Hubble Space Telescope Snapshot Survey for Resolved Companions of Galactic Cepheids: Final Results

TL;DR

This study used Hubble Space Telescope imaging to identify companions of 70 galactic Cepheids, revealing a high incidence of inner binaries linked to outer companions, suggesting complex dynamical formation scenarios for these systems.

Contribution

First comprehensive HST survey of Cepheid companions within 5 arcseconds, linking inner binary presence to outer companions and proposing dynamical interaction models for system formation.

Findings

16 probable companions identified among 70 Cepheids

Inner binaries are more common in systems with close companions

Companion colors change at around 2000 AU separation

Abstract

Cepheids in multiple systems provide information on the outcome of the formation of massive stars. They can also lead to exotic end-stage objects. This study concludes our survey of 70 galactic Cepheids using the {\it Hubble Space Telescope\} (\HST) Wide Field Camera~3 (WFC3) with images at two wavelengths to identify companions closer than $5\arcsec$. In the entire WFC3 survey we identify 16 probable companions for 13 Cepheids. The seven Cepheids having resolved candidate companions within $2"$ all have the surprising property of themselves being spectroscopic binaries (as compared with a 29\% incidence of spectroscopic binaries in the general Cepheid population). That is a strong suggestion that an inner binary is linked to the scenario of a third companion within a few hundred~AU\null. This characteristic is continued for more widely separated companions. Under a model where the outer companion is formed first, it is unlikely that it can anticipate a subsequent inner binary. Rather it is more likely that a triple system has undergone dynamical interaction, resulting in one star moving outward to its current location. {\it Chandra\} and {\it Gaia\} data as well as radial velocities and \HSTSTIS and {\it IUE\} spectra are used to derive properties of the components of the Cepheid systems. The colors of the companion candidates show a change in distribution at approximately 2000~AU separations, from a range including both hot and cool colors for closer companions, to only low-mass companions for wider separations.