Evolving pulsation of the slowly rotating magnetic β Cep star ξ1 CMa

Abstract

Recent BRITE-Constellation space photometry of the slowly rotating, magnetic β Cep pulsator ξ 1 CMa permits a new analysis of its pulsation properties. Analysis of the twocolour BRITE data reveals the well-known single pulsation period of 0.209 d, along with its first and second harmonics. A similar analysis of SMEI and TESS observations yields compatible results, with the higher precision TESS observations also revealing several low-amplitude modes with frequencies below 5 d −1 ; some of these are likely g modes. The phase lag between photometric and radial velocity maxima -equal to 0.334 cycles -is significantly larger than the typical value of 1/4 observed in other large-amplitude β Cep stars. The phase lag, as well as the strong dependence of phase of maximum light on wavelength, can be reconciled with seismic models only if the dominant mode is the fundamental radial mode. We employ all published photometric and radial velocity measurements, spanning over a century, to evaluate the stability of the pulsation period. The O − C diagram exhibits a clear parabolic shape consistent with a mean rate of period change P = 0.34±0.02 s/cen. The residuals from the best-fit parabola exhibit scatter that is substantially larger than the uncertainties. In particular, dense sampling obtained during the past ∼20 years suggests more complex and rapid period variations. Those data cannot be coherently phased with the mean rate of period change, and instead require P ∼ 0.9 s/cen. We examine the potential contributions of binarity, stellar evolution, and stellar rotation and magnetism to understand the apparent period evolution.

Variability of OB stars from TESS southern Sectors 1–13 and high-resolution IACOB and OWN spectroscopy

“…HD 46328 (ξ 1 CMa, B0.5 IV, Lesh 1968) is a well-studied high-amplitude β Cep star, with a single pulsation period of P = 0.209 d (McNamara 1955;Shobbrook 1973b;Heynderickx 1992;Heynderickx et al 1994;Saesen et al 2006;Shultz et al 2017Shultz et al , 2018Wade et al 2020b). Its one-sector TESS data set was most recently studied by Wade et al (2020b), who recover this period again and use it to constrain the period change over the last 100 yr. They also note the possible presence of g modes.…”

Space Photometry with Brite-Constellation

“…Building on work by Pigulski [71], Jerzykiewicz [72] and Shultz et al [69], BRITE-Constellation photometry (BLb, BHr, BTr) of ξ 1 CMa was employed by Wade et al [73], as one of the most recent anchor points to monitor the evolution of its pulsation period. Combining over one century of photometric and radial velocity monitoring, they concluded that the period evolution of ξ 1 CMa consists of a secular period lengthening of roughly 0.3 s/century that can be satisfactorily understood as a consequence of expansion due to stellar evolution.…”

Confirmation of ξ1 CMa’s ultra-slow rotation: magnetic polarity reversal and a dramatic change in magnetospheric UV emission lines

“…We calculated RV measurements (Table A1) from the individual subexposures of the spectropolarimetric sequences, following the multiline procedure described by . The RVs from the 2019 observations were published by Wade et al (2020). To calculate pulsation phase, we adopt the nonlinear ephemeris (P 0 = 0.2095763(1) d and Ṗ = 0.0096(5) s yr −1 ) from , consistent with the short-term period evolution described by Wade et al (2020).…”

“…The RVs from the 2019 observations were published by Wade et al (2020). To calculate pulsation phase, we adopt the nonlinear ephemeris (P 0 = 0.2095763(1) d and Ṗ = 0.0096(5) s yr −1 ) from , consistent with the short-term period evolution described by Wade et al (2020).…”

“…Radial velocity (RV) measurements obtained from the intensity spectra (that is, from the individual subexposures of the spectropolarimetric sequences) in 2020. RVs in 2019 were reported byWade et al (2020).…”