322 research outputs found
A multi-wavelength view on the dusty Wolf-Rayet star WR 48a
We present results from the first attempts to derive various physical
characteristics of the dusty Wolf-Rayet star WR 48a based on a multi-wavelength
view of its observational properties. This is done on the basis of new optical
and near-infrared spectral observations and on data from various archives in
the optical, radio and X-rays. The optical spectrum of WR 48a is acceptably
well represented by a sum of two spectra: of a WR star of the WC8 type and of a
WR star of the WN8h type. The strength of the interstellar absorption features
in the optical spectra of WR 48a and the near-by stars D2-3 and D2-7 (both
members of the open cluster Danks 2) indicates that WR 48a is located at a
distance of ~4 kpc from us. WR 48a is very likely a thermal radio source and
for such a case and smooth (no clumps) wind its radio emission suggests a
relatively high mass-loss rate of this dusty WR star (dM/dt = a few x 10^(-4)
solar masses per year). Long timescale (years) variability of WR 48a is
established in the optical, radio and X-rays. Colliding stellar winds likely
play a very important role in the physics of this object. However, some
LBV-like (luminous blue variable) activity could not be excluded as well.Comment: Accepted for publication in MNRAS; 16 pages, 16 figures, 6 table
Long-term Stellar Variability in the Galactic Centre Region
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.We report the detection of variable stars within a 11.5' x 11.5' region near the Galactic centre (GC) that includes the Arches and Quintuplet clusters, as revealed by the VISTA Variables in the Via Lactea (VVV) survey. There are 353 sources that show Ks-band variability, of which the large majority (81%) correspond to red giant stars, mostly in the asymptotic giant branch (AGB) phase. We analyze a population of 52 red giants with long-term trends that cannot be classified into the typical pulsating star categories. Distances and extinctions are calculated for 9 Mira variables, and we discuss the impact of the chosen extinction law on the derived distances. We also report the presence of 48 new identified young stellar object (YSO) candidates in the region.Peer reviewe
Imaging of the Stellar Population of IC10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope
We present adaptive optics (AO) images of the central starburst region of the
dwarf irregular galaxy IC10. The Keck 2 telescope laser guide star was used to
achieve near diffraction-limited performance at H and K' (Strehls of 18% and
32%, respectively). The images are centered on the putative Wolf-Rayet (W-R)
object [MAC92]24. We combine our AO images with F814W data from HST. By
comparing the K' vs. [F814W]-K' color-magnitude diagram (CMD) with theoretical
isochrones, we find that the stellar population is best represented by at least
two bursts of star formation, one ~ 10 Myr ago and one much older (150-500
Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This
population represents an OB association with a half-light radius of about 3 pc.
We resolve the W-R object [MAC92]24 into at least six blue stars. Four of these
components have near-IR colors and luminosities that make them robust WN star
candidates. By matching the location of C-stars in the CMD with those in the
SMC we derive a distance modulus for IC10 of about 24.5 mag. and a foreground
reddening of E(B-V) = 0.95. We find a more precise distance by locating the tip
of the giant branch in the F814W, H, and K' luminosity functions. We find a
weighted mean distance modulus of 24.48 +/- 0.08. The systematic error in this
measurement, due to a possible difference in the properties of the RGB
populations in IC10 and the SMC, is +/- 0.16 mag.Comment: 13 pages, 13 figures, ApJ in pres
Discovery of new Milky Way star cluster candidates in the 2MASS Point Source Catalog IV. Follow-up observations of cluster candidates in the Galactic plane
Nearly 500 cluster candidates have been reported by searches based on the new
all-sky near infrared surveys. The true nature of the majority of these objects
is still unknown. This project aims to estimate the physical parameters of some
of the candidates in order to use them as probes of the obscured star formation
in the Milky Way.
Here we report deep near infrared observations of four objects, discovered by
our search based on the 2MASS Point Source Catalog (Ivanov et al. 2002;
Borissova et al. 2003). CC04 appears to be a few million year old cluster. We
estimate its distance and extinction, and set a limit on the total mass. CC08
contains red supergiants, indicating a slightly older age of about 7-10 Myr.
The suspected cluster nature of CC13 was not confirmed. CC14 appears to be an
interesting candidate with double-tail-like morphology but our data doesn't
allow us to derive a firm conclusion about the nature of this object.
We found no supermassive star clusters similar to the Arches or the
Quintuplet (Mtot >= 10**4 Msolar) among the dozen confirmed clusters studied so
far in this series of papers, indicating that such objects are not common in
the Milky Way.Comment: 6 pages, 8 pages; accepted for publication in Astronomy and
Astrophysic
Discovery of New Milky Way Star Clusters Candidates in the 2MASS Point Source Catalog
A systematic search of the 2MASS point source catalog, covering 47% of the
sky, was carried out aiming to reveal any hidden globular clusters in our
Galaxy. Eight new star clusters were discovered by a search algorithm based on
finding peaks in the apparent stellar surface density, and a visual inspection
of their vicinities yielded additional two. They all are concentrated toward
the Galactic plane and are hidden behind up to Av=20 mag which accounts for
their late discovery. The majority of new clusters are associated with H II
regions or unidentified IRAS sources suggesting that they are young, probably
similar to Arches or open clusters. Only one candidate has morphology similar
to a globular cluster and the verification of its nature will require deeper
observations with higher angular resolution than the 2MASS data.Comment: 4 pages, 3 figures (1 is a jpeg file), acccepted for publication in
Astronomy and Astrophysic
Chandra and HST Confirmation of the Luminous and Variable X-ray Source IC 10 X-1 as a Possible Wolf-Rayet, Black-Hole Binary
We present a Chandra and HST study of IC 10 X-1, the most luminous X-ray
binary in the closest starburst galaxy to the Milky Way. Our new hard X-ray
observation of X-1 confirms that it has an average 0.5-10 keV luminosity of
1.5e38 erg/s, is strongly variable (a factor of ~2 in >3 ks), and is spatially
coincident (within 0.'23 +/-0.'30) with the Wolf-Rayet (WR) star [MAC92] 17A in
IC 10. The spectrum of X-1 is best fit by a power law with photon index of ~1.8
and a thermal plasma with kT~1.5 keV, although systematic residuals hint at
further complexity. Taken together, these facts suggest that X-1 may be a black
hole belonging to the rare class of WR binaries; it is comparable in many ways
to Cyg X-3. The Chandra observation also finds evidence for extended X-ray
emission co-spatial with the large non-thermal radio superbubble surrounding
X-1.Comment: ApJL in press (Oct 2003), 4 pages, 4 figures (w/ fig1 at severely
reduced quality), latest emulateapj.cls use
Discovery of > 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue HB
We report on the discovery of a large number of RR Lyrae variable stars in
the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which
places it among the top three most RR Lyrae-rich globular clusters known.
Likely members of the cluster in our studied field, from our preliminary number
counts, include about 130 fundamental-mode (RRab) pulsators, with =
0.548 d, and about 75 first-overtone (RRc) pulsators, with = 0.300 d.
The average periods and the position of the RRab variables with well-defined
light curves in the Bailey diagram both suggest that the cluster is of
Oosterhoff type I. However, the morphology of the cluster's horizontal branch
(HB) is strikingly similar to that of the Oosterhoff type II globular cluster
M15 (NGC 7078), with a dominant blue HB component and a very extended blue
tail. Since M15 and M62 differ in metallicity by about one dex, we conclude
that metallicity, at a fixed HB type, is a key parameter determining the
Oosterhoff status of a globular cluster and the position of its variables in
the Bailey diagram.Comment: 5 pages, 4 figures. ApJ Letters, in pres
Properties of the solar neighbor WISE J072003.20-084651.2
The severe crowding towards the Galactic plane suggests that the census of
nearby stars in that direction may be incomplete. Recently, Scholz reported a
new M9 object at an estimated distance d~7 pc (WISE J072003.20-084651.2;
hereafter WISE0720) at Galactic latitude b=2.3 degr.
Our goals are to determine the physical characteristics of WISE0720, its
kinematic properties, and to address the question if it is a binary object, as
suggested in the discovery paper.
Optical and infrared spectroscopy from the Southern African Large Telescope
and Magellan, respectively, and spectral energy distribution fitting were used
to determine the spectral type of WISE0720. The measured radial velocity,
proper motion and parallax yielded its Galactic velocities. We also
investigated if WISE0720 may show X-ray activity based on archival data.
Our spectra are consistent with spectral type L0+/-1. We find no evidence for
binarity, apart for a minor 2-sigma level difference in the radial velocities
taken at two different epochs. The spatial velocity of WISE0720 does not
connect it to any known moving group, instead it places the object with high
probability in the old thin disk or in the thick disk. The spectral energy
distribution fit hints at excess in the 12 and 22 micron WISE bands which may
be due to a redder companion, but the same excess is visible in other late type
objects, and it more likely implies a shortcoming of the models (e.g., issues
with the effective wavelengths of the filters for these extremely cool objects,
etc.) rather than a disk or redder companion. The optical spectrum shows some
Halpha emission, indicative of stellar activity. Archival X-ray observations
yield no detection.Comment: A&A, accepted; 9 pages, 6 figure
Molecular Gas in the Low Metallicity, Star Forming Dwarf IC 10
We present a complete survey of CO 1->0 emission in the Local Group dwarf
irregular IC 10. The survey, conducted with the BIMA interferometer, covers the
stellar disk and a large fraction of the extended HI envelope with the
sensitivity and resolution necessary to detect individual giant molecular
clouds (GMCs) at the distance of IC 10 (950 kpc). We find 16 clouds with a
total CO luminosity of 1 x 10^6 K km s^-1 pc^2, equivalent to 4 x 10^6 Msun of
molecular gas using the Galactic CO-to-H2 conversion factor. Observations with
the ARO 12m find that BIMA may resolve out as much as 50% of the CO emission,
and we estimate the total CO luminosity as 2.2 x 10^6 K km s^-1 pc^2. We
measure the properties of 14 GMCs from high resolution OVRO data. These clouds
are very similar to Galactic GMCs in their sizes, line widths, luminosities,
and CO-to-H2 conversion factors despite the low metallicity of IC 10 (Z ~ 1/5
Zsun). Comparing the BIMA survey to the atomic gas and stellar content of IC 10
we find that most of the CO emission is coincident with high surface density
HI. IC 10 displays a much higher star formation rate per unit molecular (H2) or
total (HI+H2) gas than most galaxies. This could be a real difference or may be
an evolutionary effect - the star formation rate may have been higher in the
recent past.Comment: 21 pages, 14 figures, Accepted to Ap
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