527 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
The catalogue of OB associations in IC 1613
We present a catalogue of OB associations in IC 1613. Using an automatic and
objective method (Battinelli's 1991 technique) 60 objects were found. The size
distribution reveals a significant peak at about 60 parsecs if a distance
modulus of 24.27 mag is assumed. Spatial distributions of the detected
associations and H II regions are strongly correlated.Comment: 5 pages, 3 figures, A&A accepte
Obscured clusters. II. GLIMPSE-C02 - A new metal rich globular cluster in the Milky Way
The estimated total number of Milky Way globulars is 160+-20. The question of
whether there are any more undiscovered globular clusters in the Milky Way is
particularly relevant with advances in near and mid-IR instrumentation. This
investigation is a part of a long-term project to search the inner Milky Way
for hidden star clusters and to study them in detail. GLIMPSE-C02 (G02) is one
of these objects, situated near the Galactic plane (l=14.129deg, b=-0.644deg).
Our analysis is based on SOFI/NTT JHKs imaging and low resolution (R~1400)
spectroscopy of three bright cluster red giants in the K atmospheric window. We
derived the metal abundance by analysis of these spectra and from the slope of
the RGB. The cluster is deeply embedded in dust and undergoes a mean reddening
of Av~24.8+-3 mag. The distance to the object is D=4.6+-0.7kpc. The metal
abundance of G02 is [Fe/H](H96)=-0.33+-0.14 and [Fe/H](CG)=-0.16+-0.12 using
different scales. The best fit to the radial surface brightness profile with a
single-mass King's model yields a core radius rc=0.70 arcmin (0.9pc), tidal
radius rt=15 arcmin (20pc), and central oncentration c=1.33. We demonstrate
that G02 is new Milky Way globular cluster, among the most metal rich globular
clusters in the Galaxy. The object is physically located at the inner edge of
the thin disk and the transition region with the bulge, and also falls in the
zone of the "missing" globulars toward the central region of the Milky Way.Comment: 5 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Obscured clusters.I. GLIMPSE30 - Young Milky Way Star Cluster Hosting Wolf-Rayet Stars
Young massive clusters are perfect astrophysical laboratories for study of
massive stars. Clusters with Wolf-Rayet (WR) stars are of special importance,
since this enables us to study a coeval WR population at a uniform metallicity
and known age. GLIMPSE30 (G30) is one of them. The cluster is situated near the
Galactic plane (l=298.756deg, b=-0.408deg) and we aimed to determine its
physical parameters and to investigate its high-mass stellar content and
especially WR stars. Our analysis is based on SOFI/NTT JsHKs imaging and low
resolution (R~2000) spectroscopy of the brightest cluster members in the K
atmospheric window. For the age determination we applied isochrone fits for MS
and Pre-MS stars. We derived stellar parameters of the WR stars candidates
using a full nonLTE modeling of the observed spectra. Using a variety of
techniques we found that G30 is very young cluster, with age t~4Myr. The
cluster is located in Carina spiral arm, it is deeply embedded in dust and
suffers reddening of Av~10.5+-1.1mag. The distance to the object is
d=7.2+-0.9kpc. The mass of the cluster members down to 2.35Msol is ~1600Msol.
Cluster's MF for the mass range of 5.6 to 31.6Msol shows a slope of
Gamma=-1.01+-0.03. The total mass of the cluster obtained by this MF down to
1Msol is about 3x10^3Msol. The spectral analysis and the models allow us to
conclude that in G30 are at least one Ofpe/WN and two WR stars. The WR stars
are of WN6-7 hydrogen rich type with progenitor masses more than 60Msol. G30 is
a new member of the exquisite family of young Galactic clusters, hosting WR
stars. It is a factor of two to three less massive than some of the youngest
super-massive star clusters like Arches, Quintuplet and Central cluster and is
their smaller analog.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
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
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
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