Wolf-Rayet Stars as Tracers of the Recent History of the Star Formation Rate

Abstract

Recently Vacca & Conti (1992) have measured the ratio of the luminosity in the broad He II λ4686 emission feature to that in the H_β emission line in fourteen starburst galaxies. They related these luminosity ratios to the relative numbers of Wolf-Rayet (type WNL) to O-type stars in these galaxies (higher is the ratio L(λ4686)/L(H_β ), higher is N_WNL /N_O ). They found that in general the number ratios are an order of magnitude larger than those expected in region of constant star formation rate. On Fig. 1 the predicted line ratios of our starbursts models (instantaneous burst of star formation at time t = 0; initial mass function dN/dM = CM ⁻²; stellar models from Meynet et al. 1993; conversion formula between the line ratios and the number ratios of WNL to O-type stars given by Vacca & Conti 1992, with η = 1) are compared with the observed values given by these authors. This figure shows that a starburst taking place about 2–3 millions years ago can account for the high observed values of L(λ4686)/L(H_β ). One sees that the effects of the age of the burst (i.e. the time elapsed since the burst) and of the metallicity are quite important. It is the hope that in a next future, it will be possible on the base of this kind of luminosity ratios to disentangle the various effects influencing the WR population resulting from a starburst.