β Cephei stars in the ASAS-3 data^*,**

II. 103 new β Cephei stars and a discussion of low-frequency modes

¹ Instytut Astronomiczny Uniwersytetu Wrocławskiego, Kopernika 11, 51-622 Wrocław, Poland e-mail: pigulski@astro.uni.wroc.pl
² Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Al. Ujazdowskie 4, 00-478 Warszawa e-mail: gp@astrouw.edu.pl

Received: 30 August 2007
Accepted: 7 November 2007

Abstract

Context.The β Cephei stars have been studied for over a hundred years. Despite this, many interesting problems related to this class of variable stars remain unsolved. Fortunately, these stars seem to be well-suited to asteroseismology. Hence, the results of seismic analysis of β Cephei stars should help us to better understand pulsations and the main sequence evolution of massive stars, particularly the effect of rotation on mode excitation and internal structure. It is therefore extremely important to increase the sample of known β Cephei stars and select targets that are useful for asteroseismology.

Aims.We analysed ASAS-3 photometry of bright early-type stars with the goal of finding new β Cephei stars. We were particularly interested in β Cephei stars that would be good for seismic analysis, i.e., stars that (i) have a large number of excited modes; (ii) show rotationally split modes; (iii) are components of eclipsing binary systems; (iv) have low-frequency modes, that is, are hybrid β Cephei/SPB stars.

Methods.Our study was made with a homogeneous sample of over 4100 stars having MK spectral type B5 or earlier. For these stars, the ASAS-3 photometry was analysed by means of a Fourier periodogram.

Results.We have discovered 103 β Cephei stars, nearly doubling the number of previously known stars of this type. Among these stars, four are components of eclipsing binaries, seven have modes equidistant or nearly equidistant in frequency. In addition, we found five β Cephei stars that show low-frequency periodic variations, very likely due to pulsations. We therefore regard them as candidate hybrid β Cephei/SPB pulsators. All these stars are potentially very useful for seismic modeling. Moreover, we found β Cephei-type pulsations in three late O-type stars and fast period changes in one, HD 168050.