Methanol enhancemant in young bipolar outflows.
Abstract
We report observations of the 2_k_-1_k_, 3_k_-2_k_, and 5_k_-4_k_ thermal lines of CH_3_OH toward young bipolar outflows. Strong emission (~>6K, in the case of L 1157) is observed toward the shocked molecular regions associated with the lobes of some outflows, whereas the emission from the cold quiescent material in the surrounding molecular core is rather weak (~<1K). We have derived the methanol abundance by combining the CH_3_OH data with new C^18^O observations. It results that methanol is enhanced by large factors toward the shocked regions (~400 toward the blue-shifted lobe of L1157). Although the shocked gas is known to be heated to about 100K, the CH_3_OH rotation temperatures have moderate values: in L 1157 T_rot_ increases from 8K in the ambient gas to 12K in the shocked region. Radiative transfer calculations, carried out to simulate the CH_3_OH excitation in different physical circumstances, confirm that in the range of physical conditions discussed here the methanol molecules are very subthermally excited, and T_rot_ is not a good meusure of the kinetic temperature (T_rot_<<T_K_). The observed methanol abundance enhancements are likely caused by processes of desorption of grain mantles in shocks.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 1995
- Bibcode:
- 1995A&A...295L..51B
- Keywords:
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- STARS: FORMATION;
- ISM: JETS AND OUTFLOWS;
- ISM: INDIVIDUAL OBJECTS: L1157;
- ISM: MOLECULES;
- ISM: ABUNDANCES;
- RADIO LINES: ISM