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1

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.

2

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.

3

A Detection Threshold in the Amplitude Spectra Calculated from TESS Time-Series Data

Baran A. S., Koen C.

Acta Astronomica

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2021

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Vol. 71, No. 2

113--121

EN

We present results of time-series data simulation. We aimed at estimating the threshold used for detecting signals in amplitude spectra, calculated from simulating TESS photometry of up to one year duration. We selected the threshold at a false alarm probability FAP=0.1% and derived S/N ratios between 4.6 and 5.7 depending on the data cadence and coverage. We also provide a formula to estimate the threshold for any FAP adopted and a given number of data points. Our result confirms that, to avoid spurious detection, space-based photometry may require substantially higher S/N than that typically being employed for ground-based data.

4

Mode Identification in a Pulsating Subdwarf B Star EPIC 212707862 Observed with K2

Bachulski S., Baran A. S., Jeffery C. S., Østensen R. H., Reed M. D., Telting J. H., Kuutma T.

Acta Astronomica

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2016

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Vol. 66, No. 4

455--467

EN

We present an analysis of K2 observations of EPIC 212707862, a pulsating subdwarf B star. We detected 13 significant frequencies from an 81 day run during Campaign 6. Our goal was to find features that could help us to identify pulsation modes. We could not identify any multiplets, which would also have enabled us to precisely derive a rotation period. Based on amplitude modulation we estimated that period to be around 80 days. We found two period-spacing sequences, and successfully identified modal degrees for 11 out of 13 detected frequencies. We assigned six of them to l=1, and another five to l=2. These results will facilitate future theoretical modeling. This star brings to 19 the number of pulsating subdwarf B stars observed with K2. Radial velocities obtained to date and the spectral energy distribution are consistent with EPIC 212707862 being a single hot subdwarf. Analysis of the spectrum gives atmospheric parameters: Teff=28 298±162 K, log g[cm/s2]=5.479±0.025 and log (n(He)/n(H))=-2.752±0.069.

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Constraining the Hydrogen Envelope Mass of a Pulsating sdB Star KIC 10670103 Through Asteroseismology

Krzesinski J., Blokesz A., Baran A. S., Bachulski S.

Acta Astronomica

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2014

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Vol. 64, No. 2

151--165

EN

In this paper we present our study of KIC 10670103, a pulsating hot subdwarf located in the Kepler field. The study is based on three years of data taken by the Kepler spacecraft during Q5-16. Using Fourier analysis, we investigate periodic signals associated with pulsations. Using asymptotic relationships and rotational multiplets we identify modal degrees. The amplitude spectrum appears to be rich in l=1 and 2 multiplets, allowing derivation of a 90 days rotation period for this star. Comparing the pattern of identified gravity mode period spacings with theoretical models, we show that KIC 10670103 is a thick-envelope sdB star.

6

Spurious Frequencies in the Kepler Short Cadence Data

Baran A. S.

Acta Astronomica

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2013

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Vol. 63, No. 2

203--224

EN

We present our search for artifacts in the Kepler short cadence data using a commonly known Fourier technique. We analyzed data on a monthly basis searching for a possible correlation between artifacts and the events attributed to the spacecraft as potential sources of the spurious frequencies. We defined a peak to be an artifact if it shows in at least two, yet preferentially most of the stars, during a given month. Besides the commonly known "long cadence comb" we found a periodic appearance of another two combs, one single artifact and very strange wide artifacts roaming between 10 c/d and 35 c/d. These artifacts evolve on a yearly basis (four of Kepler's rolls) and we may only speculate that their sources are in the reaction wheels since they are the only moving parts, or temperature variation. The orientation of the spacecraft is likely excluded from the possible sources. More resources are needed to provide a definite explanation of the artifacts.

7

Mt. Suhora Survey - Searching for Pulsating M Dwarfs. III

Baran A. S., Winiarski M., Siwak M., Fox-Machado L., Kozieł-Wierzbowska D., Krzesinski J., Dróżdz M., Winans A.

Acta Astronomica

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2013

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Vol. 63, No. 1

41--52

EN

We present our final report on the search for pulsating M dwarfs. We used moderate (<1 m) ground-based telescopes. Our detection was limited to 1 ppt, which is roughly 0.1% of flux variation. We employed both the Fourier technique and Phase Dispersion Minimization (PDM) method. Our data analyses revealed no detection of pulsations in M dwarfs down to the above amplitude level. The results of our survey cannot reject or confirm the theoretical calculations suggesting the M dwarfs can pulsate, however, if the pulsations exist in these stars their amplitudes must be lower than 1 ppt. As a by-product of our search we found several new variable stars, including pulsating stars, binary systems and flare stars.

8

Detection of Multiplets of Degree ℓ=3 and ℓ=4 in the Subdwarf-B Pulsator KIC 10139564

Baran A. S., Østensen R. H.

Acta Astronomica

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2013

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Vol. 63, No. 1

79--90

EN

We present up-to-date results on the short-period sdB pulsator in the Kepler field, KIC 10139564, including 27 months of near continuous photometric data. With this extended dataset we significantly lower the detection threshold, which allows us to detect multiplet components that were missing in our previous analysis based on 15 months of data. We conclude that two regions in the amplitude spectrum of KIC 10139564 contain modes of degree ℓ=3 and ℓ=4, respectively. We also identify many combination frequencies, in both the sub- and super-Nyquist regions, as well as several pulsation modes which show strong frequency or amplitude variability.

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Mt. Suhora Survey - Searching for Pulsating M Dwarfs. II

Krzesinski J., Baran A. S., Winiarski M., Fox-Machado L., Dróżdż M., Siwak M., Kozieł-Wierzbowska D.

Acta Astronomica

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2012

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Vol. 62, No. 2

201--212

EN

We present our report on the second part of our ongoing project which aims at finding stellar pulsations in early type M dwarfs. Our search is based on time-series photometric data acquired mainly at Mt. Suhora Observatory, though, several stars have been observed from other sites. Data of 49 M dwarfs are included in this paper. We analyzed the data by means of the Fourier technique as well as Phase Dispersion Minimization method. We still cannot confirm any pulsating M dwarf at the model-predicted period of 23-40 min. In addition to our main goal we searched for flare activities, planetary transits and other types of brightness changes. We did find several new variable stars, including pulsating stars, binary systems and flare stars, however, only flaring M dwarfs are presented in this paper.

10

Mode Identification of Three Pulsating Subdwarf B Stars via Multiplets and Period Spacing

Baran A. S., Winans A.

Acta Astronomica

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2012

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Vol. 62, No. 4

343--355

EN

We present an analysis of three pulsating subdwarf B stars residing in short period binaries discovered by the Kepler spacecraft. We followed the analysis presented in Baran. Most of the peaks we detected were assigned to mode degrees and, in the case of the multiplet components, the azimuthal numbers. We confirmed the spin rates for all three stars to be tens of days rather than hours as the binary frequencies may suggest. Using the average period spacings of each object we produced an échelle diagram which shows a common feature at a period of nearly 5000 s. The period spacing between consecutive overtones is quite regular, which confirms that the mode trapping is not significant as previously predicted by the models.

11

Mode Identification in the Pulsating Subdwarf B Star KIC 2697388

Baran A. S.

Acta Astronomica

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2012

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Vol. 62, No. 2

179--200

EN

We present our results on mode identification in the pulsating subdwarf B star KIC 2697388 observed with the Kepler spacecraft. We detected 148 frequencies of which five were attributed to p-modes while 122, to g-modes. The remaining 21 frequencies are also likely g-modes. We used multiplets and asymptotic period spacing to constrain degrees of 89 peaks to either l=1 or 2. Using splittings in multiplets we derived the rotation period of this star to be nearly 45 days. The average period spacing between l=1 and 2 overtones are nearly 240 s and 139 s, respectively. Our results show that combining tentative identification based on the presence of multiplets and asymptotic period spacing is indeed useful in mode identification. Mode degree consistently inferred from independent methods make the results reliable and will help to construct accurate models of subdwarf B stars.

12

Mt. Suhora Survey - Searching for Pulsating M Dwarfs in Kepler Public Dataset

Baran A. S., Fox-Machado L., Lykke J., Nielsen M., Telting J. H.

Acta Astronomica

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2011

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Vol. 61, No. 4

325--343

EN

We present our analysis of Kepler short cadence public data to search for stellar pulsations in M dwarfs. Theoretical calculations predict that these stars may suffer from a fundamental radial mode driven by the nuclear ε mechanism. First results published thus far show no significant signal around the expected frequency of 36 c/d down to 1 ppt (parts per thousand). In this paper we include new results obtained from a sample of 86 stars, although a real number of M dwarfs after spectral classification turned out to be small. Unluckily, we have not detected any significant signal typically down to 1-10 ppm (parts per million) in M dwarfs and since the sample of these stars consists of 6 objects only, we cannot make any strong conclusion if the ε mechanism drives perturbation in radius to propagate to a detectable amplitude at the surface. As a by-product of our work we provide a spectral classification of 86 Kepler objects, along with their variability and thus far unknown artifacts residing in Kepler photometry.

13

Mt. Suhora Survey - Searching for Pulsating M Dwarfs. I

Baran A. S., Winiarski M., Krzesiński J., Fox-Machado L., Kawaler S. D., Dróżdż M., Faltenbacher A., Thompson M. A., Reed M. D.

Acta Astronomica

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2011

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Vol. 61, No. 1

37--58

EN

We present the first results of our M dwarf survey in search for stellar pulsation in low mass main sequence stars. Theoretical calculations predict that ε mechanism might drive a fundamental radial mode in these stars and therefore pulsations could be observed photometrically. Although M dwarfs are known for their flare and spot activity they have not yet been the subject of dedicated time-series surveys for pulsation. In this work we include the light curves and amplitude spectra of 46 M dwarfs, which have been observed during the first two years of our survey. We did not detect any pulsations yet. As a by-product of our search, we describe the light curves of some flare M dwarfs. The survey will last for two more years and during that period more than a hundred of M0-M4 type main sequence stars will be observed.