Physics
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt.......230h&link_type=abstract
Ph.D. Thesis, University of Heidelberg, (1994)
Physics
7
Scientific paper
Our poor knowledge about the physical properties of outflows from young stars (e.g. outflow velocity, collimation, excitation) on scales of a few 100 AU distance from the source was the reason for conducting a long-slit spectroscopic survey of the forbidden emission lines from 38 classical T Tauri stars in the optical. The forbidden emission lines which are thought to be excited by internal shocks in the outflowing gas are often the only diagnostic mean to investigate these outflows since these stars have no extended jet or Herbig-Haro objects. More than half of the T Tauri stars of the survey are located within nearby star formation regions (d <= 200 pc) and for 14 objects spatial information on the properties of the forbidden emission line regions have been obtained by taking long-slit spectra at four to six slit positions which allowed an approximate determination of the outflow direction. As a surprising result of this survey all but one of these 14 T Tauri stars show spatially extended forbidden emission line regions. In most cases this is probably the result of a bipolar jet. The typical spatial extent in the outflow direction is of the order of 50 -- 500 AU. The opening angles of the outflows derived from radial velocity and velocity dispersion data suggest values between 10 and 20 degrees at distances between a few 100 and 1000 AU and values up to 2 -- 3 times higher at scales below 200 AU. This means that extended and partially well collimated outflows are a typical property of presumably most of the classical T Tauri stars. Interestingly in 50% of all bipolar outflows from T Tauri stars and embedded sources the outflow velocity of the blueshifted and redshifted part differs about a factor of 1.4 to 2.6. Probably the flow is accelerated to intrinsically different velocities on opposite sides of the star. However, an asymmetric pressure and density distribution of the surrounding circumstellar matter could also cause similar effects. A detailed study of the double-peaked profile of the forbidden emission lines gives strong evidence that the two velocity components originate in spatially different regions. The high-velocity component is probably formed in a well collimated jet whereas there are indications that the low-velocity component originates within a rotating line formation region presumably associated with a disk wind. For seven young stars of the survey spectra of at least three epochs are available. From these observations and from data in the literature it was possible to show that temporal variations in the forbidden emission lines are probably a typical property of many T Tauri stars. Studying such variations gives an additional interesting diagnostic mean for these outflows. Due to the relatively small extent of the emission regions of some forbidden emission lines (r <= 30 AU) and the high outflow velocities (≅ 200 km s-1) it is in principle possible that strong variations in the emission properties of the outflows occur within a few months. Note that so far radial velocity studies along extended jets and Herbig-Haro objects provided information only on time scales of the order of 10 to 1000 years. In studying position-velocity maps obtained from spectra with epoch differences of a few months up to a few years significant changes in the forbidden emission line flux, radial velocity, velocity dispersion and in the spatial properties of the high- and low-velocity component have been observed in seven TTSs. The different temporal variation of the two velocity components gives additional support to the idea that the two components in the double-peaked profile of the forbidden emission lines are formed in two physically separated regions.
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