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Register a new userK2 space photometry reveals rotational modulation and stellar pulsations in chemically peculiar A and B stars
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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. 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. 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. 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. The discovery of pulsations in a non-negligible fraction of chemically peculiar stars make these stars high-priority targets for spectropolarimetric campaigns.
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We report the results of an observational study aimed at searching for magnetic pulsating hot stars suitable for magneto-asteroseismology. A sample of sixteen chemically peculiar stars was selected and analysed using both high-resolution spectropolarimetry with ESPaDOnS and K2 high-precision space photometry. For all stars, we derive the effective temperature, surface gravity, rotational and non-rotational line broadening from our spectropolarimetric data. High-quality K2 light curves were obtained for thirteen of the sixteen stars and revealed rotational modulation, providing accurate rotation periods. Two stars show evidence for roAp pulsations, and one star shows signatures of internal gravity waves or unresolved g-mode pulsations. We confirm the presence of a large-scale magnetic field for eleven of the studied stars, of which nine are first detections. Further, we report one marginal detection and four non-detections. Two of the stars with a non-detected magnetic field show rotational modulation due to surface abundance inhomogeneities in the K2 light curve, and we confirm that the other two are chemically peculiar. Thus, these five stars likely host a weak (undetected) large-scale magnetic field.
Light curves and periodograms of 160 B stars observed by the TESS space mission and 29 main-sequence B stars from Kepler and K2 were used to classify the variability type. There are 114 main-sequence B stars in the TESS sample, of which 45 are classified as possible rotational variables. This confirms previous findings that a large fraction (about 40 percent) of A and B stars may exhibit rotational modulation. Gaia DR2 parallaxes were used to estimate luminosities, from which the radii and equatorial rotational velocities can be deduced. It is shown that observed values of the projected rotational velocities are lower than the estimated equatorial velocities for nearly all the stars, as they should be if rotation is the cause of the light variation. We conclude that a large fraction of main-sequence B stars appear to contain surface features which cannot likely be attributed to abundance patches.
We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5yr. All four stars are found to be multiperiodic. The strongest secondary mode with frequency f_2 has an amplitude of a few mmag, 20 - 45 times lower than the main radial mode with frequency f_1. The two oscillations have a period ratio of P_2/P_1 = 0.612 - 0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is nonradial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P_2/P_1 ~ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f_2 at ~1/2 f_2 and at ~3/2 f_2. This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f_2 and its subharmonics are variable on a timescale of 10 - 200d. The dominant radial mode also shows variations on the same timescale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1mmag, that must correspond to nonradial g-modes. Such modes never before have been observed in RR Lyrae variables.
This paper presents the catalogue and the method of determination of averaged quadratic effective magnetic fields B_e for 596 main sequence and giant stars. the catalogue is based on measurements of the stellar effective (or mean longitudinal) magnetic field strengths B_e, which were compiled from the existing literature. We analysed the properties of 352 chemically peculiar A and B stars in the catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, and all ApSr type stars. We have found, that the number distribution of all chemically peculiar (CP) stars vs. averaged magnetic field strength is described by a decreasing exponential function. Relations of this type hold also for stars of all the analysed subclasses of chemical peculiarity. The exponential form of the above distribution function can break down below about 100 Gs, the latter value representing approximately the resolution of our analysis for A type stars.
We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant rho Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency of $f_{rmrot}=0.0373$d$^{-1}$ and its harmonics. This dominant frequency corresponds with a rotation period of 26.8d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability ($<1.5 $d$^{-1}$) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 days. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20km s$^{-1}$ in the H$alpha$ line, as well as photospheric velocity variations of a few km s$^{-1}$ at frequencies in the range 0.2 to 0.6 d$^{-1}$ in the SiIII 4567AA line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling super-inertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why $rho$~Leo is an excellent target to study how the observed photospheric variability propagates into the wind.
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