K2 space photometry reveals rotational modulation and stellar pulsations in chemically peculiar A and B stars

Abstract

Context. The physics of magnetic hot stars and how a large-scale magnetic field affects their interior properties is largely unknown. Few studies have combined high-quality observations and modelling of magnetic pulsating stars, known as magneto-asteroseismology, primarily because of the dearth of detected pulsations in stars with a confirmed and well-characterised large-scale magnetic field. Aims. We aim to characterise observational signatures of rotation and pulsation in chemically peculiar candidate magnetic stars using photometry from the K2 space mission. Thus, we identify the best candidate targets for ground-based, optical spectropolarimetric follow-up observations to confirm the presence of a large-scale magnetic field. Methods. We employed customised reduction and detrending tools to process the K2 photometry into optimised light curves for a variability analysis. We searched for the periodic photometric signatures of rotational modulation caused by surface abundance inhomogeneities in 56 chemically peculiar A and B stars. Furthermore, we searched for intrinsic variability caused by pulsations (coherent or otherwise) in the amplitude spectra of these stars. Results. The rotation periods of 38 chemically peculiar stars are determined, 16 of which are the first determination of the rotation period in the literature. We confirm the discovery of high-overtone roAp pulsation modes in HD 177765 and find an additional 3 Ap and Bp stars that show evidence of high-overtone pressure modes found in roAp stars in the form of possible Nyquist alias frequencies in their amplitude spectra. Furthermore, we find 6 chemically peculiar stars that show evidence of intrinsic variability caused by gravity or pressure pulsation modes. Conclusions. The discovery of pulsations in a non-negligible fraction of chemically peculiar stars make these stars high-priority targets for spectropolarimetric campaigns to confirm the presence of their expected large-scale magnetic field. The ultimate goal is to perform magneto-asteroseismology and probe the interior physics of magnetic pulsating stars.

New and improved rotational periods of magnetic CP stars from ASAS-3, KELT, and MASCARA data

“…However, analyzing photometric time-series data is an efficient means of deriving information on the rotational period, in particular as they are much easier acquired than spectroscopic time series. Many studies have successfully exploited the wealth of archival photometric observations provided by numerous past and on-going sky surveys (Paunzen & Maitzen 1998;Wraight et al 2012;Bernhard et al 2015a,b;Hümmerich et al 2016;Bowman et al 2018;Hümmerich et al 2018;David-Uraz et al 2019;Sikora et al 2019), which has greatly enhanced our knowledge of the rotational properties of mCP stars (e.g. Netopil et al 2017).…”

The accelerating rotation of the magnetic He-weak star HD 142990

“…Note that the STEREO data are not publicly available, but are included here for completeness. (1983( ), c: Bohlender et al (1993d: Høg et al (2000); e: van Leeuwen (2007); f : Manfroid et al (1995); g: Sterken et al (1995); h: This work; i: Wraight et al (2012); j: Bowman et al (2018); k: Shultz et al (2018). extensive and well-sampled datasets spanning decades are available. A spindown rate qualitatively compatible with predictions from magnetospheric braking models was reported by Townsend et al (2010) for the magnetic B2 star σ Ori E. Curiously, the rotational period of the B6 star CU Vir has been observed to decrease with time, i.e.…”

“…Spin-up was also reported by Mikulášek et al (2011) for the magnetic B2 star HD 37776, a suspicion later confirmed by Mikulášek (2016) with additional observations. HD 142990 (V 913 Sco; B5 V) is a mCP He-weak star (Nissen 1974) distinguished by rapid rotation (Prot ∼ 0.98 d; Catalano & Leone 1996;Shultz et al 2018;Bowman et al 2018) and a strong (several kG; Shultz 2016) surface magnetic field. The star shows extremely weak ultraviolet and Hα emission originating in its circumstellar magnetosphere (Shore et al 2004).…”

Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) – I. First-light observations of known magnetic B and A stars

“…Twelve out of the nineteen known magnetic B and A stars observed in the first two sectors of TESS show periodograms displaying a main frequency peak and at least one harmonic -this is a characteristic signature of rotational modulation (e.g. Bowman et al 2018), understood to be due to the presence of an oblique dipolar magnetic field -with an additional star exhibiting only a fundamental peak, but whose light curve still appears to be rotationally modulated. Other targets showing this behaviour in the periodogram of their TESS light-curves should be considered as promising magnetic candidates.…”

“…1, to account for the average difference in noise levels in the high and low frequency regimes (i.e. 'pink noise') often found in oversampled timeseries data (see Schwarzenberg-Czerny 2003;Degroote et al 2009;Holdsworth et al 2018;Bowman et al 2018). Finally, we perform a Lomb-Scargle analysis (Lomb 1976;Scargle 1982) of these detrended light curves of our nineteen targets by using the LombScargle class in the astropy.stats Python package.…”