Discovery of photometric variability of a new variable star with some peculiarities

ATel #13392; Mannucci M. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy), Santangelo M. M.M. (OAC - Osservatorio Astronomico di Capannori, Lucca, Italy), Montigiani N. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy)
on 9 Jan 2020; 08:46 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

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We report the discovery of the photometric variability of a new variable star with some peculiarities in the light curve: we cross-identify it as USNO A2.0 1125-17013138 = USNO B1.0 1143-0498670 = 2MASS 20465351+2422543. Its J2000 coordinates are: R.A. = 20h 46m 53.52s Decl. = +24d 22'54.3"; it is listed as member of the open cluster Alessi 12. Its g magnitude from PANSTARRS DR2 is 16.033 and from our data its range is about 0.16 magnitudes. We discovered its photometric variability by means of 592 unfiltered CCD frames taken in 7 nights in a time span of 50 days in September-October 2019 with the 0.35-m f/10 Schmidt-Cassegrain of OAMH. Differential CCD synthetic aperture photometry was performed using USNO B1.0 1143-0498677, 0498603, 0498730, 0498842 and 1144-0502896 as comparison stars. USNO A2.0 1125-17013138 is not listed as variable star in the following catalogs and surveys: GCVS, NSV, AAVSO-VSX, ASAS-SN, Catalina, Linear, PanSTARRS DR2, NSVS, SDSS, ASAS3, TASS, MOTESS-GNAT, SuperWASP, and on IBVS from 5500 to 6200 and on OEJV, PZ and PZP of the last years. ATLAS survey lists it as dubious variable. We have analyzed our time series of USNO A2.0 1125-17013138 with the software Period 04 (using Deeming's DFT + least squares), PerSea 2.7 and 2.6 (using Schwarzenberg-Czerny's POP-Periodic Orthogonal Polynomials sometimes referred to as Multi-Harmonic ANOVA) and ATSA (Santangelo et al., 2007, AN 328, 55) using many algorithms (Scargle, Vanicek, POP, DCDFT, AoV, Residualgram, PDM, Penfold and more). With Period 04, ATSA and PerSea we have found the same following period P1 = 0.4179 +/- 0.0001 d (namely f1 = 2.393 c/d) and full peak-to-peak amplitude A1 = 0.151 +/- 0.001 mag. However our photometric data can be very well fitted by a 2*P1 = 0.8358 d periodicity too. Prewhitening for the above mentioned frequency f1 lead to no other significant periodicities. With ATSA the statistical significance of this f1 frequency was computed in a parametric way (with Scargle's periodogram) with p-value << 0.001, and in a non-parametric way by means of Monte-Carlo simulations (with Shrager's LPG3 periodogram), with p-value < 0.001. Our period P1 or 2*P1 is confirmed from a time series analysis of the photometric data from ZTF DR2 too. USNO A2.0 1125-17013138 was not measured in APASS and is not in the SDSS footprint, so its V magnitude and spectral class are unknown. From the parallax measured by Gaia a distance d = 0.55 kpc can be derived. From 2MASS JHK magnitudes we get J-H = +0.557 and H-K = +0.163. From the 3D dust extinction map of PANSTARRS (Bayestar 2019) and IRSA NASA/IPAC extinction tables and maps we derived E(g-r)=+0.12 and E(B-V)=+0.163 (at d = 0.55 kpc) and from PANSTARRS DR2 we get (g-r)=+0.776. So, using equations from the ADPS, we get the following colour indexes corrected for the interstellar reddening (g-r)o=+0.656 and (J-H)o=+0.504 and (H-Ks)o=+0.133. From E(B-V)=+0.163 a value of Av=+0.507 can be derived. Moreover g and r can be transformed in B and V getting (B-V)=+1.040. So (B-V)o=+0.877. All these intrinsic colour indexes lead to a spectral class around K1 V. Given the distance d = 0.55 kpc from Gaia, and the above mentioned Av and B and V derived from g and r, then Mv around +6 can be derived which again is compatible with spectral class around early K dwarf. Although the light curve folded with P1 = 0.4179 d resembles that of an RRc variable star with a bump around phase 0.45, the distance given by Gaia and Av given as above lead to an absolute magnitude and to a possible spectral class incompatible with an RRc. So plausible classifications with period 2*P1 are: an EW with inclination of about 40-50 degrees and some scatter just before the secondary minimum, or otherwise an ELL variable. Plausible classifications with period P1 are: an RS with a bump at phase 0.45 or a BYDra-type variable star. This small scatter or bump was more visible in October 2019, so it may be transient.