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β Cep: a Magnetic Be Star?

Published online by Cambridge University Press:  15 February 2018

H.F. Henrichs ,
F. Bauer ,
G.M. Hill ,
L. Kaper ,
J.S. Nichols-Bohlin  and
P.M. Veen
H.F. Henrichs
Affiliation:
Astronomical Institute Univ. of Amsterdam
F. Bauer
Affiliation:
Universitäts-Sternwarte, München
G.M. Hill
Affiliation:
Dep. of Astronomy, Univ. of Western Ontario
L. Kaper
Affiliation:
Astronomical Institute Univ. of Amsterdam
J.S. Nichols-Bohlin
Affiliation:
CSC/NASA, Greenbelt
P.M. Veen
Affiliation:
Astronomical Institute Univ. of Amsterdam

Abstract

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Besides the well-known pulsation period of 4h34m, the B1 IV star β Cep has a very significant period of 6 or possibly 12 days in the equivalent width of the ultraviolet resonance lines. This was discovered by Fishel and Sparks (1972) with the OAO-2 satellite, and later confirmed with IUE data. Until now, no explanation has been put forward for this period.

We propose that the UV periodicity arises from a 12 day rotational period of the star and that the stellar wind is modulated by an oblique dipolar magnetic field at the surface.

Support for this hypothesis is given by the striking similarity between the UV-line behavior of β Cep and of known rotating magnetic B stars, for example the B2 V helium-strong star HD 184927 (Barker et al. 1982), and by the measured magnetic field strength of B ± σ = (810 ± 170) G for β Cep itself by Rudy and Kemp (1978). A rotational period of 12 days corresponds well with an adopted radius between 6 and 10 R⊙, given the reported values of 20 - 43 km/s for vsini.

To verify our hypothesis we carried out new magnetic field measurements simultaneously with UV spectroscopy. We confirm the 12 day UV period in the equivalent width of the stellar wind lines of CIV, Si III, Si IV and N V (see Figs. 1 and 2), but find a lower and likely variable field strength (Fig. 3), which is consistent with a 12 day period, but not conclusive.

It remains puzzling why our new magnetic field measurements show a lower field than in 1978. It is interesting to recall the recent discovery (Mathias et al. 1991) that β Cep has entered a new Be phase (July 1990), when Hα turned into emission (see Fig. 4). This opens the suggestion that the lower magnetic field is related to the emission phase. Because the UV period is still the same, the field must still be strong enough to modulate the wind. A possibly higher equivalent width of the 1991 UV data with respect to the 1979 data might also be related to this transition, but this needs to be confirmed.

The star β Cep appears to be the only star in its class which shows this wind variability and in this respect β Cep is an exceptional β Cep star.

Type
Beyond the Classical Instability Strip
Information
International Astronomical Union Colloquium , Volume 139: New Perspectives on Stellar Pulsation and Pulsating Variable Stars , 1993 , pp. 186 - 187
Copyright
Copyright © Cambridge University Press 1993

References

Barker, P., Brown, D., Bolton, C. and Landstreet, J., 1982, in Advances in UV Astronomy, NASA CP-2238, Kondo, Y., Mead, J.M. and Chapman, R.D. (Eds.), p. 589 Google Scholar
Fishel, D., Sparks, W.M., 1972, in The Scientifique Results From the Orbiting Astronomical Observatory (OAO-2), NASA SP-310, p. 475 Google Scholar
Kaper, L., Henrichs, H.F. and Mathias, Ph., 1992, OHP Newsletter, Feb.Google Scholar
Mathias, P., Gillet, D. and Kaper, L., 1991, in Proc. ESO Workshop “Nature and Diagnostics of OB star Variability”, D. Baade (Ed.)Google Scholar
Rudy, R.J. and Kemp, J.C., 1978, MNRAS 183, 595 CrossRefGoogle Scholar