AAS 204th Meeting, June 2004
Session 7 A Walk Through the HR Diagram
Poster, Monday, May 31, 2004, 9:20am-6:30pm, Ballroom

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[7.10] Cepheid Masses: FUSE Observations of S Mus B

N. R. Evans (SAO), D. Massa (NASA's GSFC, SGT, Inc), A. W. Fullerton (Univ. of Victoria), G. Sonneborn, R. Iping (NASA's GSFC)

Measurement of Cepheid masses has been a goal both in order to understand these primary distance indicators and also as a benchmark for stellar evolution calculations of high mass stars. The approach is to use ultraviolet spectroscopy (IUE, HST GHRS, and HST STIS) to measure the orbital velocity amplitude of the hot companion of a binary Cepheid. This amplitude plus the Cepheid orbital velocity amplitude from the ground provides the mass ratio between the Cepheid and the companion. The mass of the companion is inferred from an ultraviolet spectral type and hence the mass of the Cepheid is determined. S Mus B is the hottest Cepheid companion known. As a benefit, the large ultraviolet flux made it the only Cepheid companion for which the velocity amplitude could be measured with the echelle mode of the HST GHRS. Unfortunately, the high temperature of S Mus B is difficult to constrain at wavelengths longer than 1200 A because of the degeneracy between temperature and reddening. Thus the companion has the most accurate velocity amplitude of any Cepheid binary, but a relatively inaccurate mass. We have now obtained a FUSE spectrum, and have identified two regions which are temperature sensitive but relatively unaffected by H2 absorption (940 A and Ly Beta wings). By comparing S Mus B with other FUSE spectra, we have determined a temperature of 17,000 K. The resultant Cepheid mass is 6.0 +/- 0.4 solar masses.

Financial assistance was provided by Chandra X-ray Center NASA Contract NAS8-39073 for NRE and FUSE grant NAG5-11946.

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