HD 108: The mystery deepens with XMM-Newton observations^*

Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, Bât. B5c, 4000 Liège, Belgium

Corresponding author: Y. Nazé, naze@astro.ulg.ac.be

Received: 1 October 2003
Accepted: 24 November 2003

Abstract

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  keV and  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.