MOST Detects Corotating Bright Spots On The Mid-O-Type Giant xi Persei

Tahina Ramiaramanantsoa [1,2], Anthony F. J. Moffat [1,2], Andre-Nicolas Chene [3,4,5], Noel D. Richardson [1,2], Huib F. Henrichs [6], Sebastien Desforges [1,2], Victoria Antoci [7], Jason F. Rowe [8], Jaymie M. Matthews [9], Rainer Kuschnig [9,10], Werner W. Weiss [10], Dimitar Sasselov [11], Slavek M. Rucinski [12] and David B. Guenther [13]

[1] Departement de physique, Universite de Montreal, C.P. 6128, Succ. Centre-Ville, Montreal, QC H3C 3J7, Canada
[2] Centre de Recherche en Astrophysique du Quebec (CRAQ), Canada
[3] Gemini Observatory, Northern Operations Centre, 670 North A'ohoku Place, Hilo, HI 96720, USA
[4] Departamento de Fisica y Astronomia, Universidad de Valparaiso, Av. Gran Bretana 1111, Playa Ancha, Casilla 5030, Chile
[5] Departamento de Astronomia, Universidad de Concepcion, Casilla 160-C, Chile
[6] Astronomical Institute 'Anton Pannekoek', University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands
[7] Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
[8] NASA Ames Research Center, Moffett Field, CA 94035, USA
[9] Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
[10] Institute for Astronomy, University of Vienna, Turkenschanzstrasse 17, A-1180 Vienna, Austria
[11] Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
[12] Department of Astronomy and Astrophysics, University of Toronto, 50 St George Street, Toronto, ON M5S 3H4, Canada
[13] Institute for Computational Astrophysics, Department of Astronomy and Physics, St Mary’s University, Halifax, NS B3H 3C3, Canada

We have used the MOST (Microvariability and Oscillations of STars) microsatellite to obtain four weeks of contiguous high-precision broad-band visual photometry of the O7.5III(n)((f)) star xi Persei in November 2011. This star is well known from previous work to show prominent DACs (discrete absorption components) on time-scales of about 2 d from UV spectroscopy and non-radial pulsation with one (l = 3) p-mode oscillation with a period of 3.5 h from optical spectroscopy. Our MOST-orbit (101.4 min) binned photometry fails to reveal any periodic light variations above the 0.1 mmag 3 sigma noise level for periods of a few hours, while several prominent Fourier peaks emerge at the 1 mmag level in the two-day period range. These longer period variations are unlikely due to pulsations, including gravity modes. From our simulations based upon a simple spot model, we deduce that we are seeing the photometric modulation of several corotating bright spots on the stellar surface. In our model, the starting times (random) and lifetimes (up to several rotations) vary from one spot to another yet all spots rotate at the same period of 4.18 d, the best-estimated rotation period of the star. This is the first convincing reported case of corotating bright spots on an O star, with important implications for drivers of the DACs (resulting from corotating interaction regions) with possible bright-spot generation via a breakout at the surface of a global magnetic field generated by a subsurface convection zone.

Reference: Published in MNRAS (June 11, 2014) 441, 910-917. DOI:10.1093/mnras/stu619
Status: Other