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1

The OGLE Collection of Variable Stars. Over 15 000 δ Scuti Stars in the Large Magellanic Cloud

Soszyński I., Pietrukowicz P., Udalski A., Skowron J., Szymański M. K., Poleski R., Skowron D. M., Kozłowski S., Mróz P., Iwanek P., Wrona M., Ulaczyk K., Rybicki K., Gromadzki M., Mróz M.

Acta Astronomica

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2023

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Vol. 73, No. 2

105--126

EN

We present the OGLE collection of δ Scuti stars in the Large Magellanic Cloud and in its foreground. Our dataset encompasses a total of 15 256 objects, constituting the largest sample of extragalactic δ Sct stars published so far. In the case of 12 δ Sct pulsators, we detected additional eclipsing or ellipsoidal variations in their light curves. These are the first known candidates for binary systems containing δ Sct components beyond the Milky Way. We provide observational parameters for all variables, including pulsation periods, mean magnitudes, amplitudes, and Fourier coefficients, as well as long-term light curves in the I- and V-bands collected during the fourth phase of the OGLE project. We construct the period-luminosity (PL) diagram, in which fundamental-mode and first-overtone δ Sct stars form two nearly parallel ridges. The latter ridge is an extension of the PL relation obeyed by first-overtone classical Cepheids. The slopes of the PL relations for δ Sct variables are steeper than those for classical Cepheids, indicating that the continuous PL relation for first-overtone δ Sct variables and Cepheids is non-linear, exhibiting a break at a period of approximately 0.5 d. We also report the enhancement of the OGLE collection of Cepheids and RR Lyr stars with newly identified and reclassified objects, including pulsators contained in the recently published Gaia DR3 catalog of variable stars. As a by-product, we estimate the contamination rate in the Gaia DR3 catalogs of Cepheids and RR Lyr variables.

2

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.

3

Blue Large-Amplitude Pulsators and Other Short-Period Variable Stars in the OGLE-IV Galactic Disk Fields

Borowicz J., Pietrukowicz P., Mróz P., Soszyński I., Udalski A., Szymański M. K., Ulaczyk K., Poleski R., Kozłowski S., Skowron J., Skowron D. M., Rybicki K., Iwanek P., Wrona M., Gromadzki M.

Acta Astronomica

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2023

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Vol. 73, No. 1

1--19

EN

Blue Large-Amplitude Pulsators (BLAPs) form a mysterious class of variable stars with typical periods of tens of minutes and amplitudes above 0.1 mag. In this work, we present results of a variability search focused on timescales shorter than 1 h, conducted in OGLE-IV Galactic disk fields containing about 1.1 billion stellar sources down to I≈20 mag. Twenty-five BLAPs have been detected, 20 of which are new discoveries. Their periods range from 8.4 min to 62.1 min. We have also found six new eclipsing binary systems with orbital periods from 38.3 min to 121.3 min and five short-period large-amplitude (> 0.17 mag in the I-band) variable stars of unknown type.

4

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.

5

The OGLE Collection of Variable Stars. Over 2600 δ Scuti Stars in the Small Magellanic Cloud

Soszyński I., Udalski A., Skowron J., Pietrukowicz P., Szymański M. K., Poleski R., Skowron D. M., Kozłowski S., Mróz P., Iwanek P., Wrona M., Ulaczyk K., Rybicki K., Gromadzki M.

Acta Astronomica

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2022

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Vol. 72, No. 4

245--256

EN

We present the first-ever collection of δ Scuti stars found over the entire area of the Small Magellanic Cloud (SMC). The sample consists of 2810 variables of which over 2600 objects belong to the SMC while the remaining stars are most likely members of the Milky Way's halo. The sample has been divided into 2733 single mode and 77 multimode pulsators. We provide observational parameters (pulsation periods, mean magnitudes, amplitudes, Fourier coefficients) of all δ Sct stars and the long-term I- and V-band time-series photometric measurements collected during the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV).

6

Photometric and Spectroscopic Investigations of the Galactic Field RRc Candidates V764 Mon and HY Com

Benkő J. M., Sódor Á., Pál A.

Acta Astronomica

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2021

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Vol. 71, No. 1

55--72

EN

By analyzing photometric and spectroscopic time series, we show that the pulsator V764 Mon, assumed to be the brightest RR Lyr star in the sky, is in fact a rapidly rotating δ Sct star with an unusually long dominant period (P1=0.29 d). Our spectroscopy confirmed the binarity of V764 Mon discovered by the Gaia satellite. In the case of HY Com, a bona fide RRc star, we present its first complete radial velocity curve. Additionally, we found that the star continues its strong phase variation reported before.

7

Over a Century of δ Ceti Variability Investigation

Śreniawska E., Kamiński K., Kamińska M. K., Tokarek J., Zgórz M.

Acta Astronomica

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2021

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Vol. 71, No. 4

297--310

EN

We present results of a 2014-2018 campaign of radial velocity measurements of δ Ceti. Combining our determination of pulsation period with historical data we conclude that the most likely explanation of observed changes is the presence of a secondary component with a minimum mass of 1.10±0.05 Mo on an orbit with a period of 169±6 yr. Consequently, we revised the intrinsic, evolutionary period change rate to be not larger than 0.018±0.004 s/century, which is significantly lower than previous estimations and is consistent with evolutionary models. We did not find any significant multiperiodic frequencies in radial velocity periodograms such as those reported in the photometric data from the MOST satellite. Using interferometric angular size of δ Ceti from JMMC Stellar Diameters Catalogue we determined several physical parameters of the star with bolometric flux method. They turned out to be consistent with most previous determinations, confirming lower mass and slower evolution of δ Ceti than obtained using different methods.

8

Over 24 000 δ Scuti Stars in the Galactic Bulge and Disk from the OGLE Survey

Soszyński I., Pietrukowicz P., Skowron J., Udalski A., Szymański M. K., Skowron D. M., Poleski R., Kozłowski S., Mróz P., Ulaczyk K., Rybicki K., Iwanek P., Wrona M., Gromadzki M.

Acta Astronomica

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2021

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Vol. 71, No. 3

189--204

EN

We present the largest collection of δ Scuti-type stars in the Milky Way. Together with the recently published OGLE collection of δ Sct variables in the inner Galactic bulge, our sample consists of 24 488 objects distributed along the Milky Way plane, over galactic longitudes ranging from about -170° to +60°. The collection data include the I- and V-band time-series photometry collected since 1997 during the OGLE-II, OGLE-III, and OGLE-IV surveys. We show the on-sky distribution of δ Sct stars in the Galactic bulge and disk, discuss their period, luminosity and amplitude distributions, present Petersen diagram for multimode pulsators, distinguish 34 δ Sct stars in eclipsing and ellipsoidal binary systems, and list probable members of globular clusters.

9

Ground-Based Observations of the ZZ Ceti Star HS~1625+1231

Kalup Cs., Bognár Zs., Sódor Á.

Acta Astronomica

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2021

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Vol. 71, No. 4

281--295

EN

We present the results of our detailed light curve analysis of the ZZ Ceti star HS 1625+1231. We collected photometric time series data at Konkoly Observatory on 14 nights, and performed Fourier analysis of these data sets. We detected 11 significant frequencies, where six of them are found to be independent pulsation modes in the 514-881 s period range. By utilizing these frequencies, we performed preliminary asteroseismic investigations to give constraints on the main physical parameters, and to derive seismic distances for the star. Finally, we compared the astrometric distance provided by the Gaia EDR3 data with those seismic distances. Our selected model, considering both the spectroscopic measurements and the distance value provided by Gaia, has Teff=11 000 K and M*=0.60 M☉.

10

Comparison of the Hartle-Thorne Model of Neutron Stars with its Kerr Approximation by using Resonant Switch Model

Stuchlík Z., Klimovičová K., Schee J., Török G., Kotrlová A.

Acta Astronomica

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2021

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Vol. 71, No. 4

311--141

EN

The Hartle-Thorne (H-T) models of slowly rotating neutron or quark stars, characterized by the mass M, dimensionless spin a, and reduced quadrupole moment q, are constructed for the observationally given rotational frequency frot=580 Hz (290 Hz) of the compact star in the atoll source 4U 1636-53, and a wide range of equations of state (EoS) giving sequences of allowed states governed by the relations a(M), q(M). These sequences are used in the framework of the resonant switch (RS) model combining pairs of geodesic oscillation models to match the data of the twin high-frequency quasi-periodic oscillations observed in the 4U 1636-53 source. The results of the matching procedure using the H-T models are compared to those based on the Kerr approximation of the exterior of the neutron stars. The best H-T matches fix the only variant of the RS model combining particular modifications of the relativistic precession model, exclude the rotation frequency frot=290 Hz, restrict the considered EoS to six of them, excluding the strange quark stars, and significantly improve precision of the matches given by any single geodesic oscillations model. The Kerr matching allows two variants of the RS model, thus, giving false information, and only three EoS, thus, giving insufficient information. Our results demonstrate that in the matching procedure, the Kerr approximation can be used only for neutron stars governed by the H-T models with q<2, implying an important restriction on the applicability of the Kerr approximation for description of the oscillatory phenomena around neutron stars. On the other hand, the RS model is sufficiently discriminating for the spacetime metric to be largely determined by fitting to the data. The ranges of the external spacetime parameters of the neutron stars related to the best H-T matches are determined to be M≈2.10-2.13 Mo, a≈0.21-0.25, q≈1.8-2.3. Most compact neutron star is predicted by the Gandolfi EoS, when M≈2.10 Mo, a≈0.21, q≈1.8, with the equatorial radius R≈10.83 km and eccentricity ε=0.03.

11

Classical Cepheids in the Milky Way

Pietrukowicz P., Soszyński I., Udalski A.

Acta Astronomica

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2021

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Vol. 71, No. 3

205--222

EN

We share the most up-to-date, carefully verified list of classical Cepheids residing in the Galaxy. Based on long-term OGLE experience in the field of variable stars, we have inspected candidates for Cepheids from surveys such as ASAS, ASAS-SN, ATLAS, Gaia, NSVS, VVV, WISE, ZTF, among others, and also known sources from the General Catalogue of Variable Stars. Only objects confirmed in the optical range as classical Cepheids are included in the list. We provide Gaia EDR3 identifications of the stars. Purity of the sample exceeds 97 per cent, while its completeness is of about 88 per cent down to a magnitude G = 18. The list contains 3352 classical Cepheids, of which 2140 stars are fundamental-mode pulsators. Basic statistics and comparison between the classical Cepheids from the Milky Way, Andromeda Galaxy (M31), and Magellanic Clouds are provided. The list is available at the OGLE Internet Data Archive.

12

On the Pulsations of the δ Scuti Star of the Binary System KIC 6629588

Liakos A.

Acta Astronomica

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2020

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Vol. 70, No. 4

265--289

EN

his work includes a comprehensive analysis of the Kepler detached eclipsing binary system KIC 6629588 that aims at the detailed study of the oscillation properties of its pulsating component. Ground-based spectroscopic observations were obtained and used to classify the components of the system. The spectroscopic results were used as constraints for the modeling of the short-cadence Kepler light curves and for the estimation of the absolute parameters of the components. Furthermore, the light curve residuals are analyzed using Fourier transformation techniques in order to search for pulsation frequencies. The primary component of the system is identified as a δ Sct star that pulsates in seven eigenfrequencies in the range of 13-22 d-1, while more than 270 combination frequencies were also detected. The absolute and the oscillation parameters of this pulsating star are compared with those of other δ Sct stars members of detached binary systems using evolutionary and correlation diagrams. Finally, the distance to the system is also estimated.

13

Additional Galactic Cepheids from the OGLE Survey

Soszyński I., Udalski A., Szymański M. K., Pietrukowicz P., Skowron J., Skowron D. M., Poleski R., Kozłowski S., Mróz P., Ulaczyk K., Rybicki K., Iwanek P., Wrona M., Gromadzki M.

Acta Astronomica

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2020

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Vol. 70, No. 2

101--119

EN

We report on the results of a comprehensive search for Cepheid variable stars in the fields photometrically monitored by the OGLE Galaxy Variability Survey. We identify 742 Cepheids not included in the first release of the OGLE Collection of Galactic Cepheids and reclassify several dozen previously published variables. The upgraded collection comprises 1974 classical, 1625 type II, and 119 anomalous Cepheids located in the area of about 3000 square degrees covering the disk and bulge of the Milky Way. We present the most interesting objects in our sample: an isolated group of long-period double-mode classical Cepheids pulsating in the fundamental mode and first overtone, the first known Galactic double-mode Cepheid with the second and third overtone excited, double-mode type II Cepheids, candidates for single-mode first-overtone BL Her stars, and Cepheids showing simultaneous eclipsing variability. We also discuss type II and anomalous Cepheids that are potential members of globular clusters.

14

A Comprehensive Study of the Eclipsing Binaries V1241 Tau and GQ Dra

Ulaş B., Gazeas K., Liakos A., Ulusoy C., Stateva I., Erkan N., Napetova M., Iliev I. K.

Acta Astronomica

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2020

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Vol. 70, No. 3

219--240

EN

We present new photometric and spectroscopic observations and analyses for the eclipsing binary systems V1241 Tau and GQ Dra. Our photometric light and radial velocity curve analyses combined with the TESS light curves show that both are conventional semi-detached binary systems. Their absolute parameters are also derived. We present the O-C analyses of the systems and we propose the most likely orbital period modulating mechanisms. Furthermore, Fourier analyses are applied to the photometric residual data of the systems to check for the pulsational behavior of the components. We conclude that the primary component of the system GQ Dra is a δ Sct type pulsator with a dominant pulsation frequency of 18.58 d-1 based on our B filter residual light curve although it can not be verified by 30-minute cadence TESS data. No satisfactory evidence of pulsational behavior for V1241 Tau was verified. Finally, the evolutionary tracks of the components of both systems are calculated. Their locations within evolutionary diagrams are compared with other Algol-type systems.

15

Over 78 000 RR Lyrae Stars in the Galactic Bulge and Disk from the OGLE Survey

Soszyński I., Udalski A., Wrona M., Szymański M. K., Pietrukowicz P., Skowron J., Skowron D., Poleski R., Kozłowski S., Mróz P., Ulaczyk K., Rybicki K., Iwanek P., Gromadzki M.

Acta Astronomica

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2019

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Vol. 69, No. 4

321--337

EN

We present an upgrade of the OGLE Collection of RR Lyrae stars in the Galactic bulge and disk. The size of our sample has been doubled and reached 78 350 RR Lyr variables, of which 56 508 are fundamental-mode pulsators (RRab stars), 21 321 pulsate solely in the first-overtone (RRc stars), 458 are classical double-mode pulsators (RRd stars), and 63 are anomalous RRd variables (including six triple-mode pulsators). For all the newly identified RR Lyr stars, we publish time-series photometry obtained during the OGLE Galaxy Variability Survey. We present the spatial distribution of RR Lyr stars on the sky, provide a list of globular clusters hosting RR Lyr variables, and discuss the Petersen diagram for multimode pulsators. We find new RRd stars belonging to a compact group in the Petersen diagram (with period ratios P 1O/PF≈0.74 and fundamental-mode periods P F≈0.44$ d) and we show that their spatial distribution is roughly spherically symmetrical around the Milky Way center.

16

Mapping the Northern Galactic Disk Warp with Classical Cepheids

Skowron D. M., Skowron J., Mróz P., Udalski A., Pietrukowicz P., Soszyński I., Szymański M. K., Poleski R., Kozłowski S., Ulaczyk K., Rybicki K., Iwanek P., Wrona M., Gromadzki M.

Acta Astronomica

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2019

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Vol. 69, No. 4

305--320

EN

We present an updated three dimensional map of the Milky Way based on a sample of 2431 classical Cepheid variable stars, supplemented with about 200 newly detected classical Cepheids from the OGLE survey. The new objects were discovered as a result of a dedicated observing campaign of the ≈280 square degree extension of the OGLE footprint of the Galactic disk during 2018-2019 observing seasons. These regions cover the main part of the northern Galactic warp that has been deficient in Cepheids so far. We use direct distances to the sample of over 2390 classical Cepheids to model the distribution of the young stellar population in the Milky Way and recalculate the parameters of the Galactic disk warp. Our data show that its northern part is very prominent and its amplitude is ≈10% larger than that of the southern part. By combining Gaia astrometric data with the Galactic rotation curve and distances to Cepheids from our sample, we construct a map of the vertical component of the velocity vector for all Cepheids in the Milky Way disk. We find large-scale vertical motions with amplitudes of 10-20 km/s, such that Cepheids located in the northern warp exhibit large positive vertical velocity (toward the north Galactic pole), whereas those in the southern warp - negative vertical velocity (toward the south Galactic pole).

17

Final Release of the OGLE Collection of Cepheids and RR Lyrae Stars in the Magellanic System. The Outer Regions

Soszyński I., Udalski A., Szymański M. K., Pietrukowicz P., Skowron J., Skowron D. M., Poleski R., Kozłowski S., Mróz P., Ulaczyk K., Rybicki K., Iwanek P., Wrona M.

Acta Astronomica

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2019

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Vol. 69, No. 2

87--99

EN

We present the final release of the OGLE collection of classical pulsators (Cepheids and RR Lyr stars) in the Large and Small Magellanic Clouds. The sky coverage has been increased from 670 to 765 square degrees compared to the previous edition of our collection. We also add some Cepheids and RR Lyr stars found by the Gaia team and reclassify three Cepheids. Ultimately, our collection consists of 9650 classical Cepheids, 343 type II Cepheids, 278 anomalous Cepheids, and 47 828 RR Lyr stars inside and toward the Magellanic System.

18

CCD Photometry, Light Curve Deconvolution, Period Analysis and Evolutionary Status of the HADS Variable V524 And

Alton K. B., Stępień K.

Acta Astronomica

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2019

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Vol. 69, No. 3

283--304

EN

New multi-color BVIc CCD photometric data of V524 And, a pulsating variable long considered to be an SX Phe-type subdwarf system were acquired during 2018. Deconvolution of precise time-series light curve data was accomplished using discrete Fourier transformation and revealed a mean fundamental mode f0 of oscillation at 10.583 d-1 along with four other harmonics from 2f0 to 5f0. Following successive pre-whitening of each residual signal, no other statistically significant frequency shared by all band-passes was resolved. Potential period changes over time were evaluated using four new times-of-maximum light derived from the present study along with other values reported in the literature. Photometric data collected during the ROTSE-I (1999) and SuperWASP (2007-2008) surveys combined with CCD-derived V-mag data acquired from the AAVSO archives produced an additional sixty-six times-of-maximum measurements. Corresponding residuals from the observed minus calculated values indicate very little change in the primary pulsation period since 1999. However, a far more interesting finding from this secular analysis suggests an underlying sinusoidal-like variation of PB=6.41 yr consistent with a putative binary partner that may be a brown dwarf (M<0.07 M☉). An investigation with PARSEC models for generating stellar tracks and isochrones provided further insight into the evolutionary status and physical character of V524 And. This intrinsic variable associated with the Galactic disk has an estimated mass around 2 M☉ with near-solar metallicity. These characteristics point to V524 And being a Population I HADS with an age approaching 1 Gyr rather than an SX Phe-type variable.

19

The OGLE Collection of Variable Stars. Type II Cepheids in the Magellanic System

Soszyński I., Udalski A., Szymański M. K., Wyrzykowski Ł., Ulaczyk K., Poleski R., Pietrukowicz P., Kozłowski S., Skowron D., Skowron J., Mróz P., Rybicki K., Iwanek P.

Acta Astronomica

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2018

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Vol. 68, No. 2

89--109

EN

We present a nearly complete collection of type II Cepheids in the Magellanic System. The sample consists of 338 objects: 285 and 53 variables in the Large and Small Magellanic Clouds, respectively. Based on the pulsation periods and light-curve morphology, we classified 118 of our type II Cepheids as BL Her, 120 as W Vir, 34 as peculiar W Vir, and 66 as RV Tau stars. For all objects, we publish time-series {\it VI} photometry obtained during the OGLE-IV survey, from 2010 to the end of 2017. We present the most interesting individual objects in our collection: 16 type II Cepheids showing additional eclipsing or ellipsoidal variability, two RV Tau variables more than 2.5 mag fainter than other stars of this type in the LMC, an RVb star that drastically decreased the amplitude of the long-period modulation, type II Cepheids exhibiting significant amplitude and period changes, and an RV Tau star which undergoes interchanges of deep and shallow minima. We show that peculiar W Vir stars have markedly different spatial distribution than other subclasses of type II Cepheids, which indicates different evolutionary histories of these objects.

20

OGLE Collection of Galactic Cepheids

Udalski A., Soszyński I., Pietrukowicz P., Szymański M. K., Skowron D. M., Skowron J., Mróz P., Poleski R., Kozłowski S., Ulaczyk K., Rybicki K., Iwanek P., Wrona M.

Acta Astronomica

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2018

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Vol. 68, No. 4

315--339

EN

We present here a new major part of the OGLE Collection of Variable Stars - OGLE Collection of Galactic Cepheids. The new dataset was extracted from the Galaxy Variability Survey images - a dedicated large-scale survey of the Galactic disk and outer bulge conducted by the OGLE project since 2013. The OGLE collection contains 2721 Cepheids of all types - classical, type II and anomalous. It more than doubles the number of known Galactic classical Cepheids. Due to the long-term monitoring and large number of epochs the selected sample is very pure, generally free from contaminating stars of other types often mimicking Cepheids. Its completeness is high at 90% level for classical Cepheids – tested using recent samples of Galactic Cepheids: ASAS-SN, ATLAS, Gaia DR2 and Wise catalog of variable stars. Our comparisons indicate that the completeness of the two latter datasets, Gaia DR2 and Wise catalog, is very low, at < I < 19.5 mag). Both these samples are severely contaminated by non-Cepheids (the purity is 67% and 56%, respectively). We also present several interesting objects found in the new OGLE Collection - multi-mode pulsators, first Galactic candidates for eclipsing systems containing Cepheid, a binary Cepheid candidate. New OGLE Collection of Galactic Cepheids is available for the astronomical community from the OGLE Internet Archive in similar form as previous parts of the OGLE Collection of Variable Stars.