arXiv subjects

R. H. D. Townsend

Publications and source records attributed to R. H. D. Townsend.

59 records · Page 4Linked to original sources

arXiv2005

The Rigidly Rotating Magnetosphere of Sigma Ori E

We attempt to characterize the observed variability of the magnetic helium-strong star sigma Ori E in terms of a recently developed rigidly rotating magnetosphere model. This model predicts the accumulation of circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic equilibrium at the intersection between magnetic and rotational equators. We find that the model can reproduce well the periodic modulations observed in the star's light curve, H alpha emission-line profile, and longitudinal field strength, confirming that it furnishes an essentially correct, quantitative description of the star's magnetically controlled circumstellar environment.

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arXiv2005

On the evidence for discs around Blue Straggler stars

Recent observations of blue stragglers by De Marco et al. (2004) have revealed continuum deficits on the blue side of the Balmer discontinuity, leading these authors to infer the presence of discs around the stars. This intriguing possibility may throw light on aspects of the mechanisms responsible for at least some of these objects; current theories of blue straggler formation invoke stellar collisions or interacting binaries, both of which appear capable of forming a circumstellar disc. However, by synthesizing photospheric spectra for models of rotating blue stragglers, we demonstrate that the Balmer jump enhancements can be wholly attributed to the influence of oblateness and gravity darkening on the formation of the continuum. Therefore, we are led to conclude that the observations of De Marco et al. can be ascribed a more prosaic explanation, that of rapid stellar rotation arising from the merger/interaction formation process.

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arXiv2005

Influence of the Coriolis force on the instability of slowly pulsating B stars

This paper explores the effect of rotation on the kappa-mechanism instability of slowly pulsating B stars. A new nonadiabatic code, that adopts the so-called `traditional approximation' to treat the Coriolis force, is used to investigate the influence exerted by rotation over the stability of stellar models covering the mass range 2.5 M_sun <= M_* <= 13.0 M_sun. The principal finding is that, for all modes considered apart from the prograde sectoral class, rotation shifts the kappa-mechanism instability toward higher luminosities and effective temperatures; these shifts are accompanied by broadenings in the extent of instability strips. Such behaviour is traced to the shortening of mode periods under the action of the Coriolis force. Instability strips associated with prograde sectoral modes behave rather differently, being shifted to marginally lower luminosities and effective temperatures under the influence of rotation. The implications of these results are discussed in the context of the observational scarcity of pulsation in B-type stars having significant rotation; various scenarios are explored to explain the apparent dichotomy between theory and observations. Furthermore, the possible significance of the findings to Be stars is briefly examined.

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arXiv2004

A Rigidly Rotating Magnetosphere Model for Circumstellar Emission from Magnetic OB Stars

We present a semi-analytical approach for modeling circumstellar emission from rotating hot stars with a strong dipole magnetic field tilted at an arbitrary angle to the rotation axis. By assuming the rigid-field limit in which material driven (e.g., in a wind outflow) from the star is forced to remain in strict rigid-body co-rotation, we are able to solve for the effective centrifugal-plus-gravitational potential along each field line, and thereby identify the location of potential minima where material is prone to accumulate. Applying basic scalings for the surface mass flux of a radiatively driven stellar wind, we calculate the circumstellar density distribution that obtains once ejected plasma settles into hydrostatic stratification along field lines. The resulting accumulation surface resembles a rigidly rotating, warped disk, tilted such that its average surface normal lies between the rotation and magnetic axes. Using a simple model of the plasma emissivity, we calculate time-resolved synthetic line spectra for the disk. Initial comparisons show an encouraging level of correspondence with the observed rotational phase variations of Balmer-line emission profiles from magnetic Bp stars like sigma Ori E.

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arXiv2000

Surface trapping and leakage of low-frequency g-modes in rotating early-type stars -- I. Qualitative analysis

A qualitative study of the surface trapping of low-frequency non-radial g-modes in rotating early-type stars is undertaken within the Cowling, adiabatic and traditional approximations. It is demonstrated that, at frequencies below a cut-off, waves cannot be fully trapped within the star, and will leak through the surface. Expressions for the cut-off frequency are derived in both the non-rotating and rotating cases; it is found from these expressions that the cut-off frequency increases with the rotation rate for all but prograde sectoral modes. The results are of possible relevance to the 53 Per and SPB classes of variable star, which exhibit pulsation frequencies of the same order of magnitude as the cut-off frequencies found for the stellar model. It is suggested that observations either of an upper limit on variability periods (corresponding to the cut-off), or of line-profile variations due to leaking modes, may permit asteroseismological studies of the outer layers of these systems.

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