CCD VI time-series of the extremely metal-poor globular cluster M92: revisiting its variable star population

Abstract

We present an analysis of VI CCD time-series photometry of the Oo II type globular cluster M92. The variable star population of the cluster is studied with the aim of revising their classifications, identifications, frequency spectra and to select indicators of the parental cluster metallicity and distance. The Fourier decomposition of RR Lyrae light curves lead to the estimation of mean [Fe/H] spec = −2.20 ± 0.18, and distance of 8.3 ± 0.2 kpc. Four new variables are reported; one RRd (V40), a multimode SX Phe (V41), a SR (V42) and one RRc (F1) that is most likely not a cluster member. The AC nature of V7 is confirmed. The double mode nature of the RRc star V11 is not confirmed and its amplitude modulations are most likely due to the Blazhko effect. Two modes are found in the known RRc variable V13. It is argued that the variable V30, previously classified as RRab is, in fact, an BL Her-type star not belonging to the cluster. Making use of the Gaia-DR2 proper motions, we identified 5012 stars in the field of the cluster, that are very likely cluster members, and for which we possess photometry, enabling the production of a refined Colour-Magnitude Diagram. This also allowed us to identify a few variable stars that do not belong to the cluster. The RR Lyrae pulsation modes on the HB are cleanly separated by the first overtone red edge, a common feature in all Oo II type clusters.

A New Visit to the Variable Stars in M56 and Its Colour-Magnitude Diagram Structure

“…Although the sample of RR Lyrae stars is small, the fact that the RRc and RRab stars are on the first overtone and fundamental sides of the first overtone blue edge (FOBE), is consistent with the mode distribution found in other OoII type clusters, e.g. Yepez et al (2020). A useful quantity to help describe the morphology of the HB is the Lee parameter (Lee 1990) defined as L = (B − R)/(B + V + R), where B is the number of stars on the blue side of the instability strip (IS) or the first overtone blue edge (FOBE), R is the number of stars on the red side of the IS or fundamental mode red edge (FMRE) and V is the number of variable stars within the IS.…”

A new visit to the variable stars in M56 and its colour magnitude diagram structure

“…Although the sample of RR Lyrae stars is small, the fact that the RRc and RRab stars are on the first overtone and fundamental sides of the First Overtone Blue Edge (FOBE), is consistent with the mode distribution found in other OoII type clusters, e.g. Yepez et al (2020). A useful quantity to help describe the morphology of the HB is the Lee parameter (Lee 1990) defined as…”

A new photometric study of M14 (NGC 6402): An interpretation of the Horizontal Branch and beyond

“…This enables to distinguish and confirm the variability of new variables found by the other methods. The above three approaches have been described in detail by Yepez et al (2020).…”

“…† E-mail: myepez@astro.unam.mx and evolutionary properties of the several families of variables are in the scope or if their implications of some of the system properties are pursued (e.g. Arellano Ferro et al (2020); Yepez et al (2020) and references therein).…”