9,981 research outputs found
Convergent null data expansions at space-like infinity of stationary vacuum solutions
We present a characterization of the asymptotics of all asymptotically flat
stationary vacuum solutions of Einstein's field equations. This
characterization is given in terms of two sequences of symmetric trace free
tensors (we call them the `null data'), which determine a formal expansion of
the solution, and which are in a one to one correspondence to Hansen's
multipoles. We obtain necessary and sufficient growth estimates on the null
data to define an absolutely convergent series in a neighbourhood of spatial
infinity. This provides a complete characterization of all asymptotically flat
stationary vacuum solutions to the field equations.Comment: 71 pages, no figure
A new Extended Main Sequence Turnoff star cluster in the Large Magellanic Cloud
We present results on the age and metallicity estimates of the poorly studied
LMC cluster SL 529, from CCD SDSS gr photometry obtained at the Gemini South
telescope with the GMOS attached. The cluster MSTO region possesses an extended
structure, with an age spread (~ 0.5 Gyr) bigger than the mean age width of
known EMSTO LMC clusters. We report for the first time a mean cluster age of
2.25 Gyr and a mean cluster metallicity of Z=0.004, which place it as the most
metal-poor and oldest cluster in the EMSTO LMC cluster group. In addition, the
cluster RC appears to be formed by two concentrations of stars - although it is
not clear whether this feature can be caused, in part, by binary interactions
and mergers -, whereas the cluster core radius of 4.2 pc is in excellent
agreement with those determined for the previously 12 known EMSTO LMC clusters.Comment: 6 pages, 6 figures, MNRAS (accepted
On the physical nature of globular cluster candidates in the Milky Way bulge
We present results from 2MASS JKs photometry on the physical reality of
recently reported globular cluster (GC) candidates in the Milky Way (MW) bulge.
We relied our analysis on photometric membership probabilities that allowed us
to distinguish real stellar aggregates from the composite field star
population. When building colour-magnitude diagrams and stellar density maps
for stars at different membership probability levels, the genuine GC candidate
populations are clearly highlighted. We then used the tip of the red giant
branch (RGB) as distance estimator, resulting heliocentric distances that place
many of the objects in regions near of the MW bulge where no GC had been
previously recognised. Some few GC candidates resulted to be MW halo/disc
objects.Metallicities estimated from the standard RGB method are in agreement
with the values expected according to the position of the GC candidates in the
Galaxy. We finally derived from the first time their structural parameters. We
found that the studied objects have core, half-light and tidal radii in the
ranges spanned by the population of known MW GCs. Their internal dynamical
evolutionary stages will be described properly when their masses are estimated.Comment: 11 pages, 7 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Bruck 88 : a young star cluster with an old age resemblance in the outskirts of the Small Magellanic Cloud
We present spectroscopic and photometric results for the Small Magellanic
Cloud (SMC) cluster Bruck 88. From the comparison of the cluster integrated
spectrum with template cluster spectra we found that the Milky Way globular
cluster template spectra are the ones which best resemble it. However, the
extracted cluster colour magnitude diagram reveals that Bruck 88 is a young
cluster (log(t) = 8.1 +- 0.1). The derived cluster age is compatible with the
presence of a Bright Red Giant (BRG) star located ~ 2.6 arcsec in the sky from
the cluster centre. We serendipitously observed HW 33, a star cluster located ~
3 arcmin to the south-east from Bruck 88. We obtained for the cluster the same
age than Bruck 88 and surprisingly, a BRG star located within the cluster
radius also appears to be compatible with the cluster age. We estimated the MK
type of the BRG star in the Bruck 88 field to be in the range G9 II/Ib - K1
III. By combining the spectrum of a star within this MK type range with a
100-150 Myr template cluster integrated spectrum, we found that a proportion
85/15 in the sense BRG/template results in a spectrum which best resembles that
of Bruck 88. This result confirms that a BRG star dominates the cluster
integrated spectrum, so that it causes the globular cluster appearance of its
integrated light.Comment: MNRAS, accepte
The star cluster frequency throughout the Large Magellanic Cloud
We address the issue about the variation of the star cluster frequency (CF)
in the Large Magellanic Cloud (LMC) in terms of the cluster spatial
distribution. We adopted the LMC regions traced by Harris & Zaritsky (2009) and
used an updated version of the cluster database compiled by Baumgardt et al.
(2013). The CFs were produced by taking into account an appropriate selection
of age bins. Since the uncertainty in a cluster's age can be large compared to
the size of the age bins, we account for the fact that a cluster could actually
reside in one of a few adjacent age bins. We confirm that there exist some
variations of the LMC CFs in terms of their spatial distributions, although
some caveats should be pointed out. 30 Doradus resulted to be the region with
the highest relative frequency of youngest clusters, while the log() = 9-9.5
(1-3 Gyr) age range is featured by cluster formation at a higher rate in the
inner regions than in the outer ones. We compared the observed CFs to
theoretical CFs, which are based on the star formation histories of the field
stars in each region of the LMC, and found the former predicting more or fewer
clusters than observed depending on the field and age range considered.Comment: 27 pages, 14 figures, MNRAS, accepte
Discovery of a loose star cluster in the Large Magellanic Cloud
We present results for an up-to-date uncatalogued star cluster projected
towards the Eastern side of the Large Magellanic Cloud (LMC) outer disc. The
new object was discovered from a search of loose star cluster in the Magellanic
Clouds' (MCs) outskirts using kernel density estimators on Washington CT1 deep
images. Contrarily to what would be commonly expected, the star cluster
resulted to be a young object (log(t /yr) = 8.45) with a slightly subsolar
metal content (Z = 0.013) and a total mass of 650Mo. Its core, half-mass and
tidal radii also are within the frequent values of LMC star clusters. However,
the new star cluster is placed at the Small Magellanic Cloud distance and at
11.3 kpc from the LMC centre. We speculate with the possibility that it was
born in the inner body of the LMC and soon after expeled into the intergalactic
space during the recent Milky Way/MCs interaction. Nevertheless, radial
velocity and chemical abundance measurements are needed to further understand
its origin, as well as extensive search for loose star clusters in order to
constrain the effectiveness of star cluster scattering during galaxy
interactions.Comment: MNRAS Letters, 5 pages, 2 figures (in press
The age-metallicity relationship in the Small Magellanic Cloud periphery
We present results from Washington CT1 photometry for eleven star fields
located in the western outskirts of the Small Magellanic Cloud (SMC), which
cover angular distances to its centre from 2 up to 13 degrees (~ 2.2 - 13.8
kpc). The colour- magnitude diagrams, cleaned from the unavoidable Milky Way
(MW) and background galaxy signatures, reveal that the most distant dominant
main sequence (MS) stellar populations from the SMC centre are located at an
angular distance of ~ 5.7 deg (6.1 kpc); no sign of farther clear SMC MS is
visible other than the residuals from the MW/background field contamination.
The derived ages and metallicities for the dominant stellar populations of the
western SMC periphery show a constant metallicity level ([Fe/H] = -1.0 dex) and
an approximately constant age value (~ 7-8 Gyr). Their age-metallicity
relationship (AMR) do not clearly differ from the most comprehensive AMRs
derived for almost the entire SMC main body. Finally, the range of ages of the
dominant stellar populations in the western SMC periphery confirms that the
major stellar mass formation activity at the very early galaxy epoch peaked ~
7-8 Gyr ago.Comment: 20 pages, 7 figures, MNRAS (accepted
Extended Main Sequence Turn-Offs in Low Mass Intermediate Age Clusters
We present an imaging analysis of four low mass stellar clusters (< 5000 Mo)
in the outer regions of the LMC in order to shed light on the extended main
sequence turn-off (eMSTO) phenomenon observed in high mass clusters. The four
clusters have ages between 1-2 Gyr and two of them appear to host eMTSOs. The
discovery of eMSTOs in such low mass clusters - > 5 times less massive than the
eMSTO clusters previously studied - suggests that mass is not the controlling
factor in whether clusters host eMSTOs. Additionally, the narrow extent of the
eMSTO in the two older (~ 2 Gyr) clusters is in agreement with predictions of
the stellar rotation scenario, as lower mass stars are expected to be
magnetically braked, meaning that their CMDs should be better reproduced by
canonical simple stellar populations. We also performed a structural analysis
on all the clusters and found that a large core radius is not a requisite for a
cluster to exhibit an eMSTO.Comment: Astronomy & Astrophyscis, 12 pages, 7 figures, accepte
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