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Astronomy & Astrophysics

HD108: the mystery deepens with XMM-Newton observations.+

Y. Nazé1,*, G. Rauw1,**, J.M. Vreux1 and M. De Becker1

1 Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium

* Research Fellow FNRS, Belgium
** Research Associate FNRS, Belgium

+ Based on observations collected at the Observatoire de Haute-Provence (France) and with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA).


     In 2001, using a large spectroscopic dataset from an extensive monitoring campaign, we discovered that the peculiar Of star HD108 displayed spectacular line variations. This strange behaviour could be attributed to a variety of models, and an investigation of the high energy properties of HD108 was crucially needed to test the predictions from these models. Our dedicated XMM-Newton observation of HD108 shows that its spectrum is well represented by a two temperature thermal plasma model with kT1 ~ 0.2 keV and kT2 ~ 1.4 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 HD108 does not present long-term flux variations either. While the dramatic line variations continue to modify HD108'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 HD108. Their properties are also discussed in this paper.

Key Words
Stars: early-type -- X-rays: stars -- Stars: winds, outflows -- Stars: individual: HD108

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