Supernovae and molecular clouds

Abstract

IN a study of the CO distribution in the Galaxy, Bash and Peters¹ concluded that there is an upper limit of 30 Myr to the age of stellar systems which contain CO. Younger systems, launched by the impulse of the density wave show abundant CO emission. Systems which have existed for longer than 30 Myr display no CO emission. Based on their initial observations, Bash and Peters predicted that there would be a correlation between the presence of CO emission and the turnoff in open clusters. Observations by Green confirm this prediction² showing CO emission in at least 24 of 28 clusters with turnoff earlier than spectral type BO and no evidence of CO emission in 35 of 38 clusters with turnoff later than BO. We suggest that there is a critical mass M_crit such that for M > M_crit stars do not evolve to an explosive phase and hence the surrounding molecular cloud is substantially unaffected by the evolving stars. As the system ages, the most massive star with M < M_crit finally evolves to an explosive endpoint and the resulting supernova dissipates the dense molecular cloud. From the cluster observations we identify M_crit ∼ 15–20M_⊙, the mass of a BO star (ref. 3 and E. Green, private communication), although there is some uncertainty in luminosity class.