Publications (87)105.29 Total impact
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Article: Millimetron—a large Russian-European submillimeter space observatory
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ABSTRACT: Millimetron is a Russian-led 12m diameter submillimeter and far-infrared space observatory which is included in the Space Plan of the Russian Federation for launch around 2017. With its large collecting area and state-of-the-art receivers, it will enable unique science and allow at least one order of magnitude improvement with respect to the Herschel Space Observatory. Millimetron will be operated in two basic observing modes: as a single-dish observatory, and as an element of a ground-space very long baseline interferometry (VLBI) system. As single-dish, angular resolutions on the order of 3 to 12arc sec will be achieved and spectral resolutions of up to a million employing heterodyne techniques. As VLBI antenna, the chosen elliptical orbit will provide extremely large VLBI baselines (beyond 300,000km) resulting in micro-arc second angular resolution.Experimental Astronomy 04/2012; 23(1):221-244. · 1.82 Impact Factor -
Article: Sensitive limits on the abundance of cold water vapor in the DM Tau protoplanetary disk
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ABSTRACT: We performed a sensitive search for the ground-state emission lines of ortho- and para-water vapor in the DM Tau protoplanetary disk using the Herschel/HIFI instrument. No strong lines are detected down to 3sigma levels in 0.5 km/s channels of 4.2 mK for the 1_{10}--1_{01} line and 12.6 mK for the 1_{11}--0_{00} line. We report a very tentative detection, however, of the 1_{10}--1_{01} line in the Wide Band Spectrometer, with a strength of T_{mb}=2.7 mK, a width of 5.6 km/s and an integrated intensity of 16.0 mK km/s. The latter constitutes a 6sigma detection. Regardless of the reality of this tentative detection, model calculations indicate that our sensitive limits on the line strengths preclude efficient desorption of water in the UV illuminated regions of the disk. We hypothesize that more than 95-99% of the water ice is locked up in coagulated grains that have settled to the midplane. Comment: 5 pages, 3 figures. Accepted for publication in the Herschel HIFI special issue of A&A07/2010; -
Article: Herschel observations of EXtra-Ordinary Sources: the present and future of spectral surveys with Herschel/HIFI
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ABSTRACT: We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band continuum in this molecule-rich source is ~20-40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify multiple transitions of HD18O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is optically thick in the Orion hot core with HDO/H2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores. Comment: 6 pages, 5 figures, accepted and to be published in Herschel HIFI special issue of A&A Letters07/2010; -
Article: A HIFI preview of warm molecular gas around chi Cyg : first detection of H2O emission toward an S-type AGB star
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ABSTRACT: A set of new, sensitive, and spectrally resolved, sub-millimeter line observations are used to probe the warm circumstellar gas around the S-type AGB star chi Cyg. The observed lines involve high rotational quantum numbers, which, combined with previously obtained lower-frequency data, make it possible to study in detail the chemical and physical properties of, essentially, the entire circumstellar envelope of chi Cyg. The data were obtained using the HIFI instrument aboard Herschel, whose high spectral resolution provides valuable information about the line profiles. Detailed, non-LTE, radiative transfer modelling, including dust radiative transfer coupled with a dynamical model, has been performed to derive the temperature, density, and velocity structure of the circumstellar envelope. We report the first detection of circumstellar H2O rotational emission lines in an S-star. Using the high-J CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho- and para-H2O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H2O abundance (relative to H2) is (1.1 +/- 0.2)E-5, much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1 +/- 0.6 is close to the high-temperature equilibrium limit, consistent with H2O being formed in the photosphere. Comment: 5 pages, 4 figures accepted for publication in A&A HIFI special issue07/2010; -
Article: Herschel-PACS spectroscopy of the intermediate mass protostar NGC7129 FIRS 2
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ABSTRACT: Aims: We present preliminary results of the first Herschel spectroscopic observations of NGC7129 FIRS2, an intermediate mass star-forming region. We attempt to interpret the observations in the framework of an in-falling spherical envelope. Methods: The PACS instrument was used in line spectroscopy mode (R=1000-5000) with 15 spectral bands between 63 and 185 microns. This provided good detections of 26 spectral lines seen in emission, including lines of H2O, CO, OH, O I, and C II. Results: Most of the detected lines, particularly those of H2O and CO, are substantially stronger than predicted by the spherical envelope models, typically by several orders of magnitude. In this paper we focus on what can be learned from the detected CO emission lines. Conclusions: It is unlikely that the much stronger than expected line emission arises in the (spherical) envelope of the YSO. The region hot enough to produce such high excitation lines within such an envelope is too small to produce the amount of emission observed. Virtually all of this high excitation emission must arise in structures such as as along the walls of the outflow cavity with the emission produced by a combination of UV photon heating and/or non-dissociative shocks. Comment: A&A Special Issue on Herschel06/2010; -
Article: Origin of the hot gas in low-mass protostars: Herschel-PACS spectroscopy of HH 46
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ABSTRACT: 'Water in Star-forming regions with Herschel' (WISH) is a Herschel Key Programme aimed at understanding the physical and chemical structure of young stellar objects (YSOs) with a focus on water and related species. The low-mass protostar HH 46 was observed with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory to measure emission in H2O, CO, OH, [OI], and [CII] lines located between 63 and 186 um. The excitation and spatial distribution of emission can disentangle the different heating mechanisms of YSOs, with better spatial resolution and sensitivity than previously possible. Far-IR line emission is detected at the position of the protostar and along the outflow axis. The OH emission is concentrated at the central position, CO emission is bright at the central position and along the outflow, and H2O emission is concentrated in the outflow. In addition, [OI] emission is seen in low-velocity gas, assumed to be related to the envelope, and is also seen shifted up to 170 km/s in both the red- and blue-shifted jets. Envelope models are constructed based on previous observational constraints. They indicate that passive heating of a spherical envelope by the protostellar luminosity cannot explain the high-excitation molecular gas detected with PACS, including CO lines with upper levels at >2500 K above the ground state. Instead, warm CO and H2O emission is probably produced in the walls of an outflow-carved cavity in the envelope, which are heated by UV photons and non-dissociative C-type shocks. The bright OH and [OI] emission is attributed to J-type shocks in dense gas close to the protostar. In the scenario described here, the combined cooling by far-IR lines within the central spatial pixel is estimated to be 2 \times 10-2 L_sun, with 60-80% attributed to J- and C-type shocks produced by interactions between the jet and the envelope. Comment: Accepted for publication in Astronomy and Astrophysics (Herschel special issue)05/2010; -
Article: Detection of Abundant CO2 Ice in the Quiescent Dark Cloud Medium toward Elias 16
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ABSTRACT: We report the first detection of solid carbon dioxide (CO2) in quiescent regions of a dark cloud in the solar neighborhood, a result that has important implications for models of ice formation and evolution in the interstellar medium. The K-type field star Elias 16 was previously known to display solid-state absorption features of H2O and CO ices arising in the Taurus Dark Cloud. Our detection of the CO2 feature at 4.27 μm in this line of sight implies a column density N(CO2)=4.6+ 1.3−0.6×1017 cm-2, equivalent to ~18% and 70% of the H2O and CO column densities, respectively. Comparison with laboratory data indicates that (unlike CO) the CO2 resides primarily in a polar (H2O-rich) component of the ices. CO2 is formed easily in the laboratory by the photolysis of ice mixtures containing CO, but the detection toward Elias 16 indicates that CO2 formation can occur in dark clouds in the absence of a local embedded source of radiation. Possible alternative mechanisms for CO2 production include grain surface reactions and energetic processing driven by the interstellar radiation field or cosmic rays.The Astrophysical Journal 01/2009; 498(2):L159. · 6.02 Impact Factor -
Chapter: Physical and chemical variations within the W3 star-forming region
04/2007: pages 254-255; -
Conference Proceeding: ESPRIT: a space interferometer concept for the far-infrared
Proc. of SPIE, Orlando; 06/2006 -
Article: The Pre-flight Performance of the Herschel-Heterodyne Instrument for the Far-Infrared (HIFI)
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ABSTRACT: The Heterodyne Instrument for the Far-Infrared (HIFI) is a high-resolution (>300.000), spectrometer for ESA's Herschel Space Observatory. The instrument is designed to provide a wide and continuous frequency coverage with velocity resolved resolution and high sensitivity. The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band for the 1410-1910 GHz range with Hot Electron Bolometer Mixers (HEBM). One frequency band is operating at a time with a single sky-pixel. The Local Oscillator (LO) subsystem consists of a Ka-band synthesiser followed by 14 chains of frequency multipliers, 2 chains for each frequency band. Each frequency band has two mixers operating on orthogonal polarisations. All mixers are designed to have noise performance close to the quantum noise limit. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarisation front-ends and provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 KHz to 1 MHz), better than < 0.1 Km/s. We will present the design and the first results of the pre-flight testing and characterization of the instrument.11/2005; 37:1197. -
Article: The ISO atlas of near-infrared stellar spectra and the IR spectral classification of late-type stars
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ABSTRACT: After the liquid Helium depletion of the Infrared Space Observatory (ISO), the InSb band 1 detectors of the Short-Wavelength Spectrometer (SWS) could be operated in some timeslots available during the engineering test programme. This time was used to record spectra of stars between 2.36 and 4.05 μm at medium resolution (~ 1500 - 2000). As the temperatures of the focal plane increased, changes in the position read-out of the grating assembly and changes in the response of the instrument could be seen. We present the calibration of these changes and the calibration accuracy of the resulting spectra. The set of spectra obtained during the post-helium phase has been combined with all SWS spectra of stellar sources measured during the nominal mission in the SWS AOT1 speed 4 observation mode. These spectra will be published in a unique atlas of more than 300 stellar spectra between 2.36 and 4.05 μm covering the entire MK classification. We present the work on the atlas and discuss how it complements the effort on the atlas of the STARTYPE programme that covers a wider spectral range at lower spectral resolution. Finally, we also discuss the first results of the work on the near-infrared classification of late-type stars and show how spectral indices in the 2.36-4.05 μm region can be used to characterise stellar properties of cool stars.10/2000; 456:147. -
Article: Infrared Observations of Hot Gas and Cold Ice toward the Low Mass Protostar Elias 29
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ABSTRACT: We have obtained the full 1-200 um spectrum of the low luminosity (36 Lsun) Class I protostar Elias 29 in the Rho Ophiuchi molecular cloud. It provides a unique opportunity to study the origin and evolution of interstellar ice and the interrelationship of interstellar ice and hot core gases around low mass protostars. We see abundant hot CO and H2O gas, as well as the absorption bands of CO, CO2, H2O and ``6.85 um'' ices. We compare the abundances and physical conditions of the gas and ices toward Elias 29 with the conditions around several well studied luminous, high mass protostars. The high gas temperature and gas/solid ratios resemble those of relatively evolved high mass objects (e.g. GL 2591). However, none of the ice band profiles shows evidence for significant thermal processing, and in this respect Elias 29 resembles the least evolved luminous protostars, such as NGC 7538 : IRS9. Thus we conclude that the heating of the envelope of the low mass object Elias 29 is qualitatively different from that of high mass protostars. This is possibly related to a different density gradient of the envelope or shielding of the ices in a circumstellar disk. This result is important for our understanding of the evolution of interstellar ices, and their relation to cometary ices.06/2000; -
Article: ISO-SWS observations of pure rotational water absorption lines toward Orion-IRc2
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ABSTRACT: First detections of thermal water vapor absorption lines have been made toward Orion IRc2 using the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). Grating spectra covering wavelengths 25-45 micron yield 19 pure rotational lines, originating from energy levels 200-750 K above ground. Fabry-Perot spectra of 5 transitions resolve the line profiles and reveal the water gas kinematics. The fact that all lines are seen in absorption is in striking contrast with data from the ISO Long Wavelength Spectrometer (LWS), where the water lines appear in emission. At least one line displays a P-Cygni type profile, which suggests that the water is located in an expanding shell centered on or near IRc2. The expansion velocity is 18 km per second, in agreement with the value inferred from water maser observations by Genzel et al. (1981). Because the continuum is intense and likely formed in or near the water-containing gas, the excitation of the observed transitions is dominated by radiative processes. A simple, generalised curve-of-growth method is presented and used to analyze the data. A mean excitation temperature of 72 K and a total water column density of 1.5e18 molecules per cm squared are inferred, each with an estimated maximum uncertainty of 20%. Combined with the molecular hydrogen column density derived from ISO observations of the pure rotational lines, and an assumed temperature of 200-350 K, the inferred water abundance is 2e-4 to 5e-4 in the warm shocked gas. This abundance is similar to that found recently by Harwit et al. (1998) toward Orion using data from the LWS, but higher than that found for most other shocked regions by, for example, Liseau et al. (1996).05/2000; -
Article: ISO-SWS observations of interstellar solid...
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ABSTRACT: We present observations of the stretching CO 2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory (ISO--SWS). Remarkable variations are seen in the absorption band profile in the di#erent lines of sight. The main feature is attributed to mixed with polar molecules such as H 2 O, and CH 3 OH. The high-mass protostars GL 2136, GL 2591, S 140 : IRS1, and W 3 : IRS5 show an additional narrow substructure at 2282 cm -1 (4.382 m), which we attribute to a polar, CH 3 OH--containing CO 2 ice, that experienced heating. This heating e#ect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO 2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing.02/2000; -
Article: The infrared spectrum of the Be star gamma Cassiopeiae
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ABSTRACT: We present the 2.4-45 micrometer ISO-SWS spectrum of the Be star gamma Cas (B0.5 IVe). The spectrum is characterised by a thermal continuum which can be well fit by a power-law S_nu ~ nu^0.99 over the entire SWS wavelength range. For an isothermal disc of ionized gas with constant opening angle, this correponds to a density gradient rho(r) ~ r^(-2.8). We report the detection of the Humphreys bound-free jump in emission at 3.4 micrometer. The size of the jump is sensitive to the electron temperature of the gas in the disc, and we find T~9000 K, i.e. much lower than the stellar effective temperature (25000-30000 K). The spectrum is dominated by numerous emission lines, mostly from HI, but also some HeI lines are detected. Several spectral features cannot be identified. The line strengths of the HI{\sc i} emission lines do not follow case B recombination line theory. The line strengths and widths suggest that many lines are optically thick and come from an inner, high density region with radius 3-5 R_star and temperature above that of the bulk of the disc material. Only the alpha, beta and gamma transitions of the series lines contain a contribution from the outer regions. The level populations deviate significantly from LTE and are highly influenced by the optically thick, local (disc) continuum radiation field. The inner disk may be rotating more rapidly than the stellar photosphere.12/1999; -
Article: ISO-SWS observations of interstellar solid 13CO2: heated ice and the Galactic 12C/13C abundance ratio
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ABSTRACT: We present observations of the stretching mode of 13CO2 ice along 13 lines of sight in the Galaxy, using the Short Wavelength Spectrometer on board of the Infrared Space Observatory. Remarkable variations are seen in the absorption band profile in the different lines of sight. The main feature is attributed to 13CO2 mixed with polar molecules such as H2O, and CH3OH. Several high-mass protostars show an additional narrow substructure at 4.382 um, which we attribute to a polar CO2 ice, that experienced heating. This heating effect is sustained by a good correlation of the strength of the substructure with dust and CO gas temperatures along the line of sight, and anti-correlation with ice abundances. Thus, our main conclusion is that interstellar CO2 ices around luminous protostars are subjected to, and altered by, thermal processing and that it may reflect the evolutionary stage of the nearby protostar. In contrast, the ices around low mass protostars and in a quiescent cloud in our sample do not show signs of thermal processing. Furthermore, we determine for the first time the Galactic 12C/13C ratio from the solid state. The 12CO2/13CO2 ratio for the local ISM (69+/-15), as well as the dependence on Galacto-centric radius, are in good agreement with gas phase (C18O, H2CO) studies. For the few individual objects for which gas phase values are available, the 12C/13C ratios derived from CO2 tend to be higher compared to CO studies (albeit with 2.5 sigma significance only). We discuss the implications of this possible difference for the chemical origin of interstellar CO2.10/1999; -
Article: The central stars of the planetary nebulae NGC 7027 and NGC 6543
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ABSTRACT: Infrared spectra of NGC 7027 and NGC 6543 ranging from 2.4 to 45 micron were obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. A first analysis of these spectra, with the aid of photo-ionization models, is presented. We report the first detection of the Ar VI 4.53 micron and Ne VI 7.65 micron lines in the spectrum of NGC 7027. When compared with older observations it is clear that the Ar VI line and possibly also other lines have increased in strength since 1981. We argue that a likely explanation for this variability is a change in the spectral energy distribution of the central star, possibly an increase in effective temperature. However, this result needs to be confirmed by further observations. We also report a non-detection of the O IV 25.9 micron line and the first detection of the Na III 7.32 micron line in the spectrum of NGC 6543. The non-detection is not expected based on a blackbody approximation for the spectrum of the central star. The ionization threshold for O(3+) is just beyond the He II limit, and the absence of this line shows that the stellar flux drops at least by a factor 350 at the He II limit. Modeling the O IV line may prove to be a valuable test for atmosphere models.06/1999; -
Article: Classification of silicate emission sources observed with SWS
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ABSTRACT: We present preliminary results from our effort to classify sources with silicate dust shells observed for the STARTYP1/2 and ZZSTARTY programs. To characterize our silicate sources, we calculate the silicate emission index, SE, developed by Sloan & Price (1995). This index is a quantative description of the shape of the 10 μm silicate emission feature as seen in the IRAS Low Resolution Spectrometer data of AGB stars. With the larger wavelength coverage and the higher spectral resolution of SWS, we can provide added details to the SE index. In particular, we detect correlations in the strength of the 4.27 μm CO2 band with the SE classification. The objective of the STARTYP1/2 and ZZSTARTY programs is to construct an ISO database for the spectral classification of the brightest representative sources. The final database will include all sources with full SWS grating scans or PHOT-S observations.02/1999; 427:401. -
Article: ISO observations of AGB and post-AGB stars
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ABSTRACT: We review some of the highlights of the ISO observing programmes dedicated to the photospheres and winds of late-type stars. The ISO mission has provided for the first time an unbiased view of the gas-phase molecular components in the extended atmospheres of cool stars. The transition region between the photosphere and the cool outer layers was detected by means of spectroscopy. Several new molecules have been discovered. Also, a very rich harvest of new solid state emission bands was discovered. The detection of crystalline silicates in AGB and post-AGB stars is one of the surprises of the ISO mission. ISO also showed that the co-existence of C-rich and O-rich circumstellar material is much more common than previously believed. Observations of AGB stars in the Magellanic Clouds show that C-rich AGB stars can reach very high luminosities.02/1999; 427:219. -
Article: ISO's view on dust in comets and circumstellar media
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ABSTRACT: The infrared spectrum of Comet Hale-Bopp obtained with the Infrared Space Observatory (ISO) presents astonishing similarities with those of the dusty disks surrounding young stars. The presence of crystalline silicates in comets, Herbig Ae/Be stars and the main-sequence star β Pictoris, is interesting in the context of the formation of comets and confirms that Herbig Ae/Be stars are evolutionary precursors of β Pictoris stars. The detection of hydrous silicates in both the spectrum of HD 142527 and in interplanetary dust particles (IDPs) strengthens the link between these dust disks and our own Solar System.02/1999; 427:607.
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Institutions
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1999
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Pisgah Astronomical Research Institute
Belgium, WI, USA
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1996
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University of Groningen
- Department of Applied Physics
Groningen, Province of Groningen, Netherlands
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