A low optical depth region in the inner disk of the Herbig Ae star HR 5999 (original) (raw)
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Astronomy and Astrophysics, 2010
We present new long-baseline spectro-interferometric observations of the Herbig Ae star HD 163296 (MWC 275) obtained in the H and K bands with the AMBER instrument at the VLTI. The observations cover a range of spatial resolutions between ∼3 and ∼12 milliarcseconds, with a spectral resolution of ∼30. With a total of 1481 visibilities and 432 closure phases, they represent the most comprehensive (u, v) coverage achieved so far for a young star. The circumstellar material is resolved at the sub-AU spatial scale and closure phase measurements indicate a small but significant deviation from point-symmetry. We discuss the results assuming that the near-infrared excess in HD 163296 is dominated by the emission of a circumstellar disk. A successful fit to the spectral energy distribution, near-infrared visibilities and closure phases is found with a model in which a dominant contribution to the H and K band emission originates in an optically thin, smooth and point-symmetric region extending from about 0.1 to 0.45 AU. At a distance of 0.45 AU from the star, silicates condense, the disk becomes optically thick and develops a puffed-up rim, whose skewed emission can account for the non-zero closure phases. We discuss the source of the inner disk emission and tentatively exclude dense molecular gas as well as optically thin atomic or ionized gas as its possible origin. We propose instead that the smooth inner emission is produced by very refractory grains in a partially cleared region, extending to at least ∼0.5 AU. If so, we may be observing the disk of HD 163296 just before it reaches the transition disk phase. However, we note that the nature of the refractory grains or, in fact, even the possibility of any grain surviving at the very high temperatures we require (∼2100−2300 K at 0.1 AU from the star) is unclear and should be investigated further.
The absence of the 10�?m?silicate feature in the isolated Herbig Ae star HD�100453
Astronomy and Astrophysics, 2002
We analyse the optical and IR spectra, as well as the spectral energy distribution (UV to mm) of the candidate Herbig Ae star HD 100453. This star is particular, as it shows an energy distribution similar to that of other isolated Herbig Ae/Be stars (HAEBEs), but unlike most of them, it does not have a silicate emission feature at 10 µm, as is shown in . We confirm the HAEBE nature of HD 100453 through an analysis of its optical spectrum and derived location in the H-R diagram. The IR spectrum of HD 100453 is modelled by an optically thin radiative transfer code, from which we derive constraints on the composition, grain-size and temperature distribution of the circumstellar dust. We show that it is both possible to explain the lack of the silicate feature as (1) a grain-size effect -lack of small silicate grains, and (2) a temperature effect -lack of small, hot silicates, as proposed by Dullemmond et al. (2001), and discuss both possibilities.
The circumstellar disc around the Herbig AeBe star HD169142
Monthly Notices of the Royal Astronomical Society, 2006
We present 7 mm and 3.5 cm wavelength continuum observations towards the Herbig AeBe star HD169142 performed with the Very Large Array (VLA) with an angular resolution of 1 arcsec. We find that this object exhibits strong (4.4 mJy), unresolved (1 arcsec) 7 mm continuum emission, being one of the brightest isolated Herbig AeBe stars ever detected with the VLA at this wavelength. No emission is detected at 3.5 cm continuum, with a 3σ upper limit of 0.08 mJy. From these values, we obtain a spectral index α 2.5 in the 3.5 cm to 7 mm wavelength range, indicating that the observed flux density at 7 mm is most likely dominated by thermal dust emission coming from a circumstellar disc. We use available photometric data from the literature to model the spectral energy distribution (SED) of this object from radio to near-ultraviolet frequencies. The observed SED can be understood in terms of an irradiated accretion disc with low mass accretion rate,Ṁ acc 10 −8 M yr −1 , surrounding a star with an age of 10 Myr. We infer that the mass of the disc is 0.04 M , and is populated by dust grains that have grown to a maximum size of 1 mm everywhere, consistent with the lack of silicate 10 µm emission. These features, as well as indications of settling in the wall at the dust destruction radius, led us to speculate that the disc of HD169142 is in an advanced stage of dust evolution, particularly in its inner regions.
Subaru Near-Infrared Imaging of Herbig Ae Stars*
Publications of the Astronomical Society of Japan, 2010
We report results of H-band (= 1.65 m) imaging observations of young intermediate-mass stars using the Subaru 8.2-m Telescope with the adaptive optics AO36 and the infrared coronagraph CIAO. The targets consist of 16 Herbig Ae/Be stars (15 Herbig Ae stars and one Herbig Be star) and four additional main-sequence stars with infrared excesses. Five protoplanetary disks have been spatially resolved around the Herbig Ae stars with ages of 2-8 Myr. The resolved disks have outer radii of several 100 AU, and their surface brightnesses range from 10 mag arcsec 2 to 18 mag arcsec 2. The images reveal various morphologies in optically thick disks: spiral arms around AB Aur, a banana-split structure with an outer arm for HD 142527, a compact circumprimary disk of HD 150193, a faint discontinuous ring around HD 163296, and an unstructured face-on disk of HD 169142. The detection of an optically thick disk in scattered light implies that it is vertically flared, and intercepts stellar light at least in the outer region where those images were obtained. However, the surface brightness distribution, the resolved structure, and other observational characteristics suggest that the disks are unlikely to be continuously flared young disks with small grains well mixed with gas. The detection rate and the disk brightness do not correlate with the stellar age and the disk mass, but there is a tendency that the brightest disks are still surrounded by longlived envelopes (AB Aur, HD 100546, HD 142527). The significant diversity of the disk structure can be attributed to the multiplicity and the initial condition of the local star-forming environments. The detections of companion candidates around our targeted stars are also reported.
8–13 μm Spectroscopy of Young Stellar Objects: Evolution of the Silicate Feature
The Astrophysical Journal, 2005
Silicate features arising from material around pre-main sequence stars are useful probes of the star and planet formation process. In order to investigate possible connections between dust processing and disk properties, 8-13 µm spectra of 34 young stars, exhibiting a range of circumstellar environments and including spectral types A to M, were obtained using the Long Wavelength Spectrometer at the W. M. Keck Observatory. The broad 9.7 µm amorphous silicate (Si-O stretching) feature which dominates this wavelength regime evolves from absorption in young, embedded sources, to emission in optically revealed stars, and to complete absence in older "debris" disk systems for both lowand intermediate-mass stars. This is similar to the evolutionary pattern seen in ISO observations of high/intermediate-mass YSOs. The peak wavelength and FWHM are centered about 9.7 µm and ∼2.3 µm, respectively, corresponding to amorphous olivine, with a larger spread in FWHM for embedded sources and in peak wavelength for disks. In a few of our objects that have been previously identified as class I low-mass YSOs, the observed silicate feature is more complex, with absorption near 9.5 µm and emission peaking around 10 µm. Although most of the emission spectra show broad classical features attributed to amorphous silicates, small variations in the shape/strength may be linked to dust processing, including grain growth and/or silicate crystallization. For some of the Herbig Ae stars in the sample, the broad emission feature has an additional bump near 11.3 µm, similar to the emission from crystalline forsterite seen in comets and the debris disk β Pictoris. Only one of the low-mass stars, Hen 3-600A, and one Herbig Ae star, HD 179218 clearly show strong, narrow emission near 11.3 µm. We study quantitatively the evidence for evolutionary trends in the 8-13 µm spectra through a variety of spectral shape diagnostics. Based on the lack of correlation between these diagnostics and broad-band infrared luminosity characteristics for silicate emission sources we conclude that although spectral signatures of dust processing are present, they can not be connected clearly to disk evolutionary stage (for optically thick disks) or optical depth (for optically thin disks). The diagnostics of silicate absorption features (other than the central wavelength of the feature), however, are tightly correlated with optical depth and thus do not probe silicate grain properties.
A family portrait of disk inner rims around Herbig Ae/Be stars
Astronomy & Astrophysics, 2020
Context. The innermost astronomical unit (au) in protoplanetary disks is a key region for stellar and planet formation, as exoplanet searches have shown a large occurrence of close-in planets that are located within the first au around their host star. Aims. We aim to reveal the morphology of the disk inner rim using near-infrared interferometric observations with milli-arcsecond resolution provided by near-infrared multitelescope interferometry. Methods. We provide model-independent reconstructed images of 15 objects selected from the Herbig AeBe survey carried out with PIONIER at the Very Large Telescope Interferometer, using the semi-parametric approach for image reconstruction of chromatic objects. We propose a set of methods to reconstruct and analyze the images in a consistent way. Results. We find that 40% of the systems (6/15) are centrosymmetric at the angular resolution of the observations. For the rest of the objects, we find evidence for asymmetric emission due to modera...
Gas and dust in the inner disk of the Herbig Ae star MWC 758
Astronomy and Astrophysics, 2008
We investigate the origin of the near-infrared emission of the Herbig Ae star MWC 758 on sub-astronomical unit (AU) scales using spectrally dispersed low resolution (R = 35) AMBER/VLTI interferometric observations in the H (1.7 µm) and K (2.2 µm) bands. We find that the K band visibilities and closure phases are consistent with the presence of a dusty disk inner rim located at the dust evaporation distance (0.4 AU) while the bulk of the H band emission arises within 0.1 AU from the central star. Comparing the observational results with theoretical model predictions, we suggest that the H band emission is dominated by an hot gaseous accretion disk.
EMISSION FROM DISKS AROUND HERBIG Ae AND T TAURI STARS W.F. Thi
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 ux of 0.3{1 Jy. The S(0) line is de nitely seen in 2 objects: GG Tau and HD 163296. The observations suggest the presence of \warm" gas at T kin 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 ux does not show a strong correlation with spectral type of the central star or continuum ux at 1.3 millimeter. Possible origins for the warm gas seen in H 2 are discussed, and comparisons with model calculations are made.
Discs and the 10- m silicate spectra of young stellar objects with non-photospheric continua
Monthly Notices of the Royal Astronomical Society, 2001
Dust emission in the non-photospheric 10-mm continua of HL Tau and Taurus-Elias 7 (Haro6-10, GV Tau) is distinguished from foreground silicate absorption using a simple disc model with radial power-law temperature and mass±density distributions based on the IR± submm model of T Tauri stars by Adams, Lada & Shu with foreground extinction. The resulting 10-mm absorption profiles are remarkably similar to those of the field star Taurus-Elias 16 obtained by Bowey, Adamson & Whittet. The fitted temperature indices are 0.44 (HL Tau) and 0.33 (Elias 7) in agreement with Boss's theoretical models of the 200±300 K region, but lower than those of IR±submm discs (0.5±0.61; Mannings & Emerson); a significant fraction of the modelled 10-mm emission of HL Tau is optically thin, whilst that of Elias 7 is optically thick. We suggest that HL Tau's optically thin component arises from silicate dust within low-density layers above an optically thick disc.
arXiv (Cornell University), 2020
Context. The innermost astronomical unit (au) in protoplanetary disks is a key region for stellar and planet formation, as exoplanet searches have shown a large occurrence of close-in planets that are located within the first au around their host star. Aims. We aim to reveal the morphology of the disk inner rim using near-infrared interferometric observations with milli-arcsecond resolution provided by near-infrared multitelescope interferometry. Methods. We provide model-independent reconstructed images of 15 objects selected from the Herbig AeBe survey carried out with PIONIER at the Very Large Telescope Interferometer, using the semi-parametric approach for image reconstruction of chromatic objects. We propose a set of methods to reconstruct and analyze the images in a consistent way. Results. We find that 40% of the systems (6/15) are centrosymmetric at the angular resolution of the observations. For the rest of the objects, we find evidence for asymmetric emission due to moderate-to-strong inclination of a disk-like structure for ∼30% of the objects (5/15) and noncentrosymmetric morphology due to a nonaxisymmetric and possibly variable environment (4/15, ∼27%). Among the systems with a disk-like structure, 20% (3/15) show a resolved dust-free cavity. Finally, we do not detect extended emission beyond the inner rim. Conclusions. The image reconstruction process is a powerful tool to reveal complex disk inner rim morphologies, which is complementary to the fit of geometrical models. At the angular resolution reached by near-infrared interferometric observations, most of the images are compatible with a centrally peaked emission (no cavity). For the most resolved targets, image reconstruction reveals morphologies that cannot be reproduced by generic parametric models (e.g., perturbed inner rims or complex brightness distributions). Moreover, the nonaxisymmetric disks show that the spatial resolution probed by optical interferometers makes the observations of the near-infrared emission (inside a few au) sensitive to temporal evolution with a timescale down to a few weeks. The evidence of nonaxisymmetric emission that cannot be explained by simple inclination and radiative transfer effects requires alternative explanations, such as a warping of the inner disks. Interferometric observations can therefore be used to follow the evolution of the asymmetry of those disks at an au or sub-au scale.