To be published in:
Astronomy & Astrophysics
HD108: the mystery deepens with XMM-Newton observations.+
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.
Stars: early-type -- X-rays: stars -- Stars: winds, outflows --
Stars: individual: HD108