New findings on the prototypical Of?p stars^*

¹ GAPHE, Département AGO, Université de Liège, Allée du 6 Août 17, Bât. B5C, B4000 Liège, Belgium e-mail: naze@astro.ulg.ac.be
² Penn State Worthington Scranton, 120 Ridge View Drive, Dunmore, PA 18512, USA
³ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
⁴ Observatoire de Haute-Provence, 04870 Saint-Michel l'Observatoire, France
⁵ Space Telescope Science Institute (Operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.) , 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 1 March 2010
Accepted: 7 June 2010

Abstract

Aims. In recent years several in-depth investigations of the three prototypical Of?p stars were undertaken. These multiwavelength studies revealed the peculiar properties of these objects (in the X-rays as well as in the optical): magnetic fields, periodic line profile variations, recurrent photometric changes. However, many questions remain unsolved.

Methods. To clarify some of the properties of the Of?p stars, we have continued their monitoring. A new xmm-Newton observation and two new optical datasets were obtained.

Results. Additional information about the prototypical Of?p trio has been found. HD 108 has now reached its quiescent, minimum-emission state for the first time in 50–60 yr. The échelle spectra of HD 148937 confirm the presence of the 7d variations in the Balmer lines and reveal similar periodic variations (though of lower amplitudes) in the He i λ 5876 and He ii λ 4686 lines, underlining its similarities with the other two prototypical Of?p stars. The new xmm-Newton observation of HD 191612 was taken at the same phase in the line modulation cycle, but at a different orbital phase from previous data. It clearly shows that the X-ray emission of HD 191612 is modulated by the 538d period and not by the orbital period of 1542d – it is thus not of colliding-wind origin. The phenomenon responsible for the optical changes appears also at work in the high-energy domain. There are problems however: our MHD simulations of the wind magnetic confinement predict both a harder X-ray flux of a much larger strength than what is observed (the modelled differential emission measure peaks at 30–40 MK, whereas the observed one peaks at 2 MK) and narrow lines (hot gas moving with velocities of 100–200 km s^-1, whereas the observed full width at half maximum is ~2000 km s^-1).