907 research outputs found
Modeling Variable Linear Polarization Produced by Co-Rotating Interaction Regions (CIRs) Across Optical Recombination Lines of Wolf-Rayet Stars
Massive star winds are structured both stochastically ("clumps") and often
coherently (Co-rotation Interaction Regions, or CIRs). Evidence for CIRs
threading the winds of Wolf-Rayet (WR) stars arises from multiple diagnostics
including linear polarimetry. Some observations indicate changes in
polarization position angle across optical recombination emission lines from a
WR star wind but limited to blueshifted Doppler velocities. We explore a model
involving a spherical wind with a single conical CIR stemming from a rotating
star as qualitative proof-of-concept. To obtain a realistic distribution of
limb polarization and limb darkening across the pseudo-photosphere formed in
the optically thick wind of a WR star, we used Monte Carlo radiative transfer
(MCRT). Results are shown for a parameter study. For line properties similar to
WR 6 (EZ CMa; HD 50896), the combination of the MCRT results, a simple model
for the CIR, and the Sobolev approximation for the line formation, we were able
to reproduce variations in both polarization amplitude and position angle
commensurate with observations. Characterizing CIRs in WR~winds has added
importance for providing stellar rotation periods since the v sin i values are
unobtainable because the pseudo-photosphere forms in the wind itself.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in MNRAS following peer revie
Polarimetric Evidence of Non-Spherical Winds
Polarization observations yield otherwise unobtainable information about the
geometrical structure of unresolved objects. In this talk we review the
evidences for non-spherically symmetric structures around Luminous Hot Stars
from polarimetry and what we can learn with this technique. Polarimetry has
added a new dimension to the study of the envelopes of Luminous Blue Variables,
Wolf-Rayet stars and B[e] stars, all of which are discussed in some detail.Comment: 8 pages, 2 encapsulated Postscript figures, uses lamuphys.sty.
Invited review to appear in IAU Coll. 169, Variable and Non-Spherical Stellar
Winds in Luminous Hot Stars, eds. B. Wolf, A.Fullerton and O. Stahl
(Springer
Abundances and Physical Conditions in the Interstellar Gas toward HD 192 639
We present a study of the abundances and physical conditions in the
interstellar gas toward the heavily reddened star HD 192639 [E_(B-V) = 0.64],
based on analysis of FUSE and HST/STIS spectra covering the range from 912 to
1361 A. This work constitutes a survey of the analyses that can be performed to
study the interstellar gas when combining data from different instruments.
Low-velocity (-18 to -8 km/s) components are seen primarily for various neutral
and singly ionized species such as C I, O I, S I, Mg II, Cl I, Cl II, Mn II, Fe
II and Cu II. Numerous lines of H2 are present in the FUSE spectra, with a
kinetic temperature for the lowest rotational levels T_(01) = (90 +/- 10) K.
Analysis of the C I fine-structure excitation implies an average local density
of hydrogen n_H = (16 +/- 3) cm^-3. The average electron density, derived from
five neutral/first ion pairs under the assumption of photoionization
equilibrium, is n_e = (0.11 +/- 0.02) cm^-3. The relatively complex component
structure seen in high-resolution spectra of K I and Na I, the relatively low
average density, and the measured depletions all suggest that the line of sight
contains a number of diffuse clouds, rather than a single dense, translucent
cloud. Comparisons of the fractions of Cl in Cl I and of hydrogen in molecular
form suggest a higher molecular fraction, in the region(s) where H2 is present,
than that derived considering the average line of sight. In general, such
comparisons may allow the identification and characterization of translucent
portions of such complex lines of sight. The combined data also show
high-velocity components near -80 km/s for various species which appear to be
predominantly ionized, and may be due to a radiative shock. A brief overview of
the conditions in this gas will be given.Comment: 37 pages, accepted for publication in Ap
The MiMeS Project: Magnetism in Massive Stars
The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration
among the foremost international researchers of the physics of hot, massive
stars, with the basic aim of understanding the origin, evolution and impact of
magnetic fields in these objects. The cornerstone of the project is the MiMeS
Large Program at the Canada-France-Hawaii Telescope, which represents a
dedication of 640 hours of telescope time from 2008-2012. The MiMeS Large
Program will exploit the unique capabilities of the ESPaDOnS spectropolarimeter
to obtain critical missing information about the poorly-studied magnetic
properties of these important stars, to confront current models and to guide
theory.Comment: 6 pages, 3 figures, proceedings of IAUS 259: Cosmic Magnetic Field
Cygnus X-3 with ISO: investigating the wind
We observed the energetic binary Cygnus X-3 in both quiescent and flaring
states between 4 and 16 microns using the ISO satellite. We find that the
quiescent source shows the thermal free-free spectrum typical of a hot, fast
stellar wind, such as from a massive helium star. The quiescent mass-loss rate
due to a spherically symmetric, non-accelerating wind is found to be in the
range 0.4-2.9 x 10E-4 solar masses per year, consistent with other infrared and
radio observations, but considerably larger than the 10E-5 solar masses per
year deduced from both the orbital change and the X-ray column density. There
is rapid, large amplitude flaring at 4.5 and 11.5 microns at the same time as
enhanced radio and X-ray activity, with the infrared spectrum apparently
becoming flatter in the flaring state. We believe non-thermal processes are
operating, perhaps along with enhanced thermal emission.Comment: Accepted for publication in MNRAS, 11 pages, 6 figure
An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei
CX Cep (WR 151) is the WR+O binary (WN5+O5V) with the second shortest period
known in our Galaxy. To examine the circumstellar matter distribution and to
better constraint the orbital parameters and mass-loss rate of the WR star, we
obtained broadband and multi-band (i.e. UBVRI) linear polarization observations
of the system. Our analysis of the phase-locked polarimetric modulation
confirms the high orbital inclination of the system (i.e. ). Using the
orbital solution of Lewis et al. (1993) we obtain masses of
and for the O and WR stars respectively, which agree with
their spectral types. A simple polarimetric model accounting for finite stellar
size effects allowed us to derive a mass-loss rate for the WR star of
. This result was remarkably independent of
the model's input parameters and favors an earlier spectral type for the WR
component (i.e. WN4). Finally, using our multi-band observations, we fitted and
subtracted from our data the interstellar polarization. The resulting constant
intrinsic polarization of is misaligned in relation to the orbital plane
(i.e. vs. ) and is the highest intrinsic
polarization ever observed for a WR star. This misalignment points towards a
rotational (or magnetic) origin for the asymmetry and contradicts the most
recent evolutionary models for massive stars (Meynet & Maeder 2003) which
predict spherically symmetric winds during the WR phase (i.e. ).Comment: 26 pages, 4 figures. Astrophysical Journal (submited
Response rates for providing a blood specimen for HIV testing in a population-based survey of young adults in Zimbabwe
<p>Abstract</p> <p>Background</p> <p>To determine differences among persons who provided blood specimens for HIV testing compared with those who did not among those interviewed for the population-based Zimbabwe Young Adult Survey (YAS).</p> <p>Methods</p> <p>Chi-square analysis of weighted data to compare demographic and behavioral data of persons interviewed who provided specimens for anonymous testing with those who did not. Prevalence estimation to determine the impact if persons not providing specimens had higher prevalence rates than those who did.</p> <p>Results</p> <p>Comparing those who provided specimens with those who did not, there was no significant difference by age, residence, education, marital status, perceived risk, sexual experience or number of sex partners for women. A significant difference by sexual experience was found for men. Prevalence estimates did not change substantially when prevalence was assumed to be two times higher for persons not providing specimens.</p> <p>Conclusion</p> <p>When comparing persons who provided specimens for HIV testing with those who did not, few significant differences were found. If those who did not provide specimens had prevalence rates twice that of those who did, overall prevalence would not be substantially affected. Refusal to provide blood specimens does not appear to have contributed to an underestimation of HIV prevalence.</p
Investigating the origin of the spectral line profiles of the Hot Wolf-Rayet Star WR 2
The hot WN star WR 2 (HD 6327) has been claimed to have many singular characteristics. To explain its unusually rounded and relatively weak emission line profiles, it has been proposed that WR 2 is rotating close to break-up with a magnetically confined wind. Alternatively, the line profiles could be explained by the dilution of WR 2’s spectrum by that of a companion. In this paper, we present a study of WR 2 using near-infrared AO imaging and optical spectroscopy and polarimetry. Our spectra reveal the presence of weak photospheric absorption lines from a ∼B 2.5-4V companion, which however contributes only ∼5–10% to the total light, suggesting that the companion is a background object. Therefore, its flux cannot be causing any significant dilution of the WR star’s emission lines. The absence of intrinsic linear continuum polarization from WR 2 does not support the proposed fast rotation. Our Stokes V spectrum was not of sufficient quality to test the presence of a moderately strong organized magnetic field but our new modelling indicates that to confine the wind the putative magnetic field must be significantly stronger than was previously suggested sufficiently strong as to make its presence implausible
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