970 research outputs found
The Hanle Effect as a Diagnostic of Magnetic Fields in Stellar Envelopes. V. Thin Lines from Keplerian Disks
This paper focuses on the polarized profiles of resonance scattering lines
that form in magnetized disks. Optically thin lines from Keplerian planar disks
are considered. Model line profiles are calculated for simple field topologies
of axial fields (i.e., vertical to the disk plane) and toroidal fields (i.e.,
purely azimuthal). A scheme for discerning field strengths and geometries in
disks is developed based on Stokes Q-U diagrams for the run of polarization
across line profiles that are Doppler broadened by the disk rotation. A
discussion of the Hanle effect for magnetized disks in which the
magnetorotational instability (MRI) is operating is also presented. Given that
the MRI has a tendency to mix the vector field orientation, it may be difficult
to detect the disk fields with the longitudinal Zeeman effect, since the
amplitude of the circularly polarized signal scales with the net magnetic flux
in the direction of the observer. The Hanle effect does not suffer from this
impediment, and so a multi-line analysis could be used to constrain field
strengths in disks dominated by the MRI.Comment: to appear in Astrophysical Journa
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 when the source functions are not equal
to reveal that line shapes can be asymmetric about line center. A parameter
study is presented and applications discussed.Comment: accepted to Revista Mexicana de Astronom\'ia y Astrof\'isic
Variability in X-ray line ratios in helium-like ions of massive stars: the radiation-driven case
Line ratios in "fir" triplets of helium-like ions have proven to be a
powerful diagnostic of conditions in X-ray emitting gas surrounding massive
stars. Recent observations indicate that these ratios can be variable with
time.
The possible causes of variation in line ratios are limited: changes in the
radiation field or changes in density, and changes in mass-loss or geometry. In
this paper, we investigate the ability of changes in the radiation field to
induce variability in the ratio R=f/i.
To isolate the radiative effect, we use a heuristic model of temperature and
radius changes in variable stars in the B and O range with low-density,
steady-state winds. We model the changes in emissivity of X-ray emitting gas
close to the star due to differences in level-pumping from available UV photons
at the location of the gas.
We find that under these conditions, variability in R is dominated by the
stellar temperature. Although the relative amplitude of variability is roughly
comparable for most lines at most temperatures, detectable variations are
limited to a few lines for each spectral type. We predict that variable values
in R due to stellar variability must follow predictable trends found in our
simulations.
Our model uses radial pulsations as a mode of stellar variability that
maximizes the amplitude of variation in R. This model is robust enough to show
which ions will provide the best opportunity for observing variability in the
f/i ratio at different stellar temperatures, and the correlation of that
variability with other observable parameters. In real systems, the effects
would be more complex than in our model, with differences in phase and
suppressed amplitude in the presence of non-radial pulsations. This suggests
that changes in R across many lines concurrently are not likely to be produced
by a variable radiation field.Comment: 10 pages, 6 figure
Microlensing of circumstellar envelopes: II. emission lines from radial and azimuthal flow during fold caustic crossings
This paper examines the line profile evolution due to bulk motion in circumstellar envelopes during microlensing fold caustic crossing events. These events have recently been shown to be a sensitive probe of stellar surface brightness profiles, thus providing a means - through both photometric and spectroscopic observations - to constrain and test stellar atmosphere models. Here it is demonstrated, through the examination of simplified line profiles, that spectroscopic studies of fold caustic crossings could also prove to be a powerful diagnostic of bulk motion in circumstellar envelopes
Profile Shapes for Optically Thick X-ray Emission Lines from Stellar Winds
We consider the consequences of appreciable line optical depth for the
profile shape of X-ray emission lines formed in stellar winds. The hot gas is
thought to arise in distributed wind shocks, and the line formation is
predominantly via collisional excitation followed by radiative decay. Such
lines are often modelled as optically thin, but the theory has difficulty
matching resolved X-ray line profiles. We suggest that for strong lines of
abundant metals, newly created photons may undergo resonance scattering,
modifying the emergent profile. Using Sobolev theory in a spherically symmetric
wind, we show that thick-line resonance scattering leads to emission profiles
that still have blueshifted centroids like the thin lines, but which are
considerably less asymmetric in appearance. We focus on winds in the
constant-expansion domain, and derive an analytic form for the profile shape in
the limit of large line and photoabsorptive optical depths. Our theory is
applied to published {\it Chandra} observations of the O star Pup.Comment: ApJ, in pres
Long-Wavelength, Free-Free Spectral Energy Distributions from Porous Stellar Winds
The influence of macroclumps for free-free spectral energy distributions
(SEDs) of ionized winds is considered. The goal is to emphasize distinctions
between microclumping and macroclumping effects. Microclumping can alter SED
slopes and flux levels if the volume filling factor of the clumps varies with
radius; however, the modifications are independent of the clump geometry. To
what extent does macroclumping alter SED slopes and flux levels? In addressing
the question, two specific types of macroclump geometries are explored: shell
fragments ("pancake"-shaped) and spherical clumps. Analytic and semi-analytic
results are derived in the limiting case that clumps never obscure one another.
Numerical calculations based on a porosity formalism is used when clumps do
overlap. Under the assumptions of a constant expansion, isothermal, and fixed
ionization wind, the fragment model leads to results that are essentially
identical to the microclumping result. Mass-loss rate determinations are not
affected by porosity effects for shell fragments. By contrast, spherical clumps
can lead to a reduction in long-wavelength fluxes, but the reductions are only
significant for extreme volume filling factors.Comment: to appear in MNRA
Spectropolarimetric Variability and Co-Rotating Structure in HD 92207
We report on low resolution (R~3000) spectropolarimetry of the A0 supergiant
star HD 92207. This star is well-known for significant spectral variability.
The source was observed on seven different nights spanning approximately 3
months in time. With a rotation period of approximately 1 year, our data covers
approximately a quarter of the star's rotational phase. Variability in the
continuum polarization level is observed over this period of time. The
polarization across the Halpha line on any given night is typically different
from the degree and position angle of the polarization in the continuum.
Interestingly, Hbeta is not in emission and does not show polarimetric
variability. We associate the changes at Halpha as arising in the wind, which
is in accord with the observed changes in the profile shape and equivalent
width of Halpha along with the polarimetric variability. For the continuum
polarization, we explore a spiral shaped wind density enhancement in the
equatorial plane of the star, in keeping with the suggestion of Kaufer etal
(1997). Variable polarization signatures across Halpha are too complex to be
explained by this simple model and will require a more intensive polarimetric
follow-up study to interpret properly.Comment: to appear in A
On the Absence of Non-thermal X-ray emission around Runaway O stars
Theoretical models predict that the compressed interstellar medium around
runaway O stars can produce high-energy non-thermal diffuse emission, in
particular, non-thermal X-ray and -ray emission. So far, detection of
non-thermal X-ray emission was claimed for only one runaway star AE Aur. We
present a search for non-thermal diffuse X-ray emission from bow shocks using
archived XMM-Newton observations for a clean sample of 6 well-determined
runaway O stars. We find that none of these objects present diffuse X-ray
emission associated to their bow shocks, similarly to previous X-ray studies
toward Oph and BD433654. We carefully investigated
multi-wavelength observations of AE Aur and could not confirm previous findings
of non-thermal X-rays. We conclude that so far there is no clear evidence of
non-thermal extended emission in bow shocks around runaway O stars.Comment: 6 pages, 2 tables, 3 figures; Accepted to ApJ Letter
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