384 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
NGC 2401: A template of the Norma-Cygnus Arm's young population in the Third Galactic Quadrant
Based on a deep optical CCD (UBV(RI)_C) photometric survey and on the
Two-Micron All-Sky-Survey (2MASS) data we derived the main parameters of the
open cluster NGC 2401. We found this cluster is placed at 6.3 0.5 kpc
(V_O - M_V = 14.0 \pm 0.2) from the Sun and is 25 Myr old, what allows us to
identify NGC 2401 as a member of the young population belonging to the
innermost side of the extension of the Norma-Cygnus spiral--arm in the Third
Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies
in the cluster area revealed it is a B0Ve star; however, we could not confirm
it is a cluster member. We also constructed the cluster luminosity function
(LF) down to and the cluster initial mass function (IMF) for all
stars with masses above M \sim 1-2 M_{\sun}. It was found that the slope of
the cluster IMF is . The presence of a probable PMS star
population associated to the cluster is weakly revealed.Comment: 10 paginas, 11 eps figures, accepted for publication in MNRA
Spiral structure of the Third Galactic Quadrant and the solution to the Canis Major debate
With the discovery of the Sagittarius dwarf spheroidal (Ibata et al. 1994), a
galaxy caught in the process of merging with the Milky Way, the hunt for other
such accretion events has become a very active field of astrophysical research.
The identification of a stellar ring-like structure in Monoceros, spanning more
than 100 degrees (Newberg et al. 2002), and the detection of an overdensity of
stars in the direction of the constellation of Canis Major (CMa, Martin et al.
2004), apparently associated to the ring, has led to the widespread belief that
a second galaxy being cannibalised by the Milky Way had been found. In this
scenario, the overdensity would be the remaining core of the disrupted galaxy
and the ring would be the tidal debris left behind. However, unlike the
Sagittarius dwarf, which is well below the Galactic plane and whose orbit, and
thus tidal tail, is nearly perpendicular to the plane of the Milky Way, the
putative CMa galaxy and ring are nearly co-planar with the Galactic disk. This
severely complicates the interpretation of observations. In this letter, we
show that our new description of the Milky Way leads to a completely different
picture. We argue that the Norma-Cygnus spiral arm defines a distant stellar
ring crossing Monoceros and the overdensity is simply a projection effect of
looking along the nearby local arm. Our perspective sheds new light on a very
poorly known region, the third Galactic quadrant (3GQ), where CMa is located.Comment: 5 pages, 2 figures. Quality of Fig 1 has been degraded to make it
smaller. Original fig. available on request. accepted for publication in
MNRAS letter
Open clusters in the Third Galactic Quadrant. II. The intermediate age open clusters NGC 2425 and NGC 2635
We analyse CCD broad band (UBV(RI)c) photometric data obtained in the fields
of the poorly studied open clusters NGC 2425 and NGC 2635. Both clusters are
found to be of intermediate age thus increasing the population of open clusters
known to be of the age of, or older than, the Hyades. More explicitly, we find
that NGC 2425 is a 2.2 Gyr old cluster, probably of solar metallicity, located
at 3.5 kpc from the Sun. NGC 2635 is a Hyades age (600 Myr) cluster located at
a distance of 4.0 kpc from the Sun. Its Colour Magnitude Diagram reveals that
it is extremely metal poor for its age and position, thus making it a very
interesting object in the context of Galactic Disk chemical evolution models.Comment: 10 pages, 13 figures. Figure quality has been quiet degraded.
Accepted by A&
The edge of the young Galactic disc
In this work we report and discuss the detection of two distant diffuse
stellar groups in the third Galactic quadrant. They are composed of young
stars, with spectral types ranging from late O to late B, and lie at
galactocentric distances between 15 and 20 kpc. These groups are located in the
area of two cataloged open clusters (VdB-Hagen~04 and Ruprecht~30), projected
towards the Vela-Puppis constellations, and within the core of the Canis Major
over-density. Their reddening and distance has been estimated analyzing their
color-color and color-magnitude diagrams, derived from deep photometry.
The existence of young star aggregates at such extreme distances from the
Galactic center challenges the commonly accepted scenario in which the Galactic
disc has a sharp cut-off at about 14 kpc from the Galactic center, and
indicates that it extends to much greater distances (as also supported by
recent detection of CO molecular complexes well beyond this distance). While
the groups we find in the area of Ruprecht~30 are compatible with the Orion and
Norma-Cygnus spiral arms, respectively, the distant group we identify in the
region of VdB-Hagen~4 lies in the external regions of the Norma-Cygnus arm, at
a galactocentric distance (20 kpc) where no young stars had been detected
so far in the optical.Comment: 45 pages, 11 eps figure, accepted for publication in the
Astrophysical Journa
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&
The intermediate-age open clusters Ruprecht 61, Czernik 32, NGC 2225 and NGC 2262
We present the first CCD photometry to of 4 fields centered on
the region of the southern Galactic star clusters Ruprecht~61, Czernik~32, NGC
2225 and NGC 2262 and of 4 displaced control fields. These clusters were never
studied before, and we provide for the first time estimates of their
fundamental parameters, namely radial extent, age, distance and reddening. We
find that the four clusters are all of intermediate age (around 1 Gyr), close
to the Sun and possess lower than solar metal abundance.Comment: 10 pages, 14 figures, in press in MNRA
NGC 2362: a Template for Early Stellar Evolution
We present UBVRI photometry for the young open cluster NGC 2362. From
analysis of the appropriate color-color and color-magnitude diagrams we derive
the fundamental parameters of the NGC 2362 cluster to be: age = 5 (+1-2) Myr,
distance = 1480 pc, E(B-V)=0.10 mag. The cluster age was independently
determined for both high mass (2.1 - 36Msun) and low mass (0.7 - 1.2Msun) stars
with excellent agreement between the ages derived using post-main sequence and
pre-main sequence evolutionary tracks for the high and low mass stars
respectively. Analysis of this cluster's color-magnitude diagram reveals a well
defined pre-main sequence (covering DeltaV ~ 9 magnitudes in V and extending
from early A stars to near the hydrogen burning limit) which makes this cluster
an ideal laboratory for pre-main sequence evolution studies.Comment: 9 pages, 3 figures, to be published in ApJ
Open clusters in the Third Galactic Quadrant : III. Alleged binary clusters
Aims: We aim to determine accurate distances and ages of eight open clusters in order to: (1) assess their possible binarity (2) provide probes to trace the structure of the Third Galactic Quadrant. Methods: Cluster reddenings, distances, ages and metallicities are derived from ZAMS and isochrone fits in UBVRI photometric diagrams. Field contamination is reduced by restricting analysis to stars within the cluster limits derived from star counts. Further membership control is done by requiring that stars have consistent positions in several diagrams and by using published spectral types. Results: The derived distances, ages and metallicities have shown that none of the analysed clusters compose binary/double systems. Of the four candidate pairs, only NGC 2383/NGC 2384 are close to each other, but have different metallicities and ages. Ruprecht 72 and Ruprecht 158 are not clusters but fluctuations of the field stellar density. Haffner 18 is found to be the superposition of two stellar groups at different distances: Haffner 18(1) at 4.5 kpc and Haffner 18(2) between 9.5 and 11.4 kpc from the Sun. The derived distances and ages have been used to situate the clusters in the Galactic context. In particular, young stellar groups trace spiral structure at large Galactocentric radii. At least two clusters formed during the last few 108 yr in an interstellar medium with less than solar abundances. Conclusions: In contrast with the LMC, double clusters are apparently rare, or even non existent, in the undisturbed environment of the Third Galactic Quadrant. This leaves open the question of whether binary clusters form more easily toward denser and more violent regions of the Milky Way such as the inner Galaxy.Facultad de Ciencias Astronómicas y Geofísica
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