2. Supernova rate in the Gould Belt
Our Galaxy produces on average SNe per century (Tammann et al. 1994), 85% of which arise from the core collapse of a massive star. This value and the stellar distribution in the Galaxy imply a local rate of 20 events Myr-1 kpc-2, in reasonable agreement with the 29 progenitors Myr-1 kpc-2 found with masses 8 M within 1 kpc from the Sun (Tammann et al. 1994). This region, however, includes the starburst Gould Belt with its 300 pc radius. Today, the Belt hosts 432 15 progenitors with masses 8 M (Comeron et al. 1994) and their maximum lifetime will imply a crude minimum rate 40 collapses Myr-1 kpc-2 in a few tens of Myr. Numerous explosions have lately occurred from the first generations of massive stars formed in the Belt. A recent-past rate can be inferred from the current stellar content of the Belt as a function of mass, for both short-lived and long-lived stars, given a few assumptions: 1) a stellar initial mass spectrum with a range of indices as measured in nearby OB associations (Scalo 1986; Massey et al. 1995); 2) lifetime estimates as modelled for stars with solar metallicity (; Schaller et al. 1992; Meynet et al. 1994) and interpolated in mass; 3) a constant birth rate for simplicity; 4) a conservative mass threshold for collapse of 8 M. Using this formalism, star counts were integrated in various spectral bands and scaled to the observed Belt star counts (Comeron et al. 1994) to eliminate the unknown birth rate and amplitude a.
The significant increase in birth rate obtained for stars 40 Myr confirms the Belt nuclear age and the validity of our simple formalism. SN yields of 21.4 0.8, 24.1 0.9, and 27.2 1.0 per Myr were obtained for indices of -2.0, -1.5, and -1.1, respectively, for a 40 Myr old Belt. The quoted error results from the uncertainty in the observed star counts. These estimates decrease by 15% with Belt age from 50 to 30 Myr. Truncating the mass spectrum at 60 or 120 M, or choosing 10 M for the collapse threshold, has impact on the results. Given the uncertainties for the Belt age and index (particularly at large masses), we infer a frequency of 20 to 27 SNe per Myr and a rate of 75 to 95 Myr-1 kpc-2 which is 3 to 5 times the local Galactic one. This high rate is valid for the past few Myr. It is consistent with the power of erg Myr-1 kpc-2 required to maintain the local cosmic-ray density (Blandford & Ostriker 1980) for a standard SN-to-cosmic-ray energy conversion efficiency of a few percent (Drury et al. 1994). It is consistent with the presence, within the Belt, of four 0.1-1 Myr radio loops (Berkhuijsen 1973) and the Local Bubble. Thirteen radio pulsars from the Princeton catalogue are found at high latitudes with distances 1 kpc and age 2 Myr, but they are too few to show a correlation with the inclined Belt or with the Galactic plane. The narrow radio beams from many more may miss the Earth.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001