What is the real nature of HD 108?

Abstract

Abstract. Since the beginning of the past century, the nature of HD 108 has been a subject of intense debate. One after another, astronomers explored its variability and attributed it either to binarity, or to changes in the stellar wind of a single star. In this article, we analyse a 30 year campaign of spectroscopic observations of this star with special emphasis on the last 15 years during which photographic plates have been replaced by CCD detectors. Our investigation of the radial velocities of HD 108 yields no significant short-or long-term period and does not confirm the published periodicities either. Though the radial velocity of HD 108 appears clearly variable, the variations cannot be explained by the orbital motion in a spectroscopic binary. However, our data reveal spectacular changes in the H i Balmer lines and some He i profiles over the years. These lines continuously evolved from P Cygni profiles to "pure" absorption lines. A similar behaviour has already been observed in the past, suggesting that these changes are recurrent. HD 108 seems to share several characteristics of Oe stars and we discuss different hypotheses for the origin of the observed long-term variations. As we are now in a transition period, a continuous monitoring of HD 108 should be considered for the next few years.

New findings on the prototypical Of?p stars

“…1 and 2). Based on past behaviour (Nazé et al 2001), it is expected that the emission will get stronger again in the future, as is seen in HD 191612. Note however that for HD 191612 the quiescent state occurs during one-third of the cycle, which would correspond to about 18 years for HD 108.…”

“…The spectrum of HD 108 has been followed intensively since 1986 (Nazé et al 2001(Nazé et al , 2004. While some (purely photospheric) lines such as He ii λ 4542 remain constant, the strength of the Balmer, He i (e.g.…”

“…In the past, stars displaying this feature were classified by Walborn (1972) in a separate category, dubbed Of?p. In recent years, this category has attracted a lot of attention, as one can see from the numerous publications (Walborn et al 2003(Walborn et al , 2004Donati et al 2006;Howarth et al 2007;Hubrig et al 2008;Nazé et al 2001Nazé et al , 2004Nazé et al , 2007Nazé et al , 2008a. They dealt with the peculiar properties of these stars (for a summary see Nazé et al 2008b) and led to a modification of the definition of these stars.…”

“…He i line profile variations: the star is brighter when the emission lines are stronger (Nazé et al 2001;Nazé et al 2008b). …”

The latest developments on Of?p stars

“…HD 108 is the first Of?p for which long-term spectral variations were reported (Andrillat et al 1973, Nazé et al 2001. However, the recurrence timescale appeared very long, about 55 yrs (Nazé et al 2006).…”