X-ray Detection of the Cluster Containing the Cepheid S Mus

Abstract

The galactic Cepheid S Muscae has recently been added to the important list of Cepheids linked to open clusters, in this case the sparse young cluster ASCC 69. Low-mass members of a young cluster are expected to have rapid rotation and X-ray activity, making X-ray emission an excellent way to discriminate them from old field stars. We have made an XMM-Newton observation centered on S Mus and identified (Table 1) a population of X-ray sources whose near-IR 2MASS counterparts lie at locations in the J, (J-K) color-magnitude diagram consistent with cluster membership at the distance of S Mus. Their median energy and X-ray luminosity are consistent with young cluster members as distinct from field stars. These strengthen the association of S Mus with the young cluster, making it a potential Leavitt Law (Period-Luminosity relation) calibrator.

arXiv comment7 pages, 1 figure. Accepted for publication on ApJL

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• A 0.9% Calibration of the Galactic Cepheid luminosity scale based on Gaia DR3 data of open clusters and Cepheids
• Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 -- II. Resolved common proper motion pairs
• Hubble Space Telescope: Snapshot Survey for Resolved Companions of Galactic Cepheids
• X-rays in Cepheids: XMM-Newton Observations of $\eta$ Aql