1,660 research outputs found
The effects of clumping on wind line variability
We review the effects of clumping on the profiles of resonance doublets. By
allowing the ratio of the doublet oscillator strenghts to be a free parameter,
we demonstrate that doublet profiles contain more information than is normally
utilized. In clumped (or porous) winds, this ratio can lies between unity and
the ratio of the f-values, and can change as a function of velocity and time,
depending on the fraction of the stellar disk that is covered by material
moving at a particular velocity at a given moment. Using these insights, we
present the results of SEI modeling of a sample of B supergiants, zeta Pup and
a time series for a star whose terminal velocity is low enough to make the
components of its Si IV 1400 doublet independent. These results are interpreted
within the framework of the Oskinova et al. (2007) model, and demonstrate how
the doublet profiles can be used to extract infromation about wind structure.Comment: 3 pages, to appear in Clumping in Hot Star Winds, W.-R. Hamann, A.
Feldmeier & L. Oskinova, eds., Potsdam: Univ.-Verl., 2007, URN:
http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-1398
Mass loss rates from mid-IR excesses in LMC and SMC O stars
We use a combination of BVJHK and Spitzer [3.6], [5.8] and [8.0] photometry
to determine IR excesses for a sample of 58 LMC and 46 SMC O stars. This sample
is ideal for determining IR excesses because the very small line of sight
reddening minimizes uncertainties due to extinction corrections. We use the
core-halo model developed by Lamers & Waters (1984a) to translate the excesses
into mass loss rates and demonstrate that the results of this simple model
agree with the more sophisticated CMFGEN models to within a factor of 2. Taken
at face value, the derived mass loss rates are larger than those predicted by
Vink et al. (2001), and the magnitude of the disagreement increases with
decreasing luminosity. However, the IR excesses need not imply large mass loss
rates. Instead, we argue that they probably indicate that the outer atmospheres
of O stars contain complex structures and that their winds are launched with
much smaller velocity gradients than normally assumed. If this is the case, it
could affect the theoretical and observational interpretations of the "weak
wind" problem, where classical mass loss indicators suggest that the mass loss
rates of lower luminosity O stars are far less than expected.Comment: 15 pages, 10 figures. Accepted for publication in MNRA
Structure and clumping in the fast wind of NGC6543
Far-UV spectroscopy from the FUSE satellite is analysed to uniquely probe
spatial structure and clumping in the fast wind of the central star of the
H-rich planetary nebula NGC6543 (HD164963). Time-series data of the unsaturated
PV 1118, 1128 resonance line P Cygni profiles provide a very sensitive
diagnostic of variable wind conditions in the outflow. We report on the
discovery of episodic and recurrent optical depth enhancements in the PV
absorption troughs, with some evidence for a 0.17-day modulation time-scale.
SEI line-synthesis modelling is used to derive physical properties, including
the optical depth evolution of individual `events'. The characteristics of
these features are essentially identical to the `discrete absorption
components' (DACs) commonly seen in the UV lines of massive OB stars. We have
also employed the unified model atmosphere code CMFGEN to explore spectroscopic
signatures of clumping, and report in particular on the clear sensitivity of
the PV lines to the clump volume filling factor. The results presented here
have implications for the downward revision of mass-loss rates in PN central
stars. We conclude that the temporal structures seen in the PV lines of NGC6543
likely have a physical origin that is similar to that operating in massive,
luminous stars, and may be related to near-surface perturbations caused by
stellar pulsation and/or magnetic fields.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
A Revised Geometry for the Magnetic Wind of theta^1 Orionis C
Theta^1 Ori is thought to be a hot analog of Bp variables because its optical
and UV line and X-ray continuum fluxes modulate regularly over the
magnetic/rotational period. A flattened magnetosphere surrounding co-rotates
with these stars, producing a periodic modulation of emission and absorption
components of the UV resonance lines, as well as of optical H and He lines. In
this paper we examine these modulations in detail and point out that the
far-blue and near-red wings of C IV and N V resonance lines exhibit
anticorrelated modulations, causing mild flux elevations at moderate redshifts
at edge-on phase (phi=0.5). However, the lines do not exhibit rest-frame
absorption features, the usual signatures of cool static disks surrounding Bp
stars. We suggest that this behavior can be explained by the existence of two
geometrically distinct wind regions separated by the local magnetic Alfven
radius. Wind streams emerging outside this point are forced outward by
radiative forces and eventually expand outward radially to infinity - this
matter produces the far-blue wing absorptions at phi=0.5. Interior streams
follow closed loops and collide at the magnetic equator with counterstreams.
There they coalesce and fall back to the star along their original field lines
- these are responsible for mild emissions at this same phase. The rapid
circulation of the interior wind component back to the star is responsible for
the absence of static disk features.Comment: 7 figure
Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
We model the detailed time-evolution of Discrete Absorption Components (DACs)
observed in P Cygni profiles of the Si IV lam1400 resonance doublet lines of
the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption
that the DACs are caused by Co-rotating Interaction Regions (CIRs) in the
stellar wind. We perform 3D radiative transfer calculations with hydrodynamic
models of the stellar wind that incorporate these large-scale density- and
velocity-structures. We develop the 3D transfer code Wind3D to investigate the
physical properties of CIRs with detailed fits to the DAC shape and morphology.
The CIRs are caused by irregularities on the stellar surface that change the
radiative force in the stellar wind. In our hydrodynamic model we approximate
these irregularities by circular symmetric spots on the stellar surface. We use
the Zeus3D code to model the stellar wind and the CIRs, limited to the
equatorial plane. We constrain the properties of large-scale wind structures
with detailed fits to DACs observed in HD 64760. A model with two spots of
unequal brightness and size on opposite sides of the equator, with opening
angles of 20 +/- 5 degr and 30 +/- 5 degr diameter, and that are 20 +/- 5 % and
8 +/- 5 % brighter than the stellar surface, respectively, provides the best
fit to the observed DACs. The recurrence time of the DACs compared to the
estimated rotational period corresponds to spot velocities that are 5 times
slower than the rotational velocity. The mass-loss rate of the structured wind
model for HD 64760 does not exceed the rate of the spherically symmetric smooth
wind model by more than 1 %. The fact that DACs are observed in a large number
of hot stars constrains the clumping that can be present in their winds, as
substantial amounts of clumping would tend to destroy the CIRs.Comment: 58 pages, 16 figures, 1 animation. Accepted for publication in The
Astrophysical Journal, Main Journal. More information and animations are
available at http://alobel.freeshell.org/hotstars.htm
Is the Wind of the Galactic Oe Star HD 155806 Magnetically Confined?
Spectropolarimetric observations of HD 155806 - the hottest Galactic Oe star
- were obtained with CFHT/ESPaDOnS to test the hypothesis that disk signatures
in its spectrum are due to magnetic channeling and confinement of its stellar
wind. We did not detect a dipole field of sufficient strength to confine the
wind, and could not confirm previous reports of a magnetic detection. It
appears that stellar magnetism is not responsible for producing the disk of HD
155806.Comment: 2 pages, 1 figure; to appear in the proceedings of IAU Symposium 27
CIR Modulation of the X-ray Flux from the O7.5 III(n)((f)) Star xi Persei?
We analyze a 162 ks HETG Chandra observation of the O7.5 III(n)((f)) star xi
Per, together with contemporaneous H alpha observations. The X-ray spectrum of
this star is similar to other single O stars, and not pathological in any way.
Its UV wind lines are known to display cyclical time variability, with a period
of 2.086 days, which is thought to be associated with co-rotating interaction
regions (CIRs). We examine the Chandra and H alpha data for variability on this
time scale. We find that the X-rays vary by about 15% over the course of the
observations and that this variability is out of phase with variable absorption
on the blue wing of the H alpha profiles (assumed to be a surrogate for the UV
absorption associated with CIRs). While not conclusive, both sets of data are
consistent with models where the CIRs are either a source of X-rays or modulate
them.Comment: Accepted by MNRAS. 9 pages, 9 figure
The changing UV and X-ray properties of the Of?p star CPD -28 2561
The Of?p star CPD -28 2561 was monitored at high energies with XMM-Newton and
HST. In X-rays, this magnetic oblique rotator displays bright and hard emission
that varies by ~55% with rotational phase. These changes occur in phase with
optical variations, as expected for magnetically confined winds; there are two
maxima and two minima in X-rays during the 73d rotational period of CPD -28
2561. However, contrary to previously studied cases, no significant hardness
variation is detected between minima and maxima, with the exception of the
second minimum which is slightly distinct from the first one. In the UV domain,
broad-band fluxes remain stable while line profiles display large variations.
Stronger absorptions at low velocities are observed when the magnetic equator
is seen edge-on, which can be reproduced by a detailed 3D model. However, a
difference in absorption at high velocities in the CIV and NV lines is also
detected for the two phases where the confined wind is seen nearly pole-on.
This suggests the presence of strong asymmetries about the magnetic equator,
mostly in the free-flowing wind (rather than in the confined dynamical
magnetosphere).Comment: 14 pages, 11 figures, accepted for publication by MNRA
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