340 research outputs found
Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530
We derive the proper motions, membership probabilities, and velocity
dispersions of stars in the regions of the young (about 2-4 Myr-old) open
clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC
6530 (the dominant cluster in the Sgr OB1 association) from photographic plate
material obtained at Shanghai Astronomical Observatory, with time baselines of
34 and 87 years, respectively. Both clusters show clear evidence of mass
segregation, but they do not exhibit any significant velocity-mass (or,
equivalently, a velocity-luminosity) dependence. This provides strong support
for the suggestion that the observed mass segregation is -- at least partially
-- due to the way in which star formation has proceeded in these complex
star-forming regions (``primordial'' mass segregation). Based on arguments
related to the clusters' published initial mass functions, in conjunction with
our new measurements of their internal velocity dispersions (35 and 8 km/s for
NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of
the dissolution of NGC 2244 on very short time-scales, while we speculate that
NGC 6530 may be more stable against the effects of internal two-body
relaxation. However, this latter object may well be destroyed by the strong
tidal field prevalent at its location in the Galactic plane in the direction of
the Galactic Center.Comment: 36 pages, 10 figures, accepted to A
Star Cluster Formation and Disruption Time-Scales - II. Evolution of the Star Cluster System in M82's Fossil Starburst
ABRIDGED: We obtain new age and mass estimates for the star clusters in M82's
fossil starburst region B, based on improved fitting methods. Our new age
estimates confirm the peak in the age histogram attributed to the last tidal
encounter with M81; we find a peak formation epoch at slightly older ages than
previously published, log(t_peak / yr) = 9.04, with a Gaussian sigma of Delta
log(t_width) = 0.273. Cluster disruption has removed a large fraction of the
older clusters. Adopting the expression for the cluster disruption time-scale
of t_dis(M)= t_dis^4 (M/10^4 Msun)^gamma with gamma = 0.62 (Paper I), we find
that the ratios between the real cluster formation rates in the pre-burst phase
(log(t/yr) <= 9.4), the burst-phase (8.4 < log(t/yr) < 9.4) and the post-burst
phase (log(t/yr) <= 8.4) are about 1:2:1/40. The mass distribution of the
clusters formed during the burst shows a turnover at log(M_cl/Msun) ~ 5.3 which
is not caused by selection effects. This distribution can be explained by
cluster formation with an initial power-law mass function of slope alpha=2 up
to a maximum cluster mass of M_max = 3 x 10^6 Msun, and cluster disruption with
a normalisation time-scale t_dis^4 / t_burst = (3.0 +/- 0.3) x 10^{-2}. For a
burst age of 1 x 10^9 yr, we find that the disruption time-scale of a cluster
of 10^4 Msun is t_dis^4 ~ 3 x 10^7 years, with an uncertainty of approximately
a factor of two. This is the shortest disruption time-scale known in any
galaxy.Comment: 14 pages including 8 postscript figures; accepted for publication in
MNRA
Open clusters: their kinematics and metellicities
We review our work on Galactic open clusters in recent years, and introduce
our proposed large program for the LOCS (LAMOST Open Cluster Survey). First,
based on the most complete open clusters sample with metallicity, age and
distance data as well as kinematic information, some preliminary statistical
analysis regarding the spatial and metallicity distributions is presented. In
particular, a radial abundance gradient of - 0.058 0.006 dex kpc
was derived, and by dividing clusters into age groups we show that the disk
abundance gradient was steeper in the past. Secondly, proper motions,
membership probabilities, and velocity dispersions of stars in the regions of
two very young open clusters are derived. Both clusters show clear evidence of
mass segregation, which provides support for the ``primordial'' mass
segregation scenarios. Based on the great advantages of the forthcoming LAMOST
facility, we have proposed a detailed open cluster survey with LAMOST (the
LOCS). The aim, feasibility, and the present development of the LOCS are
briefly summarized.Comment: 7 pages, 4 figures, submitted to Proceeding of IAU Symposium No.248:
"A Giant Step:from Milli- to Micro-arcsecond Astrometry
Cluster and nebular properties of the central star-forming region of NGC 1140
We present new high spatial resolution HST/ACS imaging of NGC 1140 and high
spectral resolution VLT/UVES spectroscopy of its central star-forming region.
The central region contains several clusters, the two brightest of which are
clusters 1 and 6 from Hunter, O'Connell & Gallagher, located within
star-forming knots A and B, respectively. Nebular analysis indicates that the
knots have an LMC-like metallicity of 12 + log(O/H) = 8.29 +/- 0.09. According
to continuum subtracted H alpha ACS imaging, cluster 1 dominates the nebular
emission of the brighter knot A. Conversely, negligible nebular emission in
knot B originates from cluster 6. Evolutionary synthesis modelling implies an
age of 5 +/- 1 Myr for cluster 1, from which a photometric mass of (1.1 +/-
0.3) x 10^6 Msun is obtained. For this age and photometric mass, the modelling
predicts the presence of ~5900 late O stars within cluster 1. Wolf-Rayet
features are observed in knot A, suggesting 550 late-type WN and 200 early-type
WC stars. Therefore, N(WR)/N(O) ~ 0.1, assuming that all the WR stars are
located within cluster 1. The velocity dispersions of the clusters were
measured from constituent red supergiants as sigma ~ 23 +/- 1 km/s for cluster
1 and sigma ~ 26 +/- 1 km/s for cluster 6. Combining sigma with half-light
radii of 8 +/- 2 pc and 6.0 +/- 0.2 pc measured from the F625W ACS image
implies virial masses of (10 +/- 3) x 10^6 Msun and (9.1 +/- 0.8) x 10^6 Msun
for clusters 1 and 6, respectively. The most likely reason for the difference
between the dynamical and photometric masses of cluster 1 is that the velocity
dispersion of knot A is not due solely to cluster 1, as assumed, but has an
additional component associated with cluster 2.Comment: 13 pages, 7 figure
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&
CIRPASS near-infrared integral-field spectroscopy of massive star clusters in the starburst galaxy NGC 1140
[ABRIDGED] We analyse near-infrared integral field spectroscopy of the
central starburst region of NGC 1140, obtained at the Gemini-South telescope
equipped with CIRPASS. Our ~1.45-1.67 um wavelength coverage includes the
bright [Fe II] emission line, as well as high-order Brackett (hydrogen) lines.
While strong [Fe II] emission, thought to originate in the thermal shocks
associated with supernova remnants, is found throughout the galaxy, both Br
12-4 and Br 14-4 emission, and weak CO(6,3) absorption, is predominantly
associated with the northern starburst region. The Brackett lines originate
from recombination processes occurring on smaller scales in (young) HII
regions. The time-scale associated with strong [Fe II] emission implies that
most of the recent star-formation activity in NGC 1140 was induced in the past
35-55 Myr. Based on the spatial distributions of the [Fe II] versus Brackett
line emission, we conclude that a galaxy-wide starburst was induced several
tens of Myr ago, with more recent starburst activity concentrated around the
northern starburst region. This scenario is (provisionally) confirmed by our
analysis of the spectral energy distributions of the compact, young massive
star clusters (YMCs) detected in new and archival broad-band HST images. The
YMC ages in NGC 1140 are all <= 20 Myr, consistent with independently
determined estimates of the galaxy's starburst age, while there appears to be
an age difference between the northern and southern YMC complexes in the sense
expected from our CIRPASS analysis. Our photometric mass estimates of the NGC
1140 YMCs, likely upper limits, are comparable to those of the highest-mass
Galactic globular clusters and to spectroscopically confirmed masses of
(compact) YMCs in other starburst galaxies.Comment: 16 pages LaTeX, incl. 6 postscript figures; accepted for publication
in MNRA
Simple Stellar Population Models as probed by the Large Magellanic Cloud Star Cluster ESO 121-SC03
The presence of blue straggler stars (BSs) in star clusters has proven a
challenge to conventional simple stellar population (SSP) models. Conventional
SSP models are based on the evolution theory of single stars. Meanwhile, the
typical locations of BSs in the colour-magnitude diagram of a cluster are
brighter and bluer than the main sequence turn-off point. Such loci cannot be
predicted by single-star evolution theory. However, stars with such properties
contribute significantly to the integrated light of the cluster. In this paper,
we reconstruct the integrated properties of the Large Magellanic Cloud cluster
ESO 121-SC03, based on a detailed exploration of the individual cluster stars,
and with particular emphasis on the cluster's BSs. We find that the integrated
light properties of ESO 121-SC03 are dramatically modified by its BS component.
The integrated spectral energy distribution (ISED) flux level is significantly
enhanced toward shorter wavelengths, and all broad-band colours become bluer.
When fitting the fully integrated ISED of this cluster based on conventional
SSP models, the best-fitting values of age and metallicity are significantly
underestimated compared to the true cluster parameters. The age underestimate
is per cent if we only include the BSs within the cluster's half-light
radius and per cent if all BSs are included. The corresponding
underestimates of the cluster's metallicity are and per cent,
respectively. The populous star clusters in the Magellanic Clouds are ideal
objects to explore the potential importance of BSs for the integrated light
properties of more distant unresolved star clusters in a statistically robust
manner, since they cover a large range in age and metallicity.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA
- …