Physics
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996sao..reptr....h&link_type=abstract
Technical Report, Smithsonian Astrophysical Observatory ,Cambridge,MA United States
Physics
Astronomical Models, Protostars, Gravitational Collapse, Mathematical Models, Stellar Gravitation, Pre-Main Sequence Stars, Molecular Clouds, Shock Waves, T Tauri Stars, Stellar Spectra, Stellar Envelopes, Stellar Physics, Astronomical Maps, Hubble Space Telescope, Interferometry, Red Shift, Time Dependence, Spectral Energy Distribution, Computerized Simulation, Contraction, Hydrostatic Pressure
Scientific paper
Recognizing that protostellar clouds are unlikely to be completely spherical, we explore some effects of initial cloud geometry by considering collapse from a sheet initially in hydrostatic equilibrium. A qualitatively different feature of sheet collapse compared with spherical contraction is the development of relatively evacuated cavities in the infalling dusty cloud, which arise because material falls in first along the shortest dimension to the central gravitating mass. We introduce a flattening parameter eta which joins our approach with that of Terebey et al. (1984), while values of eta = 0 corresponds exactly to that of Terebey et al., what values of eta is approximately 2-3 are indicated for typical times during the collapse of the self-gravitating isothermal sheet. This analytic models reproduces the main features of our previous numerical time-dependent simulations. We performed detailed radiative transfer calculations which suggest that these collapse cavities can naturally explain the morphological appearance of many reflection nebulae around young stars on small distance scales without requiring initially diverging outflows. Sheet collapse models can simultaneously explain small-scale reflection nebula morphologies and dust envelope emission properties of many young stellar objects more easily than the standard spherical collapse models. The sheet collapse picture suggests that protostars, i.e. young stellar objects still accreting a large fraction of their mass from infalling envelopes, may be optically visible over a substantial range of system inclinations to the line of sight. These results may be especially relevant to cases where fragmentation and collapse has been triggered by an external impulse, such as a shock wave. We showed how many properties of the flat-spectrum T Tauri star HL Tau can be interpreted in terms of flattened protostellar cloud collapse. Overall, our model explains the main features of the flattened infalling gas cloud found by Hayashi et al. (1993) from CO-13 interferometric mapping, the scattered light nebulae observed bv Beckwith et al. (1989) and Beckwith & Birk (1995), the redshifted C2 absorption found by Grasdalen et al. (1989), and the flat-spectral energy distribution (Calvet et al. 1994). The model cannot reproduce all of the fine detail seen in the Hubble Space Telescope image of HL Tau (Stapelfeldt et al., 1995), but these observations are more consistent with an infall model than with a disk.
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