Multiple short-lived stellar prominences on O stars: The O6.5I(n)fp starλCephei

Abstract

Context. Most O-type stars and many B stars show unexplained cyclical variability in their spectral lines, i.e., modulation on the rotational timescale, but not strictly periodic. The variability occurs in the so-called discrete absorption components (DACs) that accelerate through the UV-wind line profiles and also in many optical lines. For such OB stars no dipolar magnetic fields have been detected with upper limits of ∼300 G. Aims. We investigate whether multiple magnetic loops on the surface rather than non-radial pulsations or a dipolar magnetic field can explain the observed cyclical UV and optical spectral line variability. Methods. We present time-resolved, high-resolution optical spectroscopy of the O6.5I(n)fp star λ Cephei. We apply a simplified phenomenological model in which multiple spherical blobs attached to the surface represent magnetic-loop structures, which we call stellar prominences, by analogy with solar prominences. We compare the calculated line profiles as a function of rotational phase, adopting a rotation period of 4.1 d, with observed relative changes in subsequent quotient spectra. Results. We identify many periodicities in spectral lines, almost none of which is stable over timescales from months to years. We show that the relative changes in various optical absorption and emission lines are often very similar. Our proposed model applied to the He ii λ4686 line can typically be fitted with 2−5 equatorial blobs with lifetimes between ∼1 and 24 h.Conclusions. Given the irregular timescales involved, we propose that the azimuthal distribution of DACs correspond to the locations of stellar prominences attached to the surface. This could explain the observed variability of optical and UV lines, and put constraints on the strength and lifetime of these structures, which can be compared with recent theoretical predictions, in which bright magnetic surface spots are formed by the action of the subsurface convection zone.

A detailed X-ray investigation of ζ Puppis

“…The behaviour remains coherent but only for a while during the lifetime of the spot, and as new spots arise, a phase shift and a change in the light curve shape can be observed. In any case, this somewhat erratic behaviour is usual for Oef stars such as ζ Pup, where cycles with similar but not perfectly identical periodicities are often found in spectroscopic time series (Rauw et al 2003(Rauw et al , 2015De Becker & Rauw 2004;Sudnik & Henrichs 2016). However, at a given date, the star should display one single behaviour.…”

“…These large-scale (>R * ) wind disturbances are believed to originate at the base of the wind from bright spots on the photosphere. The presence of such spots has also been inferred from optical variations, for example modelling of the photometry of ξ Per and ζ Pup (Ramiaramanantsoa et al 2014(Ramiaramanantsoa et al , 2018 and modelling of the He ii λ4686 profile of λ Cep (Sudnik & Henrichs 2016). But, until recently, CIRs were not considered to be a potential source of X-rays since models predicted only a velocity kink and not a large jump (e.g.…”

Similar but different: the varied landscape of Onfp/Oef stars variability

“…Jankov et al 2000;Rauw et al 2008) or spots rotating with the star (e.g. Sudnik & Henrichs 2016), lines formed in the stellar wind can display variability as a result of moving small-scale clumps (e.g. Eversberg et al 1998;Lépine & Moffat 2008) or large-scale structures, either magnetically confined winds (e.g.…”

“…Another common feature is the presence of significant spectroscopic variability, especially in this He ii λ 4686 line, e.g. λ Cep (Kaper et al 1997;Rauw et al 2015;Sudnik & Henrichs 2016), BD+60 • 2522 (Rauw et al 2003), or HD 192 281, HD 14 442 andHD 14 434 (De Becker &Rauw 2004). Usually, the timescale as well as the pattern of the spectral variability strongly change from one epoch to the other (Kaper et al 1999;Rauw et al 2003;Sudnik & Henrichs 2016).…”

Subphotospheric fluctuations in magnetized radiative envelopes: contribution from unstable magnetosonic waves

“…Similarly, about 10% of all A-and B-type stars are chemically peculiar and host surface magnetic fields 300 G (e.g., Landstreet et al 2007;Aurière et al 2007;. In addition to large-scale, stable fields, rapid variability in Oto A-type stars suggests that smaller-scale fields and fluctuations, presumably originating below the stellar surface, are present in a larger fraction of intermediate-and highmass stars (e.g., Ramiaramanantsoa et al 2014;Sudnik & Henrichs 2016;Ramiaramanantsoa et al 2018).…”