Tidal disruption rates of stars in observed galaxies

We derive the rates of capture N˙ of main-sequence turn-off stars by the central massive black hole in a sample of galaxies from Magorrian et al. The disruption rates are smaller than previously believed with ˙ N ∼ 10-4–10-7 per galaxy. A correlation between ˙N and black hole mass M is exploited to estimate the rate of tidal disruptions in the local Universe. Assuming that all or most galaxies have massive black holes in their nuclei, this rate should be dominated by sub-L* galaxies. The rate of tidal disruptions could be high enough to be detected in supernova (or similar) monitoring campaigns – we estimate the rate of tidal disruptions to be 0.01–0.1 times the supernova rate. We have also estimated the rates of disruption of red giants, which may be significant (˙ N ≥ 10-4 yr-1 per galaxy) for M ≥ 108 M⊙, but are likely to be harder to observe – only of order 10-4 times the supernova rate in the local Universe. In calculating capture rates, we advise caution when applying scaling formulae by other authors, which are not applicable in the physical regime spanned by the galaxies considered here.