Resolved Companions of Cepheids: Testing the Candidates with X-Ray Observations

TL;DR

This study used X-ray observations to test whether candidate resolved companions of Cepheids are young stars, confirming only one as a likely physical companion or cluster member, and suggesting some X-ray emissions originate from the Cepheids themselves.

Contribution

First X-ray survey of candidate Cepheid companions, providing a method to confirm physical association and distinguishing between cluster members and true companions.

Findings

Most candidate companions are not X-ray sources, indicating they are not young stars.

One candidate companion (S Nor #4) is likely a cluster member, not a bound companion.

X-ray emissions in some cases originate from the Cepheids themselves or close unresolved companions.

Abstract

We have made {\it XMM-Newton\/} observations of 14 Galactic Cepheids that have candidate resolved ($\geq$5$\arcsec$) companion stars based on our earlier {\it HST\/} WFC3 imaging survey. Main-sequence stars that are young enough to be physical companions of Cepheids are expected to be strong X-ray producers in contrast to field stars. {\it XMM-Newton\/} exposures were set to detect essentially all companions hotter than spectral type M0 (corresponding to 0.5 $ M_\odot$.) The large majority of our candidate companions were not detected in X-rays, and hence are not confirmed as young companions. One resolved candidate (S~Nor \#4) was unambiguously detected, but the Cepheid is a member of a populous cluster. For this reason, it is likely that S~Nor \#4 is a cluster member rather than a gravitationally bound companion. Two further Cepheids (S~Mus and R~Cru) have X-ray emission that might be produced by either the Cepheid or the candidate resolved companion. A subsequent {\it Chandra} observation of S Mus shows that the X-rays are at the location of the Cepheid/spectroscopic binary. R Cru and also V659 Cen (also X-ray bright) have possible companions closer than 5$\arcsec$ (the limit for this study) which are the likely source of X-rays. One final X-ray detection (V473 Lyr) has no known optical companion, so the prime suspect is the Cepheid itself. It is a unique Cepheid with a variable amplitude.