Discovery of photometric variability of a new low-amplitude multiperiodic DSCT variable star

ATel #12458; 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 31 Jan 2019; 08:34 UT
Credential Certification: Filippo Mannucci (filippo@arcetri.astro.it)

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We report the discovery of the photometric variability of a new low-amplitude multiperiodic DSCT variable star: we cross-identify it as 2MASS 06535374+1201175 = Gaia DR2 3351439848102117376. Its J2000 coordinates are: R.A. = 06h53m53.730s Decl. = +12d 01'17.54". 2MASS 06535374+1201175 has V = 14.72 from APASS; from our data its range is about 0.06 magnitudes. We discovered its variability by means of 908 unfiltered CCD frames taken in 10 nights in a time span of 25 days with the 0.35-m f/10 Schmidt-Cassegrain of OAMH. Differential CCD synthetic aperture photometry was performed using UCAC4 511-031855 and UCAC4 511-031826 as comparison stars. 2MASS 06535374+1201175 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 many OEJV. No periods were detected in 959 ASAS-SN data points due to their large measurement errors. We have analyzed our time series of 2MASS 06535374+1201175 with the software Period 04 (using Deeming's DFT + least squares), PerSea 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, DFT, AoV, Residualgram, PDM, and many more). With Period 04, ATSA and PerSea 2.6 we have found the same following frequencies and full peak-to-peak amplitudes : f1 = 14.1561 +/- 0.0009 c/d (namely P1 = 0.070641 +/- 0.000004 d) and A1 = 0.0186 +/- 0.0004 mag, f2 = 18.5089 +/- 0.0018 c/d and A2 = 0.0100 +/- 0.0004 mag, f3 = 13.2144 +/- 0.0018 c/d and A3 = 0.0101 +/- 0.0004 mag. More frequencies could be present, so in order to get more evidence we will collect new data to be analyzed in a future paper. The errors in frequencies, amplitudes and phases were derived by means of Monte-Carlo simulations with Period 04, and by means of analytical formulae with ATSA and they are in agreement. With ATSA the statistical significance of each of these frequencies was computed in a parametric way (with Scargle's periodogram) for f1 with p-value << 0.001, and in a non-parametric way for f2 and f3 by means of MC simulations (with Shrager's LPG3 periodogram), both with p-values < 0.001. The f2 and f3 were checked also with some algorithms of ATSA (Vanicek, Scargle, POP, PDM, DCDFT, Lafler & Kinman and more). The f2 and f3 frequencies were derived in two ways, obtaining the same results: first with Vanicek's algorithm iteratively assigning the previous frequency as known constituent, and then independently by iteratively performing a prewhitening for a frequency and re-analyzing the residuals. We have obtained very similar results with the prewhitening utility in Period 04 too. From APASS we get B-V=+0.539, and from NED based on Schlafly's maps we get Av=0.544; so if the total to selective extinction ratio is 3.1 we obtain E(B-V) = +0.175 from which we derive (B-V)o = +0.364 which suggests a spectral class around F2 IV. Given the distance d = 2.7 kpc from Gaia, and the above mentioned Av and V, then Mv = 2.02 can be derived which is compatible with spectral class from A7 V to F5 III, in agreement with F2 IV from (B-V)o colour. The light curve shape, periods, amplitudes and possible spectral class derived from (B-V)o are fully compatible with a low-amplitude multiperiodic DSCT-type variable star. The f1/f2 frequency ratio is 0.7648 +/- 0.0001, which is close to the canonical value of the period ratio of the radial fundamental and first overtone of a DSCT variable star. An alternative classification with f = (f1)/2 and EW light curve is ruled out by the intra-night variability that shows the seemingly irregular variations and fast light curve modulations which are typical of multiperiodic DSCT stars. Moreover a period P = 2/(f1) = 0.141 d would be under the short-period limit (0.21 - 0.22 d) of contact binaries.