New findings on the prototypical Of?p stars
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).
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2010
- DOI:
- 10.1051/0004-6361/201014333
- arXiv:
- arXiv:1006.2054
- Bibcode:
- 2010A&A...520A..59N
- Keywords:
-
- X-rays: stars;
- stars: individual: HD 108;
- stars: individual: HD 148937;
- stars: individual: HD 191612;
- stars: emission-line;
- Be;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 10 pages, 8 figures (2 in jpg), accepted for publication by A&