New Variable Stars in Open Cluster Berkeley 59

A. A. Popov^#1, A. M. Zubareva^#2,3

#1. Kourovka Astronomical Observatory of Ural Federal University, Yekaterinburg, Russia; #2. Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia; #3. Sternberg Astronomical Institute, Lomonosov Moscow State University, Moscow, Russia.

Received:   30.09.2022;   accepted:   22.12.2022
(E-mail for contact: apopov66@gmail.com)

Comments:


2. We observed only one minimum. HJDmin = 2459677.195.

5. MinII = 12^m.39 R.

9. MinII = 13^m.49: R. Eccentric orbit. MinII–MinI = 0.33 P . Period 3^d.604 is possible.

14. MinII = 14^m.96 R. D = 0.11 P.

17. We observed only one minimum. HJDmin = 2459495.36.

19. MinII = 16^m.48 R. The star is listed in the ZTF catalog of periodic variable stars (Chen et al. 2020) as EW.

20. MinII = 11^m.21 R. D = 0.09 P.

Remarks:
Kourovka Planet Search (KPS, Burdanov et al. 2016; Burdanov et al. 2018) is a project aimed at finding new transiting exoplanets using the Master-II-URAL telescope. Our pilot observations were obtained during short and bright summer nights of 2012 at the Kourovka Astronomical Observatory of the Ural Federal University. In 2017, the project was renamed as the Galactic Plane eXoplanet Survey (GPX, Benni 2017). We observed the field of the Open Cluster (OC) Berkeley 59 centered at α = 00h 02m 14.0s, δ = +67d 25m 00s (J2000.0).

The Master-II-URAL consists of two parallel optical telescopes (40-cm aperture, 1:2.5 focal ratio) installed on the same mount and equipped with two Peltier-cooled Apogee Alta U16M CCD cameras. The image scale is 1.85''/px. Observations can be performed simultaneously in two filters of the Johnson– Cousins BVRI photometric system (Lipunov et al. 2010). All observations with the MASTER- II-Ural telescope were made in automatic mode. Prior to each observation night, dark frames were obtained with the necessary CCD temperature; under good weather conditions, every morning the telescope obtained twilight sky flat-field frames in the appropriate filter.

Observations of the OC Berkeley 59 were carried out during October 2021 – April 2022 for 44 nights. 5000 fits files were obtained in R and V bands with 50-second exposures.

Astrometric reductions of all frames were performed using the Astrometry.net application (Lang et al. 2010). All objects were identified using 2MASS catalogue (Skrutskie et al. 2006). Initial photometric reductions and aperture photometry were performed in the IRAF package (Tody 1986). We used the Astrokit console application (Burdanov et al. 2014) for data post-processing. The program performs high- precision differential CCD photometry for thousands of stars and uses Robust Median Statistic criterion (Rose & Hintz 2007) to search for variable-star candidates. The photometric precision for stars from 11 to 16.5 mag was 0.008–0.08 mag, and 0.006–0.09 mag for V and R bands respectively.

From our initial sample of 5000 stars, we selected 39 variable objects whose light curves were inspected by eye. Variability has not been previously reported for 20 out of 39 objects in the international variable star index (VSX) database. To determine periods of variability, we used the light curve analysis tool by Kirill Sokolovsky. This application implements Lafler & Kinman (1965) and Deeming (1975) methods to search for periods as well as transforms Julian Dates to Heliocentric Julian Dates.

In this paper, we provide figures that consist of two panels. Star's instrumental magnitude as a function of Heliocentric Julian Date is given in the left panel and phase folded light curve is given in the right panel. When we could not define a period, we provide only the light curve as a function of Heliocentric Julian Date. In the figures, we used red color for R band, green color for V band data. For the sake of visibility, we shifted stars' magnitudes in V band by (V-R) value. Color indices are provided on top of each figure.

Acknowledgements:

This work was supported by Ministry of Science and Higher Education of the Russian Federation under the grant 075-15-2020-780.

The authors wish to thank Dr. Kirill Sokolovsky for providing his online light curve analysis tool.

This research made use of Aladin (Bonnarel et al. 2000), SIMBAD database (operated at the Centre de Données astronomiques de Strasbourg), the International Variable Star Index (VSX) database (operated at AAVSO, Massachusetts, USA).

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