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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

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).

3

New Short-Period δ Scuti Stars in OGLE-IV Fields toward the Galactic Bulge

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

Acta Astronomica

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2022

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Vol. 72, No. 3

161--170

EN

We report the classification of 24 puzzling short-period variable stars located in OGLE-IV Galactic bulge fields. The stars are low-amplitude (<0.05 mag) multi-periodic objects with dominant periods between 22 min and 54 min whose type could not have been unambiguously established based on photometry only. A low-resolution spectroscopic follow-up observations have shown that all the objects are main sequence A/F-type stars. Thus, all the variables are δ Sct-type pulsators. We have added them to the OGLE-IV Collection of Variable Stars.

4

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.

5

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.

6

The Clusters AgeS Experiment (CASE). Variable Stars in the Field of the Globular Cluster NGC 3201

Rozyczka M., Pych W., Thompson I. B., Mazur B.

Acta Astronomica

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2020

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Vol. 70, No. 4

291--299

EN

We report the detection of 20 new variables in the field of NGC 3201. Among them there are 15 confirmed cluster members. Follow-up observations are desirable for two blue stragglers with variable light curves (probably resulting from magnetic activity), and for a yellow straggler coinciding with a Chandra X-ray source. Two of the eclipsing binaries, newly detected at the turnoff in cluster's CMD, and an eclipsing binary on the red giant branch, are potentially suitable for the determination of age and distance of NGC 3201.

7

Over 10 000 δ Scuti Stars toward the Galactic Bulge from OGLE-IV

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

Acta Astronomica

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2020

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Vol. 70, No. 4

241--263

EN

We present a collection of 10 111 genuine δ Sct-type pulsating variable stars detected in the OGLE-IV Galactic bulge fields. In this sample, 9835 variables are new discoveries. For most of the stars photometric data cover the whole decade 2010-2019. We illustrate a huge variety of light curve shapes of δ Sct variables. Long-term observations have allowed us to spot objects with evident period, amplitude, and mean brightness variations. Our analysis indicates that about 28% of the stars are single-mode pulsators. Fourteen δ Sct stars show additional eclipsing or ellipsoidal binary modulation. We report significant attenuation or even disappearance of the pulsation signal in six sources. The whole set of variables is a mix of objects representing various Milky Way's populations, with the majority of stars from the Galactic bulge. There are also representatives of the Sagittarius Dwarf Spheroidal Galaxy. Some of the newly detected variables could be SX Phe-type stars residing in globular clusters. The collection, including full V- and I-band time-series data, is available to the astronomical community from the OGLE On-line Data Archive.

8

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.

9

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.

10

The Cluster AgeS Experiment (CASE). Variable Stars in the Field of the Globular Cluster M10

Rozyczka M., Narloch W., Schwarzenberg-Czerny A., Thompson I. B., Poleski R., Pych W.

Acta Astronomica

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2018

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Vol. 68, No. 3

237--258

EN

The field of the globular cluster M10 (NGC 6254) was monitored between 1998 and 2015 in a search for variable stars. V-band light curves were derived for 40 variables or likely variables, most of which are new detections. Proper motions obtained within the CASE project indicate that 18 newly detected variables and 14 previously known ones are members or likely members of the cluster, including one RRc-type, three type II Cepheids, and 14 SX Phe-type pulsators, one contact binary, and six semi-regular red giants. As a by-product of the search we discovered a candidate binary comprised of main sequence stars with the record-short orbital period of 0.042 d. We also confirmed the photometric variability of the red straggler M10-VLA1 hinted at by Shishkovsky, who discovered this object spectroscopically. In Appendix 1 we show that CASE proper motion measurements are in a good agreement with those retrieved from the Gaia archive, while Appendix 2 presents evidence for low frequency γ Dor-type oscillations in SX Phe stars belonging to M10.

11

Hybrid Pulsations and Tidal Splitting detected in the Kepler Eclipsing and Spotted Binary System KIC 6048106

Samadi Ghadim A., Lampens P., Jassur D. M.

Acta Astronomica

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2018

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Vol. 68, No. 4

425--447

EN

We present a new asteroseismic analysis of KIC 6048106, a Kepler Algol-type eclipsing binary star in a circularized orbit with Porb=1.559361±0.000036 d. Based on a physical model for the binary and its corresponding set of fundamental parameters (Teff=7033±187 K, M1=1.55±0.11 M☉, R1=1.58±0.12 R☉ and Teff=4522±103 K, M2=0.33±0.07 M☉, R2=1.78±0.16 R☉, respectively for the primary and the secondary component), we obtained the residual light curve after removal of the full binary model, including a 290-d activity cycle for the secondary component. In this work, we used the method of Fourier analysis of the residual light curve in combination with least squares optimization for the frequency analysis. We detected seven dominant, independent gravity (g) modes as well as 34 low-amplitude acoustic (p) modes. The g modes in the range of 1.96–2.85 d-1 have a mean spacing of ΔΠmean=1517.92±131.54 s. Though of much lower amplitude, additional significant frequencies were detected in the intervals 7.49–15.2 d-1 and 19–22.5 d-1 (i.e., in the p mode region), with corresponding dominant modes νmax1=11.745±0.001 d-1 and νmax2=20.960±0.002 d-1 From its position in the HR diagram, we conclude that the primary component is the source of the detected hybrid pulsations. Consequently, the pulsation constants, Q, of the high frequencies cover the range of 0.028–0.064 d. Furthermore, ν43 (19.037±0.002 d-1) might correspond to the fundamental radial mode (Q=0.033±0.007 d). The other frequencies in the range of 19–22.5 d-1 could be radial or non-radial overtone modes. Moreover, the low-amplitude p modes show an equidistant splitting by forb, which we interpret as tidal splitting following theoretical predictions.

12

A Low-Resolution Spectroscopic Exploration of Puzzling OGLE Variable Stars

Pietrukowicz P., Latour M., Angeloni R., di Mille F., Soszyński I., Udalski A., Germanà C.

Acta Astronomica

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2015

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Vol. 65, No. 1

63--79

EN

We present the results of a spectroscopic follow-up of various puzzling variable objects detected in the OGLE-III Galactic disk and bulge fields. The sample includes mainly short-period multi-mode pulsating stars that could not have been unambiguously classified as either δ Sct or β Cep type stars based on photometric data only, also stars with irregular fluctuations mimicking cataclysmic variables and stars with dusty shells, and periodic variables displaying brightenings in their light curves that last for more than half of the period. The obtained low-resolution spectra show that all observed short-period pulsators are of δ Sct type, the stars with irregular fluctuations are young stellar objects, and the objects with regular brightenings are A type stars or very likely Ap stars with strong magnetic field responsible for the presence of bright caps around magnetic poles on their surface. We also took spectra of objects designated OGLE-GD-DSCT-0058 and OGLE-GD-CEP-0013. An estimated effective temperature of 33 000 K in OGLE-GD-DSCT-0058 indicates that it cannot be a δ Sct type variable. This very short-period (0.01962 d) high-amplitude (0.24 mag in the I-band) object remains a mystery. It may represent a new class of variable stars. The spectrum of OGLE-GD-CEP-0013 confirms that this is a classical Cepheid despite a peculiar shape of its light curve. The presented results will help in proper classification of variable objects in the OGLE Galaxy Variability Survey.

13

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IX. NGC 457

Moździerski D., Pigulski A., Kopacki G., Kołaczkowski Z., Stęślicki M.

Acta Astronomica

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2014

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Vol. 64, No. 2

89--114

EN

We present results of a BVIC variability survey in the young open cluster NGC 457 based on observations obtained during three separate runs spanning almost 20 years. In total, we found 79 variable stars, of which 66 are new. The BVIC photometry was transformed to the standard system and used to derive cluster parameters by means of isochrone fitting. The cluster is about 20 Myr old, the mean reddening amounts to about 0.48 mag in terms of the color excess E(B-V). Depending on the metallicity, the isochrone fitting yields a distance between 2.3 kpc and 2.9 kpc, which locates the cluster in the Perseus arm of the Galaxy. Using the complementary Hα photometry carried out in two seasons separated by over 10 years, we find that the cluster is very rich in Be stars. In total, 15 stars in the observed field of which 14 are cluster members showed Hα in emission either during our observations or in the past. Most of the Be stars vary in brightness on different time scales including short-period variability related most likely to g-mode pulsations. A single-epoch spectrum of NGC 457-6 shows that this Be star is presently in the shell phase. The inventory of variable stars in the observed field consists of a single β Cep-type star, NGC 457-8, 13 Be stars, 21 slowly pulsating B stars, seven δ Sct stars, one γ Dor star, 16 unclassified periodic stars, 8 eclipsing systems and a dozen of stars with irregular variability, of which six are also B-type stars. As many as 45 variable stars are of spectral type B which is the largest number in all open clusters presented in this series of papers. The most interesting is the discovery of a large group of slowly pulsating B stars which occupy the cluster main sequence in the range between V=11 mag and 14.5 mag, corresponding to spectral types B3 to B8. They all have very low amplitudes and about half show pulsations with frequencies higher than 3 d-1. We argue that these are most likely fast-rotating slowly pulsating B stars, observed also in other open clusters.

14

Variable Stars in the Globular Cluster NGC 288: [Fe/H] and Distance

Arellano Ferro A., Bramich D. M., Giridhar S., Figuera Jaimes R., Kains N., Kuppuswamy K.

Acta Astronomica

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2013

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Vol. 63, No. 4

429--451

EN

A search for variable stars in the globular cluster NGC 288 was carried out using a time-series of CCD images in the V and I filters. The photometry of all stellar sources in the field of view of our images, down to V≈19 mag, was performed using difference image analysis (DIA). For stars of ≈15 mag, measurement accuracies of ≈8 mmag and ≈10 mmag were achieved for V and I respectively. Three independent search strategies were applied to the 5525 light curves but no new variables were found above the threshold limits characteristic of our data set. The use of older data from the literature combined with the present data allowed the refinement of the periods of all known variables. Fourier light curve decomposition was performed for the RRab and the RRc stars to obtain an estimate of [Fe/H]ZW=-1.62±0.02 (statistical) ±0.14 (systematic). A true distance modulus of 14.768±0.003 mag (statistical) ±0.042 mag (systematic), or a distance of 8.99±0.01 kpc (statistical) ±0.17 kpc (systematic) was calculated from the RRab star. The RRc star predicts a discrepant distance about one kiloparsec shorter but it is possibly a Blazhko variable. An independent distance from the P-L relationship for SX Phe stars leads to a distance of 8.9±0.3 kpc.The SX Phe stars V5 and V9 are found to be double mode pulsators.

15

Large Variety of New Pulsating Stars in the OGLE-III Galactic Disk Fields

Pietrukowicz P., Dziembowski W. A., Mróz P., Soszyński I., Udalski A., Poleski R., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Kozłowski S., Skowron J.

Acta Astronomica

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2013

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Vol. 63, No. 4

379--404

EN

We present the results of a search for pulsating stars in the 7.12 deg2 OGLE-III Galactic disk area in the direction tangent to the Centaurus Arm. We report the identification of 20 Classical Cepheids, 45 RR Lyr type stars, 31 Long-Period Variables, such as Miras and Semi-Regular Variables, one pulsating white dwarf, and 58 very likely δ Sct type stars. Based on asteroseismic models constructed for one quadruple-mode and six triple-mode δ Sct type pulsators, we estimated masses, metallicities, ages, and distance moduli to these objects. The modeled stars have masses in the range 0.9-2.5 MSun and are located at distances between 2.5 kpc and 6.2 kpc. Two triple-mode and one double-mode pulsators seem to be Population II stars of the SX Phe type, probably from the Galactic halo. Our sample also includes candidates for Type II Cepheids and unclassified short-period (P<0.23 d) multi-mode stars which could be either δ Sct or β Cep type stars. One of the detected variables is a very likely δ Sct star with an exceptionally high peak-to-peak I-band amplitude of 0.35 mag at the very short period of 0.0196 d. All reported pulsating variable stars but one object are new discoveries. They are included in the OGLE-III Catalog of Variable Stars. Finally, we introduce the on-going OGLE-IV Galactic Disk Survey, which covers more than half of the Galactic plane. For the purposes of future works on the spiral structure and star formation history of the Milky Way, we have already compiled a list of known Galactic Classical Cepheids.