28 research outputs found
Structural parameters of star clusters: relations among light, mass and star-count radial profiles and the dependence on photometric depth
Structural parameters of model star clusters are measured in radial profiles
built from number-density, mass-density and surface-brightness distributions,
assuming as well different photometric conditions. Star clusters of different
ages, structure and mass functions are modelled by assuming that the radial
distribution of stars follows a pre-defined analytical form. Near-infrared
surface brightness and mass-density profiles result from mass-luminosity
relations taken from a set of isochrones. Core, tidal and half-light, half-mass
and half-star count radii, together with the concentration parameter, are
measured in the three types of profiles, which are built under different
photometric depths. While surface-brightness profiles are almost insensitive to
photometric depth, radii measured in number-density and mass-density profiles
change significantly with it. Compared to radii derived with deep photometry,
shallow profiles result in lower values. This effect increases for younger
ages. Radial profiles of clusters with a spatially-uniform mass function
produce radii that do not depend on depth. With deep photometry, number-density
profiles yield radii systematically larger than those derived from
surface-brightness ones. In general, low-noise surface-brightness profiles
result in uniform structural parameters that are essentially independent of
photometric depth. For less-populous star clusters, those projected against
dense fields and/or distant ones, which result in noisy surface-brightness
profiles, this work provides a quantitative way to estimate the intrinsic radii
by means of number-density profiles built with depth-limited photometry.Comment: 10 pages and 9 figures. Accepted by A&
Constraining the star formation rate in the Solar neighbourhood with star clusters
This paper investigates the star formation rate (SFR) in the Solar
neighbourhood. First, we build the local age distribution function (ADF) with
an updated sample of 442 star clusters located at less than 1\,kpc from the
Sun. Next, we define the SFR, compute the individual mass evolution of a
population of artificial clusters covering the broad range of parameters
observed in actual clusters, and assume 100\,\ms\ as the low-mass limit for
effective cluster observation. This leads to a simulated ADF, which is compared
to the low-noise Solar neighbourhood ADF. The best match corresponds to a
non-constant SFR presenting two conspicuous excesses for ages \,Myr and
between 220-600\,Myr (the local starburst). The average formation rate is
\bar{SFR}\approx(2500\pm500)\,\mmy, corresponding to the average surface
formation rate \bar{\ssfr}\approx(790\pm160)\,\mmk. These values are
consistent with the formation rate inferred from embedded clusters (ECs), but
much lower (\la16%) than that implied by field stars. Both the local
starburst and the recent star formation period require
to be described. The simulations show that
of the clusters created in the Solar neighbourhood do not survive
the first 10\,Myr, which is consistent with the rate of EC dissolution.Comment: Accepted by MNRA
FSR584 - a new globular cluster in the Galaxy?
We investigate the nature of the recently catalogued star cluster candidate
FSR584, which is projected in the direction of the molecular cloud W3 and may
be the nearest globular cluster to the Sun. 2MASS CMDs, the stellar radial
density profile, and proper motions are employed to derive fundamental and
structural parameters. The CMD morphology and the radial density profile show
that FSR584 is an old star cluster. With proper motions, the properties of
FSR584 are consistent with a metal-poor globular cluster with a well-defined
turnoff and evidence of a blue horizontal-branch. FSR584 might be a
Palomar-like halo globular cluster that is moving towards the Galactic plane.
The distance from the Sun is approx 1.4kpc, and it is located at approx 1kpc
outside the Solar circle. The radial density profile is characterized by a core
radius of rc=0.3+/-0.1 pc. However, we cannot exclude the possibility of an old
open cluster. Near-infrared photometry coupled to proper motions support the
scenario where FSR584 is a new globular cluster in the Galaxy. The absorption
is A_V=9.2+/-0.6$, which makes it a limiting object in the optical and explains
why it has so far been overlookedComment: Astronomy and Astrophysics, accepted. 6 pages and 6 figure
Star cluster evolution in barred disc galaxies. I. Planar periodic orbits
The dynamical evolution of stellar clusters is driven to a large extent by
their environment. Several studies so far have considered the effect of tidal
fields and their variations, such as, e.g., from giant molecular clouds,
galactic discs, or spiral arms. In this paper we will concentrate on a tidal
field whose effects on star clusters have not yet been studied, namely that of
bars. We present a set of direct N-body simulations of star clusters moving in
an analytic potential representing a barred galaxy. We compare the evolution of
the clusters moving both on different planar periodic orbits in the barred
potential and on circular orbits in a potential obtained by axisymmetrising its
mass distribution. We show that both the shape of the underlying orbit and its
stability have strong impact on the cluster evolution as well as the morphology
and orientation of the tidal tails and the sub-structures therein. We find that
the dissolution time-scale of the cluster in our simulations is mainly
determined by the tidal forcing along the orbit and, for a given tidal forcing,
only very little by the exact shape of the gravitational potential in which the
cluster is moving.Comment: 15 pages, 17 figures, 5 tables; accepted for publication in MNRAS.
Complementary movies can be be found at this http URL
http://lam.oamp.fr/research/dynamique-des-galaxies/scientific-results/star-cluster-evolution
Characterisation of 15 overlooked Ruprecht clusters with ages within 400Myr and 3Gyr
We derive fundamental, structural, and photometric parameters of 15
overlooked Ruprecht (hereafter Ru) star clusters by means of 2MASS photometry
and field-star decontamination. Ru\,1, 10, 23, 26, 27, 34, 35, 37, 41, 54, 60,
63, 66, and 152 are located in the third Galactic quadrant, while Ru\,174 is in
the first. With the constraints imposed by the field-decontaminated
colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs), we
derive ages in the range 400\,Myr --- 1\,Gyr, except for the older Ru\,37, with
\,Gyr. Distances from the Sun are within \rm1.5\la\ds(kpc)\la8.0. The
RDPs are well-defined and can be described by a King-like profile for most of
the radial range, except for Ru\,23, 27, 41, 63, and 174, which present a
conspicuous stellar density excess in the central region. The clusters dwell
between (or close to) the Perseus and Sagittarius-Carina arms. We derive
evidence in favour of cluster size increasing with distance to the Galactic
plane (\zgc), which is consistent with a low frequency of tidal stress
associated with high-|\zgc| regions. The clusters are rather faint even in
the near-infrared, with apparent integrated \jj\ magnitudes within 6.4\la
m_J\la9.8, while their absolute magnitudes are -6.6\la M_J\la-2.6.
Extrapolation of the relation between and , derived for globular
clusters, suggests that they are low-luminosity optical clusters, with -5\la
M_V\la-1.Comment: The paper contains 11 figures and 3 tables. Accepted by MNRAS
Open cluster survival within the solar circle: Teutsch145 and Teutsch146
Teutsch145 and Teutsch146 are shown to be open clusters (OCs) orbiting well
inside the Solar circle, a region where several dynamical processes combine to
disrupt most OCs on a time-scale of a few 10^8yrs. BVI photometry from the
GALILEO telescope is used to investigate the nature and derive the fundamental
and structural parameters of the optically faint and poorly-known OCs
Teutsch145 and 146. These parameters are computed by means of field-star
decontaminated colour-magnitude diagrams (CMDs) and stellar radial density
profiles (RDPs). Cluster mass estimates are made based on the intrinsic mass
functions (MFs). We derive the ages 200+100-50Myr and 400+/-100Myr, and the
distances from the Sun 2.7+/-0.3kpc and 3.8+/-0.2kpc, respectively for
Teutsch145 and 146. Their integrated apparent and absolute magnitudes are m_V ~
12.4, m_V ~ 13.3, M_V ~- 5.6 and M_V ~- 5.3. The MFs (detected for stars with
m>1Msun) have slopes similar to Salpeter's IMF. Extrapolated to the H-burning
limit, the MFs would produce total stellar masses of ~1400Msun, typical of
relatively massive OCs. Both OCs are located deep into the inner Galaxy and
close to the Crux-Scutum arm. Since cluster-disruption processes are important,
their primordial masses must have been higher than the present-day values. The
conspicuous stellar density excess observed in the innermost bin of both RDPs
might reflect the dynamical effects induced by a few 10^8yrs of external tidal
stress.Comment: 8 pagas with 9 figs. Accepted by MNRA
Structures in surface-brightness profiles of LMC and SMC star clusters: evidence of mergers?
The LMC and SMC are rich in binary star clusters, and some mergers are
expected. It is important to characterize single clusters, binary clusters and
candidates to mergers. We selected a sample of star clusters in each Cloud with
this aim. Surface photometry of 25 SMC and 22 LMC star clusters was carried
with the ESO Danish 1.54 m telescope. 23 clusters were observed for the first
time for these purposes. We fitted Elson, Fall and Freeman (1987, EFF) profiles
to the data, deriving structural parameters, luminosities and masses. We also
use isophotal maps to constrain candidates to cluster interactions.} {The
structural parameters, luminosities and masses presented good agreement with
those in the literature. Three binary clusters in the sample have a double
profile. Four clusters (NGC 376, K 50, K 54 and NGC 1810) do not have
companions and present as well important deviations from EFF profiles. The
present sample contains blue and red Magellanic clusters. Extended EFF profiles
were detected in some blue clusters. We find evidence that important deviations
from the body of EFF profiles might be used as a tool to detect cluster
mergers.Comment: 16 pages and 8 figures. Accepted by A&
The old Galactic open clusters FSR1716 and Czernik23
Open clusters older than Gyr are rare in the Galaxy. Affected by a
series of mass-decreasing processes, the stellar content of most open clusters
dissolves into the field in a time-scale shorter than Gyr. In this
sense, improving the statistics of old objects may provide constraints for a
better understanding of the dynamical dissolution of open clusters. Isochrone
fits indicate that FSR 1716 is more probably an old ( Gyr) and absorbed
(\aV=6.3\pm0.2) open cluster, located kpc inside the Solar
circle in a contaminated central field. However, we cannot rule out the
possibility of a low-mass, loose globular cluster. Czernik 23 is shown to be an
almost absorption-free open cluster, Gyr old, located about 2.5 kpc
towards the anti-centre. In both cases, Solar and sub-Solar ()
metallicity isochrones represent equally well the stellar sequences. Both star
clusters have a low mass content (\la200 \ms) presently stored in stars.
Their relatively small core and cluster radii are comparable to those of other
open clusters of similar age. These structural parameters are probably
consequence of the several Gyrs of mass loss due to stellar evolution, tidal
interactions with the disk (and bulge in the case of FSR 1716), and possibly
giant molecular clouds. Czernik 23, and especially FSR 1716, are rare examples
of extreme dynamical survivors. The identification of both as such represents
an increase of to the known population of open clusters older than
Gyr in the Galaxy.Comment: 15 pages and 13 figures. Accepted by A&
Old open clusters in the inner Galaxy: FSR1744, FSR89 and FSR31
We establish the nature and derive fundamental and structural parameters of
the recently catalogued objects FSR1744, FSR89 and FSR31. This work intends to
provide clues to constrain the Galactic tidal disruption efficiency, improve
statistics of the open cluster parameter space, and better define their
age-distribution function inside the Solar circle. Properties of the objects
are investigated by means of 2MASS colour-magnitude diagrams and stellar radial
density profiles built with field star decontaminated photometry.
Diagnostic-diagrams for structural parameters are used to help disentangle
dynamical from high-background effects affecting such centrally projected open
clusters. FSR1744, FSR89 and FSR31 are Gyr-class OCs located at Galactocentric
distances 4.0 - 5.6kpc. Compared to nearby OCs, they have small core and
limiting radii. With respect to the small number of OCs observed in the inner
Galaxy, the emerging scenario in the near-infrared favours disruption driven by
dynamical evolution rather than observational limitations associated with
absorption and/or high background levels. Internally, the main processes
associated with the dynamical evolution are, e.g. mass loss by stellar
evolution, mass segregation and evaporation. Externally they are, e.g. tidal
stress from the disk and bulge, and interactions with giant molecular clouds.
FSR1744, FSR89 and FSR31 have structural parameters consistent with their
Galactocentric distances, in the sense that tidally induced effects may have
accelerated the dynamical evolution.Comment: 12 pages and 13 figs; A&A, accepted, July 9, 200
Hierarchical structures in the Large and Small Magellanic Clouds
We investigate the degree of spatial correlation among extended structures in
the LMC and SMC. To this purpose we work with sub-samples characterised by
different properties such as age and size, taken from the updated catalogue of
Bica et al. or gathered in the present work. The structures are classified as
star clusters or non-clusters (basically, nebular complexes and their stellar
associations). The radius distribution functions follow power-laws
() with slopes and maximum radius () that
depend on object class (and age). Non-clusters are characterised by
and R_{max}\la472 pc, while young clusters (age \la10
Myr) have and R_{max}\la15 pc, and old ones (age \ga600
Myr) have and R_{max}\la40 pc. Young clusters present a
high degree of spatial self-correlation and, especially, correlate with
star-forming structures, which does not occur with the old ones. This is
consistent with the old clusters having been heavily mixed up, since their ages
correspond to several LMC and SMC crossing times. On the other hand, with ages
corresponding to fractions of the respective crossing times, the young clusters
still trace most of their birthplace structural pattern. Also, small clusters
( pc), as well as small non-clusters ( pc), are spatially
self-correlated, while their large counterparts of both classes are not. The
above results are consistent with a hierarchical star-formation scenario for
the LMC and SMC.Comment: Accepted by MNRA