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1

Sounding Interiors of Four Pulsating Subdwarf B Stars with Stellar Pulsations

Baran A., Sanjayan S.

Acta Astronomica

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2023

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Vol. 73, No. 1

21--34

EN

We present the results of asteroseismic modeling of four pulsating sdB stars KIC 2438324, KIC 2569576, KIC 2991403 and KIC 11159657. We used fully evolutionary models and this is the first attempt of seismic modeling of sdB stars using the MESA models and GYRE. We adopted an already reported list of periods identified with dipole modes. Additionally, we used two spectroscopic parameters, i.e., Teff and log g, and, only for KIC 2438324 and KIC 2569576, we used cluster parameters, i.e., total age and metallicity, to further filter the outcome of matching observed and theoretical pulsation periods. In the case of a spectroscopically constrained approach we derived a unique solution only for KIC 2438324. For two stars we derived unique input parameters except for the hydrogen and progenitor mass, respectively for KIC 2569576 and KIC 2991403. All input parameters except for the convective core and progenitor masses are unique for KIC 11179657. Except for KIC 11179657, our fits, measured by a relative period difference (ΔP/P), are well within 1%. These results sound very promising and more asteroseismic modeling of sdB stars should be done in future.

2

Variable Star Population in the Open Cluster NGC 6819 Observed by the Kepler Spacecraft

Sanjayan S., Baran A. S., Németh P., Kinemuchi K.

Acta Astronomica

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2022

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Vol. 72, No. 4

267--295

EN

We present the list of variable stars we found in the Kepler superstamp data covering approximately nine arcminutes from the central region of NGC 6819. This is a continuation of our earlier work. We classified the variable stars based on the variability type and we established their cluster membership based on the available Gaia Data Release 3 astrometry. Our search revealed 385 variable stars but only 128 were found to be cluster members. In the case of eclipsing and contact binaries we calculated the mid-times of eclipses and derived ephemerides. We searched for eclipse timing variation using the observed minus calculated diagrams. Only five objects show significant orbital period variation. We used isochrones calculated within the MESA Isochrones and Stellar Tracks project and derived the average age (2.54 Gyr), average distance (2.3 kpc) and iron content [Fe/H]=-0.01(2), of NGC 6819. We confirm this distance by the one derived from Gaia astrometry of the cluster members with membership probabilities greater than 0.9.

3

Variable Star Population in the Open Cluster NGC 6791 Observed by the Kepler Spacecraft

Sanjayan S., Baran A. S., Németh P., Kinemuchi K., Ostrowski J., Sahoo S. K.

Acta Astronomica

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2022

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Vol. 72, No. 2

77--102

EN

We present the list of variable stars we found in the Kepler superstamp data covering approximately 9' from the central region of NGC 6791. We classified the variable stars based on the variability type and we established their cluster membership based on the available Gaia Early Data Release 3 astrometry, by means of the Bayesian Gaussian mixture models. In total we found 278 variable objects, among which 17 binaries, 45 pulsators, 62 rotational and five unclassified variables are cluster members. The remaining 28 binaries, 25 pulsators, 83 rotational, four unclassified and nine unidentified variables are either not members or their membership is not established. In the case of eclipsing binaries we calculated the mid-times of eclipses and derived ephemerides. We searched for eclipse timing variation by means of the observed minus calculated diagrams. Only three objects show significant orbital period variation. Independently of a recently published report, we found 119 new variables. We used isochrones calculated within the MIST project and derived the age (8.91 Gyr), average distance (4134 pc) and iron content [Fe/H] (0.26-0.28), of NGC 6791. Using the cluster members with membership probabilities greater than 0.9, we calculated the distance to the cluster of 4123(31) pc, which agrees with the result from our isochrone fitting.