244 research outputs found
Modeling X-ray Emission Line Profiles from Massive Star Winds - A Review
The Chandra and XMM-Newton X-ray telescopes have led to numerous advances in
the study and understanding of astrophysical X-ray sources. Particularly
important has been the much increased spectral resolution of modern X-ray
instrumentation. Wind-broadened emission lines have been spectroscopically
resolved for many massive stars. This contribution reviews approaches to the
modeling of X-ray emission line profile shapes from single stars, including
smooth winds, winds with clumping, optically thin versus thick lines, and the
effect of a radius-dependent photoabsorption coefficient.Comment: to appear in Advances in Space Researc
Faraday Rotation Distributions from Stellar Magnetism in Wind-Blown Bubbles
Faraday rotation is a valuable tool for detecting magnetic fields. Here the
technique is considered in relation to wind-blow bubbles. In the context of
spherical winds with azimuthal or split monopole stellar magnetic field
geometries, we derive maps of the distribution of position angle (PA) rotation
of linearly polarized radiation across projected bubbles. We show that the
morphology of maps for split monopole fields are distinct from those produced
by the toroidal field topology; however, the toroidal case is the one most
likely to be detectable because of its slower decline in field strength with
distance from the star. We also consider the important case of a bubble with a
spherical sub-volume that is field-free to approximate crudely a "swept-up"
wind interaction between a fast wind (or possibly a supernova ejecta shell)
overtaking a slower magnetized wind from a prior state of stellar evolution.
With an azimuthal field, the resultant PA map displays two arc-like features of
opposite rotation measure, similar to observations of the supernova remnant
G296.5+10.0. We illustrate how PA maps can be used to disentangle Faraday
rotation contributions made by the interstellar medium versus the bubble.
Although our models involve simplifying assumptions, their consideration leads
to a number of general robust conclusions for use in the analysis of radio
mapping datasets.Comment: Astrophysical Journal, accepte
A Report on the X-ray Properties of the tau Sco Like Stars
An increasing number of OB stars have been shown to possess magnetic fields.
Although the sample remains small, it is surprising that the magnetic and X-ray
properties of these stars appear to be far less correlated than expected. This
contradicts model predictions, which generally indicate that the X-rays from
magnetic stars to be harder and more luminous than their non-magnetic
counterparts. Instead, the X-ray properties of magnetic OB stars are quite
diverse.
Sco is one example where the expectations are better met. This bright
main sequence, early B star has been studied extensively in a variety of
wavebands. It has a surface magnetic field of around 500 G, and Zeeman Doppler
tomography has revealed an unusual field configuration. Furthermore, tau Sco
displays an unusually hard X-ray spectrum, much harder than similar,
non-magnetic OB stars. In addition, the profiles of its UV P Cygni wind lines
have long been known to possess a peculiar morphology.
Recently, two stars, HD 66665 and HD 63425, whose spectral types and UV wind
line profiles are similar to those of Sco, have also been determined to
be magnetic. In the hope of establishing a magnetic field - X-ray connection
for at least a sub-set of the magnetic stars, we obtained XMM-Newton EPIC
spectra of these two objects. Our results for HD 66665 are somewhat
inconclusive. No especially strong hard component is detected; however, the
number of source counts is insufficient to rule out hard emission. longer
exposure is needed to assess the nature of the X-rays from this star. On the
other hand, we do find that HD 63425 has a substantial hard X-ray component,
thereby bolstering its close similarity to tau Sco.Comment: MNRAS, accepte
Polarization Light Curve Modeling of Corotating Interaction Regions in the Wind of the Wolf-Rayet Star WR 6
The intriguing WN4b star WR6 has been known to display epoch-dependent
spectroscopic, photometric and polarimetric variability for several decades. In
this paper, we set out to verify if a simplified analytical model in which
Corotating Interaction Regions (CIRs) threading an otherwise spherical wind is
able to reproduce the many broadband continuum light curves from the literature
with a reasonable set of parameters.
We modified the optically thin model we developed in Ignace, St-Louis &
Proulx-Giraldeau (2015) to approximately account for multiple scattering and
used it to fit 13 separate datasets of this star. By including two CIRs in the
wind, we obtained reasonable fits for all datasets with coherent values for the
inclination of the rotation axis () and for its orientation in
the plane of the sky, although in the latter case we obtained two equally
acceptable values ( and ) from the
polarimetry.
Additional line profile variation simulations using the Sobolev approximation
for the line transfer allowed us to eliminate the solution.
With the adopted configuration ( and ), we
were able to reproduce all datasets relatively well with two CIRs located near
the stellar equator and always separated by in longitude. The
epoch-dependency comes from the fact that these CIRs migrate along the surface
of the star. Density contrasts smaller than a factor of two and large opening
angles for the CIR () were found to best reproduce the
type of spectroscopic variability reported in the literature.Comment: 15 pages, 5 figures,5 table
Asymmetric Shapes of Radio Recombination Lines from Ionized Stellar Winds
Recombination line profile shapes are derived for ionized spherical stellar winds at radio wavelengths. It is assumed that the wind is optically thick owing to free-free opacity. Emission lines of arbitrary optical depth are obtained assuming that the free-free photosphere forms in the outer, constant expansion portion of the wind. Previous works have derived analytic results for isothermal winds when the line and continuum source functions are equal. Here, semi-analytic results are derived for unequal source functions to reveal that line shapes can be asymmetric about line center. A parameter study is presented and applications discussed
Long-term polarization observations of Mira variable stars suggest asymmetric structures
Mira and semi-regular variable stars have been studied for centuries but
continue to be enigmatic. One unsolved mystery is the presence of polarization
from these stars. In particular, we present 40 years of polarization
measurements for the prototype o Ceti and V CVn and find very different
phenomena for each star. The polarization fraction and position angle for Mira
is found to be small and highly variable. On the other hand, the polarization
fraction for V CVn is large and variable, from 2 - 7 %, and its position angle
is approximately constant, suggesting a long-term asymmetric structure. We
suggest a number of potential scenarios to explain these observations.Comment: 2 pages, 1 figure, poster presented at IAU Symposium 301, Precision
Asteroseismology, August 2013, Wroclaw, Polan
Non-radially pulsating stars as microlensing sources
We study the microlensing of Non-Radially Pulsating (NRP) stars. Pulsations
are formulated for stellar radius and temperature using spherical harmonic
functions with different values of l,m. The characteristics of the microlensing
light curves from NRP stars are investigated in relation to different pulsation
modes. For the microlensing of NRP stars, the light curve is not a simple
multiplication of the magnification curve and the intrinsic luminosity curve of
the source star, unless the effect of finite source size can be ignored. Three
main conclusions can be drawn from the simulated light curves. First, for modes
with and when the viewing inclination is more nearly pole-on, the
stellar luminosity towards the observer changes little with pulsation phase. In
this case, high-magnification microlensing events are chromatic and can reveal
the variability of these source stars. Second, some combinations of pulsation
modes produce nearly degenerate luminosity curves (e.g., (l,m)=(3,0), (5,0)).
The resulting microlensing light curves are also degenerate, unless the lens
crosses the projected source. Finally, for modes involving m=1, the stellar
brightness centre does not coincide with the coordinate centre, and the
projected source brightness centre moves in the sky with pulsation phase. As a
result of this time-dependent displacement in the brightness centroid, the time
of the magnification peak coincides with the closest approach of the lens to
the brightness centre as opposed to the source coordinate centre. Binary
microlensing of NRP stars and in caustic-crossing features are chromatic.Comment: 14 pages, 23 figures, accepted for publication by MNRA
The strange evolution of the Large Magellanic Cloud Cepheid OGLE-LMC-CEP1812
Classical Cepheids are key probes of both stellar astrophysics and cosmology
as standard candles and pulsating variable stars. It is important to understand
Cepheids in unprecedented detail in preparation for upcoming GAIA, JWST and
extremely-large telescope observations. Cepheid eclipsing binary stars are
ideal tools for achieving this goal, however there are currently only three
known systems. One of those systems, OGLE-LMC-CEP1812, raises new questions
about the evolution of classical Cepheids because of an apparent age
discrepancy between the Cepheid and its red giant companion. We show that the
Cepheid component is actually the product of a stellar merger of two main
sequence stars that has since evolved across the Hertzsprung gap of the HR
diagram. This post-merger product appears younger than the companion, hence the
apparent age discrepancy is resolved. We discuss this idea and consequences for
understanding Cepheid evolution.Comment: 5 pages, 3 figures, accepted to A&
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