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The Mass of the Cepheid Binary V636 Scorpii

1998; IOP Publishing; Volume: 505; Issue: 2 Linguagem: Inglês

10.1086/306177

1538-4357

Erika Böhm‐Vitense, Nancy Remage Evans, Kenneth G. Carpenter, Michael D. Albrow, P. L. Cottrell, Richard D. Robinson, Bernhard Beck‐Winchatz,

Astronomy and Astrophysical Research

The mass-luminosity relation for Cepheids depends on the degree of mixing in their main-sequence progenitors. Masses of Cepheids can be inferred by using different aspects of pulsation theory. These methods have in the past led to diverging results, showing that something was wrong in either evolution theory or pulsation theory or both. For some binary Cepheids dynamical masses, which are independent of pulsation and evolution theories, can be determined. V636 Sco is one of a handful of Cepheid binaries whose companions are bright enough in the ultraviolet that orbital radial velocities can be measured. We have here attempted to determine the radial velocity of the companion V636 Sco B to the Cepheid V636 Sco A by means of two Hubble Space Telescope (HST) spectra taken at phases of minimum and maximum Cepheid orbital velocities. The ratio of the orbital velocity of the companion and the Cepheid (which is the inverse of their mass ratio) came out to be 1.25 ± 0.17. V636 Sco B has a spectral type B9.5 V, for which we expect a stellar mass of 2.5 M☉. For the Cepheid we thus determine a mass of 3.1 ± 0.4 M☉, which is surprisingly low. If true, such a low mass would indicate an extremely large amount of excess mixing, which is very unlikely.