1. Phase-resolved observations of a peculiar O star
Abstract: Recently, two peculiar stars with spectral type Of?p, HD108 and HD191612, were found to present recurrent line profile variations, with a period of about 60 years for the former and 538 days for the latter. These changes are correlated with photometric variations. To investigate further the behavior of these stars, we have undertaken an observing campaign to obtain phase-resolved observations of the shorter-period star, HD These high quality X-ray data and high-resolution visible spectroscopy now enable to understand many characteristics of this peculiar type of star.
3. Visible data
Echelle spectroscopy covering nearly the whole cycle of HD was obtained at the Haute-Provence Observatory (France). The H and HeI behave as predicted (see figure on the left) but variations are also seen for metallic and HeII lines (see figure on the right): while the EWs are rather constant, the RVs increase with time. HeII4686 displays a composite behavior: it generally follows the H/HeI lines, but when the low-emission state, it behaves like the metallic and other HeII lines (e.g. increase in RV).
Ephemeris: T0= d, P=538d
1. HD and the Of?p stars
Of?p : question mark because doubt that these stars are normal Of supergiants
Principal defining characteristic : C III  4647, 4650, 4651 strength comparable to that of N III  4634, 4640, 4642
Spectral peculiarities : presence of sharp absorption/emission components and P Cygni profiles at some H and He lines
Only three such stars in our Galaxy : HD148937, HD108 (varying P~55yrs), HD (varying, P~1.5 yrs)
HD : detection of a magnetic field B* of -1.50.2 kG, old version of 1 Ori C ?
2. XMM-Newton observations
The X-ray observations were carefully planned within the 538d period: the first two datasets were taken near the maximum emission phase in the visible, the last one was obtained during a ‘quiescent’ phase and the the third one samples an intermediate phase (see figure below).
EW>0 for absorption, EW<0 for emission, i.e. larger absorption towards the top of the figures, larger emission to the bottom.
These figures show, from top to bottom, the mean line profile, the temporal variance spectrum, and the amplitude and phase of a sine wave of period 538d fitted to the data.The TVS and amplitude curves for H, HeI lines is gaussian, but two peaks appear for metallic and HeII lines. The behavior in phase also differs between these two groups.
4. Conclusions
The phase-resolved observing campaign of HD has enabled to discover the detailed X-ray properties of this star (including the temporal variations in the X-ray domain) but also to detect the small variations of the metallic and HeII absorption lines.
Although HD shares some similarities with 1 Ori C, many observational characteristics (especially the X-ray properties) of this Of?p star are at odds with the magnetic oblique rotator scenario.
References:
Walborn et al., 2003, ApJ, 588, 1025 (report on the variations of HD191612)
Walborn et al., 2004, ApJ, 617, L61 (discovery of the period of HD191612)
Nazé et al., 2001, A&A,372, 195 (discovery of the long-term behavior of HD108)
Nazé et al., 2004, A&A, 417, 667 (analysis of the X-ray properties of HD108 in its quiet phase)
Nazé et al., 2006, IBVS, 5693 (precisions on the yrs period)
Properties of the X-ray emission: (similar to those found for HD108)
Thermal spectrum with two components (a dominant one at 0.2 and another one at 1-2 keV, see figure above)
Count rate MOS1 from 0.1170.003 cts/s (Rev 975) to 0.002 cts/s (Rev 1068)
Unabsorbed flux of about erg/cm²s
Slight overluminosity : log(LX/LBOL) between –6.2 and –6.1 (canonical value: -6.9)
High resolution spectroscopy : spectrum similar to “normal” O stars (broad lines, few high ionization lines)
Yaël Nazé (Institut d’Astrophysique et de Géophysique, Université de Liège & FNRS, Belgium)