36 research outputs found
Constraints on porosity and mass loss in O-star winds from modeling of X-ray emission line profile shapes
We fit X-ray emission line profiles in high resolution XMM-Newton and Chandra
grating spectra of the early O supergiant Zeta Pup with models that include the
effects of porosity in the stellar wind. We explore the effects of porosity due
to both spherical and flattened clumps. We find that porosity models with
flattened clumps oriented parallel to the photosphere provide poor fits to
observed line shapes. However, porosity models with isotropic clumps can
provide acceptable fits to observed line shapes, but only if the porosity
effect is moderate. We quantify the degeneracy between porosity effects from
isotropic clumps and the mass-loss rate inferred from the X-ray line shapes,
and we show that only modest increases in the mass-loss rate (<~ 40%) are
allowed if moderate porosity effects (h_infinity <~ R_*) are assumed to be
important. Large porosity lengths, and thus strong porosity effects, are ruled
out regardless of assumptions about clump shape. Thus, X-ray mass-loss rate
estimates are relatively insensitive to both optically thin and optically thick
clumping. This supports the use of X-ray spectroscopy as a mass-loss rate
calibration for bright, nearby O stars.Comment: 20 pages, 20 figures. Accepted by Ap
An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars
Slowly rotating magnetic massive stars develop "dynamical magnetospheres"
(DM's), characterized by trapping of stellar wind outflow in closed magnetic
loops, shock heating from collision of the upflow from opposite loop
footpoints, and subsequent gravitational infall of radiatively cooled material.
In 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these
three components is spatially complex and temporally variable, making it
difficult to derive observational signatures and discern their overall scaling
trends.Within a simplified, steady-state analysis based on overall conservation
principles, we present here an "analytic dynamical magnetosphere" (ADM) model
that provides explicit formulae for density, temperature and flow speed in each
of these three components -- wind outflow, hot post-shock gas, and cooled
inflow -- as a function of colatitude and radius within the closed (presumed
dipole) field lines of the magnetosphere. We compare these scalings with
time-averaged results from MHD simulations, and provide initial examples of
application of this ADM model for deriving two key observational diagnostics,
namely hydrogen H-alpha emission line profiles from the cooled infall, and
X-ray emission from the hot post-shock gas. We conclude with a discussion of
key issues and advantages in applying this ADM formalism toward derivation of a
broader set of observational diagnostics and scaling trends for massive stars
with such dynamical magnetospheres.Comment: 15 pages, 11 figures, accepted for MNRA
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