Publications (17)66.15 Total impact
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Article: ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: towards an understanding of dust processing
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ABSTRACT: We present Infrared Space Observatory (ISO) spectra of fourteen isolated Herbig Ae/Be (HAEBE) stars, to study the characteristics of their circumstellar dust. These spectra show large star-to-star differences, in the emission features of both carbon-rich and oxygen-rich dust grains. The IR spectra were combined with photometric data ranging from the UV through the optical into the sub-mm region. We defined two key groups, based upon the spectral shape of the infrared region. The derived results can be summarized as follows: (1) the continuum of the IR to sub-mm region of all stars can be reconstructed by the sum of a power-law and a cool component, which can be represented by a black body. Possible locations for these components are an optically thick, geometrically thin disc (power-law component) and an optically thin flared region (black body); (2) all stars have a substantial amount of cold dust around them, independent of the amount of mid-IR excess they show; (3) also the near-IR excess is unrelated to the mid-IR excess, indicating different composition/location of the emitting material; (4) remarkably, some sources lack the silicate bands; (5) apart from amorphous silicates, we find evidence for crystalline silicates in several stars, some of which are new detections; (6) PAH bands are present in at least 50% of our sample, and their appearance is slightly different from PAHs in the ISM; (7) PAH bands are, with one exception, not present in sources which only show a power-law continuum in the IR; their presence is unrelated to the presence of the silicate bands; (8) the dust in HAEBE stars shows strong evidence for coagulation; this dust processing is unrelated to any of the central star properties (such as age, spectral type and activity). Comment: 15 pages, accepted by A&A12/2000; -
Article: The Vega phenomenon around G dwarfs
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ABSTRACT: In this paper, we present and discuss photometric infrared measurements taken with ISOPHOT at 60 mu m for a sample of 30 G dwarfs, using C100 3X3 minimaps. This sample was selected from the set of more than 1600 G dwarfs which will be scrutinized for radial-velocity variations by the Geneva group, aiming at the detection of extra-solar planets. In our sample, 5 stars display an infrared excess at 60 mu m. We have compared our results with those of Habing et al. (1999, in prep.), who have investigated the incidence and the survival of remnant disks around main-sequence stars. They concluded that most stars which arrive on the main-sequence still possess a disk, and that this disk then decays in the next 400 Myr. The 5 stars in our sample which have an infrared excess, are probably older than 3 Gyr, however, from which we suggest that the disks around cool stars may survive longer than those among earlier-type objects. For two stars in our sample, the fractional luminosity of the disk is significantly higher than for typical Vega-type stars, and approaches the exceptional value observed for beta Pic. To investigate the correlation between an infrared excess and planets/companion stars, we combine our results with the first results of the CORALIE survey. None of the until now detected radial-velocity variables display an infrared excess. That the stars for which an infrared excess is found lack a companion, can however not be concluded at the present stage. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASAAstronomy and Astrophysics 04/2000; 357:533-542. · 4.59 Impact Factor -
Article: ISO Spectroscopy of Circumstellar Dust surrounding Isolated Herbig Ae/Be Stars
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ABSTRACT: Herbig Ae/Be stars -- hereafter HAEBE stars -- are assumed to be the evolutionary precursors of Vegalike stars. In isolated HAEBE stars the IR excess is produced uniquely by the circumstellar disk which can be seen as an early evolutionary stage of a forming planetary system. We present and discuss ISO spectra of isolated HAEBE stars, with the aim of identifying the main dust components and linking them with observations of solar-system dust.Thermal Emission Spectroscopy and Analysis of Dust, Disks, and Regoliths. 02/2000; 196:53-61. -
Article: Low-temperature crystallization of silicate dust in circumstellar disks.
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ABSTRACT: Silicate dust in the interstellar medium is observed to be amorphous, yet silicate dust in comets and interplanetary dust particles is sometimes partially crystalline. The dust in disks that are thought to be forming planets around some young stars also appears to be partially crystalline. These observations suggest that as the dust goes from the precursor clouds to a planetary system, it must undergo some processing, but the nature and extent of this processing remain unknown. Here we report observations of highly crystalline silicate dust in the disks surrounding binary red-giant stars. The dust was created in amorphous form in the outer atmospheres of the red giants, and therefore must be processed in the disks to become crystalline. The temperatures in these disks are too low for the grains to anneal; therefore, some low-temperature process must be responsible. As the physical properties of the disks around young stars and red giants are similar, our results suggest that low-temperature crystallization of silicate grains also can occur in protoplanetary systems.Nature 11/1999; 401(6753):563-5. · 36.28 Impact Factor -
Article: The ISO spectrum of the young star HD 142527
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ABSTRACT: We present a detailed analysis of SWS (2.35-45 mu m) and LWS (43-200 mu m) spectra obtained with the Infrared Space Observatory (ISO) (Kessler et al. 1996) of the dusty circumstellar disk surrounding the isolated young Fe star HD 142527. Two dust populations can be clearly discriminated: a warm component which is dominated by very strong silicate emission at 10 mu m, but in which C-rich dust is observable as well, and a cool component, of which the spectrum is dominated by O-rich dust features (C-rich dust not cause obvious features in the far infrared). Besides silicates, crystalline water-ice and hydrous silicates - which are also found in interplanetary dust particles - are present in the cold circumstellar environment. The ISO spectrum of HD 142527 differs markedly from that of HD 100546 (Malfait et al. 1998b) and other objects in a similar evolutionary stage and with a similar broad-band energy distribution. No clear correlations between the spectral dust signatures and the stellar parameters can be found at the present stage. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASAAstronomy and Astrophysics 04/1999; 345:181-186. · 4.59 Impact Factor -
Article: EMISSION FROM DISKS AROUND HERBIG Ae AND T TAURI STARS W.F. Thi
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ABSTRACT: We present the initial results of a deep ISO-SWS survey for the low J pure rotational emission lines of H 2 toward a number of Herbig Ae and T Tauri stars. The objects are selected to be as isolated as possible from molecular clouds, with a spectral energy distribution characteristic of a circumstellar disk. For most of them the presence of a disk has been established directly by millimeter interferometry. The S(1) line is detected in most sources with a peak flux of 0.3--1 Jy. The S(0) line is definitely seen in 2 objects: GG Tau and HD 163296. The observations suggest the presence of "warm" gas at T kin ß 100 K with a mass of a few % of the total gas + dust mass, derived assuming a gas-to-dust ratio of 100:1. The S(1) peak flux does not show a strong correlation with spectral type of the central star or continuum flux at 1.3 millimeter. Possible origins for the warm gas seen in H 2 are discussed, and comparisons with model calculations are made. Key words: molecular hydrogen; Herbi...04/1999; -
Article: H2 emission from disks around Herbig Ae and T Tauri stars
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ABSTRACT: We present the initial results of a deep ISO-SWS survey for the low J pure rotational emission lines of H2 toward a number of Herbig Ae and T Tauri stars. The objects are selected to be as isolated as possible from molecular clouds, with a spectral energy distribution characteristic of a circumstellar disk. For most of them the presence of a disk has been established directly by millimeter interferometry. The S(1) line is detected in most sources with a peak flux of 0.3-1 Jy. The S(0) line is definitely seen in 2 objects: GG Tau. The observations suggest the presence of ``warm'' gas at Tkin~ 100 K with a mass of a few % of the total gas + dust mass, derived assuming a gas-to-dust ratio of 100:1. The S(1) peak flux does not show a strong correlation with spectral type of the central star or continuum flux at 1.3 millimeter. Possible origins for the warm gas seen in H2 are discussed, and comparisons with model calculations are made.02/1999; 427:767. -
Article: SWS observations of the β Pictoris dust disk
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ABSTRACT: We present in this paper a full SWS scan of the β Pictoris infrared spectrum obtained with the Short Wavelength Spectrometer on board the ISO spacecraft. The data, spanning a range from 2.4 to 45 μm can be divided into two subparts. The stellar emission produced by the star photosphere (2.4-4 μm) is shown and compared to stars atmosphere models in order to derive some physical properties of the star such as effective temperature and radius. The dust emission (5-45 μm) arises from heated dust grains in orbit around the star and arranged geometrically in a disk. We carefully examine this part and search for dust features predicted by β Pictoris dust disk models. If some crystalline silicate features are undoubtedly present in the spectrum, they cannot be attributed to crystalline forsterite. The subclass of crystalline silicate emitting these features cannot, according to the current knowledge in this field, be identified. Once again, the β02/1999; 427:385. -
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. -
Article: The spectrum of the young star HD 100546 observed with the Infrared Space Observatory
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ABSTRACT: It is generally assumed that planets form in the dusty disks that surround young stars (Beckwith & Sargent 1996). The Infrared Space Observatory (Kessler et al. 1996) now enables us to determine the characteristics of these disks with unprecedented spectral resolution and signal-to-noise. We present here ISO spectra of the disk that surrounds the young star HD 100546. A remarkable variety of emission features of carbon- and oxygen-rich dust occurs. Most prominent are a series of emission features that can be attributed to silicates in crystalline form, mostly forsterite. In the interstellar medium and HII regions the silicate dust is mostly amorphous, but crystalline silicates are found in comets, meteorites and interplanetary dust particles. The forsterite features of HD 100546 are astonishingly similar to those observed in the ISO spectrum of Comet Hale-Bopp (Crovisier et al. 1997), strengthening the hypothesis that the disk around HD 100546 contains a huge swarm of comets (Grady et al. 1997). We argue that the crystallisation process occurs during the early evolution of the circumstellar disks of young stars and speculate about the formation of an Oort cloud around HD 100546. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASAAstronomy and Astrophysics 03/1998; 332:L25-L28. · 4.59 Impact Factor -
Article: An ultraviolet, optical and infrared study of Herbig Ae/Be stars
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ABSTRACT: We have selected a list of 45 Herbig Ae/Be-type candidates on the base of their IRAS colors and their spectral types. We propose the presence of a broad infrared excess as a defining criterion for these stars, rather than the detection of circumstellar nebulosity. In this way, our selection also includes more evolved young stars, that are no longer embedded in their star-forming region. A few objects in our sample are well-known Herbig Ae/Be stars, others are new. New optical and near-infrared photometric observations, as well as ultraviolet ones, are presented. The position of the objects in several color-color diagrams, as well as their de-reddened energy distributions, permit a reliable classification. Three objects probably are binaries with a cool secondary, 9 appear to be related to the Vega-type stars and 33 objects can be classified as genuine Herbig Ae/Be stars. The majority of the Herbig Ae/Be stars have a dusty environment consisting of a distinct hot and cool component. These isolated Herbig Ae/Be stars suggest an evolution from embedded Herbig Ae/Be stars to beta Pictoris-like main-sequence stars, an evolution in which planet formation may play an important role. Based on observations obtained at the European Southern Observatory, La Silla, Chile; and at the Swiss Telescope, La Silla, Chile; and at the Swiss Telescope, Hochalpine Forschungsstation Jungfraujoch, Switzerland, and with the International Ultraviolet Explorer (IUE)Astronomy and Astrophysics 02/1998; 331:211-223. · 4.59 Impact Factor -
Article: HD 139614, HD 142666 and HD 144432: evidence for circumstellar disks
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ABSTRACT: The morphology of the material surrounding Herbig Ae/Be stars is still a topic of debate. Distinct models with different geometry and optical thickness are found to be compatible with the observed IR excess. This indicates that it is not possible to resolve this question by only looking at the fluxes. Therefore, this paper compares three known Herbig Ae/Be stars with very similar energy distributions but with different photometric behaviour, showing different spectral line profiles and circumstellar extinction. These three Herbig Ae/Be stars are studied both photometrically and spectroscopically. It is found that the parameters from the Kurucz atmosphere model, fitting their spectral energy distributions, are very alike. Notwithstanding this similarity in spectral energy distribution, HD 142666 has several indications of CS material in the line of sight, while HD 139614 and HD 144432 do not show any of these indications. Therefore the inner dust shell of these stars is assumed to be not spherical, but rather flattened. The observed differences are then interpreted as a difference in CS disk inclination with respect to the observer. As a result, it is concluded that at least some of the Herbig Ae/Be stars are surrounded by disks and also that modeling energy distributions should be done while having other restraining observations in mind. Based on observations collected at the European Southern Observatory, Chile; and at the Swiss Telescope, La Silla, ChileAstronomy and Astrophysics 12/1997; 329:131-136. · 4.59 Impact Factor -
Article: Comet Hale-Bopp, Circumstellar Dust, and the Interstellar Medium
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ABSTRACT: The infrared spectrum of comet Hale-Bopp, as observed with the Infrared Space Observatory (ISO), presents astonishing similarities with those of the dusty disks surrounding young stars. The prominent spectral features of crystalline silicates that occur in comets, have been observed in the circumstellar media of young and old stars, yet not in the interstellar medium. We suggest from the analogy with the young-star spectra that the crystalline nature of the silicates in comet Hale-Bopp dates from the early evolution of the solar system, and also that an Oort Cloud or Kuiper Belt is presently forming around young stars observed by ISO.Earth Moon and Planets 08/1997; 79(1):265-274. · 0.67 Impact Factor -
Article: ISO Observations of Isolated Herbig AE/BE Stars
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ABSTRACT: We report on SWS and LWS observations of the circumstellar disks of young stars of a few solar masses. The ISO spectra of these objects present a diversity of emission features of carbon-rich and oxygen-rich grains. The similarity of the forsterite spectra observed for Comet Hale-Bopp and the Haebe star HD100546 is particularly striking and provides a new argument that huge comet swarms are formed in the disks surrounding young stars. While the data suggest that the formation of crystalline silicates in the dust disks essentially occurs when a Haebe star has already reached the main sequence, no clear correlation with stellar age only is apparent.Astrophysics and Space Science 12/1996; 255(1):25-33. · 1.69 Impact Factor -
Article: SWS observations of young main-sequence stars with dusty circumstellar disks.
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ABSTRACT: We present SWS full-scan observations of three objects that are thought to be in a stage of evolution between the youngest, embedded, Herbig Ae/Be stars and Beta Pictoris, a young main-sequence star with a circumstellar disk. The 8-12μm spectra of all three stars cannot be understood in terms of purely amorphous silicates, but require the presence of crystalline silicates in different amounts. Around two objects both oxygen-rich and carbon-rich dust particles are present: the spectrum of HD 100546 displays the full set of UIR features; in the spectrum of HD 142527 both the 3.29 and 3.42μm emission features are observed, as well as a strong 3.51μm feature. The spectrum of HD 100546 is extremely rich in silicate features in the spectral range from 20 to 45μm; some of these features strongly suggest the presence of appreciable amounts of crystalline silicates.Astronomy and Astrophysics 10/1996; 315:L245-L248. · 4.59 Impact Factor -
Article: ISO Observations of Isolated Herbig Ae/Be Stars (Contributed Talk)
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Article: ADONIS Detection and Models of the Pre-Main-Sequence Disk of HD 100546 (Contributed Talk)
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1999
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Universiteit van Amsterdam
- Astronomical Institute Anton Pannekoek
Amsterdam, North Holland, Netherlands
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