1998A&A...330..901C

Detections of CO, CS, SO, C₂H, HCO⁺, HCN, HNC, H₂CO, and C₃H₂ are reported from LIRS 36, a star-forming region in the Small Magellanic Cloud. C¹⁸O, NO, CH₃OH, and most notably CN have not been detected, while the rare isotopes ¹³CO and, tentatively, C³⁴S 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)/I_CO conversion factor is ≃4.8x10²¹cm^–2(K.km/s)^–1, slightly larger than the local Galactic disk value. The CO (1-0) beam averaged column density then becomes N(H₂)≃3.7x10²¹cm^–2 and the density n(H₂)≃100cm^–3. A comparison with X-values from Rubio et al. (1993A&A...271....9R) shows that on small scales (R≃10pc) 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(¹³CO)/I(C¹⁸O) line intensity ratio indicates an underabundance of ¹²C¹⁸O relative to ¹³C¹⁶O w.r.t. Galactic clouds. I(HCO⁺)/I(HCN) and I(HCN)/I(HNC) line intensity ratios are >1 and trace a warm (T_kin>10K) 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 n(H₂)=10⁵-10⁷cm^–3. 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 H₂CO 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.