High-resolution X-ray spectroscopy of tau Scorpii (B0.2V) with XMM-Newton

We report the analysis of the first high-resolution X-ray spectrum of the B0.2V star τ Scorpii obtained with the Reflection Grating Spectrometers (rg

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Hot star polarimetric variability and the nature of wind inhomogeneities

The problem is addressed of how much hot star polarisation variability can result from density redistribution processes within the wind as opposed to localised enhancement of stellar mass loss rate, such as ejections of wind inhomogeneities. For optically thin electron scattering, we present a theory for the relative polarisation arising from particle redistribution and consider several specific cases relevant to interpreting observations of wind variability. It is concluded that, allowing for partial cancellation of the contribution from compressed and evacuated regions, density redistribution internal to the wind can produce significant polarisation but only for processes that redistribute wind material over relatively large radial or angular scales. This conclusion favors extended spatial structures (e.g., from strong radiatively driven shocks) over localised condensations (e.g., from radiative instabilities)

A Spitzer IRS survey of Wolf-Rayet stars at 10–20 microns

We report on a mid-infrared survey of Wolf-Rayet stars using the Spitzer IRS/SH instrument for types WN4-WN7 and WC4-WC7. Strong emission line spectra are seen, including forbidden emission lines. Surprisingly, the WN stars appear to show broad emission “bumps” that may be consistent with silicate dust grains

A magnetically torqued disk model for Be stars

Despite extensive study, the mechanisms by which Be star disks acquire high densities and angular momentum while displaying variability on many timescales are still far from clear. In this paper, we discuss how magnetic torquing may help explain disk formation with the observed quasi-Keplerian ( as opposed to expanding) velocity structure and their variability. We focus on the effects of the rapid rotation of Be stars, considering the regime where centrifugal forces provide the dominant radial support of the disk material. Using a kinematic description of the angular velocity, v(phi)(r), in the disk and a parametric model of an aligned field with a strength B(r), we develop analytic expressions for the disk properties that allow us to estimate the stellar surface field strength necessary to create such a disk for a range of stars on the main sequence. The fields required to form a disk are compared with the bounds previously derived from photospheric limiting conditions. The model explains why disks are most common for main-sequence stars at about spectral class B2 V. The earlier type stars with very fast and high-density winds would require unacceptably strong surface fields ( gt 10(3) G) to form torqued disks, while the late B stars ( with their low mass-loss rates) tend to form disks that produce only small fluxes in the dominant Be diagnostics. For stars at B2 V the average surface field required is about 300 G. The predicted disks provide an intrinsic polarization and a flux at Halpha comparable to observations. The radial extent of our dense quasi-Keplerian disks is compatible with typical estimates. We also discuss whether the effect on field containment of the time-dependent accumulation of matter in the flux tubes/disk can help explain some of the observed variability of Be star disks