Ultralow-amplitude RR Lyrae Stars in M4

Abstract

We report evidence for a new class of variable star, which we dub millimagnitude RR Lyrae (mmRR). From K2 observations of the globular cluster M4, we find that out of 24 horizontal branch stars not previously known to be RR Lyrae variables, two show photometric variability with periods and shapes consistent with those of first overtone RR Lyrae variables. The variability of these two stars, however, have amplitudes of only one part in a thousand, which is $\sim$200 times smaller than for any RR Lyrae variable in the cluster, and much smaller than any known RR Lyrae variable generally. The periods and amplitudes are: 0.33190704 d with 1.0 mmag amplitude and 0.31673414 d with 0.3 mmag amplitude. The stars lie just outside the instability strip, one blueward and one redward. The star redward of the instability strip also exhibits significant multi-periodic variability at lower frequencies. We examine potential blend scenarios and argue that they are all either physically implausible or highly improbable. Stars such as these are likely to shed valuable light on many aspects of stellar physics, including the mechanism(s) that set amplitudes of RR Lyrae variables.

8 pages, 4 figures, 1 table; accepted to ApJ Letters

References in corpus (10)

• The Gaia mission
• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• The K2 Mission: Characterization and Early results
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• A Technique for Extracting Highly Precise Photometry for the Two-Wheeled Kepler Mission
• Measuring the rotation period distribution of field M-dwarfs with Kepler
• Gaia Data Release 1: Catalogue validation
• Gaia data release 1, the photometric data
• Image Subtraction Reduction of Open Clusters M35 & NGC 2158 In The K2 Campaign-0 Super-Stamp

Cited by in corpus (1)

• A Catalog of Periodic Variables in Open Clusters M35 and NGC 2158