Model infrared spectra of passively heated proto-planetary disks surrounding intermediate-mass pre-main-sequence stars

Astronomy and Astrophysics (Impact Factor: 4.48). 03/2009; 496(3). DOI: 10.1051/0004-6361/200809547

ABSTRACT Aims. We study theoretical spectra at mid-infrared (5-40 ${\mu {\rm m}}$) wavelengths of proto-planetary disks surrounding intermediate-mass pre-main-sequence stars. Observations show a wide range of spectral shapes and a rich variety in strength and shape of dust resonances. These strong variations in spectral shape reflect differences in the nature and spatial distribution of dust particles in the disk. The aim of this study is to establish what model parameters influence the mid-IR spectra of planet-forming disks.Methods. A grid of models of passively heated proto-planetary disks is used to calculate the infrared spectrum. We use hydrostatic equilibrium disk models and radiative transfer to calculate the emerging spectrum. We focus on the effects that different disk geometries (flaring, self-shadowed) and dust mineralogy have on the emerging 5-40 ${\mu {\rm m}}$ spectrum. We adopt four scenarios for the radial and vertical distribution of crystalline silicate dust.Results. In our model, the 23.5 ${\mu {\rm m}}$ forsterite band is more sensitive to emission from regions $<$30 AU, while the 33.5 ${\mu {\rm m}}$ forsterite band probes regions up to 50 AU. The 23.5 ${\mu {\rm m}}$ band strength does not depend on the degree of flaring of the disk, while the 33.5 ${\mu {\rm m}}$ band does. Only models with a substantial abundance ($>$5 percent) of crystalline silicates at a long distance from the star ($>$20-50 AU) show detectable emission in the 33.5 ${\mu {\rm m}}$ forsterite band. The carbon-dust abundance affects the strength of the dust resonances in the 10 ${\mu {\rm m}}$ spectral region, but not in the 30 ${\mu {\rm m}}$ region.

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    ABSTRACT: Context. We present a study of the extreme T Tauri star Th 28, a young stellar object in the Lupus 3 cloud whose spectrum displays all the varieties of signposts associated with early stellar activity. Th 28 is the driving source of a fast jet, making it a very promising target to study the disk-jet connection. Aims: We try to identify and investigate the different structural components that contribute to the different emission lines in the spectrum of the Th 28 central source. Methods: We obtained high-resolution visible spectroscopy with the UVES spectrograph at the VLT, using the resolved profiles of both permitted and forbidden lines as tracers of gas with different kinematic and physical properties, complemented with other observations from the literature. Results: We identify four distinct structural components that contribute to the visible emission-line spectrum of Th 28. The first one, dominating most of the permitted and forbidden lines, is probably associated with the origin of the outflow that in the past produced the Herbig-Haro objects seen to the west of Th 28. The second one is an uncollimated stellar wind characterized by high excitation and temperature, as shown by the broad profile of the intense [OIII] lines. The third component, traced only by permitted lines, appears as a redshifted tail extending up to radial velocities of +450 km s-1, which we attribute to magnetospheric accretion. From the latter component we obtain a rough estimate of 0.6-0.9 M_&sun; for the mass of the central object. Using published equivalent widths of the CaII triplet lines, we estimate an accretion rate of 4.2-6.3 × 10-8M_&sun; yr-1, comparable to the values inferred for other T Tauri stars of similar mass. The last component, which appears most clearly in the [SII] lines and is hardly seen in any lines other than those of [OI], displays signatures that we interpret as coming from rotation, perhaps formed in a disk atmosphere. Following this interpretation, we estimate a disk central hole of 0.7 AU radius. Conclusions: Even though spatially unresolved, the spectrum of Th 28 is rich in emission lines whose shapes provide important information on the distinct structures where they form. In this way we were able to identify the existence of a stellar wind, an accretion flow, the basis of a collimated outflow, and possibly a rotating disk atmosphere, obtaining rough estimates of basic parameters of the central source and its environment that should be useful for further modeling. Based on observations carried out with the ESO Very Large Telescope, Cerro Paranal, Chile, under proposals 71.C-0429(C, D); and on material retrieved from the ESO Science Archive.
    Astronomy and Astrophysics 02/2010; 511. DOI:10.1051/0004-6361/200912959 · 4.48 Impact Factor
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    ABSTRACT: We study the hydrostatic density structure of the inner disc rim around Herbig Ae stars using the thermo-chemical hydrostatic code prodimo. We compare the spectral energy distributions (SEDs) and images from our hydrostatic disc models to that from prescribed density structure discs. The 2D continuum radiative transfer in prodimo includes isotropic scattering. The dust temperature is set by the condition of radiative equilibrium. In the thermal-decoupled case, the gas temperature is governed by the balance between various heating and cooling processes. The gas and dust interact thermally via photoelectrons, radiatively, and via gas accommodation on grain surfaces. As a result, the gas is much hotter than in the thermo-coupled case, where the gas and dust temperatures are equal, reaching a few thousands K in the upper disc layers and making the inner rim higher. A physically motivated density drop at the inner radius (‘soft-edge’) results in rounded inner rims, which appear ring-like in near-infrared images. The combination of lower gravity pull and hot gas beyond ∼1 au results in a disc atmosphere that reaches a height over radius ratio z/r of 0.2, while this ratio is 0.1 only in the thermo-coupled case. This puffed-up disc atmosphere intercepts larger amount of stellar radiation, which translates into enhanced continuum emission in the 3–30 μm wavelength region from hotter grains at ∼500 K. We also consider the effect of disc mass and grain size distribution on the SEDs self-consistently feeding those quantities back into the gas temperature, chemistry and hydrostatic equilibrium computation.
    Monthly Notices of the Royal Astronomical Society 03/2011; 412(2):711 - 726. DOI:10.1111/j.1365-2966.2010.17741.x · 5.23 Impact Factor

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