HD108: the mystery deepens with XMM-Newton observations

Abstract

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 kT_1\sim0.2 keV and kT_2sim1.4 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.