The pulsating magnetosphere of the extremely slowly rotating magnetic β Cep star ξ1 CMa

Abstract

ξ1 CMa is a monoperiodically pulsating, magnetic β Cep star with magnetospheric X-ray emission which, uniquely amongst magnetic stars, is clearly modulated with the star's pulsation period. The rotational period P rot has yet to be identified, with multiple competing claims in the literature. We present an analysis of a large ESPaDOnS dataset with a 9-year baseline. The longitudinal magnetic field B z shows a significant annual variation, suggesting that P rot is at least on the order of decades. The possibility that the star's Hα emission originates around a classical Be companion star is explored and rejected based upon VLTI AMBER and PIONIER interferometry, indicating that the emission must instead originate in the star's magnetosphere and should therefore also be modulated with P rot . Period analysis of Hα EWs measured from ESPaDOnS and CORALIE spectra indicates P rot > 30 yr. All evidence thus supports that ξ 1 CMa is a very slowly rotating magnetic star hosting a dynamical magnetosphere. Hα also shows evidence for modulation with the pulsation period, a phenomenon which we show cannot be explained by variability of the underlying photospheric line profile, i.e. it may reflect changes in the quantity and distribution of magnetically confined plasma in the circumstellar environment. In comparison to other magnetic stars with similar stellar properties, ξ 1 CMa is by far the most slowly rotating magnetic B-type star, is the only slowly rotating B-type star with a magnetosphere detectable in Hα (and thus, the coolest star with an optically detectable dynamical magnetosphere), and is the only known early-type magnetic star with Hα emission modulated by both pulsation and rotation.

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

“…Jerzykiewicz (1999), in his summary of period evolution of β Cep stars, cites a rate of period change of 0.37 ± 0.05 s/cen reported by Pigulski (1992a). Neilson & Ignace (2015) used those results to test the influence of rotation and convective core overshoot on models of massive star evolution, finding that the measured rate of period change of ξ 1 CMa was in good agreement with that predicted by models under the constraints applied by the physical parameters of Shultz (2016); Shultz et al (2017).…”

“…pulsation period than the combined 8 minute exposure-plusreadout time of a full sequence (0.004% vs. 0.027%), therefore individual spectra were used for RV measurements, thus yielding 32 measurements in 2018 and 76 measurements in 2019. RVs were measured from the weighted means of the centres-of-gravity across multiple unblended spectral lines, using the same method and line list described by Shultz et al (2017). FROM 1906FROM -2017 To investigate the behaviour of the pulsation period of ξ 1 CMa we have constructed an O − C diagram using all available spectroscopic and photometric observations.…”

“…ξ 1 CMa (HR 2387 = HD 46328 = HIP 31125 = MCW 441 = ADS 5176A) is a bright (V = 4.3 mag), apparently single, E-mail: wade-g@rmc.ca B0.5 subgiant located near the middle of its main sequence evolution (e.g. Niemczura & Daszyńska-Daszkiewicz 2005;Shultz et al 2017). It has long been known to exhibit largeamplitude β Cep radial pulsations with a period of roughly P = 0.20958 d (4.77 d −1 ; e.g.…”

“…Hubrig et al (2009) reported the star to be magnetic. Shultz et al (2017) performed a detailed determination of the star's physical parameters, finding T eff = 27±1 kK, log g = 3.78 ± 0.07, and an age of 11.1 ± 0.7 Myr. At an inferred mass of 14.2 ± 0.4 M , this implies that ξ 1 CMa has completed three-quarters of its main sequence evolution.…”

“…Analysis of high resolution spectropolarimetry obtained between 2000 -2017 led Shultz et al (2017) to conclude that the star's rotation period is remarkably long, over 30 years. In particular, Shultz et al (2017) (see as well Shultz et al 2018) demonstrated that previous claims of much shorter rotation periods were unable to explain the magnetic observations. Shultz et al (2018) discovered the presence of unexpected crossover signatures in Stokes V profiles of ξ 1 CMa obtained near the phase of null longitudinal field.…”