314 research outputs found
Global survey of star clusters in the Milky Way IV. 63 new open clusters detected by proper motions
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey,
Schmeja et al. applied photometric filters to the 2MASS to find new cluster
candidates that were subsequently confirmed or rejected by the MWSC pipeline.
To further extend the MWSC census, we aimed at discovering new clusters by
conducting an almost global search in proper motion catalogues as a starting
point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4
for comparison and verification of the proper motions. For 441 circular proper
motion bins (radius 15 mas/yr) within 50 mas/yr, the sky outside a thin
Galactic plane zone (5) was binned in small areas ('sky
pixels') of 0.250.25 deg. Sky pixels with enhanced numbers of stars
with a certain common proper motion in both catalogues were considered as
cluster candidates. After visual inspection of the sky images, we built an
automated procedure that combined these representations of the sky for
neighbouring proper motion subsamples after a background correction. RESULTS:
About half of our 692 candidates overlapped with known clusters (46 globular
and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or
the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open
clusters confirmed by the MWSC pipeline. They occupy predominantly the two
inner Galactic quadrants and have apparent sizes and numbers of high-probable
members slightly larger than those of the typically small MWSC clusters,
whereas their other parameters (ages, distances, tidal radii) fall in the
typical ranges. As our search aimed at finding compact clusters, we did not
find new very nearby (extended) clusters. (abridged)Comment: 14 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
Global survey of star clusters in the Milky Way II. The catalogue of basic parameters
Although they are the main constituents of the Galactic disk population, for
half of the open clusters in the Milky Way reported in the literature nothing
is known except the raw position and an approximate size. The main goal of this
study is to determine a full set of uniform spatial, structural, kinematic, and
astrophysical parameters for as many known open clusters as possible. On the
basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing
pipeline to determine kinematic and photometric membership probabilities for
stars in a cluster region. For an input list of 3784 targets from the
literature, we confirm that 3006 are real objects, the vast majority of them
are open clusters, but associations and globular clusters are also present. For
each confirmed object we determined the exact position of the cluster centre,
the apparent size, proper motion, distance, colour excess, and age. For about
1500 clusters, these basic astrophysical parameters have been determined for
the first time. For the bulk of the clusters we also derived the tidal radius.
We estimated additionally average radial velocities for more than 30% of the
confirmed clusters. The present sample (called MWSC) reaches both the central
parts of the Milky Way and its outer regions. It is almost complete up to 1.8
kpc from the Sun and also covers neighbouring spiral arms. However, for a small
subset of the oldest open clusters () we found some evidence
of incompleteness within about 1 kpc from the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Spectroscopic and photometric studies of white dwarfs in the Hyades
The Hyades cluster is known to harbour ten so-called classical white dwarf
members. Numerous studies through the years have predicted that more than twice
this amount of degenerate stars should be associated with the cluster. Using
the PPMXL catalog of proper motions and positions, a recent study proposed 17
new white dwarf candidates. We review the membership of these candidates by
using published spectroscopic and photometric observations, as well as by
simulating the contamination from field white dwarfs. In addition to the ten
classical Hyades white dwarfs, we find six white dwarfs that may be of Hyades
origin and three more objects that have an uncertain membership status due to
their unknown or imprecise atmospheric parameters. Among those, two to three
are expected as field stars contamination. Accurate radial velocity
measurements will confirm or reject the candidates. One consequence is that the
longstanding problem that no white dwarf older than ~340 Myr appears to be
associated with the cluster remains unsolved.Comment: 14 pages, 9 figures, accepted for publication in the Astronomy and
Astrophysics journa
Why Simple Stellar Population models do not reproduce the colours of Galactic open clusters
(...) We search for an explanation of the disagreement between the observed
integrated colours of 650 local Galactic clusters and the theoretical colours
of present-day SSP models. We check the hypothesis that the systematic offsets
between observed and theoretical colours, which are and
, are caused by neglecting the discrete nature of the
underlying mass function. Using Monte Carlo simulations, we construct
artificial clusters of coeval stars taken from a mass distribution defined by
an Salpeter initial mass function (IMF) and compare them with corresponding
"continuous-IMF" SSP models. If the discreteness of the IMF is taken into
account, the model fits the observations perfectly and is able to explain
naturally a number of red "outliers" observed in the empirical colour-age
relation. We find that the \textit{systematic} offset between the continuous-
and discrete-IMF colours reaches its maximum of about 0.5 in for a
cluster mass at ages , and diminishes
substantially but not completely to about one hundredth of a magnitude at at cluster masses . At younger ages, it is still
present even in massive clusters, and for it is
larger than 0.1 mag in . Only for very massive clusters () with ages is the offset small (of the order of 0.04
mag) and smaller than the typical observational error of colours of
extragalactic clusters.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letters, revised version after language editing and with an
additional reference to Cervino and Luridiana (2004
PPM-Extended (PPMX) - a catalogue of positions and proper motions
Aims: We build a catalogue PPM-Extended (PPMX) on the ICRS system which is
complete down to a well-defined limiting magnitude and contains the best
presently available proper motions to be suited for kinematical studies in the
Galaxy.
Methods: We perform a rigorous weighted least-squares adjustment of
individual observations, spread over more than a century, to determine mean
positions and proper motions. The stellar content of PPMX is taken from GSC 1.2
supplemented by catalogues like ARIHIP, PPM and Tycho-2 at the bright end. All
observations have been weighted according to their individual accuracy. The
catalogue has been screened towards rejecting false entries in the various
source catalogues.
Results: PPM-Extended (PPMX) is a catalogue of 18,088,920 stars containing
astrometric and photometric information. Its limiting magnitude is about 15.2
in the GSC photometric system. PPMX consists of three parts: a) a survey
complete down to R_U = 12.8 in the magnitude system of UCAC2; b) additional
stars of high-precision proper motions, and c) all other stars from GSC 1.2
identified in 2MASS. The typical accuracy of the proper motions is 2mas/y for
66 percent of the survey stars (a) and the high-precision stars (b), and about
10 mas/y for all other stars. PPMX contains photometric information from
ASCC-2.5 and 2MASS.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Open star clusters in the Milky Way - Comparison of photometric and trigonometric distance scales based on Gaia TGAS data
The global survey of star clusters in the Milky Way (MWSC) is a comprehensive
list of 3061 objects that provides, among other parameters, distances to
clusters based on isochrone fitting. The Tycho-Gaia Astrometric Solution (TGAS)
catalogue, which is a part of Gaia data release 1 (Gaia DR1), delivers accurate
trigonometric parallax measurements for more than 2 million stars, including
those in star clusters. We compare the open cluster photometric distance scale
with the measurements given by the trigonometric parallaxes from TGAS to
evaluate the consistency between these values. The average parallaxes of
probable cluster members available in TGAS provide the trigonometric distance
scale of open clusters, while the photometric scale is given by the distances
published in the MWSC. Sixty-four clusters are suited for comparison as they
have more than 16 probable members with parallax measurements in TGAS. We
computed the average parallaxes of the probable members and compared these to
the photometric parallaxes derived within the MWSC. We find a good agreement
between the trigonometric TGAS-based and the photometric MWSC-based distance
scales of open clusters, which for distances less than 2.3 kpc coincide at a
level of about 0.1 mas with no dependence on the distance. If at all, there is
a slight systematic offset along the Galactic equator between and
galactic longitude.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics, 4
pages, 4 figure
A deep all-sky census of the Hyades
On the basis of the PPMXL catalogue we perform an all-sky census of the
Hyades down to masses of about 0.2 m_sun in a region up to 30 pc from the
cluster centre. We use the proper motions from PPMXL in the convergent point
method to determine probable kinematic members. From 2MASS photometry and CMC14
r'-band photometry, we derive empirical colour-absolute magnitude diagrams and,
finally, determine photometric membership for all kinematic candidates. This is
the first deep (r' < 17) all-sky survey of the Hyades allowing a full
three-dimensional analysis of the cluster. The survey is complete down to at
least M_{K_s} = 7.3 or 0.25 m_sun. We find 724 stellar systems co-moving with
the bulk Hyades space velocity, which represent a total mass of 435 m_sun. The
tidal radius is about 9 pc, and 275 m_sun (364 systems) are gravitationally
bound. This is the cluster proper. Its mass density profile is perfectly fitted
by a Plummer model with a central density of 2.21 m_sun*pc^-3 and a core radius
of r_co = 3.10 pc, while the half-mass radius is r_h = 4.1 pc. There are
another 100 m_sun in a volume between one and two tidal radii (halo), and
another 60 m_sun up to a distance of 30 pc from the centre. Strong mass
segregation is inherent in the cluster. The present-day luminosity and mass
functions are noticeably different in various parts of the cluster (core,
corona, halo, and co-movers). They are strongly evolved compared to presently
favoured initial mass functions. The analysis of the velocity dispersion of the
cluster shows that about 20% of its members must be binaries. As a by-product,
we find that presently available theoretical isochrones are not able to
adequately describe the near-infrared colour-absolute magnitude relation for
those cluster stars that are less massive than about 0.6 m_sun.Comment: 14 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
The PPMXL catalog of positions and proper motions on the ICRS. Combining USNO-B1.0 and 2MASS
USNO-B1.0 and 2MASS are the most widely used full-sky surveys. However, 2MASS
has no proper motions at all, and USNO-B1.0 published only relative, not
absolute (i.e. on ICRS) proper motions. We performed a new determination of
mean positions and proper motions on the ICRS system by combining USNO-B1.0 and
2MASS astrometry. This catalog is called PPMXL {VO-access to the catalog is
possible via http://vo.uni-hd.de/ppmxl}, and it aims to be complete from the
brightest stars down to about full-sky. PPMXL contains about 900
million objects, some 410 million with 2MASS photometry, and is the largest
collection of ICRS proper motions at present. As representative for the ICRS we
chose PPMX. The recently released UCAC3 could not be used because we found
plate-dependent distortions in its proper motion system north of -20
declination. UCAC3 served as an intermediate system for . The resulting typical individual mean errors of the proper motions
range from 4 mas/y to more than 10 mas/y depending on observational history.
The mean errors of positions at epoch 2000.0 are 80 to 120 mas, if 2MASS
astrometry could be used, 150 to 300 mas else. We also give correction tables
to convert USNO-B1.0 observations of e.g. minor planets to the ICRS system.Comment: 9 pages, 9 figure
Towards absolute scales of radii and masses of open clusters
Aims: In this paper we derive tidal radii and masses of open clusters in the
nearest kiloparsecs around the Sun. Methods: For each cluster, the mass is
estimated from tidal radii determined from a fitting of three-parametric King's
profiles to the observed integrated density distribution. Different samples of
members are investigated. Results: For 236 open clusters, all contained in the
catalogue ASCC-2.5, we obtain core and tidal radii, as well as tidal masses.
The distributions of the core and tidal radii peak at about 1.5 pc and 7 - 10
pc, respectively. A typical relative error of the core radius lies between 15%
and 50%, whereas, for the majority of clusters, the tidal radius was determined
with a relative accuracy better than 20%. Most of the clusters have tidal
masses between 50 and 1000 , and for about half of the clusters, the
masses were obtained with a relative error better than 50%.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
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