10 research outputs found
X-ray, UV and optical analysis of supergiants: Ori
We present a multi-wavelength (X-ray to optical) analysis, based on non-local
thermodynamic equilibrium photospheric+wind models, of the B0 Ia-supergiant:
~Ori. The aim is to test the consistency of physical parameters, such
as the mass-loss rate and CNO abundances, derived from different spectral
bands. The derived mass-loss rate is
1.610 M yr where
is the volume filling factor. However, the S IV
1062,1073 profiles are too strong in the models; to fit the
observed profiles it is necessary to use 0.01. This value is a
factor of 5 to 10 lower than inferred from other diagnostics, and implies
M yr. The discrepancy could
be related to porosity-vorosity effects or a problem with the ionization of
sulfur in the wind. To fit the UV profiles of N V and O VI it was necessary to
include emission from an interclump medium with a density contrast
() of 100. X-ray emission in H-He like and Fe L
lines was modeled using four plasma components located within the wind. We
derive plasma temperatures from to K, with
lower temperatures starting in the outer regions (R3-6 R), and a
hot component starting closer to the star (R2.9 R). From X-ray
line profiles we infer M yr. The
X-ray spectrum (0.1 kev) yields an X-ray luminosity , consistent with the superion line profiles.
X-ray abundances are in agreement with those derived from the UV and optical
analysis: Ori is slightly enhanced in nitrogen and depleted in
carbon and oxygen, evidence for CNO processed material.Comment: 33 pages, 25 figures. Accepted for publication in MNRA
Modeling broadband X-ray absorption of massive star winds
We present a method for computing the net transmission of X-rays emitted by
shock-heated plasma distributed throughout a partially optically thick stellar
wind from a massive star. We find the transmission by an exact integration of
the formal solution, assuming that the emitting plasma and absorbing plasma are
mixed at a constant mass ratio above some minimum radius, below which there is
assumed to be no emission. This model is more realistic than either the slab
absorption associated with a corona at the base of the wind or the exospheric
approximation that assumes that all observed X-rays are emitted without
attenuation from above the radius of optical depth unity. Our model is
implemented in XSPEC as a pre-calculated table that can be coupled to a
user-defined table of the wavelength dependent wind opacity. We provide a
default wind opacity model that is more representative of real wind opacities
than the commonly used neutral interstellar medium (ISM) tabulation.
Preliminary modeling of \textit{Chandra} grating data indicates that the X-ray
hardness trend of OB stars with spectral subtype can largely be understood as a
wind absorption effect.Comment: 9 pages, 9 figures. Includes minor corrections made in proof
Measuring mass-loss rates and constraining shock physics using X-ray line profiles of O stars from the Chandra archive
We quantitatively investigate the extent of wind absorption signatures in the X-ray grating spectra of all non-magnetic, effectively single O stars in the Chandra archive via line profile fitting. Under the usual assumption of a spherically symmetric wind with embedded shocks, we confirm previous claims that some objects show little or no wind absorption. However, many other objects do show asymmetric and blueshifted line profiles, indicative of wind absorption. For these stars, we are able to derive wind mass-loss rates from the ensemble of line profiles, and find values lower by an average factor of 3 than those predicted by current theoretical models, and consistent with Hα if clumping factors of fcl ≈ 20 are assumed. The same profile fitting indicates an onset radius of X-rays typically at r ≈ 1.5R*, and terminal velocities for the X-ray emitting wind component that are consistent with that of the bulk wind. We explore the likelihood that the stars in the sample that do not show significant wind absorption signatures in their line profiles have at least some X-ray emission that arises from colliding wind shocks with a close binary companion. The one clear exception is ζ Oph, a weak-wind star that appears to simply have a very low mass-loss rate. We also reanalyse the results from the canonical O supergiant ζ Pup, using a solar-metallicity wind opacity model and find M^˙=1.8×10−6 M_ ⊙yr^−1, consistent with recent multiwavelength determinations
A mass-loss rate determination for zeta Puppis from the quantitative analysis of X-ray emission line profiles
We fit every emission line in the high-resolution Chandra grating spectrum of
zeta Pup with an empirical line profile model that accounts for the effects of
Doppler broadening and attenuation by the bulk wind. For each of sixteen lines
or line complexes that can be reliably measured, we determine a best-fitting
fiducial optical depth, tau_* = kappa*Mdot/4{pi}R_{\ast}v_{\infty}, and place
confidence limits on this parameter. These sixteen lines include seven that
have not previously been reported on in the literature. The extended wavelength
range of these lines allows us to infer, for the first time, a clear increase
in tau_* with line wavelength, as expected from the wavelength increase of
bound-free absorption opacity. The small overall values of tau_*, reflected in
the rather modest asymmetry in the line profiles, can moreover all be fit
simultaneously by simply assuming a moderate mass-loss rate of 3.5 \pm 0.3
\times 10^{-6} Msun/yr, without any need to invoke porosity effects in the
wind. The quoted uncertainty is statistical, but the largest source of
uncertainty in the derived mass-loss rate is due to the uncertainty in the
elemental abundances of zeta Pup, which affects the continuum opacity of the
wind, and which we estimate to be a factor of two. Even so, the mass-loss rate
we find is significantly below the most recent smooth-wind H-alpha mass-loss
rate determinations for zeta Pup, but is in line with newer determinations that
account for small-scale wind clumping. If zeta Pup is representative of other
massive stars, these results will have important implications for stellar and
galactic evolution.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society. 17 pages, including 14 figures (7 color