Volume 588, April 2016
|Number of page(s)||10|
|Section||Stellar structure and evolution|
|Published online||17 March 2016|
University of Vienna, Institute for Astrophysics,
2 Leibniz-Institut für Astrophysik (AIP), Kleine Strasse 9, 14482 Potsdam, Germany
3 Astronomical Institute, University of Wrocław, 50-137 Wroclaw, Poland
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
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.
Key words: stars: chemically peculiar / asteroseismology / stars: oscillations / stars: rotation / stars: individual:αCir / starspots
Based on data collected by the BRITE-Constellation satellite mission, built, launched and operated thanks to support from the Austrian Aeronautics and Space Agency, the University of Vienna, the Canadian Space Agency (CSA), the Foundation for Polish Science & Technology (FNiTP MNiSW), and National Centre for Science (NCN).
The light curves and the reduced data for α Circinus are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/588/A54
© ESO, 2016
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