Molecular abundances in the Magellanic Clouds
III. LIRS 36, a star-forming region in the Small Magellanic Cloud *
Y.-N. Chin 1, 2,
C. Henkel 3, 4,
T.J. Millar 5,
J.B. Whiteoak 6, 7 and
M. Marx-Zimmer 2
Received 17 April 1997 / Accepted 16 October 1997
Detections of CO, CS, SO, C2 H, HCO , HCN, HNC, H2 CO, and C3 H2 are reported from LIRS 36, a star-forming region in the Small Magellanic Cloud. C18 O, NO, CH3 OH, and most notably CN have not been detected, while the rare isotopes 13 CO and, tentatively, C34 S are seen. This is so far the most extensive molecular multiline study of an interstellar medium with a heavy element depletion exceeding a factor of four.
The X = N (H2)/ conversion factor is ()-1, slightly larger than the local Galactic disk value. The CO (1-0) beam averaged column density then becomes N (H2) and the density ) 100 . A comparison with X -values from Rubio et al. (1993a) shows that on small scales (R 10 pc) X -values are more similar to Galactic disk values than previously anticipated, favoring a neutral interstellar medium of predominantly molecular nature in the cores. The I (13 CO)/I (C18 O) line intensity ratio indicates an underabundance of 12 C18 O relative to 13 C16 O w.r.t. Galactic clouds. I (HCO )/I (HCN) and I (HCN)/I (HNC) line intensity ratios are 1 and trace a warm ( 10 K) molecular gas exposed to a high ionizing flux. Detections of the CS J =2-1, 3-2, and 5-4 lines imply the presence of a high density core with ) = . In contrast to star-forming regions in the LMC, the CN 1-0 line is substantially weaker than the corresponding ground rotational transitions of HCN, HNC, and CS. CO, CS, HCO , and H2 CO fractional abundances are a factor 10 smaller than corresponding values in Galactic disk clouds. Fractional abundances of HCN, HNC, and likely CN are even two orders of magnitude below their `normal', Galactic disk values. The CN/CS abundance ratio is 1. Based on chemical model calculations, we suggest that this is because of the small metallicity of the SMC, which affects the destruction of CN but not CS, and because of the high molecular core density which also favors CN destruction.
Key words: ISM: molecules galaxies: abundances galaxies: ISM Magellanic Clouds radio lines: ISM
Send offprint requests to: Y.-N. Chin, ASIAA, Taiwan, email@example.com
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998