158,543 research outputs found
The photometric evolution of dissolving star clusters I: First predictions
We calculated the broad-band photometric evolution of unresolved star
clusters, including the preferential loss of low-mass stars due to mass
segregation. The stellar mass function of a cluster evolves due to three
effects: (a) the evolution of massive stars; (b) early tidal effects reduce the
mass function independently of the stellar mass; (c) after mass segregation has
completed, tidal effects preferentially remove the lowest-mass stars from the
cluster. Results: (1) During the first ~40% of the lifetime of a cluster the
cluster simply gets fainter due to the loss of stars by tidal effects. (2)
Between ~40 and ~80% of its lifetime the cluster gets bluer due to the loss of
low-mass stars. This will result in an underestimate of the age of clusters if
standard cluster evolution models are used (0.15 -- 0.5 dex). (3) After ~80% of
the total lifetime of a cluster it will rapidly get redder. This is because
stars at the low-mass end of the main sequence, which are preferentially lost,
are bluer than the AGB stars that dominate the light at long wavelengths,
resulting in an age overestimate. (4) Clusters with mass segregation and the
preferential loss of low-mass stars evolve along almost the same tracks in
colour-colour diagrams as clusters without mass segregation. Therefore it will
be difficult to distinguish this effect from that due to the cluster age for
unresolved clusters, unless the total lifetime of the clusters can be
estimated. (5) The changes in the colour evolution of unresolved clusters due
to the preferential loss of low-mass stars will affect the determination of the
SFHs. (6) The preferential loss of low-mass stars might explain the presence of
old (~13 Gyr) clusters in NGC 4365 which are photometrically disguised as
intermediate-age clusters (2 - 5 Gyr). [Abridged]Comment: accepted for publication in A&
Open clusters: III. Fundamental parameters of B stars in NGC 6087, NGC 6250, NGC 6383 and NGC 6530. B type stars with circumstellar envelopes
Context. Stellar physical properties of star clusters are poorly known. Aims.
Our goals are to perform a spectrophotometric study of the B star population in
open clusters to derive accurate stellar parameters, search for the presence of
circumstellar envelopes, and discuss the characteristics of these stars.
Methods. The BCD spectrophotometric system is a powerful tool to obtain
fundamental parameters and infer the main properties of open clusters: distance
modulus, color excess, and age. We inspected the Balmer discontinuity to seek
circumstellar disks and identify Be-star candidates. High resolution spectra in
the H region are used to confirm the Be nature. Results. We provide
Teff , log g, Mv , Mbol and spectral types for a sample of 68 stars in the
field of the open clusters NGC 6087, NGC 6250, NGC 6383, and NGC 6530, as well
as the cluster distances, ages and reddening. Then, based on a sample of 230 B
stars in the direction of the 11 open clusters studied along this series of
three papers, we report 6 new Be stars, 4 blue straggler candidates, and 15
B-type stars (called Bdd) with a double Balmer discontinuity. We also find that
the majority of the Be stars are dwarfs and present a maximum at the spectral
type B2-B4 in young and intermediate-age open clusters. Another maximum of Be
stars is observed at the spectral type B6-B8 in open clusters older than 40
Myr, where the population of Bdd stars also becomes relevant. Conclusions. Our
results support previous statements that the Be phenomenon is present along the
whole main sequence band and occurs in very different evolutionary states. We
find clear evidence of an increase of stars with circumstellar envelopes with
cluster age. The Be phenomenon reaches its maximum in clusters of intermediate
age and the number of B stars with circumstellar envelopes (Be+Bdd stars) is
also high for the older clusters
The role of stellar collisions for the formation of massive stars
We use direct N-body simulations of gas embedded star clusters to study the
importance of stellar collisions for the formation and mass accretion history
of high-mass stars. Our clusters start in virial equilibrium as a mix of gas
and proto-stars. Proto-stars then accrete matter using different mass accretion
rates and the amount of gas is reduced in the same way as the mass of stars
increases. During the simulations we check for stellar collisions and we
investigate the role of these collisions for the build-up of high-mass stars
and the formation of runaway stars. We find that a significant number of
collisions only occur in clusters with initial half-mass radii r_h < 0.1 pc.
After emerging from their parental gas clouds, such clusters end up too compact
compared to observed young, massive open clusters. In addition, collisions lead
mainly to the formation of a single runaway star instead of the formation of
many high mass stars with a broad mass spectrum. We therefore conclude that
massive stars form mainly by gas accretion, with stellar collisions only
playing a minor role if any at all. Collisions of stars in the pre-main
sequence phase might however contribute to the formation of the most massive
stars in the densest star clusters and possibly to the formation of
intermediate-mass black holes with masses up to a few 100 Msun.Comment: 10 pages, 8 figures, MNRAS in pres
Towards ensemble asteroseismology of the young open clusters Chi Persei and NGC 6910
As a result of the variability survey in Chi Persei and NGC6910, the number
of Beta Cep stars that are members of these two open clusters is increased to
twenty stars, nine in NGC6910 and eleven in Chi Persei. We compare pulsational
properties, in particular the frequency spectra, of Beta Cep stars in both
clusters and explain the differences in terms of the global parameters of the
clusters. We also indicate that the more complicated pattern of the variability
among B type stars in Chi Persei is very likely caused by higher rotational
velocities of stars in this cluster. We conclude that the sample of pulsating
stars in the two open clusters constitutes a very good starting point for the
ensemble asteroseismology of Beta Cep-type stars and maybe also for other
B-type pulsators.Comment: 4 pages, Astronomische Nachrichten, HELAS IV Conference, Arecife,
Lanzarote, Feb 2010, submitte
Blue Straggler Stars in Galactic Open Clusters and the effect of field star contamination
We investigate the distribution of Blue Straggler stars in the field of three
open star clusters. The main purpose is to highlight the crucial role played by
general Galactic disk fore-/back-ground field stars, which are often located in
the same region of the Color Magnitude Diagram as Blue Straggler stars. We
analyze photometry taken from the literature of 3 open clusters of
intermediate/old age rich in Blue Straggler stars, and which are projected in
the direction of the Perseus arm, and study their spatial distribution and the
Color Magnitude Diagram. As expected, we find that a large portion of the Blue
Straggler population in these clusters are simply young field stars belonging
to the spiral arm. This result has important consequences on the theories of
the formation and statistics of Blue Straggler stars in different population
environments: open clusters, globular clusters or dwarf galaxies. As previously
emphasized by many authors, a detailed membership analysis is mandatory before
comparing the Blue Straggler population in star clusters against theoretical
models. Moreover, these sequences of young field stars (blue plumes) are
potentially powerful tracers of Galactic structure which require further
consideration.Comment: 11 pages, 4 figurs, in press as Research Note in A&
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