planetary Nebula (PN) that lies at an undetermined distance behind the Serpens Molecular Cloud. This illustrates the potential of X-shooter as an efficient tool for constraining the nature of faint sources with unknown spectral properties or colors. Comment: 4 pages, 5 figures, accepted by A&A12/2010; -
Article: Spectrally resolved pure rotational lines of water in protoplanetary disks
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ABSTRACT: We present ground-based high resolution N-band spectra (\Delta v = 15 km/s) of pure rotational lines of water vapor in two protoplanetary disks surrounding the pre-main sequence stars AS 205N and RNO 90, selected based on detections of rotational water lines by the Spitzer IRS. Using VISIR on the Very Large Telescope, we spectrally resolve individual lines and show that they have widths of 30-60 km/s, consistent with an origin in Keplerian disks at radii of ~1 AU. The water lines have similar widths to those of the CO at 4.67 micron, indicating that the mid-infrared water lines trace similar radii. The rotational temperatures of the water are 540 and 600K in the two disks, respectively. However, the lines ratios show evidence of non-LTE excitation, with low-excitation line fluxes being over-predicted by 2-dimensional disk LTE models. Due to the limited number of observed lines and the non-LTE line ratios, an accurate measure of the water ortho/para ratio is not available, but a best estimate for AS 205N is ortho/para = 4.5 +/- 1.0, apparently ruling out a low-temperature origin of the water. The spectra demonstrate that high resolution spectroscopy of rotational water lines is feasible from the ground, and further that ground-based high resolution spectroscopy is likely to significantly improve our understanding of the inner disk chemistry recently revealed by recent Spitzer observations. Comment: Accepted for publication in ApJL09/2010; -
Article: The c2d Spitzer Spectroscopic Survey of Ices Around Low-mass Young Stellar Objects. IV. NH3 and CH3OH
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ABSTRACT: NH3 and CH3OH are key molecules in astrochemical networks leading to the formation of more complex N- and O-bearing molecules, such as CH3CN and CH3OCH3. Despite a number of recent studies, little is known about their abundances in the solid state. This is particularly the case for low-mass protostars, for which only the launch of the Spitzer Space Telescope has permitted high-sensitivity observations of the ices around these objects. In this work, we investigate the ~8-10 μm region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 μm silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH3- and CH3OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ~9 μm in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH3 ν2 umbrella mode and derive abundances with respect to water between ~2% and 15%. Simultaneously, we also revisited the case of CH3OH ice by studying the ν4 C-O stretch mode of this molecule at ~9.7 μm in 16 objects, yielding abundances consistent with those derived by Boogert et al. based on a simultaneous 9.75 and 3.53 μm data analysis. Our study indicates that NH3 is present primarily in H2O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH3OH being in an almost pure methanol ice, or mixed mainly with CO or CO2, consistent with its formation through hydrogenation on grains. Finally, we use our derived NH3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10%-20% of nitrogen is locked up in known ices.The Astrophysical Journal 07/2010; 718(2):1100. · 6.02 Impact Factor -
Article: A Spitzer survey of mid-infrared molecular emission from protoplanetary disks I: Detection rates
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ABSTRACT: We present a Spitzer InfraRed Spectrometer search for 10-36 micron molecular emission from a large sample of protoplanetary disks, including lines from H2O, OH, C2H2, HCN and CO2. This paper describes the sample and data processing and derives the detection rate of mid-infrared molecular emission as a function of stellar mass. The sample covers a range of spectral type from early M to A, and is supplemented by archival spectra of disks around A and B stars. It is drawn from a variety of nearby star forming regions, including Ophiuchus, Lupus and Chamaeleon. In total, we identify 22 T Tauri stars with strong mid-infrared H2O emission. Integrated water line luminosities, where water vapor is detected, range from 5x10^-4 to 9x10^-3 Lsun, likely making water the dominant line coolant of inner disk surfaces in classical T Tauri stars. None of the 5 transitional disks in the sample show detectable gaseous molecular emission with Spitzer upper limits at the 1% level in terms of line-to-continuum ratios (apart from H2). We find a strong dependence on detection rate with spectral type; no disks around our sample of 25 A and B stars were found to exhibit water emission, down to 1-2% line-to-continuum ratios, in the mid-infrared, while almost 2/3 of the disks around K stars show sufficiently intense water emission to be detected by Spitzer. Some Herbig Ae/Be stars show tentative H2O/OH emission features beyond 20 micron at the 1-2 level, however, and one of them shows CO2 in emission. We argue that the observed differences between T Tauri disks and Herbig Ae/Be disks is due to a difference in excitation and/or chemistry depending on spectral type and suggest that photochemistry may be playing an important role in the observable characteristics of mid-infrared molecular line emission from protoplanetary disks. Comment: 19 pages, accepted for publication in ApJ06/2010; -
Article: A Spitzer Survey of Protoplanetary Disk Dust in the Young Serpens Cloud: How do Dust Characteristics Evolve with Time?
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ABSTRACT: We present Spitzer InfraRed Spectrograph (IRS) mid-infrared (5-35 μm) spectra of a complete flux-limited sample (≥3 mJy at 8 μm) of young stellar object (YSO) candidates selected on the basis of their infrared colors in the Serpens Molecular Cloud. Spectra of 147 sources are presented and classified. Background stars (with slope consistent with a reddened stellar spectrum and silicate features in absorption), galaxies (with redshifted polycyclic aromatic hydrocarbon (PAH) features), and a planetary nebula (with high ionization lines) amount to 22% of contamination in this sample, leaving 115 true YSOs. Sources with rising spectra and ice absorption features, classified as embedded Stage I protostars, amount to 18% of the sample. The remaining 82% (94) of the disk sources are analyzed in terms of spectral energy distribution shapes, PAHs, and silicate features. The presence, strength, and shape of these silicate features are used to infer disk properties for these systems. About 8% of the disks have 30/13 μm flux ratios consistent with cold disks with inner holes or gaps, and 3% of the disks show PAH emission. Comparison with models indicates that dust grains in the surface of these disks have sizes of at least a few μm. The 20 μm silicate feature is sometimes seen in the absence of the 10 μm feature, which may be indicative of very small holes in these disks. No significant difference is found in the distribution of silicate feature shapes and strengths between sources in clusters and in the field. Moreover, the results in Serpens are compared with other well-studied samples: the c2d IRS sample distributed over five clouds and a large sample of disks in the Taurus star-forming region. The remarkably similar distributions of silicate feature characteristics in samples with different environment and median ages—if significant—imply that the dust population in the disk surface results from an equilibrium between dust growth and destructive collision processes that are maintained over a few million years for any YSO population irrespective of environment.The Astrophysical Journal 04/2010; 714(1):778. · 6.02 Impact Factor -
Article: A new raytracer for modeling AU-scale imaging of lines from protoplanetary disks
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ABSTRACT: The material that formed the present-day Solar System originated in feeding zones in the inner Solar Nebula located at distances within ~20 AU from the Sun, known as the planet-forming zone. Meteoritic and cometary material contain abundant evidence for the presence of a rich and active chemistry in the planet-forming zone during the gas-rich phase of Solar System formation. It is a natural conjecture that analogs can be found amoung the zoo of protoplanetary disks around nearby young stars. The study of the chemistry and dynamics of planet formation requires: 1) tracers of dense gas at 100-1000 K and 2) imaging capabilities of such tracers with 5-100 (0.5-20 AU) milli-arcsec resolution, corresponding to the planet-forming zone at the distance of the closest star-forming regions. Recognizing that the rich infrared (2-200 micron) molecular spectrum recently discovered to be common in protoplanetary disks represents such a tracer, we present a new general raytracing code, RADLite, that is optimized for producing infrared line spectra and images from axisymmetric structures. RADLite can consistently deal with a wide range of velocity gradients, such as those typical for the inner regions of protoplanetary disks. The code is intended as a backend for chemical and excitation codes, and can rapidly produce spectra of thousands of lines for grids of models for comparison with observations. Such radiative transfer tools will be crucial for constraining both the structure and chemistry of planet-forming regions, including data from current infrared imaging spectrometers and extending to the Atacama Large Millimeter Array and the next generation of Extremely Large Telescopes, the James Webb Space Telescope and beyond. Comment: 14 pages, accepted for publication in ApJ08/2009; -
Article: High-precision C17O, C18O and C16O measurements in young stellar objects: analogues for CO self-shielding in the early solar system
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ABSTRACT: Using very high resolution (lambda / Dlambda ~ 95000) 4.7 micron fundamental and 2.3 micron overtone ro-vibrational CO absorption spectra obtained with the CRIRES infrared spectrometer on the Very Large Telescope (VLT), we report detections of four CO isotopologues -- C16O, 13CO, C18O and the rare species, C17O -- in the circumstellar environment of two young protostars, VV CrA and Reipurth 50. We argue that the observed CO absorption lines probe a protoplanetary disk in VV CrA, and a protostellar envelope in Reipurth 50. All CO line profiles are spectrally resolved, permitting direct calculation of CO oxygen isotopologue ratios with 5-10% accuracy. The ro-vibrational level populations for all species can be reproduced by assuming that CO absorption arises in two temperature regimes. For both objects, 12C/13C are on the order of 100, nearly twice the expected interstellar medium (ISM) ratio. The derived oxygen abundance ratios for the VV CrA disk show a significant mass-independent deficit of C17O and C18O relative to C16O compared to ISM baseline abundances. The Reipurth 50 envelope shows no clear differences in oxygen CO isotopologue ratios compared with the local ISM. A mass-independent fractionation can be interpreted as being due to selective photodissociation of CO in the disk surface due to self-shielding. The deficits in C17O$ and C18O$ in the VV CrA protoplanetary disk are consistent with an analogous origin of the 16O variability in the solar system by isotope selective photodissociation, confirmation of which may be obtained via study of additional sources. The higher fractionation observed for the VV CrA disk compared with the Reipurth 50 envelope is likely due to a combination of disk geometry, grain growth, and vertical mixing processes. [Abstract abridged] Comment: 13 pages, accepted for publication in the Astrophysical Journal06/2009;
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2011–2012
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Space Telescope Science Institute
Baltimore, MD, USA
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2008–2011
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California Institute of Technology
- Division of Geological and Planetary Sciences
Pasadena, CA, USA
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