Ultralow-amplitude RR Lyrae Stars in M4

Joshua J. Wallace, Joel D. Hartman, Gáspár A. Bakos, Waqas Bhatti

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6 Scopus citations


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, has amplitudes of only one part in a thousand, which is ∼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 day with 1.0 mmag amplitude, and 0.31673414 day 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.

Original languageEnglish (US)
Article numberL7
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Jan 1 2019

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • globular clusters: individual (M4)
  • stars: horizontal-branch
  • stars: individual (Gaia DR2 6045466571386703360, Gaia DR2 6045478558624847488)
  • stars: oscillations (including pulsations)
  • stars: peculiar (except chemically peculiar)
  • stars: variables: RR Lyrae


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