A new study of the variable star population in the Hercules globular cluster (M13; NGC 6205)★

Abstract

We present the results from V I CCD time-series photometry of the globular cluster M13 (NGC 6205). From the Fourier decomposition of the light curves of RRab and RRc stars we found an average metallicity of [Fe/H]zw = -1.58 ± 0.09. The distance to the cluster was estimated as 7.1 ± 0.1 kpc from independent methods related to the variable star families RR Lyrae, SX Phe and W Virginis, from the luminosity of the theoretical ZAHB and from the orbit solution of a newly discovered contact binary star. The RR Lyrae pulsation modes are segregated by the red edge of the first overtone instability strip in this OoII type cluster. A membership analysis of 52,800 stars in the field of the cluster is presented based on Gaia-DR2 proper motions which enabled the recognition of 23,070 likely cluster members, for 7,630 of which we possess V I photometry. The identification of member stars allowed the construction of a clean CMD and a proper ZAHB and isochrone fitting, consistent with a reddening, age and distance of 0.02 mag, 12.6 Gyrs and 7.1 kpc respectively. We report seven new variables; one RRc, two SX Phe stars, three SR and one contact binary. V31 presents double-mode nature and we confirm V36 as RRd. Fifteen variable star candidates are also reported. The analysis of eighteen stars in the field of the cluster, reported as RR Lyrae from the Gaia-DR2 data base reveals that at least seven are not variable. We noted the presence of a high velocity star in the field of the cluster.

Axion and neutrino bounds improved with new calibrations of the tip of the red-giant branch using geometric distance determinations

“…One obvious concern is if the brightest star in the list of V13 is indeed on the RGB and not an AGB contamination. In this context, we notice that stars on the upper RGB or AGB tend to be long-period variables with periods of 30-100 days [41][42][43][44][45][46]. For M5, this question was studied explicitly by Wehrung and Layden [47].…”

Dwarfs or Giants? Stellar Metallicities and Distances from ugrizG Multiband Photometry

“…Predicted mean distances and metallicities for dwarfs and giants in the 4 globular clusters in Figure 18 as derived by our algorithm, compared to literature values for the clusters. Literature values from: Harris (2010) (1), Deras et al (2019) (2) and Hernitschek et al (2019) 5272 and 5466, respectively. This removes the (significant) effect of crowding on the input photometry.…”

NGC 1261: A Time-Series Vi Study of Its Variable Stars

“…This approach has been regularly used for RR Lyrae stars in a large number of clusters (e.g., Arellano Ferro et al (2018a); Deras et al (2019) and references therein). To avoid a lengthy repetition, we refer the reader to § 4 of Deras et al (2019) for the specific calibrations.…”

“…This approach has been regularly used for RR Lyrae stars in a large number of clusters (e.g., Arellano Ferro et al (2018a); Deras et al (2019) and references therein). To avoid a lengthy repetition, we refer the reader to § 4 of Deras et al (2019) for the specific calibrations. Particularly, their equations 4 and 5 describe the calibrations for RRab stars of Jurcsik & Kovács (1996) and Kovács & Walker (2001), which render values of [Fe/H] and M V , with standard deviations of 0.14 dex and 0.04 mag respec-tively, and to equations 6 and 7, that give the calibrations for RRc stars of Morgan et al (2007) and Kovács & Kanbur (1998) with standard deviations of 0.14 dex and 0.042 mag respectively.…”

“…Other Oo I clusters with a similarly red HB where the RRc-RRab distribution is yet to be explored are: NGC 362, Rup 106 and NGC 6712. In Rup 106, however, no RRc stars are known, and NGC 6712 is an interesting borderline case, since either a few RRab are sitting in the either-or region, or the FORE is shifted to the red by only about 0.04 mag (Deras 2019; private communication), which is the uncertainty in the location of the FORE. We have found that in all Oo II clusters the modes are always segregated.…”