Detection of Neutral Carbon in the M 31 Dark Cloud D478

Abstract

Emission from the 492 GHz CI tranition was detected towards the dark cloud D478 in M31. Using existing 12CO and 13CO measurements, models for the gas properties of D478 are discussed. The observed CO and C line ratios can be explained by two-component models (dense cores and tenuous envelopes); single-density models appear less likely. The models indicate temperatures T(kin) = 10 K. The beam-averaged C column density is 0.3 - 0.8 times that of CO, whereas the total carbon to hydrogen ratio N(C)/N(H) = 5-3 times 10**-4. The resulting CO-to-H2 conversion factor X is about half that of the Solar Neighbourhood. With temperatures of about 10 K and projected mass densities of 5-10 M(sun)/pc**2 there appears to be no need to invoke the presence of very cold and very massive clouds. Rather, D478 appears to be comparable to Milky Way dark cloud complexes such as the Taurus-Auriga dark cloud complex.