Massive pulsating stars observed by BRITE-Constellation

I. The triple system β Centauri (Agena)^⋆

¹ Instytut Astronomiczny, Uniwersytet Wrocławski, Kopernika 11, 51-622 Wrocław, Poland
e-mail: pigulski@astro.uni.wroc.pl
² Instytut Automatyki, Politechnika Śląska, Akademicka 16, 44-100 Gliwice, Poland
³ Institut für Astrophysik, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
⁴ Département de physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Québec, H3C 3J7, et Centre de recherche en astrophysique du Québec (CRAQ), Canada
⁵ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4, Canada
⁶ Centrum Astronomiczne im. M. Kopernika, Polska Akademia Nauk, Bartycka 18, 00-716 Warszawa, Poland
⁷ Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario, K7K 7B4, Canada
⁸ Institut für Kommunikationsnetze und Satellitenkommunikation, Technische Universität Graz, Inffeldgasse 12, 8010 Graz, Austria
⁹ Dept. of Physics & Astronomy, The University of British Columbia, 6224 Agricultural Road, Vancouver, B.C., V6T 1Z1, Canada
¹⁰ Centrum Badań Kosmicznych, Polska Akademia Nauk, Bartycka 18A, 00-716 Warszawa, Poland
¹¹ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstrasse 25/8, 6020 Innsbruck, Austria

Received: 1 December 2015
Accepted: 14 January 2016

Abstract

Context. Asteroseismology of massive pulsating stars of β Cep and SPB types can help us to uncover the internal structure of massive stars and understand certain physical phenomena that are taking place in their interiors. We study β Centauri (Agena), a triple system with two massive fast-rotating early B-type components which show p- and g-mode pulsations; the system’s secondary is also known to have a measurable magnetic field.

Aims. This paper aims to precisely determine the masses and detect pulsation modes in the two massive components of β Cen with BRITE-Constellation photometry. In addition, seismic models for the components are considered and the effects of fast rotation are discussed. This is done to test the limitations of seismic modeling for this very difficult case.

Methods. A simultaneous fit of visual and spectroscopic orbits is used to self-consistently derive the orbital parameters, and subsequently the masses, of the components. Time-series analysis of BRITE-Constellation data is used to detect pulsation modes and derive their frequencies, amplitudes, phases, and rates of frequency change. Theoretically-predicted frequencies are calculated for the appropriate evolutionary models and their stability is checked. The effects of rotational splitting and coupling are also presented.

Results. The derived masses of the two massive components are equal to 12.02 ± 0.13 and 10.58 ± 0.18 M_⊙. The parameters of the wider, A–B system, presently approaching periastron passage, are constrained. Analysis of the combined blue- and red-filter BRITE-Constellation photometric data of the system revealed the presence of 19 periodic terms, of which eight are likely g modes, nine are p modes, and the remaining two are combination terms. It cannot be excluded that one or two low-frequency terms are rotational frequencies. It is possible that both components of β Cen are β Cep/SPB hybrids. An attempt to use the apparent changes of frequency to distinguish which modes originate in which component did not succeed, but there is potential for using this method when more BRITE data become available.

Conclusions. Agena seems to be one of very few rapidly rotating massive objects with rich p- and g-mode spectra, and precisely known masses. It can therefore be used to gain a better understanding of the excitation of pulsations in relatively rapidly rotating stars and their seismic modeling. Lacking proper mode identification, the pulsation frequencies found in β Cen cannot yet be used to constrain the internal structure of the components, but it may be possible to achieve this in the future with the use of spectroscopy and spectropolarimetry. In particular, these kinds of data can be used for mode identification since they provide new radial velocities. In consequence, they may help to improve the orbital solution, derive more precise masses, magnetic field strength and geometry, inclination angles, and reveal rotation periods. They may also help to assign pulsation frequencies to components. Finally, the case studied here illustrates the potential of BRITE-Constellation data for the detection of rich-frequency spectra of small-amplitude modes in massive pulsating stars.