RESOLVED COMPANIONS of CEPHEIDS: TESTING the CANDIDATES with X-RAY OBSERVATIONS

Nancy Remage Evans, Ignazio Pillitteri, Scott Wolk, Margarita Karovska, Evan Tingle, Edward Guinan, Scott Engle, Howard E. Bond, Gail H. Schaefer, Brian D. Mason

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Abstract

We have made XMM-Newton observations of 14 Galactic Cepheids that have candidate resolved (≥5″) companion stars based on our earlier HST Wide Field Camera 3 (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. XMM-Newton exposures were set to detect essentially all companions hotter than spectral type M0 (corresponding to 0.5 M). 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 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″ (the limit for this study) which are the likely sources 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. The 14 stars that we observed with XMM constitute 36% of the 39 Cepheids found to have candidate companions in our HST/WFC3 optical survey. No young probable binary companions were found with separations of ≥5″ or 4000 au.

Original languageEnglish (US)
Article number108
JournalAstronomical Journal
Volume151
Issue number4
DOIs
StatePublished - Apr 2016

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Evans, N. R., Pillitteri, I., Wolk, S., Karovska, M., Tingle, E., Guinan, E., Engle, S., Bond, H. E., Schaefer, G. H., & Mason, B. D. (2016). RESOLVED COMPANIONS of CEPHEIDS: TESTING the CANDIDATES with X-RAY OBSERVATIONS. Astronomical Journal, 151(4), [108]. https://doi.org/10.3847/0004-6256/151/4/108