2003MNRAS.340..197D

We obtain new age and mass estimates for the star clusters in the fossil starburst region B of M82, based on improved fitting methods. Our new age estimates confirm the peak in the age histogram attributed to the last tidal encounter with M81; we find a peak formation epoch at slightly older ages than previously published, log(t_peak.yr^–1) = 9.04, with a Gaussian σ of Δ log(t_width) = 0.273. The actual duration of the burst of cluster formation may have been shorter because uncertainties in the age determinations may have broadened the peak. Our improved mass estimates confirm that the (initial) masses of the M82 B clusters with V ≤ 22.5 mag are mostly in the range 10⁴-10⁶ M_☉, with a median mass of M_cl=1.08x10⁵ M_☉. The formation history of the observed clusters shows a steady decrease towards older ages. This indicates that cluster disruption has removed a large fraction of the older clusters. Adopting the expression for the cluster disruption time-scale of t_dis(M)=t₄^dis(M/10⁴M_☉)^γ with γ≃0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase [log(t yr^–1) ≥ 9.4], the burst phase [8.4 < log(t yr^–1) < 9.4] and the post-burst phase [log(t yr^–1) ≤ 8.4] are approximately {formmu11}. The formation rate during the burst may have been higher if the actual duration of the burst was shorter than adopted. The mass distribution of the clusters formed during the burst shows a turnover at log (M_cl/ M_☉) ≃ 5.3 that is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope α= 2 up to a maximum cluster mass of M_max= 3x10⁶ M_☉and cluster disruption with a normalization time-scale t^dis₄/t_burst=(3.0±0.3)x10^–2. For a burst age of 1x10⁹ yr, we find that the disruption time-scale of a cluster of 10⁴ M_☉is t^dis₄∼ 3x10⁷yr, with an uncertainty of approximately a factor of 2. This is the shortest disruption time-scale known in any (disc region of a) galaxy.