2,356 research outputs found
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
Mass fluxes for hot stars
In an attempt to understand the extraordinarily small mass-loss rates of
late-type O dwarfs, mass fluxes in the relevant part of (T_{eff}, g)-space are
derived from first principles using a previously-described code for
constructing moving reversing layers. From these mass fluxes, a weak-wind
domain is identified within which a star's rate of mass loss by a
radiatively-driven wind is less than that due to nuclear burning. The five
weak-wind stars recently analysed by Marcolino et al. (2009) fall within or at
the edge of this domain. But although the theoretical mass fluxes for these
stars are ~ 1.4 dex lower than those derived with the formula of Vink et al.
(2000), the observed rates are still not matched, a failure that may reflect
our poor understanding of low-density supersonic outflows.
Mass fluxes are also computed for two strong-wind O4 stars analysed by Bouret
et al. (2005). The predictions agree with the sharply reduced mass loss rates
found when Bouret et al. take wind clumping into account.Comment: Accepted by A&A; 6 pages, 5 figures; minor changes from v
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
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
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
On the nature of the bi-stability jump in the winds of early-type supergiants
We study the origin of the observed bi-stability jump in the terminal
velocity of the winds of supergiants near spectral type B1. To this purpose, we
have calculated a grid of wind models and mass-loss rates for these stars. The
models show that the mass-loss rate 'jumps' by a factor of five around spectral
type B1. Up to now, a theoretical explanation of the observed bi-stability jump
was not yet provided by radiation driven wind theory. The models demonstrate
that the subsonic part of the wind is dominated by the line acceleration due to
Fe. The elements C, N and O are important line drivers in the supersonic part
of the wind. We demonstrate that the mass-loss rate 'jumps' due to an increase
in the line acceleration of Fe III below the sonic point. Finally, we discuss
the possible role of the bi-stability jump on the mass loss during typical
variations of Luminous Blue Variable stars.Comment: Accepted by A&A, 19 pages Latex, 10 figure
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