Discovery of photometric variability of a new EA-type eclipsing binary star with sharp minima

ATel #14063; Montigiani N. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy), Santangelo M. M.M. (OAC - Osservatorio Astronomico di Capannori, Lucca, Italy), Mannucci M. (OAMH - Osservatorio Astronomico Margherita Hack, Firenze, Italy)
on 5 Oct 2020; 06:57 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

Tweet

We report the discovery of the photometric variability of a new EA-type variable star with sharp minima, lying only 21 arcseconds from the EW variable ZTF J190344.60-045333.1. We cross-identify the new EA as USNO B1.0 0851-0471938 = Gaia DR2 4254427889968886912 = 2MASS 19034362-0453477. USNO B1.0 0851-0471938 has J2000.0 coordinates (from Gaia DR2) R.A. = 19h03m43.619s Decl. = -04d53p47.73s; from our data in Rc band its mean magnitude is 15.74 and its range is about 0.25 magnitudes. Coefficients of the colour equations were derived in a night in the same season using Landolt's equatorial standards. We discovered its photometric variability using 1220 Rc band CCD frames taken in 23 nights in a time span of 61 days with a SBIG ST-10XME CCD camera at the 0.35-m f/10 Schmidt-Cassegrain of OAMH. Differential CCD synthetic aperture photometry of USNO B1.0 0851-0471938 was performed using USNO B1.0 0851-0472028, 0850-0487927, and 0850-0488885 as comparison and check stars. USNO B1.0 0851-0471938 is not listed as a variable star in the following catalogs and surveys: GCVS, NSV, AAVSO-VSX, ASAS-SN, Atlas, ZTF, CSDR2(=CRTS+CSS), Linear, PanSTARRS1 DR2, Gaia DR2, NSVS, SDSS, ASAS, TASS, MOTESS-GNAT, SuperWASP, and on IBVS from 5500 to 6300 and on OEJV from 2013. We have analyzed our time series of this new EA variable with the software PerSea 2.7, ATSA (Santangelo et al., 2007, AN 328, 55), and Period 04. With ATSA by means of PDM, AoV phase binning method, POP = Periodic Orthogonal Polynomials known also as multiharmonic ANOVA, Scargle, residualgram, Vanicek, and many more algorithms and with PerSea (with POP and AoVTr algorithms) applied to our photometric data of USNO B1.0 0851-0471938 we have clearly detected the following period P = 6.2267 +/- 0.0002 d which corresponds to the frequency f = 0.160600 c/d. Actually, most of these algorithms detect its 2f harmonic 0.321200 c/d (with p-value < 0.001 using Scargle algorithm), but this is a common situation when analyzing EA light curves. The different algorithms and software used are in very good agreement with each other. There is a chance that 0.321200 c/d could be the correct frequency of an EA without a secondary minimum (like the EA star NY Cep) but we regard this as a less likely option. We barely detected a period of 6.2254 +/- 0.0029 d (f = 0.16063 c/d) in 356 ZTF data of USNO B1.0 0851-0471938; actually here too we mostly detected its harmonic 2f with PerSea 2.7 using the POP algorithm and with ATSA using the PDM, L-Davies, POP, Penfold, SVM, and many more algorithms. Probably this EA star was not classified as a variable by ZTF due to the very small number of ZTF data in its minima. For USNO B1.0 0851-0471938, from Gaia we get d = 1.32 kpc, from APASS DR10 we get (B-V) = +1.366, and from NED extinction calculator we get Av = 1.786; so if the total to selective extinction ratio is 3.1 we obtain E(B-V) = +0.576 from which we derive (B-V)o = +0.790 which would be compatible with a spectral class around G9 V or G2 III. From Pan-STARRS1 DR2 we get (g-r) = +0.944, and from Bayestar2019 extinction calculator we get E(g-r) = +0.41 (for d = 1.32 kpc); so we derive (g-r)o = +0.534 which would be compatible with a spectral class around G7 V or G1 III. For USNO B1.0 0851-0471938 the light curve shape, period, amplitude and possible spectral classes derived from (B-V)o and (g-r)o are clearly compatible with an EA-type eclipsing binary star with sharp minima. If we call Delta(phi)min1 the phase span (width) of the primary minimum and Delta(phi)min2 the phase span of the secondary minimum in the phase diagram, then we get the following values for USNO B1.0 0851-0471938: Delta(phi)min1 = Delta(phi)min2 = 0.030. We have made a (not complete) search in the scientific literature among a few hundreds of EA stars for light curves with Delta(phi)min 1 and 2 sharper than 0.030, and we have found very few such variables; namely only slightly less than 5 percent of this sample of EA.