Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992a%26a...254..315m&link_type=abstract
Astronomy and Astrophysics, Vol.254, NO. FEB(I), P. 315, 1992
Astronomy and Astrophysics
Astrophysics
151
Stellar Winds, Molecular Outflows, Sio Chemistry
Scientific paper
We present observations and limited mapping of the J = 2 - 1, J = 3 - 2 and J = 5 - 4 lines of SiO toward the molecular outflows in L1448, 81, NGC 2071 and Cepheus A. We have also measured a single transition of SO, H13CO+, H13CN and HN13C toward these sources. The SiO profiles are very different from those of the other molecules. In contrast to the narrow lines (0.8 - 3km s-1) observed in H13CO+, H13CN and HN13C, typical of the ambient quiescent gas in these clouds, the SiO profiles exhibit remarkable broad linewidths (7 to 50 km s-1). At most the SiO shows very weak emission in the ambient component. The SO profiles are a mixture of strong emission from the narrow ambient component and a weaker broad line similar to that of SiO. The broad SiO emission is observed in the velocity range of the high velocity wings of CO in these molecular outflows. However, the SiO/CO intensity ratio in L 1448 increases as the radial velocity with respect to the narrow ambient component increases. The different line- widths, radial velocities and spatial distribution of the SiO emission with respect to other molecules, indicate the presence of at least two different regions along the line of sight: The quiescent gas characterized by narrow lines and the "shocked gas" defined by the large linewidths of SiO. Any comparison of the SiO abundances with those of other molecules must take into account this two-component structure.
The SiO abundances in the cold (14 K) quiescent gas are < 4-12 Similar SiO abundance is also found toward the quiescent warm gas (60 K) around Cep A. The SiO abundances in the shocked gas are larger than those in the quiescent gas by two orders of magnitude with the largest enhancement (103) in L1448. The enhancement increases up to 105 for the extreme velocities of the SiO emission. At the terminal velocities of the high velocity gas near the exciting star of the main molecular outflow in L1448, the SiO abundance, ≍2 10-6, is a factor of 100 larger than that measured toward the very luminous star IRc2 in Orion A. This implies that at least 3O/o of silicon is in gas phase. We briefly discuss the chemistry of SiO in molecular clouds. The large enhancements of the SiO abundances (up to 106) in the high velocity gas and the sizes of the SiO emission regions could be explained if SiO is formed in shocked layers, in which the shock waves have partially destroyed the grains, ejecting silicon into the gas phase.
Bachiller Rafael
Fuente Asuncion
Martín-Pintado Jesús
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