A&A Supplement series, Vol. 126, December II 1997, 437-469
Received December 2, 1996; accepted April 15, 1997
G.A. Hirth
- R. Mundt - J. Solf
Send offprint request: R. Mundt
Max-Planck-Institut für Astronomie, Königstuhl 17,
D-69117 Heidelberg, Germany
Thüringer Landessternwarte Tautenburg,
Sternwarte 5, D-07778 Tautenburg, Germany
We have carried out a long-slit spectroscopic survey for
38 T Tauri stars (TTSs) to study the spatial and kinematic
properties of their forbidden emission line (FEL) regions.
With these observations we hope to provide more insight
into the complex physical structure of the outflows from
young stars on the smallest spatial scales observable by
long-slit spectroscopy. Due to the differential nature of the
observational method, information on the spatial properties
(offset from the stellar continuum and spatial width) on
sub-arcsec (sub-seeing) scales can be obtained. For most
TTSs the [OI] 
, 6363,
[NII] 
and [SII] 
, 6731
lines have been investigated at a typical spatial resolution
of 1.5'' and a velocity resolution of 
.
A sub-sample of 9 closeby stars (Haro 6-10, XZ Tau, UZ Tau E,
HN Tau, DO Tau,
DP Tau, UY Aur, RW Aur and V536 Aql)
has been extensively studied and the direction of their outflows
has been approximately determined by taking spectra at
several slit position angles, if not known from emission-line CCD
imaging. The spatial and kinematic properties of the FEL regions
of these 9 TTSs are described in detail. Together with 3
additional stars discussed in the literature, a
sample of 12 stars provides the basis for the following main
results of our survey: The so-called
high-velocity component (HVC) of the FELs (or gas of
high velocity which presumably
represents in many cases a HVC being blended with emission
of lower velocity) is generally
spatially more extended than the so-called low-velocity
component (LVC, or gas near the stellar velocity).
In the [SII] 
line the centroid of the high-velocity
gas is located typically at distances of 0.6'' from the TTS while for
the low-velocity gas this value is smaller on average by more than a
factor of 3. Comparing the spatial properties of the
high-velocity gas among the investigated FELs, it turns out
that the largest spatial width and the largest offset
of the centroid from the star is usually observed
in the [NII] line, while the emission region is most
compact in the [OI] 
line. In [OI] the centroid
of the high-velocity gas is typically offset by only 0.2'' from
the star whereas in [SII] and [NII] 3 and 3.5 times larger average values
have been measured, respectively. In
the case of the low-velocity gas, the smallest offset of the
emission centroid is also observed in [OI] (typically 0.1'' in
[OI] and 0.2'' in [SII]).
Our data provide additional support for the model of Kwan & Tademaru (1988, 1995) according to which the HVC observed in the FELs of many TTSs is formed in a well-collimated jet, while the LVC represents gas from a physically distinct flow component (possibly a disk wind or a disk corona). The larger spatial extent of the high-velocity gas in [SII] and [NII] compared to that in [OI] is most probably the result of a jet decreasing in density with increasing distance from the source combined with an increase in excitation. The decrease of the electron density with distance is rather obvious for a jet with diverging stream lines, but why the electron temperature increases is unclear.
keywords: stars: pre-main-sequence -- stars: variable -- stars: emission-line -- ISM: jets and outflows