2,122 research outputs found
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
Theoretical and Observational Agreement on Mass Dependence of Cluster Life Times
Observations and N-body simulations both support a simple relation for the
disruption time of a cluster as a function of its mass of the form: t_dis = t_4
* (M/10^4 Msun)^gamma. The scaling factor t_4 seems to depend strongly on the
environment. Predictions and observations show that gamma ~ 0.64 +/- 0.06.
Assuming that t_dis ~ M^0.64 is caused by evaporation and shocking implies a
relation between the radius and the mass of a cluster of the form: r_h ~
M^0.07, which has been observed in a few galaxies. The suggested relation for
the disruption time implies that the lower mass end of the cluster initial mass
function will be disrupted faster than the higher mass end, which is needed to
evolve a young power law shaped mass function into the log-normal mass function
of old (globular) clusters.Comment: 2 pages, to appear in "The Formation and Evolution of Massive Young
Star Clusters", 17-21 November 2003, Cancun (Mexico
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
The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters
We use archival Hubble Space Telescope observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M 51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of ~7.3 × 8.1 kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). We show the critical dependence of the age distribution on the sample selection, and confirm that using a constant mass cut-off, above which the sample is complete for the entire age range of interest, is essential. In particular, in this sample we are complete only for masses above 5× 104~M? for the last 1 Gyr. Using this dataset we find: i) that the cluster formation rate seems to have had a large increase ~50-70 Myr ago, which is coincident with the suggested second passage of its companion, NGC 5195; ii) a large number of extremely young (<10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next ~10 Myr; and iii) that the distribution of cluster sizes can be well approximated by a power-law with exponent, -? = -2.2 ± 0.2, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters
On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud
We re-analyze the age distribution (dN/dt) of star clusters in the Small
Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud
Photometric Survey. For ages younger than 3x10^9 yr the dN/dt distribution can
be approximated by a power-law distribution, dN/dt propto t^-beta, with
-beta=-0.70+/-0.05 or -beta=-0.84+/-0.04, depending on the model used to derive
the ages. Predictions for a cluster population without dissolution limited by a
V-band detection result in a power-law dN/dt distribution with an index of
~-0.7. This is because the limiting cluster mass increases with age, due to
evolutionary fading of clusters, reducing the number of observed clusters at
old ages. When a mass cut well above the limiting cluster mass is applied, the
dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is
of small importance in shaping the dN/dt distribution and incompleteness causes
dN/dt to decline. The reason that no (mass independent) infant mortality of
star clusters in the first ~10-20 Myr is found is explained by a detection bias
towards clusters without nebular emission, i.e. cluster that have survived the
infant mortality phase. The reason we find no evidence for tidal (mass
dependent) cluster dissolution in the first Gyr is explained by the weak tidal
field of the SMC. Our results are in sharp contrast to the interpretation of
Chandar et al. (2006), who interpret the declining dN/dt distribution as rapid
cluster dissolution. This is due to their erroneous assumption that the sample
is limited by cluster mass, rather than luminosity.Comment: 8 pages, 4 figures, accepted for publication in Ap
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
Constraining star cluster disruption mechanisms
Star clusters are found in all sorts of environments and their formation and
evolution is inextricably linked to the star formation process. Their eventual
destruction can result from a number of factors at different times, but the
process can be investigated as a whole through the study of the cluster age
distribution. Observations of populous cluster samples reveal a distribution
following a power law of index approximately -1. In this work we use M33 as a
test case to examine the age distribution of an archetypal cluster population
and show that it is in fact the evolving shape of the mass detection limit that
defines this trend. That is to say, any magnitude-limited sample will appear to
follow a dN/dt=1/t, while cutting the sample according to mass gives rise to a
composite structure, perhaps implying a dependence of the cluster disruption
process on mass. In the context of this framework, we examine different models
of cluster disruption from both theoretical and observational standpoints.Comment: To appear in the proceedings of IAU Symposium 266: "Star Clusters:
Basic Galactic Building Blocks Throughout Time And Space", eds. R. de Grijs
and J. Lepin
Star clusters in the solar neighborhood: a solution to Oort's problem
In 1958 Jan Oort remarked that the lack of old clusters in the solar
neighborhood (SN) implies that clusters are destroyed on a timescale of less
than a Gyr. This is much shorter than the predicted dissolution time of
clusters due to stellar evolution and two-body relaxation in the tidal field of
the Galaxy. So, other (external) effects must play a dominant role in the
destruction of star clusters in the solar neighborhood. We recalculated the
survival time of initially bound star clusters in the solar neighborhood taking
into account: (1) stellar evolution, (2) tidal stripping, (3) perturbations by
spiral arms and (4) encounters with giant molecular clouds (GMCs). We find that
encounters with GMCs are the most damaging to clusters. The resulting predicted
dissolution time of these combined effects, t_dis=1.7 (Mi/10^4 M_sun)^0.67 Gyr
for clusters in the mass range of 10^2 < M < 10^5 M_sun, is very similar to the
disruption time of t_dis=1.3+/-0.5 (M/10^4 M_sun)^0.62 Gyr that was derived
empirically from a mass limited sample of clusters in the solar neighborhood
within 600 pc. The predicted shape of the age distribution of clusters agrees
very well with the observed one. The comparison between observations and theory
implies a surface star formation rate (SFR) near the sun of 3.5x10^-10 M_sun
yr^-1 pc^-2 for stars in bound clusters with an initial mass in the range of
10^2 to 3x10^4 M_sun. This can be compared to a total SFR of 7-10x10^-10 M_sun
yr^-1 pc^-2 derived from embedded clusters or 3-7x10^-9 M_sun yr^-1 pc^-2
derived from field stars. This implies an infant mortality rate of clusters in
the solar neighborhood between 50% and 95%, in agreement with the results of a
study of embedded clusters.Comment: 12 pages, 6 figures, To appear in "Mass loss from stars and the
evolution of stellar clusters". Proc. of a workshop held in honour of
H.J.G.L.M. Lamers, Lunteren, The Netherlands. Eds. A. de Koter, L. Smith and
R. Waters (San Francisco: ASP
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