Molecular abundances in the magellanic clouds .1. A multiline study of five cloud cores

Nine H II regions of the LMC were mapped in (CO)-C-13(1-0) and three in (CO)-C-12(1-0) to study the physical properties of the interstellar medium in the Magellanic Clouds. For N113 the molecular core is found to have a peak position which differs from that of the associated H II region by 20 ''. Toward this molecular core the (CO)-C-12 and (CO)-C-13 peak T-MB line temperatures of 7.3 K and 1.2 K are the highest so far found in the Magellanic Clouds. The molecular concentrations associated with N113, N44BC, N159HW, and N214DE in the LMC and LIRS 36 in the SMC were investigated in a variety of molecular species to study the chemical properties of the interstellar medium. I(HCO+)/I(HCN) and I(HCN)/I(HNC) intensity ratios as well as lower limits to the I((CO)-C-13)/I((CO)-O-18) ratio were derived for the rotational 1-0 transitions. Generally, HCO+ is stronger than HCN, and HCN is stronger than HNC. The high relative HCO+ intensities are consistent with a high ionization flux from supernovae remnants and young stars, possibly coupled with a large extent of the HCO+ emission region. The bulk of the HCN arises from relatively compact dense cloud cores. Warm or shocked gas enhances HCN relative to HNC. From chemical model calculations it is predicted that I(HCN)/I(HNC) close to one should be obtained with higher angular resolution (less than or similar to 30 '') toward the cloud cores. Comparing virial masses with those obtained from the integrated CO intensity provides an H-2 mass-to-CO luminosity conversion factor of 1.8 x 10(20) mol cm(-2) (K km s(-1))(-1) for N113 and 2.4 x 10(20) mol cm(-2) (K km s(-1))(-1) for N44BC. This is consistent with values derived for the Galactic disk.

DENSE GAS IN NEARBY GALAXIES .6. A LARGE C-12/C-13 RATIO IN A NUCLEAR STARBURST ENVIRONMENT

Toward the starburst nucleus of NGC 253, C-12/C-13 line intensity ratios from six carbon bearing molecules (CO, CN, CS, HCN, HCO+, and HNC) are used to confine the possible range of carbon and oxygen isotope ratios. A detailed analysis yields C-12/C-13 approximately 40 and O-16/O-18 approximately 200. Also reported are first detections of (CS)-C-13 and of the 0(0) - 1(-1) E line of methanol (CH3OH) in an extragalactic source.