44,711 research outputs found
UPMASK: unsupervised photometric membership assignment in stellar clusters
We develop a method for membership assignment in stellar clusters using only
photometry and positions. The method, UPMASK, is aimed to be unsupervised, data
driven, model free, and to rely on as few assumptions as possible. It is based
on an iterative process, principal component analysis, clustering algorithm,
and kernel density estimations. Moreover, it is able to take into account
arbitrary error models. An implementation in R was tested on simulated clusters
that covered a broad range of ages, masses, distances, reddenings, and also on
real data of cluster fields. Running UPMASK on simulations showed that it
effectively separates cluster and field populations. The overall spatial
structure and distribution of cluster member stars in the colour-magnitude
diagram were recovered under a broad variety of conditions. For a set of 360
simulations, the resulting true positive rates (a measurement of purity) and
member recovery rates (a measurement of completeness) at the 90% membership
probability level reached high values for a range of open cluster ages
( yr), initial masses (M_{\sun}) and
heliocentric distances ( kpc). UPMASK was also tested on real data
from the fields of the open cluster Haffner~16 and of the closely projected
clusters Haffner~10 and Czernik~29. These tests showed that even for moderate
variable extinction and cluster superposition, the method yielded useful
cluster membership probabilities and provided some insight into their stellar
contents. The UPMASK implementation will be available at the CRAN archive.Comment: 12 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
A comparison of evolutionary tracks for single Galactic massive stars
In this paper, we compare the currently available evolutionary tracks for
Galactic massive stars. Our main goal is to highlight the uncertainties on the
predicted evolutionary paths. We compute stellar evolution models with the
codes MESA and STAREVOL. We compare our results with those of four published
grids of massive stellar evolution models (Geneva, STERN, Padova and FRANEC
codes). We first investigate the effects of overshooting, mass loss,
metallicity, chemical composition. We subsequently focus on rotation. Finally,
we compare the predictions of published evolutionary models with the observed
properties of a large sample of Galactic stars. We find that all models agree
well for the main sequence evolution. Large differences in luminosity and
temperatures appear for the post main sequence evolution, especially in the
cool part of the HR diagram. Depending on the physical ingredients, tracks of
different initial masses can overlap, rendering any mass estimate doubtful. For
masses between 7 and 20 Msun, we find that the main sequence width is slightly
too narrow in the Geneva models including rotation. It is (much) too wide for
the (STERN) FRANEC models. This conclusion is reached from the investigation of
the HR diagram and from the evolution of the surface velocity as a function of
surface gravity. An overshooting parameter alpha between 0.1 and 0.2 in models
with rotation is preferred to reproduce the main sequence width. Determinations
of surface abundances of carbon and nitrogen are partly inconsistent and cannot
be used at present to discriminate between the predictions of published tracks.
For stars with initial masses larger than about 60 Msun, the FRANEC models with
rotation can reproduce the observations of luminous O supergiants and WNh
stars, while the Geneva models remain too hot.Comment: 17 pages, 12 figures. Accepted by Astronomy & Astrophysic
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