EDP Sciences
Free access
Volume 421, Number 1, July I 2004
Page(s) 203 - 213
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20035612

A&A 421, 203-213 (2004)
DOI: 10.1051/0004-6361:20035612

Magnetically controlled circumstellar plasma in the helium-weak stars

S. N. Shore1, D. A. Bohlender2, C. T. Bolton3, P. North4 and G. M. Hill5

1  Department of Physics "Enrico Fermi", University of Pisa, via Buonarroti 2, Pisa 56127, Italy
2  Herzberg Institute of Astrophysics, National Research Council of Canada, 5071 West Saanich Road, Victoria, BC, Canada V9E 2E7
    e-mail: david.bohlender@nrc-cnrc.gc.ca
3  David Dunlap Observatory, University of Toronto, PO Box 360, Richmond Hill, Ontario L4C 4Y6 Canada
    e-mail: bolton$@$astro.utoronto.ca
4  Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire, 1290 Chavannes-des-Bois, Switzerland
    e-mail: Pierre.North@obs.unige.ch
5  W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, USA
    e-mail: ghill@keck.hawaii.edu

(Received 2 November 2003 / Accepted 30 March 2004 )

We report the discovery of variable $\ion{C}{iv}$ line profiles in the magnetic helium-weak stars HD 142301, HD 142990, and HD 144334, all of which are members of the Sco-Cen association, and the hot helium-weak star HD 175362. These stars display behavior that is similar to HD 5737 and HD 79158, two helium-weak stars for which magnetically confined plasma has previously been reported. The present study thus extends the $\ion{C}{iv}$ survey to all of the stars for which strong radio emission has been detected. No $\ion{C}{iv}$ absorption has been detected for two other Sco-Cen chemically peculiar stars, HD 144844 and 146001, nor has it been detected from archival spectra of HD 28843, HD 49333, or HD 124224. The reported detections are the first helium-weak stars that do not belong to the sn subclass that have been found to display such UV line profile variations. It appears that the restriction of this phenomenon to the extreme helium-weak subgroup was a selection effect and that the occurrence of magnetically trapped corotating plasma is more widespread among the upper main sequence magnetic spectrum variables. We show model calculations for the $\ion{C}{iv}$ variability that clarify the plasmaspheric structure. Supporting H $\alpha$ observations show no evidence of line profile variability for HD 142301 and HD 144334, while HD 142990 appears to have extremely weak emission at certain phases, and the profile of HD 124224 shows some changing asymmetry.

Key words: stars: chemically peculiar -- stars: magnetic fields -- stars: atmospheres -- circumstellar matter

Offprint request: S. N. Shore, shore@df.unipi.it

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