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تسجيل مستخدم جديدInvestigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
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CONTEXT - Low-mass stars form with disks in which the coagulation of grains may eventually lead to the formation of planets. It is not known when and where grain growth occurs, as models that explain the observations are often degenerate. A way to break this degeneracy is to resolve the sources under study. AIMS - To find evidence for the existence of grains of millimetre sizes in disks around in T Tauri stars, implying grain growth. METHODS - The Australia Telescope Compact Array (ATCA) was used to observe 15 southern T Tauri stars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 millimetre. The five Lupus sources were also observed with the Submillimeter Array (SMA) at 1.4 millimetre. Our new data are complemented with data from the literature to determine the slopes of the spectral energy distributions in the millimetre regime. RESULTS - Ten sources were detected at better than 3sigma with the ATCA, with sigma ~1-2 mJy, and all sources that were observed with the SMA were detected at better than 15sigma, with sigma ~4 mJy. Six of the sources in our sample are resolved to physical radii of ~100 AU. Assuming that the emission from such large disks is predominantly optically thin, the millimetre slope can be related directly to the opacity index. For the other sources, the opacity indices are lower limits. Four out of six resolved sources have opacity indices <~1, indicating grain growth to millimetre sizes and larger. The masses of the disks range from < 0.01 to 0.08 MSun, which is comparable to the minimum mass solar nebula. A tentative correlation is found between the millimetre slope and the strength and shape of the 10-micron silicate feature, indicating that grain growth occurs on similar (short) timescales in both the inner and outer disk.
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In this article we present the results from mid-infrared spectroscopy of a sample of 14 T Tauri stars with silicate emission. The qualitative analysis of the spectra reveals a correlation between the strength of the silicate feature and its shape sim
ilar to the one which was found recently for the more massive Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical spectra of amorphous olivine with different grain sizes suggests that this correlation is indicating grain growth in the disks of T Tauri stars. Similar mechanisms of grain processing appear to be effective in both groups of young stars.
Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studi
es of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar-mass stars limited primarily to the 10 um region. The observed 10 and 20 um silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 um indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 um ratios and require even larger grains emitting at 20 um than at 10 um. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 um feature strength vs. shape trend is not correlated with age or Halpha equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength vs. shape trend is related to spectral type, however, with M stars showing significantly flatter 10 um features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation, stellar/disk winds, etc.).
We report detection of continuum emission at 850 and 450 micron from disks around four Classical T Tauri stars in the MBM 12 (L1457) young association. Using a simple model we infer masses of 0.0014-0.012 M_sun for the disk of LkHa 263 ABC, 0.005-0.0
21 M_sun for S18 ABab, 0.03-0.18 M_sun for LkHa 264 A, and 0.023-0.23 M_sun for LkHa 262. The disk mass found for LkHa 263 ABC is consistent with the 0.0018 M_sun inferred from the scattered light image of the edge-on disk around component C. Comparison to earlier 13CO line observations indicates CO depletion by up to a factor 300 with respect to dark-cloud values. The spectral energy distributions (SED) suggest grain growth, possibly to sizes of a few hundred micron, but our spatially unresolved data cannot rule out opacity as an explanation for the SED shape. Our observations show that these T Tauri stars are still surrounded by significant reservoirs of cold material at an age of 1-5 Myr. We conclude that the observed differences in disk mass are likely explained by binary separation affecting the initial value. With available accretion rate estimates we find that our data are consistent with theoretical expectations for viscously evolving disks having decreased their masses by ~30%.
Context: T Tauri stars have X-ray luminosities ranging from L_X = 10^28-10^32 erg/s. These luminosities are similar to UV luminosities (L_UV 10^30-10^31 erg/s) and therefore X-rays are expected to affect the physics and chemistry of the upper layers
of their surrounding protoplanetary disks. Aim: The effects and importance of X-rays on the chemical and hydrostatic structure of protoplanetary disks are investigated, species tracing X-ray irradiation (for L_X >= 10^29 erg/s) are identified and predictions for [OI], [CII] and [NII] fine structure line fluxes are provided. Methods: We have implemented X-ray physics and chemistry into the chemo-physical disk code ProDiMo. We include Coulomb heating and H2 ionization as heating processes and primary and secondary ionization due to X-rays in the chemistry. Results: X-rays heat up the gas causing it to expand in the optically thin surface layers. Neutral molecular species are not much affected in their abundance and spatial distribution, but charged species such as N+, OH+, H2O+ and H3O+ show enhanced abundances in the disk surface. Conclusions: Coulomb heating by X-rays changes the vertical structure of the disk, yielding temperatures of ~ 8000 K out to distances of 50 AU. The chemical structure is altered by the high electron abundance in the gas in the disk surface, causing an efficient ion-molecule chemistry. The products of this, OH+, H2O+ and H3O+, are of great interest for observations of low-mass young stellar objects with the Herschel Space Observatory. [OI] (at 63 and 145 mic) and [CII] (at 158 mic) fine structure emission are only affected for L_X > 10^30 erg/s.
Our aim is to determine the presence and location of the emission from polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young stars with disks using large aperture telescopes. VLT-VISIR N-band spectra and VLT-ISAAC and VLT-
NACO L-band spectra of 29 sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 micron PAH features. Spatial-extent profiles of the features and the continuum emission are derived and used to associate the PAH emission with the disks. The results are discussed in the context of recent PAH-emission disk models. The 3.3, 8.6, and 11.2 micron PAH features are detected toward a small fraction of the T Tauri stars, with typical upper limits between 1E-15 and 5E-17 W/m^2. All 11.2 micron detections from a previous Spitzer survey are confirmed with (tentative) 3.3 micron detections, and both the 8.6 and the 11.2 micron features are detected in all PAH sources. For 6 detections, the spatial extent of the PAH features is confined to scales typically smaller than 0.12-0.34, consistent with the radii of 12-60 AU disks at their distances (typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one or more of the PAH features are more extended than the hot dust continuum of the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is confirmed as smaller than for the large grains. For HD 100546, the 3.3 micron emission is confined to a small radial extent of 12 +- 3 AU, most likely associated with the outer rim of the gap in this disk. Gaps with radii out to 10-30 AU may also affect the observed PAH extent for other sources. For both Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2 micron features are consistent with larger (>= 100 carbon atoms) PAHs.
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