TY - JOUR
T1 - Ultralow-amplitude RR Lyrae Stars in M4
AU - Wallace, Joshua J.
AU - Hartman, Joel D.
AU - Bakos, Gáspár A.
AU - Bhatti, Waqas
N1 - Funding Information:
We thank A. Vanderburg for assistance with the roll decorrelation and W. Pych for providing CASE light curves that were useful in our initial vetting. G.Á.B. thanks J. Jurcsik, G. Kovács, and L. Molnár for useful discussions while at Konkoly Observatory. This research includes data collected by the K2 mission, funding for which is provided by the NASA Science Mission directorate. The K2 data were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-Hubble Space Telescope data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts. This research includes data from the European Space Agency (ESA) mission Gaia(https:// www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC,https://www. cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of the SIMBAD database (operated at CDS, Strasbourg, France), NASA’s Astrophysics Data System Bibliographic Services, and observations from the LCOGT network.
Publisher Copyright:
© 2019. The American Astronomical Society. All rights reserved..
PY - 2019/1/1
Y1 - 2019/1/1
N2 - 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.
AB - 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.
KW - globular clusters: individual (M4)
KW - stars: horizontal-branch
KW - stars: individual (Gaia DR2 6045466571386703360, Gaia DR2 6045478558624847488)
KW - stars: oscillations (including pulsations)
KW - stars: peculiar (except chemically peculiar)
KW - stars: variables: RR Lyrae
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U2 - 10.3847/2041-8213/aaf8ac
DO - 10.3847/2041-8213/aaf8ac
M3 - Article
AN - SCOPUS:85059855366
SN - 2041-8205
VL - 870
JO - Astrophysical Journal Letters
JF - Astrophysical Journal Letters
IS - 1
M1 - L7
ER -