488 research outputs found
Stellar Photometry of the Globular Cluster NGC 6229. I. Data Reduction and Morphology of the Brighter Part of the CMD
BV CCD photometry of the central (1.5 arcmin x 2.0 arcmin) part of the mildly
concentrated outer-halo globular cluster NGC 6229 is presented. The data
reduction in such a crowded field was based on a wavelet transform analysis.
Our larger dataset extends the previous results by Carney et al. (1991, AJ,
101, 1699) for the outer and less crowded fields of the cluster, and confirms
that NGC 6229 has a peculiar color-magnitude diagram for its position in the
Galaxy. In particular, NGC 6229's horizontal branch (HB) presents several
interesting features, among which stand out: a well populated and very extended
blue tail; a rather blue overall morphology, with (B-R)/(B+V+R) = 0.24+/-0.02;
a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808;
and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to be
identified in the Galactic outer halo. A low value of the R parameter is
confirmed, suggestive of a low helium abundance or of the presence of a quite
substantial population of extreme HB stars fainter than our photometric limit
(~ 2.5 mag below the RR Lyrae level in V). Twelve new possible variable stars
were found in the central part of the cluster. The morphology of the red giant
branch (RGB) also seems to be peculiar. In particular, the RGB luminosity
function ``bump'' is not a prominent feature and has only been tentatively
identified, on the basis of a comparison with a previously reported detection
for M3 (NGC 5272). Finally, we compare the properties of NGC 6229 with those
for other outer-halo globular clusters, and call attention to what appears to
be a bimodal HB distribution for the outer-halo cluster population, where
objects with very red or very blue HB types are much more frequently found than
clusters with intermediate HB types.Comment: 31 pages, LaTeX, uses AASTeX v4.0, 11 postscript figures and 7
postscript tables pasted into text. To appear in The Astronomical Journal
(Feb. 1997 issue
Connection between orbital modulation of H-alpha and gamma-rays in the Be/X-ray binary LSI+61303
We studied the average orbital modulation of various parameters (gamma-ray
flux, H-alpha emission line, optical V band brightness) of the radio- and
gamma-ray emitting Be/X-ray binary LSI+61303. Using the Spearman rank
correlation test, we found highly significant correlations between the orbital
variability of the equivalent width of the blue hump of the H-alpha and
Fermi-LAT flux with a Spearman p-value 2e-5, and the equivalent widths ratio
EW_B/EW_R and Fermi-LAT flux with p-value 9e-5. We also found a significant
anti-correlation between Fermi-LAT flux and V band magnitude with p-value
7.10^{-4}.
All these correlations refer to the average orbital variability, and we
conclude that the H-alpha and gamma-ray emission processes in LSI+61303 are
connected. The possible physical scenario is briefly discussed.Comment: accepted as a Letter in Astronomy and Astrophysic
Properties of massive stars in four clusters of the VVV survey
The evolution of massive stars is only partly understood. Observational
constraints can be obtained from the study of massive stars located in young
massive clusters. The ESO Public Survey VISTA Variables in the Via Lactea (VVV)
discovered several new clusters hosting massive stars. We present an analysis
of massive stars in four of these new clusters. Our aim is to provide
constraints on stellar evolution and to better understand the relation between
different types of massive stars. We use the radiative transfer code CMFGEN to
analyse K-band spectra of twelve stars with spectral types ranging from O and B
to WN and WC. We derive the stellar parameters of all targets as well as
surface abundances for a subset of them. In the Hertzsprung-Russell diagram,
the Wolf-Rayet stars are more luminous or hotter than the O stars. From the
log(C/N) - log(C/He) diagram, we show quantitatively that WN stars are more
chemically evolved than O stars, WC stars being more evolved than WN stars.
Mass loss rates among Wolf-Rayet stars are a factor of 10 larger than for O
stars, in agreement with previous findings.Comment: paper accepted in New Astronom
The Spatial Structure of Young Stellar Clusters. III. Physical Properties and Evolutionary States
We analyze the physical properties of stellar clusters that are detected in
massive star-forming regions in the MYStIX project--a comparative,
multiwavelength study of young stellar clusters within 3.6 kpc that contain at
least one O-type star. Tabulated properties of subclusters in these regions
include physical sizes and shapes, intrinsic numbers of stars, absorptions by
the molecular clouds, and median subcluster ages. Physical signs of dynamical
evolution are present in the relations of these properties, including
statistically significant correlations between subcluster size, central
density, and age, which are likely the result of cluster expansion after gas
removal. We argue that many of the subclusters identified in Paper I are
gravitationally bound because their radii are significantly less than what
would be expected from freely expanding clumps of stars with a typical initial
stellar velocity dispersion of ~3 km/s for star-forming regions. We explore a
model for cluster formation in which structurally simpler clusters are built up
hierarchically through the mergers of subclusters--subcluster mergers are
indicated by an inverse relation between the numbers of stars in a subcluster
and their central densities (also seen as a density vs. radius relation that is
less steep than would be expected from pure expansion). We discuss implications
of these effects for the dynamical relaxation of young stellar clusters.Comment: Accepted for publication in The Astrophysical Journal ; 48 pages, 13
figures, and 6 table
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