A Report on the X-ray Properties of the tau Sco Like Stars

An increasing number of OB stars have been shown to possess magnetic fields. Although the sample remains small, it is surprising that the magnetic and X-ray properties of these stars appear to be far less correlated than expected. This contradicts model predictions, which generally indicate that the X-rays from magnetic stars to be harder and more luminous than their non-magnetic counterparts. Instead, the X-ray properties of magnetic OB stars are quite diverse. $\tau$ Sco is one example where the expectations are better met. This bright main sequence, early B star has been studied extensively in a variety of wavebands. It has a surface magnetic field of around 500 G, and Zeeman Doppler tomography has revealed an unusual field configuration. Furthermore, tau Sco displays an unusually hard X-ray spectrum, much harder than similar, non-magnetic OB stars. In addition, the profiles of its UV P Cygni wind lines have long been known to possess a peculiar morphology. Recently, two stars, HD 66665 and HD 63425, whose spectral types and UV wind line profiles are similar to those of $\tau$ Sco, have also been determined to be magnetic. In the hope of establishing a magnetic field - X-ray connection for at least a sub-set of the magnetic stars, we obtained XMM-Newton EPIC spectra of these two objects. Our results for HD 66665 are somewhat inconclusive. No especially strong hard component is detected; however, the number of source counts is insufficient to rule out hard emission. longer exposure is needed to assess the nature of the X-rays from this star. On the other hand, we do find that HD 63425 has a substantial hard X-ray component, thereby bolstering its close similarity to tau Sco.Comment: MNRAS, accepte

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 $\zeta$ Pup.Comment: ApJ, in pres

A propelling neutron star in the enigmatic Be-star γ\gamma~Cassiopeia

The enigmatic X-ray emission from the bright optical star, $\gamma$ Cassiopeia, is a long-standing problem. $\gamma$ Cas is known to be a binary system consisting of a Be-type star and a low-mass ($M\sim 1\,M_\odot$) companion of unknown nature orbiting in the Be-disk plane. Here we apply the quasi-spherical accretion theory onto a compact magnetized star and show that if the low-mass companion of $\gamma$ Cas is a fast spinning neutron star, the key observational signatures of $\gamma$ Cas are remarkably well reproduced. Direct accretion onto this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed that emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from $\gamma$ Cas. We suggest that $\gamma$ Cas and its analogs constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of $\gamma$ Cas and its analogs should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from $\gamma$ Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.Comment: 6 pages, accepted for publication in MNRA

Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates

Context: The mass-loss rate is a key parameter of massive stars. Adequate stellar atmosphere models are required for spectral analyses and mass-loss determinations. Present models can only account for the inhomogeneity of stellar winds in the approximation of small-scale structures that are optically thin. This treatment of ``microclumping'' has led to reducing empirical mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be optically thick in spectral lines. We investigate how this ``macroclumping'' impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR) model atmosphere code is generalized in the ``formal integral'' to account for clumps that are not necessarily optically thin. Results: Optically thick clumps reduce the effective opacity. This has a pronounced effect on the emergent spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the optically thin H-alpha line is not affected by wind porosity, but that the PV resonance doublet becomes significantly weaker when macroclumping is taken into account. The reported discrepancies between resonance-line and recombination-line diagnostics can be resolved entirely with the macroclumping modeling without downward revision of the mass-loss rate. Conclusions: Mass-loss rates inferred from optically thin emission, such as the H-alpha line in O stars, are not influenced by macroclumping. The strength of optically thick lines, however, is reduced because of the porosity effects. Therefore, neglecting the porosity in stellar wind modeling can lead to underestimating empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex

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 $\gamma$-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 $\zeta$ Oph and BD$+$43$^{\circ}$3654. 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