G. A. J. Hussain — Page 4

arXiv2003

Coronal element abundances of the post-common envelope binary V471 Tauri with ASCA

We report on ASCA observations of the coronally active companion star in the post-common envelope binary V471 Tauri. Evolutionary calculations indicate that there should be no peculiar abundances on the companion star resulting from the common envelope epoch. Indeed, we find no evidence for peculiar abundances, although uncertainties are high. We find that a single-temperature plasma model does not fit the data. Two-temperature models with decoupled elemental abundances suggest that Fe is underabundant compared to the Hyades photospheric mean. In the absence of a measurement of photospheric Ne abundance in the cluster, we find Ne is overabundant compared to the solar photospheric value. This is indicative of the inverse first ionization potential effect. Differences between coronal and photospheric abundances are believed to result from the fractionation of ionized and neutral material in the upper atmosphere of the star.

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arXiv2002

The coronal topology of the rapidly rotating K0 dwarf, AB Doradus I. Using surface magnetic field maps to model the structure of the stellar corona

We re-analyse spectropolarimetric data of AB Dor taken in 1996 December using a surface imaging code that can model the magnetic field of the star as a non-potential current-carrying magnetic field. We find that a non-potential field needs to be introduced in order to fit the dataset at this epoch. This non-potential component takes the form of a strong unidirectional azimuthal field of a similar strength to the radial field. This azimuthal field is concentrated around the boundary of the dark polar spot recovered at the surface of the star using Doppler imaging. As polarization signatures from the center of starspots are suppressed, it is unclear whether or not this non-potential component genuinely represents electric current at the unspotted surface or whether it results from the preferred detection of horizontal field in starspot penumbrae. This model contains 20% more energy than the corresponding potential field model at the surface. This amount of free energy drops to under 1% about 1R* above the photosphere. We use these surface maps to model the coronal structure of the star. The mixed radial polarities at the pole in the surface maps support closed coronal loops in the high latitude regions, indicating that a component of the X-ray emission may originate in this area. Assuming that the field remains closed out to 5R*, we find stable surfaces where prominences may form out to the observed distances using this coronal model.

astro-ph↗

arXiv2001

Modeling the corona of AB Doradus

We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ZDI, which can sometimes suffer from the suppression of one vector component due to the effects of stellar inclination, poor phase coverage or lack of flux from dark areas on the surface. Defining a relationship beween the different vector components allows information from one component to compensate for reduced information in another. We present simulations demonstrating the capability of this technique and discuss its prospects.

astro-ph↗

arXiv2000

Mapping potential fields on the surfaces of rapidly rotating stars

We present a technique that combines Zeeman Doppler imaging (ZDI) principles with a potential field mapping prescription in order to gain more information about the surface field topology of rapid rotators. This technique is an improvement on standard ZDI, which can sometimes suffer from the suppression of one vector component due to the effects of stellar inclination, poor phase coverage or lack of flux from dark areas on the surface. Defining a relationship beween the different vector components allows information from one component to compensate for reduced information in another. We present simulations demonstrating the capability of this technique and discuss its prospects.

astro-ph↗