Detection of neutral carbon in the M 31 dark cloud D 478

Emission from the ³P₁-³ P₀ [CI] transition at 492 GHz has been detected towards the dark cloud D 478 in the Local Group galaxy M 31. Using literature detections of the lower ¹²CO and ¹³CO transitions, models for the gas distribution in D 478 are discussed. The observed CO and C line ratios can be explained by two-component models (high density cores and low-density envelopes); single-density models appear less likely. The models indicate kinetic temperatures of the order of T_k ≈ 10 K. The beam-averaged column density of neutral carbon is 0.3-0.8 times that of CO, whereas the total carbon to hydrogen ratio N_C/N_H is 5-3 × 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⊙ pc^-2 there appears to be no need to invoke the presence of very cold and very massive clouds. Rather, D 478 appears to be comparable to Milky Way dark cloud complexes at the higher metallicity expected from its central location in M 31. In particular, several similarities between D 478 and the Galactic Taurus-Auriga dark cloud complex may be noted.