139 research outputs found
Wind ionization structure of the short-period eclipsing LMC Wolf-Rayet binary BAT99-129: preliminary results
BAT99-129 is a rare, short-period eclipsing Wolf-Rayet binary in the Large
Magellanic Cloud. We present here medium-resolution NTT/EMMI spectra that allow
us to disentangle the spectra of the two components and find the orbital
parameters of the binary. We also present VLT/FORS1 spectra of this binary
taken during the secondary eclipse, i.e. when the companion star passes in
front of the Wolf-Rayet star. With these data we are able to extract, for the
first time in absolute units for a WR+O binary, the sizes of the line emitting
regions.Comment: 6 pages, 5 figures, to appear in proc. of "Close Binaries in the 21st
Century: New Opportunities and Challenges", 2005 - Corrected Figure
The Wolf-Rayet stars in the Large Magellanic Cloud: A comprehensive analysis of the WN class
Aims: Following our comprehensive studies of the WR stars in the Milky Way,
we now present spectroscopic analyses of almost all known WN stars in the LMC.
Methods: For the quantitative analysis of the wind-dominated emission-line
spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By
fitting synthetic spectra to the observed spectral energy distribution and the
available spectra (ultraviolet and optical), we obtain the physical properties
of 107 stars. Results: We present the fundamental stellar and wind parameters
for an almost complete sample of WN stars in the LMC. Among those stars that
are putatively single, two different groups can be clearly distinguished. While
12% of our sample are more luminous than 10^6 Lsun and contain a significant
amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate
the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the
few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars,
descended directly from the main sequence at very high initial masses, the bulk
of WN stars have gone through the red-supergiant phase. According to their
luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate
from initial masses between 20 and 40 Msun. This mass range is similar to the
one found in the Galaxy, i.e. the expected metallicity dependence of the
evolution is not seen. Current stellar evolution tracks, even when accounting
for rotationally induced mixing, still partly fail to reproduce the observed
ranges of luminosities and initial masses. Moreover, stellar radii are
generally larger and effective temperatures correspondingly lower than
predicted from stellar evolution models, probably due to subphotospheric
inflation.Comment: 17+46 pages; 10+54 figures; v2: typos corrected, space-saving layout
for appendix C, published in A&
Spectroscopy of SMC Wolf-Rayet Stars Suggests that Wind-Clumping does not Depend on Ambient Metallicity
The mass-loss rates of hot, massive, luminous stars are considered a decisive
parameter in shaping the evolutionary tracks of such stars and influencing the
interstellar medium on galactic scales. The small-scale structures (clumps)
omnipresent in such winds may reduce empirical estimates of mass-loss rates by
an evolutionarily significant factor of >=3. So far, there has been no direct
observational evidence that wind-clumping may persist at the same level in
environments with a low ambient metallicity, where the wind-driving opacity is
reduced. Here we report the results of time-resolved spectroscopy of three
presumably single Population I Wolf-Rayet stars in the Small Magellanic Cloud,
where the ambient metallicity is ~1/5 Z_Sun.We detect numerous small-scale
emission peaks moving outwards in the accelerating parts of the stellar
winds.The general properties of the moving features, such as their velocity
dispersions,emissivities and average accelerations, closely match the
corresponding characteristics of small-scale inhomogeneities in the winds of
Galactic Wolf-Rayet stars.Comment: 9 pages, 3 figures; accepted by ApJ Letter
Two new Wolf-Rayet stars in the LMC
We report the discovery of two previously unknown WN3 stars in the Large
Magellanic Cloud. Both are bright (15th magnitude), isolated, and located in
regions covered in earlier surveys, although both are relatively weak-lined. We
suggest that there may be remaining undiscovered WNE stars in
the LMC
Revisiting a fundamental test of the disc instability model for X-ray binaries
We revisit a core prediction of the disc instability model (DIM) applied to
X-ray binaries. The model predicts the existence of a critical mass transfer
rate, which depends on disc size, separating transient and persistent systems.
We therefore selected a sample of 52 persistent and transient neutron star and
black hole X-ray binaries and verified if observed persistent (transient)
systems do lie in the appropriate stable (unstable) region of parameter space
predicted by the model. We find that, despite the significant uncertainties
inherent to these kinds of studies, the data are in very good agreement with
the theoretical expectations. We then discuss some individual cases that do not
clearly fit into this main conclusion. Finally, we introduce the transientness
parameter as a measure of the activity of a source and show a clear trend of
the average outburst recurrence time to decrease with transientness in
agreement with the DIM predictions. We therefore conclude that, despite
difficulties in reproducing the complex details of the lightcurves, the DIM
succeeds to explain the global behaviour of X-ray binaries averaged over a long
enough period of time.Comment: 12 pages, 4 figures. Accepted for publication in MNRAS. Version 2:
some typos corrected and references adde
Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. I. The nature of the companion star
Context: The INTEGRAL hard X-ray observatory has revealed an emerging
population of highly obscured X-ray binary systems through multi-wavelength
observations. Previous studies have shown that many of these sources are
high-mass X-ray binaries hosting neutron stars orbiting around luminous and
evolved companion stars. Aims: To better understand this newly-discovered
population, we have selected a sample of sources for which an accurate
localisation is available to identify the stellar counterpart and reveal the
nature of the companion star and of the binary system. Methods: We performed an
intensive study of a sample of thirteen INTEGRAL sources, through
multi-wavelength optical to NIR photometric and spectroscopic observations,
using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate
astrometry and identified candidate counterparts for which we give the optical
and NIR magnitudes. We detected many spectral lines allowing us to determine
the spectral type of the companion star. We fitted with stellar black bodies
the mid-infrared to optical spectral energy distributions of these sources.
From the spectral analysis and SED fitting we identified the nature of the
companion stars and of the binary systems. (abridged).Comment: A&A in press; The official date of acceptance is 15/12/2007; 25
pages, 6 figures, 8 tables. New version with language editing required by
edito
Spitzer SAGE-SMC Infrared Photometry of Massive Stars in the Small Magellanic Cloud
We present a catalog of 5324 massive stars in the Small Magellanic Cloud
(SMC), with accurate spectral types compiled from the literature, and a
photometric catalog for a subset of 3654 of these stars, with the goal of
exploring their infrared properties. The photometric catalog consists of stars
with infrared counterparts in the Spitzer, SAGE-SMC survey database, for which
we present uniform photometry from 0.3-24 um in the UBVIJHKs+IRAC+MIPS24 bands.
We compare the color magnitude diagrams and color-color diagrams to those of
the Large Magellanic Cloud (LMC), finding that the brightest infrared sources
in the SMC are also the red supergiants, supergiant B[e] (sgB[e]) stars,
luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less
infrared excess, the red supergiants being less dusty and the sgB[e] stars
being on average less luminous. Among the objects detected at 24 um are a few
very luminous hypergiants, 4 B-type stars with peculiar, flat spectral energy
distributions, and all 3 known luminous blue variables. We detect a distinct Be
star sequence, displaced to the red, and suggest a novel method of confirming
Be star candidates photometrically. We find a higher fraction of Oe and Be
stars among O and early-B stars in the SMC, respectively, when compared to the
LMC, and that the SMC Be stars occur at higher luminosities. We estimate
mass-loss rates for the red supergiants, confirming the correlation with
luminosity even at the metallicity of the SMC. Finally, we confirm the new
class of stars displaying composite A & F type spectra, the sgB[e] nature of
2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue
variable with cold dust.Comment: 23 pages, 17 figures, 5 tables, accepted for publication in the
Astronomical Journa
The variability plane of accreting compact objects
Recently, it has been shown that soft-state black hole X-ray binaries and
active galactic nuclei populate a plane in the space defined by the black hole
mass, accretion rate and characteristic frequency. We show that this plane can
be extended to hard-state objects if one allows a constant offset for the
frequencies in the soft and the hard state. During a state transition the
frequencies rapidly move from one scaling to the other depending on an
additional parameter, possibly the disk-fraction. The relationship between
frequency, mass and accretion rate can be further extended by including weakly
accreting neutron stars. We explore if the lower kHz QPOs of neutron stars and
the dwarf nova oscillations of white dwarfs can be included as well and discuss
the physical implications of the found correlation.Comment: Accepted for publication in MNRA
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
OGLE-2005-BLG-071Lb, the Most Massive M-Dwarf Planetary Companion?
We combine all available information to constrain the nature of
OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first
in a high-magnification event. These include photometric and astrometric
measurements from Hubble Space Telescope, as well as constraints from higher
order effects extracted from the ground-based light curve, such as microlens
parallax, planetary orbital motion and finite-source effects. Our primary
analysis leads to the conclusion that the host of Jovian planet
OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/-
0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~
103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup,
lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has
an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate
model less favored by \Delta\chi^2 = 2.1 (or 2.2, depending on the sign of the
impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a
smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium
temperature T ~ 71 K. These results from the primary analysis suggest that
OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that
is hosted by an M dwarf. However, the formation of such high-mass planetary
companions in the outer regions of M-dwarf planetary systems is predicted to be
unlikely within the core-accretion scenario. There are a number of caveats to
this primary analysis, which assumes (based on real but limited evidence) that
the unlensed light coincident with the source is actually due to the lens, that
is, the planetary host. However, these caveats could mostly be resolved by a
single astrometric measurement a few years after the event.Comment: 51 pages, 12 figures, 3 tables, Published in Ap
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