Researchers have presented the first long-term photometric variability survey of the intermediate-age open cluster NGC 559. Time-series V band photometric observations on 40 nights taken over more than three years with three different telescopes are analyzed to search for variable stars in the cluster. They investigated the data for the periodicity analysis and revealed 70 variable stars including 67 periodic variables in the target field, all of them are newly discovered.
Located at a distance of about 7,900 light years away from the Earth, NGC 559 is an open cluster estimated to be around 224 million years old. Previous observations of NGC 559 have identified 542 member stars and found that the cluster has a reddening at a level of 0.82 mag, and that its radius containing half the members is about 4.86 arcmin.
In the new study, study detected 70 new variable stars, out of which 67 are periodic variables with periodicities ranging from three hours to 41 days. The vast majority of the newfound periodic variables have periods below one day and most of them have relatively small amplitude of variability down to 0.02 mag level.
Their membership analysis of the periodic variables revealed that 30 of them belong to the cluster, with estimated masses between 1.72 and 3.6 solar masses, and remaining 37 are identified as field variables. Out of the 67 periodic variables, 48 are short-period (P<1 day) variables and 19 are long-period (P>1 day) variables. The variable stars have periodicity between 3 hours to 41 days and their brightness ranges from V = 10.9 to 19.3 mag.
The periodic variables belonging to the cluster are then classified into different variability types on the basis of observational properties such as shape of the light curves, periods, amplitudes, as well as their positions in the Hertzsprung-Russell (H-R) diagram. As a result, researchers identified one Algol type eclipsing binary, one possible blue straggler star, 3 slowly pulsating B type stars, 5 rotational variables, 11 non-pulsating variables, 2 FKCOM variables and remaining 7 are characterized as miscellaneous variables. They also identified three Eclipsing Binary stars (EBs) belonging to the field star population. The PHOEBE package is used to analyse the light curve of all four EBs in order to determine the parameters of the binary systems such as masses, temperatures and radii.
References: Yogesh C. Joshi, Ancy Anna John, Jayanand Maurya, Alaxendra Panchal, Brijesh Kumar, Santosh Joshi, “Variable stars in the field of intermediate-age open cluster NGC 559”, pp. 1-15, 2020. arXiv:2009.06997 [astro-ph.SR] arxiv.org/abs/2009.06997 link: https://arxiv.org/abs/2009.06997