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

Abstract

We have conducted an imaging survey with the Hubble Space Telescope Wide Field Camera~3 (WFC3) of 70 Galactic Cepheids, typically within 1~kpc, with the aim of finding resolved physical companions. The WFC3 field typically covers the 0.1 pc area where companions are expected. In this paper, we identify 39 Cepheids having candidate companions, based on their positions in color--magnitude diagrams, and having separations $\ge$5$"$ from the Cepheids. We use follow-up observations of 14 of these candidates with XMM-Newton, and of one of them with ROSAT, to separate X-ray-active young stars (probable physical companions) from field stars (chance alignments). Our preliminary estimate, based on the optical and X-ray observations, is that only 3\% of the Cepheids in the sample have wide companions. Our survey easily detects resolved main-sequence companions as faint as spectral type K\null. Thus the fact that the two most probable companions (those of FF~Aql and RV~Sco) are earlier than type K is not simply a function of the detection limit. We find no physical companions having separations larger than 4,000~AU in the X-ray survey. Two Cepheids are exceptions in that they do have young companions at significantly larger separations ($\delta$~Cep and S~Nor), but both belong to a cluster or a loose association, so our working model is that they are not gravitationally bound binary members, but rather cluster/association members. All of these properties provide constraints on both star formation and subsequent dynamical evolution. The low frequency of true physical companions at separations $>5"$ is confirmed by examination of the subset of the nearest Cepheids and also the density of the fields.

arXiv commentAccepted for publication in AJ

References in corpus (23)

• PARSEC: stellar tracks and isochrones with the PAdova and TRieste Stellar Evolution Code
• Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-Main Sequence Stars
• Binary interaction dominates the evolution of massive stars
• A Survey of Stellar Families: Multiplicity of Solar-Type Stars
• The Dartmouth Stellar Evolution Database
• A Hipparcos census of the nearby OB associations
• Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
• Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations
• From binaries to multiples II: hierarchical multiplicity of F and G dwarfs
• Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator delta Cephei
• Direct Detection of the Close Companion of Polaris with the Hubble Space Telescope
• Cepheid Parallaxes and the Hubble Constant
• Formation of the Widest Binaries from Dynamical Unfolding of Triple Systems
• The formation of very wide binaries during the star cluster dissolution phase
• The MACHO Project LMC Variable Star Inventory: I. Beat Cepheids - Conclusive Evidence for the Excitation of the Second Overtone in Classical Cepheids
• Cepheids in Open Clusters: An 8-D All-sky Census
• Binary Cepheids: Separations and Mass Ratios in $5M_{\odot }$ Binaries
• The Search for Low-mass Companions of B Stars in the Carina Nebula Cluster Trumpler 16
• Binary Properties from Cepheid Radial Velocities (CRaV)
• A search for open cluster Cepheids in the Galactic plane
• Massive Star Multiplicity: The Cepheid W Sgr
• V440 Per: the longest period overtone Cepheid
• X-ray Detection of the Cluster Containing the Cepheid S Mus

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• On Cepheid Distance Scale Bias due to Stellar Companions and Cluster Populations
• Vetting Galactic Leavitt Law Calibrators using Radial Velocities: On the Variability, Binarity, and Possible Parallax Error of 19 Long-period Cepheids
• Study of changes in the pulsation period of 148 Galactic Cepheid variables
• Homing in on Polaris: A 7 M${}_{\odot }$ first-overtone Cepheid entering the instability strip for the first time
• On the dynamical evolution of Cepheid multiplicity in star clusters and its implications for B-star multiplicity at birth
• X-rays in Cepheids: XMM-Newton Observations of $\eta$ Aql
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