J/A+A/703/A286 KIC 2831097 times of maximum brightness (Poretti+, 2025)
The elusive chase of the first RR Lyr star in a binary system:
the case of KIC 2831097.
Poretti E., Le Borgne J.F., Correa M., Sodor A., Rainer M., Audejean M.,
Denoux E., Esseiva N., Faine J., Fumagalli F., Naves R., Klotz A.
<Astron. Astrophys. 703, A286 (2025)>
=2025A&A...703A.286P 2025A&A...703A.286P (SIMBAD/NED BibCode)
ADC_Keywords: Stars, variable ; Photometry, CCD ; Optical
Keywords: stars: distances - stars: general -
stars: individual: KIC 2831097 - stars: individual: Kepler-1601 -
stars: Population II - stars: distances
Abstract:
The lack of RR Lyr stars in binary systems is an atypical fact when
looking at other classes of variables. Therefore, the detection of an
RR Lyr variable in a binary system has become a challenge for
observers. The RR Lyr variable KIC 2831097 was one of the most
promising candidates.
Indeed, the phases of the maximum brightness in the Kepler photometry
showed a regular variation superimposed to a parabolic trend. These
variations in the times of maximum brightness were interpreted as a
possible light-time travel effect (LTTE) in a wide binary and a fast
evolutionary change in the period.
We planned two spectroscopic runs with the FIES instrument mounted at
the Nordic Optical Telescope to test the binarity hypothesis.
The observations were programmed at the predicted quadratures of the
orbit, when the two mean radial velocities should differ by about
100km/s. The GEOS collaboration complemented the spectroscopic survey
by a photometric one. We also analysed Gaia time series and intensive
TESS photometry. The radial velocity curves obtained at the
quadratures show the same mean radial velocity (-203km/s), ruling out
the possibility of a LTTE. The constant mean radial velocity, the
[Fe/H] content determined from ground-based CCD observations and the
astrometric parameters supplied by Gaia allow us to infer that KIC
2831097 is a single, high-velocity, metal-poor RRc star belonging to
the galactic halo.
We revisited Kepler photometry, detecting a weak Blazhko effect
consisting in oscillation of only 1.1% of the period, in about 50 d.
We also analysed the TESS photometry of Kepler-1601, whose photometry
is contaminated by KIC 2831097. We collected 116 maxima, from the
ground-based campaign, 103 maxima from TESS, and 4 maxima from Gaia.
In total, we have 3624 times of maximum brightness. Linear ephemerides
cannot fit the whole dataset, but only parts of them. The period shows
a tendency to decrease in value, as if it were an evolutionary effect,
but not at a constant rate.
The spectroscopic and photometric campaigns performed in the framework
of a professional-amateur project failed to confirm that KIC 2831097
belongs to a binary system: the chase remains open. As an RR Lyr
variable, KIC 2831097 shows an intriguing evolution of the period.
Description:
Heliocentric times of maximum brightness of KIC 2831097 are listed.
They have been observed by Kepler, GEOS collaboration (ground-based
CCD network), Gaia and TESS. In total, 3624 times of maximum
brightness. They are listed giving uncertainties, filter, and
observer.
Objects:
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RA (2000) DE Designation(s)
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19 02 06.08 +38 04 40.8 KIC 2831097 = 2MASS J19020609+3804407
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 49 3624 Times of maximum brightness
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 12 F12.4 d Tmax Observed time of maximum brightness (HJD)
16- 21 F6.4 d e_Tmax Uncertainty on Tmax
25- 30 A6 --- Filter Filter used (G BVR gr clean)
33- 49 A17 --- Obs Observer
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Acknowledgements:
Ennio Poretti, ennio.poretti(at)inaf.it
(End) Ennio Poretti [INAF-OA Brera, Italia], Patricia Vannier [CDS] 05-Sep-2025