The roAp starα Circinus as seen by BRITE-Constellation

Abstract

We report on an analysis of high-precision, multi-colour photometric observations of the rapidly-oscillating Ap (roAp) star α Cir. These observations were obtained with the BRITE-Constellation, which is a coordinated mission of five nanosatellites that collects continuous millimagnitude-precision photometry of dozens of bright stars for up to 180 days at a time in two colours (≈Johnson B and R). BRITE stands for BRight Target Explorer. The object α Cir is the brightest roAp star and an ideal target for such investigations, facilitating the determination of oscillation frequencies with high resolution. This star is bright enough for complementary interferometry and time-resolved spectroscopy. Four BRITE satellites observed α Cir for 146 d or 33 rotational cycles. Phasing the photometry according to the 4.4790 d rotational period reveals qualitatively different light variations in the two photometric bands. The phased red-band photometry is in good agreement with previously-published WIRE data, showing a light curve symmetric about phase 0.5 with a strong contribution from the first harmonic. The phased blue-lband data, in contrast, show an essentially sinusoidal variation. We model both light curves with Bayesian Photometric Imaging, which suggests the presence of two large-scale, photometrically bright (relative to the surrounding photosphere) spots. We also examine the high-frequency pulsation spectrum as encoded in the BRITE photometry. Our analysis establishes the stability of the main pulsation frequency over the last ≈20 yr, confirms the presence of frequency f 7 , which was not detected (or the mode not excited) prior to 2006, and excludes quadrupolar modes for the main pulsation frequency.

TESS cycle 1 observations of roAp stars with 2-min cadence data

“…They also provided a precise rotation period of 4.4790 d. Later, Bruntt et al (2009) used 84 d of WIRE observations to refine the separation of the modes in this star to be 30.2 µHz. Weiss et al (2016) and Weiss et al (2020) used 4 BRITE satellites to observe TIC 402546736 for a total of 146 d, providing them data to model the spot configuration of the star, and confirm the previously tentative detection of some low amplitude modes.…”

BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant ζ Puppis unveils the photospheric drivers of its small- and large-scale wind structures

“…Patterns of two consecutive dips are common in the nonsinusoidally modulated light curves of cool stars having two dominant dark surface spots (e.g. McQuillan et al 2013, Davenport et al 2015, whereas patterns of two consecutive bumps are common in the phased light curves of stars exhibiting bright surface chemical inhomogeneities (Bruntt et al 2009;Bernhard et al 2015a,b;Weiss et al 2016). Over the past decade, an increasing number of light curves of massive OB stars have been seen to show low-frequency non-sinusoidal signal best explained as arising from rotational modulation rather than pulsations as the corresponding phased light curves show a pattern of two consecutive bumps and the periodograms show prominent peaks at the harmonics of the fundamental frequency (De Cat & Aerts 2002;Degroote et al 2010Degroote et al , 2011Balona 2016).…”

Space Photometry with Brite-Constellation

“…α Cir is the brightest rapidly oscillating (roAp) star with a magnetic field. It was discovered in 1981 by Kurtz and Cropper [107] and since then, many publications dealt with photometric and spectroscopic properties, including the magnetic field (see, e.g., Holdsworth and Brunsden [108] and Weiss et al [109,110]).…”