70 research outputs found
Decoding of the light changes in eclipsing Wolf-Rayet binaries I. A non-classical approach to the solution of light curves
We present a technique to determine the orbital and physical parameters of
eclipsing eccentric Wolf-Rayet + O-star binaries, where one eclipse is produced
by the absorption of the O-star light by the stellar wind of the W-R star. Our
method is based on the use of the empirical moments of the light curve that are
integral transforms evaluated from the observed light curves. The optical depth
along the line of sight and the limb darkening of the W-R star are modelled by
simple mathematical functions, and we derive analytical expressions for the
moments of the light curve as a function of the orbital parameters and the key
parameters of the transparency and limb-darkening functions. These analytical
expressions are then inverted in order to derive the values of the orbital
inclination, the stellar radii, the fractional luminosities, and the parameters
of the wind transparency and limb-darkening laws. The method is applied to the
SMC W-R eclipsing binary HD 5980, a remarkable object that underwent an
LBV-like event in August 1994. The analysis refers to the pre-outburst
observational data. A synthetic light curve based on the elements derived for
the system allows a quality assessment of the results obtained.Comment: Accepted for publication in Astronomy & Astrophysic
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
Structure and Feedback in 30 Doradus I: Observations
We have completed a a new optical imaging and spectrophotometric survey of a
140 x 80 pc region of 30 Doradus centered on R136, covering key optical
diagnostic emission lines including \Ha, \Hb, \Hg, [O III]
4363, 4959, 5007, [N II] 6548, 6584, [S II]
6717, 6731 [S III] 6312 and in some locations [S III]
9069. We present maps of fluxes and intensity ratios for these lines,
and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on
ionization fronts. The final science-quality spectroscopic data products are
available to the public. Our analysis of the new data finds that, while stellar
winds and supernovae undoubtedly produce shocks and are responsible for shaping
the nebula, there are no global spectral signatures to indicate that shocks are
currently an important source of ionization. We conclude that the considerable
region covered by our survey is well described by photoionization from the
central cluster where the ionizing continuum is dominated by the most massive O
stars. We show that if 30 Dor were viewed at a cosmological distance, its
integrated light would be dominated by its extensive regions of lower
surface-brightness rather than by the bright, eye-catching arcs.Comment: 42 pages, 16 figures, Accepted for publication in ApJ
A Search for Wolf-Rayet Stars in the Small Magellanic Cloud
We conducted an extensive search for Wolf-Rayet stars (W-Rs) in the SMC,
using the same interference filter imaging techniques that have proved
successful in finding W-Rs in more distant members of the Local Group.
Photometry of some 1.6 million stellar images resulted in some 20 good
candidates, which we then examined spectroscopically. Two of these indeed
proved to be newly found W-Rs, bringing the total known in the SMC from 9 to
11. Other finds included previously unknown Of-type stars (one as early as
O5f?p)),the recovery of the Luminous Blue Variable S18, and the discovery of a
previously unknown SMC symbiotic star. More important, however, is the fact
that there does not exist a significant number of W-Rs waiting to be discovered
in the SMC. The number of W-Rs in the SMC is a factor of 3 lower than in the
LMC (per unit luminosity), and we argue this is the result of the SMC's low
metallicity on the evolution of the most massive stars.Comment: Accepted by Astrophysical Journal. Postscript version available via
ftp.lowell.edu/pub/massey/smcwr.ps.gz Revised version contains slightly
revised spectral types for the Of stars but is otherwise unchange
Extinction law variations and dust excitation in the spiral galaxy NGC 300
We investigate the origin of the strong radial gradient in the
ultraviolet-to-infrared ratio in the spiral galaxy NGC 300, and emphasize the
importance of local variations in the interstellar medium geometry, concluding
that they cannot be neglected with respect to metallicity effects. This
analysis is based upon a combination of maps from GALEX and Spitzer, and from
the ground (UBVRI, Halpha and Hbeta). We select ionizing stellar clusters
associated with HII regions of widely varying morphologies, and derive their
fundamental parameters from population synthesis fitting of their spectral
energy distributions, measured to eliminate local backgrounds accurately. From
these fits, we conclude that the stellar extinction law is highly variable in
the line of sight of young clusters of similar ages. In the particular model
geometry that we consider most appropriate to the sampled regions, we checked
that our findings are not significantly altered by the correct treatment of
radiative transfer effects. The variations are systematic in nature: extinction
laws of the Milky Way or LMC type are associated with compact HII regions (the
compacity being quantified in two different ways), while clusters surrounded by
diffuse HII regions follow extinction laws of the 30 Doradus or SMC type. The
Calzetti starburst attenuation law, although most often degenerate with the 30
Doradus extinction law, overpredicts ionizing photon fluxes by large amounts.
We also find that the extinction law variations are correlated with the column
density of dust species emitting in the near- and mid-infrared. Finally, we
briefly discuss the nebular to stellar extinction ratios, and the excitation of
aromatic band carriers, invalidating their claimed association with cold dust.Comment: accepted for publication in ApJ -- figure 6 abridged her
Reduced Wolf-Rayet Line Luminosities at Low Metallicity
New NTT/EMMI spectrophotometry of single WN2-5 stars in the Magellanic Clouds
are presented, from which HeII 4686 line luminosities have been derived, and
compared with observations of other Magellanic Cloud WR stars. SMC WN3-4 stars
possess line luminosities which are a factor of 4 times lower than LMC
counterparts, incorporating several binary SMC WN3-4 stars. Similar results are
found for WN5-6 stars, despite reduced statistics, incorporating observations
of single LMC WN5-9 stars. CIV 5808 line luminosities of carbon sequence WR
stars in the SMC and IC1613 (both WO subtypes) are a factor of 3 lower than LMC
WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are
also obtained for the sole LMC WO star. We demonstrate how reduced line
luminosities at low metallicity follow naturally if WR winds are Z-dependent,
as recent results suggest. We apply mass loss-Z scalings to atmospheric non-LTE
models of Milky Way and LMC WR stars to predict the wind signatures of WR stars
in the metal-poor star forming WR galaxy IZw18. WN HeII 4686 line luminosities
are 7-20 times lower than in Z-rich counterparts of identical bolometric
luminosity, whilst WC CIV 5808 line luminosities are 3-6 times lower.
Significant He^+ Lyman continuum fluxes are predicted for Z-poor early-type WR
stars. Consequently, our results suggest the need for larger population of WR
stars in IZw18 than is presently assumed, particularly for WN stars,
potentially posing a severe challenge to evolutionary models at very low Z.
Finally, reduced wind strengths from WR stars at low Z impacts upon the
immediate circumstellar environment of long duration GRB afterglows,
particularly since the host galaxies of high-redshift GRBs tend to be Z-poor.Comment: 14 pages, 12 figures, accepted for A&A, revision fixes error with Eqn
Thermal and Non-thermal X-Rays from the LMC Super Bubble 30 Dor C
We report on the discovery of thermal and non-thermal X-rays from the shells
of the super bubble (SB) 30 Dor C in the Large Magellanic Cloud (LMC). The
X-ray morphology is a nearly circular shell with a radius of about 40 pc, which
is bright on the northern and western sides. The spectra of the shells are
different from region to region. The southern shell shows clear emission lines,
and is well fitted with a model of a thin-thermal plasma (kT = 0.21keV) in
non-equilibrium ionization (NEI) plus a power-law component. This thermal
plasma is located inside of the H alpha emission, which is the outer edge of
the shell of the SB. The northern and western sides of the SB are dim in H
alpha emission, but are bright in non-thermal (power-law) X-rays with a photon
index of 2.1-2.9. The non-thermal X-ray shell traces the outer boundary of the
radio shell. These features of thin-thermal and non-thermal X-rays are similar
to those of SN 1006, a prototype of synchrotron X-ray shell, but the
non-thermal component of 30 Dor C is about ten-times brighter than that of SN
1006. 30 Dor C is the first candidate of an extragalactic SB, in which
energetic electrons are accelerating in the shell. The age is much older than
that of SN 1006, and hence the particle acceleration time in this SB may be
longer than those in normal shell-like SNRs. We found point-like sources
associated with some of tight star clusters. The X-ray luminosity and spectrum
are consistent with those of young clusters of massive stars. Point-like
sources with non-thermal spectra are also found in the SB. These may be
background objects (AGNs) or stellar remnants (neutron stars or black holes).Comment: 11 pages, 6 figures, Accepted for publication in ApJ, the paper with
full resolution images in
http://www-cr.scphys.kyoto-u.ac.jp/member/bamba/Paper/30DorC.pd
Discovery and quantitative spectral analysis of an Ofpe/WN9 (WN11) star in the Sculptor spiral galaxy NGC 300
We have discovered an Ofpe/WN9 (WN11 following Smith et al.) star in the
Sculptor spiral galaxy NGC 300, the first object of this class found outside
the Local Group, during a recent spectroscopic survey of blue supergiant stars
obtained at the ESO VLT. The light curve over a five-month period in late 1999
displays a variability at the 0.1 mag level. The intermediate resolution
spectra (3800-7200 A) show a very close resemblance to the Galactic LBV AG Car
during minimum. We have performed a detailed non-LTE analysis of the stellar
spectrum, and have derived a chemical abundance pattern which includes H, He,
C, N, O, Al, Si and Fe, in addition to the stellar and wind parameters. The
derived stellar properties and the He and N surface enrichments are consistent
with those of other Local Group WN11 stars in the literature, suggesting a
similar quiescent or post-LBV evolutionary status.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication in the
Astrophysical Journal Letter
A large Wolf-Rayet population in NGC300 uncovered by VLT-FORS2
We have detected 58 Wolf-Rayet candidates in the central region of the nearby
spiral galaxy NGC 300, based on deep VLT-FORS2 narrow-band imaging. Our survey
is close to complete except for heavily reddened WR stars. Of the objects in
our list, 16 stars were already spectroscopically confirmed as WR stars by
Schild & Testor and Breysacher et al., to which 4 stars are added using low
resolution FORS2 datasets. The WR population of NGC300 now totals 60,a
threefold increase over previous surveys, with WC/WN>1/3, in reasonable
agreement with Local Group galaxies for a moderately sub-solar metallicity. We
also discuss the WR surface density in the central region of NGC 300. Finally,
analyses are presented for two apparently single WC stars - #29 (alias WR3,
WC5) and #48 (alias WR13, WC4) located close to the nucleus, and at a
deprojected radius of 2.5 kpc, respectively. These are among the first models
of WR stars in galaxies beyond the Local Group, and are compared with early WC
stars in our Galaxy and LMC.Comment: 12 pages, 12 figures, submitted to A&A (includes aa.cls) - version
with higher resolution finding charts available from
ftp://ftp.star.ucl.ac.uk/pub/pac/ngc300.ps.g
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