Astron. Astrophys. 324, 203-210 (1997)
We have investigated 4 dense ammonia cores (L1521D, L1521F, L1524
and L1507A) in the TMC, increasing the number of known such objects to
20 in this prototypical dark cloud complex. The main results of our
study can be summarized as follows:
- The ammonia observations reveal clumps with sizes of 0.06-0.09
pc and yield kinetic temperatures between 7.9 and 9.9 K. The actual
intrinsic linewidths are between 0.17 and 0.27 km s-1,
while the thermal linewidths are around 0.15 km s-1.
Therefore, the non-thermal velocity component is rather small. The
ratio between the intrinsic and thermal linewidth is about 1.3,
indicating that only low-mass stars can eventually form in these
NH3 cores. This is also supported by the association of two
of the NH3 cores with low luminosity IRAS counterparts
- The NH3 data show that the measured (1,1) excitation
temperatures are only slightly smaller than the rotational
temperatures suggesting that the (1,1) inversion transition is close
to being thermalised. From the peak NH3 emission hydrogen
densities (between 0.6 and 19.9 104 cm-3) have
been estimated. Moreover, from the average of all observed positions
inside the FWHP contour of the ammonia cores, the core masses (0.2 -
1.0 ) have been estimated. The derived hydrogen
densities show no remarkable difference between NH3 cores
with or without IRAS counterparts. The low M values place the
four objects towards the edge of the M distribution for ammonia
cores in the TMC, which ranges from fractions to few tens of solar
masses, with a median value of 4 .
- Estimates of the turbulent, thermal and gravitational energy have
been derived. The thermal energy is always larger than the turbulent
one. The four cores may be close to equilibrium.
- Emission at 24.0 GHz, caused by the J=9-8 transition of
HC5 N, has been detected towards the ammonia peak positions
of the four molecular cores. This increases the number of known such
objects from 11 to 15 in the TMC (cf. Benson & Myers 1980, 1983).
The derived HC5 N column densities range between 1.6
1012 and 9.2 1012 cm-2, in agreement
with results of previous HC5 N observations in the TMC.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998