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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

Candidates for Transiting Planets in OGLE-IV Galactic Bulge Fields

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

Acta Astronomica

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2023

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Vol. 73, No. 2

127--158

EN

We present results of a search for transiting exoplanets in 10-yr long photometry with thousands of epochs taken in the direction of the Galactic bulge. This photometry was collected in the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV). Our search covered ≈222 000 stars brighter than I=15.5 mag. Selected transits were verified using a probabilistic method. The search resulted in 99 high-probability candidates for transiting exoplanets. The estimated distances to these targets are between 0.4 kpc and 5.5 kpc, which is a significantly wider range than for previous transit searches. The planets found are Jupiter-size, with the exception of one (named OGLE-TR-1003b) located in the hot Neptune desert. If the candidate is confirmed, it can be important for studies of highly irradiated intermediate-size planets. The existing long-term, high-cadence photometry of our candidates increases the chances of detecting transit timing variations at long timescales. Selected candidates will be observed by the future NASA flagship mission, the Nancy Grace Roman Space Telescope, in its search for Galactic bulge microlensing events, which will further enhance the photometric coverage of these stars.

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

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).

5

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.

6

Wide-Orbit Exoplanets are Common. Analysis of Nearly 20 Years of OGLE Microlensing Survey Data

Poleski R., Skowron J., Mróz P., Udalski A., Szymański M. K., Pietrukowicz 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. 1

1--23

EN

We use nearly 20 yr of photometry obtained by the OGLE survey to measure the occurrence rate of wide-orbit (or ice giant) microlensing planets, i.e., with separations from ≈5 a.u. to ≈15 a.u. and mass-ratios from 10-4 to 0.033. In a sample of 3112 events we find six previously known wide-orbit planets and a new microlensing planet or brown dwarf OGLE-2017-BLG-0114Lb, for which close and wide orbits are possible and close orbit is preferred. We run extensive simulations of the planet detection efficiency, robustly taking into account the finite-source effects. We find that the extrapolation of the previously measured rate of microlensing planets significantly under-predicts the number of wide-orbit planets. On average, every microlensing star hosts 1.4+0.9_-0.6 ice giant planets.

7

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.

8

Properties of the Milky Way's Old Populations Based on Photometric Metallicities of the OGLE RR Lyrae Stars

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

Acta Astronomica

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2020

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Vol. 70, No. 2

121--139

EN

We have used photometric data on almost 91 000 fundamental-mode RR Lyr stars (type RRab) detected by the OGLE survey to investigate properties of old populations in the Milky Way. Based on their metallicity distributions, we demonstrate that the Galaxy is built from three distinct old components: halo, bulge, and disk. The distributions reach their maxima at approximately [Fe/H]J95=-1.2 dex, -1.0 dex, and -0.6 dex on the Jurcsik's metallicity scale, respectively. We find that, very likely, the entire halo is formed from infalling dwarf galaxies. It is evident that halo stars penetrate the inner regions of the Galactic bulge. We estimate that about one-third of all RR Lyr stars within the bulge area belong in fact to the halo population. The whole old bulge is dominated by two populations, A and B, represented by a double sequence in the period--amplitude (Bailey) diagram. The boundary in iron abundance between the halo and the disk population is at about [Fe/H]J95=-0.8 dex. Using Gaia DR2 for RRab stars in the disk area, we show that the observed dispersion of proper motions along the Galactic latitude decreases smoothly with the increasing metal content excluding a bump around [Fe/H]J95=-1.0 dex.

9

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.

10

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.

11

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.

12

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).

13

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.

14

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.

15

OGLE-2017-BLG-1434Lb: Eighth q<1×10-4 Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

Udalski A., Ryu Y.-H., Sajadian S., Gould A., Mróz P., Poleski R., Szymański M. K., Skowron J., Soszyński I., Kozłowski S., Pietrukowicz P., Ulaczyk K., Pawlak M., Rybicki K., Iwanek P., Albrow M. D., Chung S. -J., Han C., Hwang K.-H., Jung Y. K., Shin I.-G., Shvartzvald Y., Yee J. C., Zang W., Zhu W., Cha S.-M., Kim D.-J., Kim H.-W., Kim S.-L., Lee C.-U., Lee D.-J, Lee Y., Park B.-G., Pogge R. W., Bozza W., Dominik M., Helling C., Hundertmark M., Jørgensen U. G., Longa-Peña P., Lowry S., Burgdorf M., Campbell-White J., Ciceri S., Evans D., Jaimes R. F., Fujii Y. I., Haikala L. K., Henning T., Hinse T. C., Mancini L., Peixinho N., Rahvar S., Rabus M., Skottfelt J., Snodgrass C., Southworth J., von Essen C.

Acta Astronomica

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2018

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Vol. 68, No. 1

1--42

EN

We report the discovery of a cold Super-Earth planet (mp=4.4±0.5 M⊕) orbiting a low-mass (M=0.23±0.03 M⊙) M dwarf at projected separation a⊥=1.18±0.10 a.u., i.e., about 1.9 times the distance the snow line. The system is quite nearby for a microlensing planet, DL=0.86±0.09 kpc. Indeed, it was the large lens-source relative parallax πrel=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax" πE∝(πrel/M)1/2 that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q<1×10-4. We apply a new planet-detection sensitivity method, which is a variant of "V/Vmax", to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d lnq ∝ qp, with p=1.05+0.78 -0.68, which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n=-0.93±0.13 at higher mass ratios q≳2×10-4. We combine our result with that of Suzuki et al. to obtain p=0.73+0.42 -0.34.

16

OGLE-2017-BLG-0373Lb: A Jovian Mass-Ratio Planet Exposes A New Accidental Microlensing Degeneracy

Skowron J., Ryu Y.-H., Hwang K.-H., Udalski A., Mróz P., Kozłowski S., Soszyński I., Pietrukowicz P., Szymański M. K., Poleski R., Ulaczyk K., Pawlak M., Rybicki K., Iwanek P., Albrow M. D., Chung S.-J., Gould A., Han C., Jung Y. K., Shin I.-G., Shvartzvald Y., Yee J. C., Zang W., Zhu W., Cha S.-M., Kim D.-J., Kim H.-W., Kim S.-L., Lee C.-U., Lee D.-J, Lee Y., Park B.-G., Pogge R. W.

Acta Astronomica

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2018

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Vol. 68, No. 1

43--61

EN

We report the discovery of microlensing planet OGLE-2017-BLG-0373Lb. We show that while the planet-host system has an unambiguous microlens topology, there are two geometries within this topology that fit the data equally well, which leads to a factor 2.5 difference in planet-host mass ratio, i.e., q=1.5×10-3 vs. q=0.6×10-3. We show that this is an "accidental degeneracy" in the sense that it is due to a gap in the data. We dub it "the caustic-chirality degeneracy". We trace the mathematical origins of this degeneracy, which should enable similar degenerate solutions to be easily located in the future. A Bayesian estimate, based on a Galactic model, yields a host mass M=0.25+0.30 -0.15 M⊙ at a distance DL=5.9+1.3 -1.95 kpc. The lens-source relative proper motion is relatively fast, μ=9 mas/yr, which implies that the host mass and distance can be determined by high-resolution imaging after about 10 years. The same observations could in principle resolve the discrete degeneracy in q, but this will be more challenging.

17

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.

18

The OGLE Collection of Variable Stars. Classical, Type II, and Anomalous Cepheids toward the Galactic Center

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

Acta Astronomica

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2017

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Vol. 67, No. 4

297--316

EN

We present a collection of classical, typeII, and anomalous Cepheids detected in the OGLE fields toward the Galactic center. The sample contains 87 classical Cepheids pulsating in one, two or three radial modes, 924 type II Cepheids divided into BL Her, W Vir, peculiar W Vir, and RV Tau stars, and 20 anomalous Cepheids - first such objects found in the Galactic bulge. Additionally, we upgrade the OGLE Collection of RR Lyr stars in the Galactic bulge by adding 828 newly identified variables. For all Cepheids and RRLyr stars, we publish time-series VI photometry obtained during the OGLE-IV project, from 2010 through 2017. We discuss basic properties of our classical pulsators: their spatial distribution, light curve morphology, period-luminosity relations, and position in the Petersen diagram. We present the most interesting individual objects in our collection: a typeII Cepheid with additional eclipsing modulation, WVir stars with the period doubling effect and the RVb phenomenon, a mode-switching RR Lyr star, and a triple-mode anomalous RRd star.

19

Searching for Potential Mergers among 22 500 Eclipsing Binary Stars in the OGLE-III Galactic Bulge Fields

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

Acta Astronomica

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2017

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Vol. 67, No. 2

115--130

EN

Inspired by the discovery of the red nova V1309 Sco (Nova Scorpii 2008) and the fact that its progenitor was a binary system with a rapidly decreasing orbital period, we have searched for period changes in OGLE binary stars. We have selected a sample of 22 462 short-period (Porb˂4 d) eclipsing binary stars observed toward the Galactic bulge by the OGLE-III survey in years 2001-2009. This dataset was extended with photometry from OGLE-II (1997-2000) and the first six years of OGLE-IV (2010--2015). For some stars, the data were supplemented with OGLE-I photometry (1992-1995). After close inspection of the whole sample we have found 56 systems with realistic period decrease and 52 systems with realistic period increase. We have also recognized 35 systems with cyclic period variations. The highest negative period change rate of -1.943×10-4 d/y has been detected in detached eclipsing binary OGLE-BLG-ECL-139622 with Porb=2.817 d, while all other found systems are contact binaries with orbital periods mostly shorter than 1.0 d. For 22 our systems with decreasing orbital period the absolute rate is higher than the value reported recently for eclipsing binary KIC 9832227. Interestingly, there is an excess of systems with high negative period change rate over systems with positive rate. We cannot exclude the possibility that some of the contact binaries with relatively long orbital period and high negative period change rate will merge in the future. However, our results rather point to the presence of tertiary companions in the observed systems and/or spot activity on the surface of the binary components.

20

OGLE-ing the Magellanic System: Three-Dimensional Structure of the Clouds and the Bridge using RR Lyrae Stars

Jacyszyn-Dobrzeniecka A. M., Skowron D. M., Mróz P., Soszyński I., Udalski A., Pietrukowicz P., Skowron J., Poleski R., Kozłowski S., Wyrzykowski Ł., Pawlak M., Szymański M. K., Ulaczyk K.

Acta Astronomica

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2017

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Vol. 67, No. 1

1--35

EN

We present a three-dimensional analysis of a sample of 22 859 type ab RR Lyr stars in the Magellanic System from the OGLE-IV Collection of RR Lyr stars. The distance to each object was calculated based on its photometric metallicity and a theoretical relation between color, absolute magnitude and metallicity. The LMC RR Lyr distribution is very regular and does not show any substructures. We demonstrate that the bar found in previous studies may be an overdensity caused by blending and crowding effects. The halo is asymmetrical with a higher stellar density in its north-eastern area, which is also located closer to us. Triaxial ellipsoids were fitted to surfaces of a constant number density. Ellipsoids farther from the LMC center are less elongated and slightly rotated toward the SMC. The inclination and position angle change significantly with the $a$ axis size. The median axis ratio is 1:1.23:1.45. The RR Lyr distribution in the SMC has a very regular, ellipsoidal shape and does not show any substructures or asymmetries. All triaxial ellipsoids fitted to surfaces of a constant number density have virtually the same shape (axis ratio) and are elongated along the line-of-sight. The median axis ratio is 1:1.10:2.13. The inclination angle is very small and thus the position angle is not well defined. We present the distribution of RR Lyr stars in the Magellanic Bridge area, showing that the Magellanic Clouds' halos overlap. A comparison of the distributions of RR Lyr stars and Classical Cepheids shows that the former are significantly more spread and distributed regularly, while the latter are very clumped and form several distinct substructures.