The roAp star α Circinus as seen by BRITE-Constellation^⋆,⋆⋆

¹ University of Vienna, Institute for Astrophysics, 1010 Vienna, Austria
e-mail: werner.weiss@univie.ac.at
² Leibniz-Institut für Astrophysik (AIP), Kleine Strasse 9, 14482 Potsdam, Germany
e-mail: hefroehlich@aip.de
³ Astronomical Institute, University of Wrocław, 50-137 Wroclaw, Poland
e-mail: pigulski@astro.uni.wroc.pl
⁴ Institute of Automatic Control, Silesian University of Technology, 44-141 Gliwice, Poland
⁵ Dept. de physique, Université de Montréal, QC H3T 1J4 Montréal, Canada
⁶ Dept. of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
⁷ Space Flight Laboratory, University of Toronto, Toronto, ON M5S, Canada
⁸ Sydney Institute for Astronomy, University of Sydney, NSW 2006, Australia
⁹ SETI Institute, Mountain View, CA 94043, USA
¹⁰ Stellar Astrophysics Centre, Aarhus University, 8000 Aarhus C, Denmark
¹¹ Graz University of Technology, 8010 Graz, Austria
¹² Astronomical Institute, Graduate School of Science, Tohoku University, Sendai, 980-8577 Miyagi Prefecture, Japan
¹³ Nicolaus Copernicus Astronomical Center, Warsaw, Poland
¹⁴ Dept. of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S, Canada
¹⁵ Space Research Center, Warsaw, Poland
¹⁶ NASA Ames Research Center, Montain View, CA 94035, USA
¹⁷ Dept. of Physics, Royal Military College of Canada, Ontario, Canada
¹⁸ Institute for Astro- and Particle Physics, University of Innsbruck, 6020 Innsbruck, Austria

Received: 19 July 2015
Accepted: 13 December 2015

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 for146 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 f₇, which was not detected (or the mode not excited) prior to 2006, and excludes quadrupolar modes for the main pulsation frequency.