Beta Cephei Stars in the LMC

Presented at the meeting of the Working Group on Active B Stars during the 25th IAU General Assembly in Sydney, Australia on 2003 July 16

Z. Kolaczkowski¹, A. Pigulski¹, I. Soszynski², A. Udalski², M. Szymanski², M. Kubiak², K. Zebrun², G. Pietrzynski², P.R. Wozniak², O. Szewczyk² and L. Wyrzykowski²

¹Wroclaw University Observatory, Kopernika 11, 51-622 Wroclaw, Poland
²Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 2003 September 29; Accepted: 2003 October 1.

The pulsations of the  Cephei stars, hot main-sequence stars, can be explained in terms of the mechanism. The driving occurs as a consequence of the opacity bump which originates from a large number of bound-bound transitions in the iron-group elements. The metallicity of the stellar material is therefore crucial for driving in  Cephei stars and it is predicted that their instability strip vanishes rapidly with low metallicities (Pamyatnykh 1999).

Observationally, this fact was confirmed by Pigulski et al. (2002) who showed that as a consequence of the metallicity gradient in the Galaxy, the lower percentage of  Cephei stars is observed in the northern open clusters in comparison with the southern ones. The study of other low-metallicity systems containing a population of young stars, like Magellanic Clouds, is very important in this context.

The first three  Cephei stars in the LMC were found by Pigulski & Kolaczkowski (2002) among ˜5200 early B-type stars from the catalogue of variable candidates published by Zebrun et al. (2001). After reprocessing the OGLE-II data, we analyzed the photometry of over 75,000 early B-type stars and found 64  Cephei stars.

The  Cephei stars in the LMC range the V magnitudes between 14 and 17.5. In comparison with Galactic  Cephei stars, they have much longer periods (median value of the pulsation period is 0.27 d in comparison with 0.17 d in the Galaxy). In addition, there are a number of stars (20 out of 64) which show also periodic changes with longer period(s), ranging from 0.35 to 0.75 d. These modes are likely g ones, which means that the stars can be regarded both as  Cephei and SPB pulsators. So far, this type of behaviour was not observed in the Galactic  Cephei stars.

Despite the large number of discovered  Cephei stars, their incidence is much lower than in the Galaxy which confirms the metallicity dependence of the pulsation mechanism. However, as far as the metallicities are concerned, a detailed comparison with the predictions of the theory can be done only after measuring metallicities spectroscopically.

References:

Pamyatnykh, A. 1999, Acta Astron., 49, 119

Pigulski, A. and Kolaczkowski, Z. 2002, A&A. 388, 88

Pigulski, A. et al. 2002, PASPC, 259, 146

Zebrun, K. et al. 2001, Acta Astron., 51, 317

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