Astron. Astrophys. 324, 203-210 (1997)

6. Conclusions

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:

1. 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 ( 10 ).
2. 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 .
3. 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.
4. 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

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