53 research outputs found
2D non-LTE Modeling for Axi-symmetric Winds. II. A Short Characteristic Solution for Radiative Transfer in Rotating Winds
We present a new radiative transfer code for axi-symmetric stellar
atmospheres and compare test results against 1D and 2D models with and without
velocity fields. The code uses the short characteristic method with
modifications to handle axi-symmetric and non-monotonic 3D wind velocities, and
allows for distributed calculations. The formal solution along a characteristic
is evaluated with a resolution that is proportional to the velocity gradient
along the characteristic. This allows us to accurately map the variation of the
opacities and emissivities as a function of frequency and spatial coordinates,
but avoids unnecessary work in low velocity regions. We represent a
characteristic with an impact-parameter vector p (a vector that is normal to
the plane containing the characteristic and the origin) rather than the
traditional unit vector in the direction of the ray. The code calculates the
incoming intensities for the characteristics by a single latitudinal
interpolation without any further interpolation in the radiation angles. Using
this representation also provides a venue for distributed calculations since
the radiative transfer can be done independently for each p.Comment: 18 pages, 12 figures, accepted for publication in A&
Sub-milliarcsecond precision spectro-astrometry of Be stars
The origin of the disks around Be stars is still not known. Further progress
requires a proper parametrization of their structure, both spatially and
kinematically. This is challenging as the disks are very small. Here we assess
whether a novel method is capable of providing these data. We obtained spectro
astrometry around the Pa beta line of two bright Be stars, alpha Col and zeta
Tau, to search for disk signatures. The data, with a pixel to pixel precision
of the centroid position of 0.3..0.4 milliarcsecond is the most accurate such
data to date. Artefacts at the 0.85 mas level are present in the data, but
these are readily identified as they were non-repeatable in our redundant
datasets. This does illustrate the need of taking multiple data to avoid
spurious detections. The data are compared with simple model simulations of the
spectro astrometric signatures due to rotating disks around Be stars. The upper
limits we find for the disk radii correspond to disk sizes of a few dozen
stellar radii if they rotate Keplerian. This is very close to observationally
measured and theoretically expected disk sizes, and this paper therefore
demonstrates that spectro-astrometry, of which we present the first such
attempt, has the potential to resolve the disks around Be stars.Comment: 6 pages, A&A accepte
Probing the circumstellar structure of Herbig Ae/Be stars
We present Halpha spectropolarimetry observations of a sample of 23 Herbig
Ae/Be stars. A change in the linear polarisation across Halpha is detected in a
large fraction of the objects, which indicates that the regions around Herbig
stars are flattened (disc-like) on small scales. A second outcome of our study
is that the spectropolarimetric signatures for the Ae stars differ from those
of the Herbig Be stars, with characteristics changing from depolarisation
across Halpha in the Herbig Be stars, to line polarisations in the Ae group.
The frequency of depolarisations detected in the Herbig Be stars (7/12) is
particularly interesting as, by analogy to classical Be stars, it may be the
best evidence to date that the higher mass Herbig stars are surrounded by
flattened structures. For the Herbig Ae stars, 9 out of 11 show a line
polarisation effect that can be understood in terms of a compact Halpha
emission that is itself polarised by a rotating disc-like circumstellar medium.
The spectropolarimetric difference between the Herbig Be and Ae stars may be
the first indication that there is a transition in the Hertzsprung-Russell
Diagram from magnetic accretion at spectral type A to disc accretion at
spectral type B. Alternatively, the interior polarised line emission apparent
in the Ae stars may be masked in the Herbig Be stars due to their higher levels
of Halpha emission.Comment: 14 pages, MNRAS accepte
Constraining GRB progenitor models by probing Wolf-Rayet wind geometries in the Large Magellanic Cloud
The favoured progenitors of long-duration gamma-ray bursts (GRBs) are rapidly
rotating Wolf-Rayet (WR) stars. However, most Galactic WR stars are slow
rotators, as stellar winds are thought to remove angular momentum. This poses a
challenge to the collapsar model. Recent observations indicate that GRBs occur
predominately in low metallicity (Z) environments, which may resolve the
problem: lower Z leads to less mass loss, which may inhibit angular momentum
removal, allowing WR stars to remain rotating rapidly until collapse. We wish
to determine whether low Z WR stars rotate on average more rapidly than
Galactic WR stars, and perform a Very Large Telescope (VLT) linear
spectropolarimetry survey of WR stars in the low Z environment of the Large
Magellanic Cloud (LMC) and compare our results with the Galactic sample. We
find that only 2 out of 13 (i.e. 15%) of LMC WR stars show line polarization
effects, compared to a similarly low fraction of ~15-20% for Galactic WR stars.
The low incidence of line polarization effects in LMC WR stars suggests that
the threshold metallicity where significant differences in WR rotational
properties occur is below that of the LMC (Z ~ 0.5 Zsun), possibly constraining
GRB progenitor channels to this upper metallicity.Comment: Accepted for A&
Properties and nature of Be stars: 29. Orbital and long-term spectral variations of \gamma\ Cassiopei\ae
A detailed analysis of more than 800 electronic high-resolution spectra of
gamma Cas, which were obtained during a time interval of over 6000 days (16.84
yrs) at several observatories, documents the smooth variations in the density
and/or extend of its circumstellar envelope. We found a clear anticorrelation
between the peak intensity and FWHM of the H alpha emission, which seems to
agree with recent models of such emission lines. The main result of this study
is a confirmation of the binary nature of the object, determination of a
reliable linear ephemeris T_{min.RV} = HJD (2452081.90.6) +
(203.520.08)*E, and a rather definitive set of orbital elements. We
clearly demonstrated that the orbit is circular within the limits of accuracy
of our measurements and has a semi-amplitude of radial-velocity curve of
4.300.09 (km/s). No trace of the low-mass secondary was found. The time
distribution of our spectra does not allow a reliable investigation of rapid
spectral variations, which are undoubtedly present in the spectra. We postpone
this investigation for a future study, based on series of dedicated whole-night
spectral observations
A representative sample of Be stars IV: Infrared Photometry and the Continuum Excess
We present infra-red (JHK) photometry of 52 isolated Be stars of spectral
types O9--B9 and luminosity classes III--V. We describe a new method of
reduction, enabling separation of interstellar reddening and circumstellar
excess. Using this technique we find that the disc emission makes a maximum
contribution to the optical (B-V) colour of a few tenths of a magnitude. We
find strong correlations between a range of emission lines (H\alpha, Br\gamma,
Br11, and Br18) from the Be stars' discs, and the circumstellar continuum
excesses. We also find that stellar rotation and disc excess are correlated.Comment: 10 pages, 9 figures, accepted for publication in Astronomy and
Astrophysics. Other papers in this series can be obtained at
http://cwis.livjm.ac.uk/astro/research/environs.htm
A cyclic bipolar wind in the interacting binary V393 Scorpii
V393 Scorpii is a Double Periodic Variable characterized by a relatively
stable non-orbital photometric cycle of 253 days. Mennickent et al. argue for
the presence of a massive optically thick disc around the more massive B-type
component and describe the evolutionary stage of the system. In this paper we
analyze the behavior of the main spectroscopic optical lines during the long
non-orbital photometric cycle. We study the radial velocity of the donor
determining their orbital elements and find a small but significant orbital
eccentricity (e = 0.04). The donor spectral features are modeled and removed
from the spectrum at every observing epoch using the light-curve model given by
Mennickent et al. We find that the line emission is larger during eclipses and
mostly comes from a bipolar wind. We find that the long cycle is explained in
terms of a modulation of the wind strength; the wind has a larger line and
continuum emissivity on the high state. We report the discovery of highly
variable chromospheric emission in the donor, as revealed by Doppler maps of
the emission lines MgII 4481 and CI 6588. We discuss notable and some novel
spectroscopic features like discrete absorption components, especially visible
at blue-depressed OI 7773 absorption wings during the second half-cycle, Balmer
double emission with V/R-curves showing "Z-type" and "S-type" excursions around
secondary and main eclipse, respectively, and H_beta emission wings extending
up to +- 2000 km/s. We discuss possible causes for these phenomena and for
their modulations with the long cycle.Comment: 19 pages, 22 figures, accepted for publication in MNRA
Axi-symmetric Models of B[e] Supergiants: I. The Effective Temperature and Mass-loss Dependence of the Hydrogen and Helium Ionization Structure
We calculate the hydrogen and helium ionization in B[e] envelopes and explore
their dependence on mass-loss and effective temperature. We also present
simulated observations of the Halpha emission line and the C IV 1550 doublet,
and study their behavior. This paper reports our first results in an ongoing
study of B[e] supergiants, and provides a glimpse on the ionization of the most
important elements in self-consistent numerical simulations.
Our newly developed 2D stellar atmosphere code, ASTAROTH, was used for the
numerical simulations. The code self-consistently solves for the continuum
radiation, non-LTE level populations, and electron temperature in axi-symmetric
stellar envelopes. Observed profiles were calculated by an auxiliary program
developed separately from ASTAROTH.
In all but one of our models, H remained fully ionized. Due to ionizations
from excited states it is much more difficult to get a H neutral disk than
indicated by previous analytical calculations. Near the poles, the ionization
is high in all models, while helium recombined in the equatorial regions for
all but our lowest mass-loss rate. Although the model parameters were not
adjusted to provide fits to any particular star, the theoretical profiles show
some features seen in the profiles of R126. These include the partially
resolved double peaked profile of Halpha, and the weak emission associated with
the UV C IV resonance line.Comment: Accepted for publication in A&
Interstellar polarization and grain alignment: the role of iron and silicon
We compiled the polarimetric data for a sample of lines of sight with known
abundances of Mg, Si, and Fe. We correlated the degree of interstellar
polarization and polarization efficiency (the ratio of to the colour
excess or extinction ) with dust phase abundances. We detect an
anticorrelation between and the dust phase abundance of iron in non
silicate - containing grains ]_\rm d, a correlation
between and the abundance of Si, and no correlation between or
and dust phase abundances. These findings can be explained if mainly
the silicate grains aligned by the radiative mechanism are responsible for the
observed interstellar linear polarization.Comment: Accepted for publication in Astronomy and Astrophysic
- …