EDP Sciences
Free access
Volume 417, Number 2, April II 2004
Page(s) 667 - 677
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20034422

A&A 417, 667-677 (2004)
DOI: 10.1051/0004-6361:20034422

HD 108: The mystery deepens with XMM-Newton observations

Y. Nazé, G. Rauw, J.-M. Vreux and M. De Becker

Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5c, 4000 Liège, Belgium
(Received 1 October 2003 / Accepted 24 November 2003 )

In 2001, using a large spectroscopic dataset from an extensive monitoring campaign, we discovered that the peculiar Of star HD 108 displayed extreme line variations. This strange behaviour could be attributed to a variety of models, and an investigation of the high energy properties of HD 108 was needed to test the predictions from these models. Our dedicated XMM-Newton observation of HD 108 shows that its spectrum is well represented by a two temperature thermal plasma model with $kT_1\sim0.2$ keV and $kT_2\sim1.4$ keV. In addition, we find that the star does not display any significant short-term changes during the XMM-Newton exposure. Compared to previous Einstein and ROSAT detections, it also appears that HD 108 does not present long-term flux variations either. While the line variations continue to modify HD 108's spectrum in the optical domain, the X-ray emission of the star appears thus surprisingly stable: no simple model is for the moment able to explain such an unexpected behaviour. Thanks to its high sensitivity, the XMM-Newton observatory has also enabled the serendipitous discovery of 57 new X-ray sources in the field of HD 108. Their properties are also discussed in this paper.

Key words: stars: early-type -- X-rays: stars -- stars: winds, outflows -- stars: individual: HD 108

Offprint request: Y. Nazé, naze@astro.ulg.ac.be

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© ESO 2004