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1

Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids

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Pietrukowicz P.

Acta Astronomica

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2003

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tom Vol. 53, No. 1

63--71

EN

We compared period changes derived from O-C diagrams for 63 classical Cepheids from our Galaxy with model calculations. We found that for Cepheids with log P>1.0 the observed changes are smaller than predicted values, except variable SZ Cas. However some of the first overtone Cepheids, particularly EU Tau and Polaris, change its period much faster than it follows from theory. Summary of the known data on period changes in Cepheids from the Galaxy and from the Magellanic Clouds (previous papers) leads to conclusion that none of the 999 Cepheids is undergoing the first crossing of the instability strip. Also the observed period changes for long-period Cepheids are a few times slower than predicted by the models. These results imply that much larger fraction of helium is burned in the Cepheid stage than it is predicted by models.

2

Period Changes of the SMC Cepheids from the Harvard, OGLE and ASAS Data

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Pietrukowicz P.

Acta Astronomica

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2002

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tom Vol. 52, No. 2

177--187

EN

Comparison of the old observations of Cepheids in the Small Magellanic Cloud from the Harvard data archive, with the recent OGLE and ASAS observations allows an estimate of their period changes. All of matched 557 Cepheids are still pulsating in the same mode. One of the Harvard Cepheid, HV 11289, has been tentatively matched to a star which is now apparently constant. Cepheids with log P>0.8 show significant period changes, positive as well as negative. We found that for many stars these changes are significantly smaller than predicted by recent model calculations. Unfortunately, there are no models available for Cepheids with periods longer than approximately 80 days, while there are observed Cepheids with periods up to 210 days.

3

Period Changes of the LMC Cepheids Determined from the Harvard, OGLE and ASAS Data

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Pietrukowicz P.

Acta Astronomica

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2001

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tom Vol. 51, No. 3

247--257

EN

Observations of Cepheids in the Large Magellanic Cloud, made over the last several decades, allow us to search for evolutionary period changes. None of the Cepheid from our sample of 378 stars stopped pulsating. Also none of them showed a large period change which could indicate mode switching. However for Cepheids with log P>0.9 we found significant period changes, positive as well as negative. A comparison between the observed period changes and theoretical predictions shows moderate agreement with some models (Bono et al. 2000), and a very large disagreement with others (Alibert et al. 1999). The large differences between the models are likely caused by the very high sensitivity of stellar evolution during core helium burning phase to even small changes in the input physics, as discovered by Lauterborn, Refsdal and Weigert (1971).

4

Hα Imaging of X-ray Sources in Selected Globular Clusters with the SOAR Telescope

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Pietrukowicz P.

Acta Astronomica

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2009

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tom Vol. 59, No. 3

291--305

EN

We present results of a search for objects with Hα excess, such as cataclysmic variables (CVs) and chromospherically active binaries (ABs), as counterparts to X-ray sources detected with Chandra satellite observatory in six Galactic globular clusters (GCs): M4, M28, M30, M71, M80, NGC 6752. Binary systems play a critical role in the evolution of GCs, serving as an internal energy source countering the tendency of GC cores to collapse. Theoretical studies predict dozens of CVs in the cores of some GCs (eg., 130 for M28, 40 for M30). A number of such binaries is also expected outside the core radius. However, few CVs are known so far in GCs. Using subtraction technique applied to images taken with the 4.1-m SOAR telescope we have found 27 objects with Hα excess in the field of the observed clusters, of which nine are likely associated with the clusters. Four are candidate CVs, four are candidate ABs, one could be either a CV or an AB. One Hα object seems to be a background galaxy, while other 17 detected objects are probably foreground or background stars.

5

Searching for Variable Stars in the Central Part of the Globular Cluster M22

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Pietrukowicz P. , Kaluzny J.

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tom Vol. 56, No. 4

371-378

EN

Time-series data taken with the Hubble Space Telescope of three fields covering the central part of the globular cluster M22 have been analyzed in search of variable objects. We report identification of eleven periodic variables of which eight are new discoveries. The sample includes five certain contact binaries as well as one SX Phe star. Two objects with periods longer than 1 day are preliminarily classified as either spotted variables of BY Dra type or ellipsoidal variables. The most unusual of the identified variables, M22_V11, has IC≈19.9 and is located far to the red of the main sequence in the cluster color-magnitude diagram. It shows variability with a period as short as P≈0.066 days or alternatively with P≈0.132 days. We propose that it may be an ellipsoidal variable harboring a degenerate component.

6

Variable Stars in the Archival HST Data of Globular Clusters M13, M30 and NGC 6712

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Pietrukowicz P. , Kaluzny J.

Acta Astronomica

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2004

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tom Vol. 54, No. 1

19--31

EN

We have analyzed archival HST/WFPC2 time-series data of the central parts of globular clusters M13, M30 and NGC 6712 in search for variable objects. Among a total of 21 identified variables there are 15 new discoveries. The sample includes nine RR Lyr stars, two SX Phe stars and seven W UMa-type contact binaries. One object is preliminarily classified as a detached eclipsing binary and another as an ellipsoidal variable.

7

Classical Cepheids in the Milky Way

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Pietrukowicz P. , Soszyński I. , Udalski A.

Acta Astronomica

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2021

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tom 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.

8

St 2-22 - Another Symbiotic Star with High-Velocity Bipolar Jets

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Tomov T. , Zamanov R. , Gałan C. , Pietrukowicz P.

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2017

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tom Vol. 67, No. 3

225--242

EN

We report the detection of high-velocity components in the wings of Hα emission line in spectra of symbiotic binary star St 2-22 obtained in 2005. This finding encouraged us to start the present investigation in order to show that this poorly-studied object is a jet-producing system. We have used high-resolution optical and low-resolution near-infrared spectra, as well as available optical and infrared photometry, to evaluate some physical parameters of the St 2-22 components and characteristics of the jets. We confirm that St 2-22 is a S-type symbiotic star. Our results demonstrate that an unnoticed outburst, similar to those in classical symbiotic systems, occurred in the first half of 2005. During the outburst, collimated bipolar jets were ejected by the hot component of St 2-22 with an average velocity of about 1700 km/s.

9

Identification of V735 Sgr as an Active Herbig Ae/Be Object

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Pietrukowicz P. , Di Mille F. , Angeloni R. , Udalski A.

Acta Astronomica

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2018

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tom Vol. 68, No. 3

259--267

EN

V735 Sgr was known as an enigmatic star with rapid brightness variations. Long-term OGLE photometry, brightness measurements in infrared bands, and recently obtained moderate resolution spectrum from the 6.5-m Magellan telescope show that this star is an active young stellar object of Herbig Ae/Be type.

10

Large Magellanic Cloud Cepheids in the ASAS Data

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Karczmarek P. , Dziembowski W. A. , Lenz P. , Pietrukowicz P. , Pojmański G.

Acta Astronomica

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2011

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tom Vol. 61, No. 4

303--318

EN

A catalog of Cepheids in the Large Magellanic Cloud (LMC) from the ASAS project is presented. It contains data on 65 fundamental mode pulsators with periods longer than about 8 days. The period-luminosity (PL) relation in the V-band does not significantly differ from the relation determined from the OGLE-III data extended toward longer periods but shows much larger scatter. For objects with periods longer than 40 d there is an evidence for a shallower PL relation. The rates of long-term period variations significant at 3σ level are found only for seven objects. The rates for 25 objects determined with the 1σ significance are confronted with the values derived from stellar evolution models. The models from various sources yield discrepant predictions. Over the whole data range, a good agreement with measurements is found for certain models but not from the same source.

11

Cluster AgeS Experiment (CASE): Dwarf Novae and a Probable Microlensing Event in the Globular Cluster M22

38%

Pietrukowicz P. , Kaluzny J. , Thompson I. B. , Jaroszyński M. , Schwarzenberg-Czerny A. , Krzeminski W. , Pych W.

Acta Astronomica

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2005

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tom Vol. 55, No. 3

261--274

EN

We report the identification of a new cataclysmic variable (denoted as CV2) and a probable microlensing event in the field of the globular cluster M22. Two outbursts were observed for CV2. During one of them superhumps with Psh=0.08875 d were present in the light curve. CV2 has an X-ray counterpart detected by XMM-Newton. A very likely microlensing event at a radius of 2.'3 from the cluster center was detected. It had an amplitude of Δ V=0.75 mag and a characteristic time of 15.9 days. Based on model considerations we show that the most likely configuration has the source in the Galactic bulge with the lens in the cluster. Two outbursts were observed for the already known dwarf nova CV1.

12

A Low-Resolution Spectroscopic Exploration of Puzzling OGLE Variable Stars

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

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2015

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tom 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

CURiuos Variables Experiment (CURVE): Variable Stars in the Metal-Poor Globular Cluster M56

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Pietrukowicz P. , Olech A. , Kędzierski P. , Złoczewski K. , Wiśniewski M. , Mularczyk K.

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2008

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tom Vol. 58, No. 2

121--130

EN

We surveyed a 6.5'×6.5' field centered on the globular cluster M56 (NGC 6779) in a search for variable stars detecting seven variables, among which two objects are new identifications. One of the new variables is an RRLyrae star, the third star of that type in M56. Comparison of the new observations and old photometric data for an RV Tauri variable V6 indicates a likely period change in the star. Its slow and negative rate of -0.005±0.003 d/yr would disagree with post-AGB evolution, however this could be a result of blue-loop evolution and/or random fluctuations of the period.

14

The Cluster AgeS Experiment (CASE). Detecting Aperiodic Photometric Variability with the Friends of Friends Algorithm

38%

Rozyczka M. , Narloch W. , Pietrukowicz P. , Thompson I. B. , Pych W. , Poleski R.

Acta Astronomica

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2018

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tom Vol. 68, No. 1

63--78

EN

We adapt the friends of friends algorithm to the analysis of light curves, and show that it can be succesfully applied to searches for transient phenomena in large photometric databases. As a test case we search OGLE-III light curves for known dwarf novae. A single combination of control parameters allows us to narrow the search to 1% of the data while reaching a ≈90% detection efficiency. A search involving ≈2% of the data and three combinations of control parameters can be significantly more effective - in our case a 100% efficiency is reached. The method can also quite efficiently detect semi-regular variability. In particular, 28 new semi-regular variables have been found in the field of the globular cluster M22, which was examined earlier with the help of periodicity-searching algorithms.

15

A New Lower Main Sequence Eclipsing Binary with Detached Components

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Rozyczka M. , Kaluzny J. , Pietrukowicz P. , Pych W. , Mazur B. , Catelan M. , Thompson I. B.

Acta Astronomica

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2009

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tom Vol. 59, No. 4

385--401

EN

We present an analysis of NGC2204-S892 - a new detached eclipsing binary composed of two late K dwarfs. Based on three photometric campaigns launched in 2008 we obtained five light curves (three in V, one in B and one in I), and derived an orbital period. We also obtained 20 VLT/UVES spectra, enabling accurate radial velocity measurements. The derived masses and radii of the components are consistent with the empirical mass-radius relationship established recently for lower main sequence stars in binary systems; in particular we find that both stars are oversized compared to theoretical models. NGC2204-S892 is very active: both components show variable emission in Hα and Hβ and are heavily spotted, causing the light curve to show appreciable changes on a timescale of weeks. Our results add to the increasing evidence that the observed inflation of the radii of K and M stars is related to high levels of magnetic activity.

16

One Thousand New Dwarf Novae from the OGLE Survey

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

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2015

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tom Vol. 65, No. 4

313--328

EN

We present one of the largest collections of dwarf novae (DNe) containing 1091 objects that have been discovered in the long-term photometric data from the Optical Gravitational Lensing Experiment (OGLE) survey. They were found in the OGLE fields toward the Galactic bulge and the Magellanic Clouds. We analyze basic photometric properties of all systems and tentatively find a population of DNe from the Galactic bulge. We identify several dozen of WZ Sge-type DN candidates, including two with superhump periods longer than 0.09 d. Other interesting objects include SU UMa-type stars with "early" precursor outbursts or a Z Cam-type star showing outbursts during standstills. We also provide a list of DNe which will be observed during the K2 Campaign 9 microlensing experiment in 2016. Finally, we present the new OGLE-IV real-time data analysis system: CVOM, which has been designed to provide continuous real time photometric monitoring of selected CVs.

17

CURious Variables Experiment (CURVE). CCD Photometry and Variable Stars in the Field of Open Cluster NGC 637

32%

Pietrukowicz P. , Olech A. , Wiśniewski M. , Kędzierski P. , Mularczyk K. , Złoczewski K. , Starczewski S. , Szaruga K.

Acta Astronomica

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2006

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tom Vol. 56, No. 3

267--275

EN

We present VI photometry for the open cluster NGC 637 which is located in the Cassiopeia region. Morphology of cluster color-magnitude diagram indicates that it is a young object with age of a few million years. The apparent distance modulus of the cluster is 13.9<(m-M)V<14.3 mag, while reddening is 0.69

18

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. Type II Cepheids in the Galactic Bulge - Supplement

32%

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

Acta Astronomica

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2013

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tom Vol. 63, No. 1

37--40

EN

We report the discovery of additional 22 RV Tau stars located in the OGLE-II and OGLE-III fields toward the Galactic bulge, increasing to 357 objects the OGLE-III catalog of type II Cepheids in the Galactic center. Four of the newly detected RV Tau stars belong to the RVb class, i.e., they show large-amplitude, long-period modulation of the mean luminosity. In the updated catalog, the relative number of RV Tau stars in the whole sample of the Galactic bulge type II Cepheids is similar to the Magellanic Cloud samples.

19

The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XIV. Classical and Type II Cepheids in the Galactic Bulge

32%

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

Acta Astronomica

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2011

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tom Vol. 61, No. 4

285--301

EN

The fourteenth part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains Cepheid variables detected in the OGLE-II and OGLE-III fields toward the Galactic bulge. The catalog is divided into two main categories: 32 classical Cepheids (21 single-mode fundamental-mode F, four first-overtone 1O, two double-mode F/1O, three double-mode 1O/2O and two triple-mode 1O/2O/3O pulsators) and 335 type II Cepheids (156 BL Her, 128 W Vir and 51 RV Tau stars). Six of the type II Cepheids likely belong to the Sagittarius Dwarf Spheroidal Galaxy. The catalog data include the time-series photometry collected in the course of the OGLE survey, observational parameters of the stars, finding charts, and cross-identifications with the General Catalogue of Variable Stars. We discuss some statistical properties of the sample and compare it with the OGLE catalogs of Cepheids in the Large and Small Magellanic Clouds. Multi-mode classical Cepheids in the Galactic bulge show systematically smaller period ratios than their counterparts in the Magellanic Clouds. BL Her in the Galactic bulge stars seem to be brighter than the linear extension of the period-luminosity relations defined by the longer-period type II Cepheids. We also show individual stars of particular interest, like two BL Her stars with period doubling.

20

Dwarf Nova V1040 Centauri and Variable Stars in its Vicinity

32%

Rutkowski A. , Pietrukowicz P. , Olech A. , Ak T. , Złoczewski K. , Poleski R. , Tappert C. , Eker Z.

Acta Astronomica

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2011

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tom Vol. 61, No. 4

345--357

EN

We present the results of a photometric campaign of the dwarf nova V1040 Cen. The light curve shows two normal outbursts with recurrence time ≈40 days and amplitude ≈2.5 mag. Quiescence data show oscillations with periods in the range ≈0.1 days (2.4 h) to ≈0.5 days (12 h) of unknown origin. We measured the orbital period of V1040 Cen to be Porb=0.060458(80) days (1.451±0.002 h). Based on the MV-Porb relation we found the distance of V1040 Cen to be 137±31 pc. In this paper we also report the detection of eleven new variable stars in the field of the monitored dwarf nova.