Massive Star Multiplicity: The Cepheid W Sgr

Abstract

We have obtained spectra of the W Sgr system with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). The spectra resolve the system into a distant companion B which is the hottest star in the system and the spectroscopic binary (A = Aa + Ab). A and B are separated by 0farcs16. We have extracted the spectra of both of these. We see no flux in the Aa + Ab spectrum which cannot be accounted for by the Cepheid, and put an upper limit on the spectral type and mass of the companion Ab of F5 V and <=1.4 M _sun. Using the orbit from HST fine guidance sensor measurements from Benedict et al., this results in an upper limit to the mass of the Cepheid of <=5.4 M _sun. We also discuss two possible distant companions. Based on the photometry from the Two Micron All Sky Survey Point Source Catalog, they are not physical companions of the W Sgr system.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA Contract NAS5-26555.