167 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
Another single hydrogen-rich Wolf-Rayet star in the SMC?
A 12th Wolf-Rayet star in the SMC has recently been discovered by Massey et
al. (2003). In order to determine its spectral type and a preliminary binary
status, we obtained 3 high signal-to-noise spectra separated in time at the
ESO-NTT. Compared to other WR stars in the SMC, SMC-WR12 appears to belong to
the subgroup of faint, single and hydrogen-rich WN stars. We discuss the
evolutionary status of WR12 and show that relatively low mass rotating
progenitors can better account for the properties of single hydrogen-rich WN
stars in the SMC.Comment: 5 pages, 2 figures, accepted by A&A, include latest correction
On the abundances of GRO J1655-40
Context: The detection of overabundances of -elements and lithium in the secondary star of a black-hole binary provides important insights about the formation of a stellar-mass black-hole. -enhancement might theoretically also be the result of pollution by the nucleosynthesis occurring during an outburst, or through spallation by the jet. Aims: We study the abundances, and their possible variations with time, in the secondary star of the runaway black-hole binary GRO J1655--40, in order to understand their origin. Methods: We present a detailed comparison between a Keck spectrum obtained in 1998 found in the literature, archival VLT-UVES data taken in 2004 and new VLT-UVES spectra obtained early 2006. We carefully determine the equivalent widths of different -elements (Mg, O, Ti, S and Si) with their associated uncertainty. We use the well-studied comparison star HD 156098 as well as synthetic spectra to match the spectrum of GRO J1655--40 in order to determine the abundances of these elements. Results: We see no significant variations of equivalent widths with time. Our fit using HD 156098 reveals that there is significant overabundance of oxygen in all our spectra, but no overabundances of any of the other -elements. Finally, we do not detect the lithium line at 6707 \AA. Conclusions: We show that there is no detected pollution in GRO J1655--40 after the burst in 2005. Moreover, we argue that uncertainties in the equivalent widths were previously underestimated by a factor of 3. Consequently, our results challenge the existence of general overabundances of -elements observed in this galactic black-hole binary, and thus the accepted interpretation that they are of supernova origin. The physical cause of the overabundance of oxygen remains unclear
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
Near-UV to near-IR disk-averaged Earth's reflectance spectra
We report 320 to 1020nm disk-averaged Earth reflectance spectra obtained from
Moon's Earthshine observations with the EMMI spectrograph on the NTT at ESO La
Silla (Chile). The spectral signatures of Earth atmosphere and ground
vegetation are observed. A vegetation red-edge of up to 9% is observed on
Europe and Africa and ~2% upon Pacific Ocean. The spectra also show that Earth
is a blue planet when Rayleigh scattering dominates, or totally white when the
cloud cover is large.Comment: Proceeding of an oral prensentation at the UAI No200 Colloquiu
The massive eclipsing LMC Wolf-Rayet binary BAT99-129. 1 Orbital parameters, hydrogen content and spectroscopic characteristics
BAT99-129 in the LMC is one among a handful of extra-galactic eclipsing
Wolf-Rayet binaries known. We present blue, medium-resolution, phase-dependent
NTT-EMMI spectra of this system that allow us to separate the spectra of the
two components of the binary and to obtain a reliable orbital solution for both
stars. We assign an O5V spectral type to the companion, and WN3(h)a to the
Wolf-Rayet component. We discuss the spectroscopic characteristics of the
system: luminosity ratio, radii, rotation velocities. We find a possible
oversynchronous rotation velocity for the O star. Surprisingly, the extracted
Wolf-Rayet spectrum clearly shows the presence of blueshifted absorption lines,
similar to what has been found in all single hot WN stars in the SMC and some
in the LMC. We also discuss the presence of such intrinsic lines in the context
of hydrogen in SMC and LMC Wolf-Rayet stars, WR+O binary evolution and GRB
progenitors. Altogether, BAT99~129 is the extragalactic counterpart of the
well-known Galactic WR binary V444 Cygni.Comment: 14 pages, 9 figures, accepted by A&A for publicatio
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&
- …