Massive Star Multiplicity: The Cepheid W Sgr

TL;DR

This study used Hubble Space Telescope spectra to analyze the W Sgr system, identifying its components, constraining the companion's properties, and determining the Cepheid's upper mass limit, while also evaluating potential distant companions.

Contribution

First direct spectral resolution of the W Sgr system components, providing new constraints on companion properties and the Cepheid's mass.

Findings

No flux detected from the Ab companion, limiting its spectral type to F5 V and mass to ≤1.4 solar masses.

Upper mass limit for the Cepheid W Sgr is ≤5.4 solar masses based on orbital data.

Two potential distant companions are not physically associated with W Sgr.

Abstract

We have obtained spectra of the W Sgr system with the STIS spectrograph on the Hubble Space Telescope. 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 0.16". 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 $\leq$1.4Msun. Using the orbit from HST FGS measurements from Benedict, et al., this results in an upper limit to the mass of the Cepheid of $\leq$5.4Msun. We also discuss two possible distant companions. Based on photometry from the 2MASS Point Source Catalog, they are not physical companions of the W Sgr system.