No description available.

New results are described for the use of the Hanle effect in the photospheres of oblique magnetic rotators and of the Zeeman effect in stellar wind emission lines.

Star formation in the Milky Way is confined to star-forming regions (OB association, HII regions, and open clusters) in the Galactic plane. It is usually assumed that these regions are found preferably along spiral arms, as is observed in other spiral galaxies. However, young early-type stars are often found at high Galactic latitudes, far away from their birthplaces in the Galactic disc. These stars are called runaway stars, and it is believed that they were ejected from their birth- places early in their lifetimes by one of two mechanisms: ejection from a binary system following the destruction of the massive companion in a supernova type II event (the binary ejection mechanism), or ejection from a dense cluster following a close gravitational encounter between two close binaries (the dynamical ejection mechanism). The aims of our study were: to improve the current understanding of the nature of high Galactic latitude runaway stars, in particular by investigating whether the theoretical ejection mechanisms could explain the more extreme cases; to show the feasibility of using high Galactic latitude stars as tracers of the spiral arms. The main technique used in this investigation was the tracing of stellar orbits back in time, given their present positions and velocities in 3D space. This technique allowed the determination of the ejection velocities, flight times and birthplaces of a sample of runaway stars. In order to obtain reasonable velocity estimates several recent catalogues of proper motion data were used. We found that the evolutionary ages of the vast majority of runaway stars is consistent with the disc ejection scenario. However, we identified three outliers which would need flight times much larger then their estimated ages in order to reach their present positions in the sky. Moreover, the ejection velocity distribution appears to be bimodal, showing evidence for two populations of runaway stars: a “low” velocity population (89 per cent of the sample), with a maximum ejection velocity of about 300 kms−1, and a “high” velocity population, with ejection velo- cities of 400 – 500 kms−1. We argue that the observed bimodality and maximum ejection velocity of 500 kms−1 can be interpreted as a natural consequence of a variation of the binary ejection mechanism. A possible connection between the “high” velocity population and the so-called hypervelocity stars is also explored, resulting in the conclusion that some stars previously identified as hypervelocity may be in fact runaway stars. The feasibility of using stars as tracers of the spiral arms was tested on a local sample, in order to obtain better quality data and larger numbers. We found that the spiral arms pattern speeds estimated from this sample (24.9±5.2 kms−1 kpc−1) and from a selected sample of runaways (22.8 ± 7.8 kms−1 kpc−1) are consistent within the errors and also consistent with other published estimates. We concluded that our estimates combined with the ones obtained in other studies suggest a value in the range 20 − 25 kms−1 kpc−1 for the pattern speed. Moreover, we concluded that an adequate representation of the spiral arms is obtained given the former pattern speed estimate, even when applied to the sample of runaway stars.

I present low-resolution (Dl 6 A° ) follow-up spectroscopy of 370 Ha emitters (12 . r . 17) identified with IPHAS, in a 100 deg2 wide section of the Galactic plane that is located between ` = (120 ; 140 ) and b = ( 1 ; +4 ). Classical Be stars are found to be the most numerous group of the observed targets ( 60%). Sixty-eight classical Be stars have also been observed at higher spectral resolution (Dl 2 4 A° ) and S/N ratio, which allows spectral typing to an estimated precision of 1 sub-type. Colour excesses were measured via spectral energy distribution fitting of flux-calibrated data. I took care to remove the circumstellar contribution to the measured colour excess, using an established scaling to the Ha equivalent widths. In doing so, this method of correction was re-evaluated and modified to better suit the data at hand. Spectroscopic parallaxes were measured constraining the luminosity class via estimates of distances to main sequence A/F stars, which are found within a few arcminutes of each classical Be star on the sky. In order to probe the structure of the outer Galactic disc, I studied the spatial distribution of 63 out of 248 classical Be stars identified. Their cumulative distribution function with respect to the distance is statistically compatible both with a smooth exponential density profile and with a simple spiral arms representation. The distribution of reddenings of classical Be stars is compared with estimates of the total Galactic reddening along their sightlines. It is expected that the measured reddenings match the integrated Galactic values, for distant stars located outside the Galactic dust layer, or they are smaller than the asymptotic values if the stars are less distant. The outcome meets expectations, and lends support to the conclusion that the measured reddenings are determined to a precision of 10%. The sample of 248 objects doubles the number of known classical Be stars in this part of the Galactic plane. Unlike the pre-existing bright sample, the new objects are seen at large distances, between 2 – 8 kpc with typical E(B V) 0:9. Only four stars are members of known clusters. Ten classical Be stars are proposed to be well beyond the putative Outer Arm, at distances larger than 8 kpc. The large sample of stars, which has been identified here, is the result of a successful selection and analysis of classical Be stars that is offered for more exploitation in future. The proposition is that GAIA observations will use the present sample of classical Be stars as a new tracer of the Galactic disc.

No description available.

Recombination line profile shapes are derived for ionized spherical stellar winds at radio wavelengths. It is assumed that the wind is optically thick owing to free-free opacity. Emission lines of arbitrary optical depth are obtained assuming that the free-free photosphere forms in the outer, constant expansion portion of the wind. Previous works have derived analytic results for isothermal winds when the line and continuum source functions are equal. Here, semi-analytic results are derived for unequal source functions to reveal that line shapes can be asymmetric about line center. A parameter study is presented and applications discussed.

We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7 III(n)((f)) star ξ Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of ξ Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N iv λ1718 and Si iv λ1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si iv λ1402, N iv λ1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180°, on the surface of the star. We note that the presence and persistence of two spots separated by 180° suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in ξ Per.

Massive star winds are complex radiation-hydrodynamic (sometimes magnetohydrodynamic) outflows that are propelled by their enormously strong luminosities. The winds are often found to be structured and variable, but can also display periodic or quasi-periodic behavior in a variety of wind diagnostics. The regular variations observed in putatively single stars, especially in UV wind lines, have often been attributed to corotating interaction regions (CIRs) like those seen in the solar wind. We present light curves for variable polarization from winds with CIR structures. We develop a model for a time-independent CIR based on a kinematical description. Assuming optically thin electron scattering, we explore the range of polarimetric light curves that result as the curvature, latitude, and number of CIRs are varied. We find that a diverse array of variable polarizations result from an exploration of cases. The net polarization from an unresolved source is weighted more toward the inner radii of the wind. Given that most massive stars have relatively fast winds compared to their rotation speeds, CIRs tend to be conical at inner radii, transitioning to a spiral shape at a few to several stellar radii in the wind. Winds with a single CIR structure lead to easily identifiable polarization signatures. By contrast allowing for multiple CIRs, all emerging from a range of azimuth and latitude positions at the star, can yield complex polarimetric behavior. Although our model is based on some simplifying assumptions, it produces qualitative behavior that we expect to be robust, and this has allowed us to explore a wide range of CIR configurations that will prove useful for interpreting polarimetric data.

No description available.

AH Cep and CW Cep are both early B-type binaries with short orbital periods of 1.8 days and 2.7 days, respectively. All four components are B0.5V types. The binaries are also double-lined spectroscopic and eclipsing. Consequently, solutions for orbital and stellar parameters make the pair of binaries ideal targets for a study of the colliding winds between two B stars. Chandra ACIS-I observations were obtained to determine X-ray luminosities. AH Cep was detected with an unabsorbed X-ray luminosity at a 90% confidence interval of erg s−1, or , relative to the combined Bolometric luminosities of the two components. While formally consistent with expectations for embedded wind shocks, or binary wind collision, the near-twin system of CW Cep was a surprising nondetection. For CW Cep, an upper limit was determined with , again for the combined components. One difference between these two systems is that AH Cep is part of a multiple system. The X-rays from AH Cep may not arise from standard wind shocks nor wind collision, but perhaps instead from magnetism in any one of the four components of the system. The possibility could be tested by searching for cyclic X-ray variability in AH Cep on the short orbital period of the inner B stars.