N-body simulations of star clusters

Two aspects of our recent N-body studies of star clusters are presented: (1) What impact does mass segregation and selective mass loss have on integrated photometry? (2) How well compare results from N-body simulations using NBODY4 and STARLAB/KIRA?Comment: 2 pages, 1 figure with 4 panels (in colour, not well visible in black-and-white; figures screwed in PDF version, ok in postscript; to see further details get the paper source). Conference proceedings for IAUS246 'Dynamical Evolution of Dense Stellar Systems', ed. E. Vesperini (Chief Editor), M. Giersz, A. Sills, Capri, Sept. 2007; v2: references correcte

New insight into the physics of atmospheres of early type stars

The phenomenon of mass loss and stellar winds from hot stars are discussed. The mass loss rate of early type stars increases by about a factor of 100 to 1000 during their evolution. This seems incompatible with the radiation driven wind models and may require another explanation for the mass loss from early type stars. The winds of early type stars are strongly variable and the stars may go through active phases. Eclipses in binary systems by the stellar winds can be used to probe the winds. A few future IUE studies are suggested

Stagnation and Infall of Dense Clumps in the Stellar Wind of tau Scorpii

Observations of the B0.2V star tau Scorpii have revealed unusual stellar wind characteristics: red-shifted absorption in the far-ultraviolet O VI resonance doublet up to +250 km/s, and extremely hard X-ray emission implying gas at temperatures in excess of 10^7 K. We describe a phenomenological model to explain these properties. We assume the wind of tau Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being confined by the ram pressure of the outflowing ambient wind. We calculate detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show these clumps will fall back towards the star with velocities of several hundred km/sec for a broad range of initial conditions. The infalling clumps produce X-ray emitting plasmas with temperatures in excess of (1-6)x10^7 K in bow shocks at their leading edge. The infalling material explains the peculiar red-shifted absorption wings seen in the O VI doublet. The required mass loss in clumps is 3% - 30% ofthe total mass loss rate. The model developed here can be generally applied to line-driven outflows with clumps or density irregularities. (Abstract Abridged)Comment: To appear in the ApJ (1 May 2000). 24 pages, including 6 embedded figure

The Star Cluster Population of M51

We present the age and mass distribution of star clusters in M51. The structural parameters are found by fitting cluster evolution models to the spectral energy distribution consisting of 8 HST-WFPC2 pass bands. There is evidence for a burst of cluster formation at the moment of the second encounter with the companion NGC5195 (50-100 Myr ago) and a hint for an earlier burst (400-500 Myr ago). The cluster IMF has a power law slope of -2.1. The disruption time of clusters is extremely short (< 100 Myr for a 10^4 Msun cluster).Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young Star Clusters", 17-21 November 2003, Cancun (Mexico

Star Cluster Formation and Disruption Time-Scales - II. Evolution of the Star Cluster System in M82's Fossil Starburst

ABRIDGED: We obtain new age and mass estimates for the star clusters in M82's fossil starburst region B, 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) = 9.04, with a Gaussian sigma of Delta log(t_width) = 0.273. Cluster disruption has removed a large fraction of the older clusters. Adopting the expression for the cluster disruption time-scale of t_dis(M)= t_dis^4 (M/10^4 Msun)^gamma with gamma = 0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase (log(t/yr) <= 9.4), the burst-phase (8.4 < log(t/yr) < 9.4) and the post-burst phase (log(t/yr) <= 8.4) are about 1:2:1/40. The mass distribution of the clusters formed during the burst shows a turnover at log(M_cl/Msun) ~ 5.3 which is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope alpha=2 up to a maximum cluster mass of M_max = 3 x 10^6 Msun, and cluster disruption with a normalisation time-scale t_dis^4 / t_burst = (3.0 +/- 0.3) x 10^{-2}. For a burst age of 1 x 10^9 yr, we find that the disruption time-scale of a cluster of 10^4 Msun is t_dis^4 ~ 3 x 10^7 years, with an uncertainty of approximately a factor of two. This is the shortest disruption time-scale known in any galaxy.Comment: 14 pages including 8 postscript figures; accepted for publication in MNRA