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Variable Stars in M13. III. The Cepheid Variables and their Relation to Evolutionary Changes in Metal-Poor BL Her Stars

Osborn W., Kopacki G., Smith H. A., Layden A., Pritzl B., Kuehn C., Anderson M.

Acta Astronomica

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2019

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Vol. 69, No. 2

101--133

EN

New CCD photometry has been combined with published and unpublished earlier observations to study the three Cepheid variables in M13: V1, V2 and V6. The light curve characteristics in B, V and IC have been determined and the periods updated. A period change analysis shows all three stars have increasing periods but for V1 and V2 the rate of period increase does not appear to be constant over the 118 years of observation. The observed rates of period increase are in good agreement with the predictions of the Pisa theoretical models with helium abundance Y=0.25. Theory suggests V1 and V6 have masses of ≈0.57 M⊙ and are in the redward-evolving final stage of the "blue loop" evolutionary phase that is produced when helium-shell ignition occurs. The larger period and period change rate for V2 indicate it has a mass of ≈0.52 M⊙. A study of eighteen metal-poor BL Her stars shows the observed period changes for such objects in general can be reasonably well explained using the predictions from horizontal branch evolutionary tracks. BL Her stars with periods less than ≈3 d and relatively large secular period change rates (dP/dt≈5-15 d/Myr) are in the evolutionary stage before He-shell ignition. The remaining cases are stars that have already experienced He-shell ignition. Moreover, an analysis of crossing time through the instability strip indicates that it is likely that few, if any, BL Her stars have a He abundance as large as Y=0.33.

2

Variable Stars in M13. II. The Red Variables and the Globular Cluster Period-Luminosity Relation

Osborn W., Layden A., Kopacki G., Smith H., Anderson M., Kelly A., McBride K., Pritzl B.

Acta Astronomica

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2017

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Vol. 67, No. 2

131--158

EN

New CCD observations have been combined with archival data to investigate the nature of the red variables in the globular cluster M13. Mean magnitudes, colors and variation ranges on the UBVIC system have been determined for the 17 cataloged red variables. 15 of the stars are irregular or semi-regular variables that lie at the top of the red giant branch in the color-magnitude diagram. Two stars are not, including one with a well-defined period and a light curve shape indicating it is an ellipsoidal or eclipsing variable. All stars redder than (V-IC)0=1.38 mag vary, with the amplitudes being larger with increased stellar luminosity and with bluer filter passband. Searches of the data for periodicities yielded typical variability cycle times ranging from 30 d up to 92 d for the most luminous star. Several stars have evidence of multiple periods. The stars' period-luminosity diagram compared to those from microlensing survey data shows that most M13 red variables are overtone pulsators. Comparison with the diagrams for other globular clusters shows a correlation between red variable luminosity and cluster metallicity.

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Wide-Field Variability Survey of the Globular Cluster M 79 and a New Period-Luminosity Relation for SX Phe Stars

Kopacki G.

Acta Astronomica

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2015

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Vol. 65, No. 1

81--106

EN

We present the results of a search for variable stars in a 26×39 arcmin² field around globular cluster M 79 (NGC 1904). The search was made by means of an extended version of image subtraction, which allows us to analyze in a uniform manner CCD frames obtained with different telescopes and cameras of different sizes and resolutions. The search resulted in finding 20 new variable stars, among which 13 are cluster members. The members include one new RR Lyr star of subtype c, three SX Phe stars, and nine variable red giants. We also show that V7 is a W Vir star with a period of 13.985 d. Revised mean periods of RRab and RRc stars, ⟨P⟩_ab=0.71 d and ⟨P⟩=0.34 d, respectively, and relative percentage of RRc stars, N_c/(N_ab+N_c)=45% confirm that M 79 belongs to the Oosterhoff II group of globular clusters. The mean V magnitude of the horizontal branch of M 79 based on ten RR Lyr stars has been estimated to be V=⟨V⟩_RR=16.11±0.03 mag. In one RRc star, V9, light changes with three close frequencies were detected, indicating excitation of nonradial modes. An SX Phe star, V18, is a double-mode pulsator with two radial modes excited, fundamental and first overtone. Moreover, we have discovered two SX Phe or δ Sct stars and one W UMa type system, all likely field objects. We also studied the period-luminosity relation for SX Phe stars. Using 62 fundamental and fundamentalized periods of radial double-mode and high-amplitude SX Phe stars known in Galactic globular clusters, we have derived the slope and zero point of this relation to be, -3.3±0.27 and 2.68±0.03 mag (at log(P/d)=-1.24), respectively.

4

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.

5

Variable Stars in the Globular Cluster M 80

Kopacki G.

Acta Astronomica

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2013

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Vol. 63, No. 1

91--107

EN

We present results of a search for variable stars in the globular cluster M 80. Application of the image subtraction method to the ground-based times-series of CCD frames resulted in finding nine new RR Lyr stars, six of the RRc and three of the RRab type, and four SX Phe variables. Revised mean period of RRab stars, hPiab=0.68 d, and relative percentage of RRc stars, Nc/(Nab+Nc)=53%, strongly confirm that M 80 belongs to the Oosterhoff II group of globular clusters. The mean V magnitude of the horizontal branch of M 80 based on the ten RR Lyr stars has been estimated to be VHB=hViRR=16.14±0.03 mag. In two pulsating stars, one of the RR Lyr type and the other of the SX Phe type, oscillations with two close frequencies were detected, indicating excitation of nonradial modes in these stars. Moreover, we discovered two W UMa or ellipsoidal systems, two periodic stars of unknown type, one of which is probably a field star, and detected light variations in three red giants of the cluster.

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New Observations and Period Change Study for the Anomalous Cepheid in M 92

Osborn W., Kopacki G., Haberstroh J.

Acta Astronomica

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2012

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Vol. 62, No. 4

377--388

EN

New observations of the variable star V7 in the globular cluster M 92 have been used to determine the parameters of its B, V and IC light curves. The star's classification as an anomalous Cepheid type is confirmed. The asymmetric shape of the light curve and position in the observed period-luminosity relation for anomalous Cepheids indicate the star is pulsating in the fundamental mode. Data from photographic plates, extending back to 1900, have been combined with published material and recent CCD observations to carry out a period change study. No evidence of a period change is found. Expected period change rates have been calculated using theoretical relations for the pulsational periods of anomalous Cepheids and evolutionary tracks for metal-poor horizontal branch stars with masses 1.0≤M/Msun≤1.6. Only models for stars close to central helium exhaustion show measurable rates, which indicates V7 is in the core helium burning phase of evolution.

7

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VIII. NGC 6834

Jerzykiewicz M., Kopacki G., Pigulski A., Kołaczkowski Z., Kim S.-L.

Acta Astronomica

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2011

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Vol. 61, No. 3

247--273

EN

We present results of a CCD variability search in the field of the young open cluster NGC 6834. We discover 15 stars to be variable in light. The brightest, a multiperiodic γ Doradus-type variable is a foreground star. The eight fainter ones, including a γ Cassiopeiae-type variable, two λ Eridani-type variables, an ellipsoidal variable, an EB-type eclipsing binary, and three variable stars we could not classify, all have E(B-V) within proper range, thus fulfilling the necessary condition to be members. One of the three unclassified variables may be a non-member on account of its large angular distance from the center of the cluster. Four of the six faintest variable stars, which include two eclipsing binaries and two very red stars showing year-to-year variations, are certain non-members. One of the remaining two faintest variable stars, an EA-type eclipsing binary may be a member, while the faintest one, a W Ursae Majoris-type variable, is probably a non-member. For 6937 stars we provide the V magnitudes and V-IC color indices on the standard system. Because of nonuniform reddening over the cluster's face, a direct comparison of these data with theoretical isochrones is not possible. We therefore obtain E(B-V) from available UBV photometry, determine the lower and upper bound of E(B-V) for NGC 6834, and then fit properly reddened Padova isochrones to the data. Assuming HDE 332843, an early-F supergiant, to be a member we obtain log(age/yr)=7.70, V0-MV=12.10 mag. For 103 brightest stars in our field we obtained the α index, a measure of the equivalent widths of the Hα line. We find Hα emission in five stars, including the γ Cas-type variable and the two λ Eri-type variables.

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Photometric Study of Variable Stars in the Open Cluster NGC 6866

Molenda-Żakowicz J., Kopacki G., Stęślicki M., Narwid A.

Acta Astronomica

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2009

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Vol. 59, No. 2

193--211

EN

We report the discovery of 19 variable stars and two blue stragglers in the field of the open cluster NGC 6866. Three of the variable stars we classify as δ Sct, two as γ Dor, four as W UMa, two as ellipsoidal variables, and one as an eclipsing binary. Seven stars show irregular variability. Two of the pulsators, δ Sct star NGC 6866-29 and γ Dor star NGC 6866-21, are multiperiodic. From an analysis of proper motions, we conclude that the δ Sct stars, one of the γ Dor stars and both blue stragglers are very probable members of the cluster. The position on the color-magnitude diagram of seven other variables suggests that they also belong to the cluster. The eclipsing binary, which we discover to be a new high velocity star, and the seven irregular variables are non-members. We discuss in detail the age and metallicity of open clusters that host γ Dor stars and we show that none of these parameters is correlated with the number of γ Dor stars in cluster.

9

Variable Stars in the Open Cluster NGC 7044

Kopacki G., Drobek D., Kołaczkowski Z., Połubek G.

Acta Astronomica

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2008

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Vol. 58, No. 4

373--388

EN

We present results of a search for variable stars in the intermediate-age open cluster NGC 7044. We found 23 variable stars in the observed field. One star turned out to be of the δ Sct type with two pulsational modes excited. From the position in the color-magnitude diagram we conclude that this star is a member of the cluster. Moreover, we found 13 eclipsing systems, of which five are W UMa stars, one is a β Lyr variable, six are β Per binaries showing detached configuration, and the last one is another probable β Per system. Using the period-luminosity-color relation for W UMa stars we established the membership of the contact binaries, finding four of them to be very probable cluster members. We estimated from these four stars an apparent distance modulus (m-M)_V of NGC 7044 to be 14.2±0.4 mag, which is smaller than previous determinations of this parameter. We were able to derive orbital period for only four β Per systems. For the remaining ones we observed only two or three eclipses. Finally, nine stars we found show irregular light changes. Most of them are red stars not belonging to the cluster. For the cluster core we determined a reddening map, which allowed us to construct a dereddened color-magnitude diagram of NGC 7044 with a narrow main sequence. By fitting a theoretical isochrone to this diagram we derived E(V-IC)=0.92 mag, (m-M)V=14.45 mag and log(τ/yr)=9.2.

10

SX Phoenicis Stars in the Globular Cluster M 92

Kopacki G.

Acta Astronomica

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2007

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Vol. 57, No. 1

49--60

EN

We present results of a search for SX Phe stars in the globular cluster M 92. In photometric reductions, we used the image subtraction method supplemented with a procedure of searching for short-period variables, based on the examination of each pixel in the CCD frame. In addition to two SX Phe stars already known in the cluster, we have found five new variables of this type. One SX Phe variable, v34, shows double-mode pulsation with a period ratio equal to 0.741 or 0.790. The second value would indicate oscillations in radial fundamental and first overtone modes. Combining our ground based data and the WFPC HST observations, we determine the average V magnitudes and (V-IC) colors for three SX Phe stars. Moreover, from an estimate of the scatter in the HST data of all observed blue stragglers we suspect three stars to be variable.

11

SX Phoenicis Stars in the Globular Cluster M13

Kopacki G.

Acta Astronomica

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2005

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Vol. 55, No. 1

85--95

EN

We present results of the search for SX Phe stars in the globular cluster M13. In photometric reductions we used the image subtraction method supplemented with procedure of searching for short-period variable stars, based on the examination of each pixel in the CCD frames. We illustrate also the ability of this approach to find variability with small signal-to-noise ratio. Using this method we were able to derive the light curves for all three SX Phe stars known in the cluster. Moreover, we discovered another star of this type. Except v37, all detected SX Phe stars have full-amplitudes in V smaller than 0.25 mag. Combining our ground-based data and the HST observations we determined the average V magnitudes for these stars. One SX Phe star, v46, shows double-mode pulsations which are very likely the radial fundamental mode and the first overtone.

12

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VI. NGC 6910

Kołaczkowski Z., Pigulski A., Kopacki G., Michalska G.

Acta Astronomica

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2004

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Vol. 54, No. 1

33--55

EN

We present results of variability search in the field of the young open cluster NGC 6910. We found four &beta Cep-type stars in the cluster, which makes it rather exceptional among the northern clusters observed in our project. Two of these &beta Cep stars show three modes with amplitudes above the detection level, in the remaining two we detect single modes. We discuss the possibility that the large number of &beta Cep stars in NGC 6910 is due to higher metallicity of the cluster. In addition, we found four other variables, one ellipsoidal or eclipsing binary and three that show irregular variations. The latter three stars have Hα in emission. We also provide VIC photometry for 139 stars down to V=16.6 mag and Hα photometry for 73 stars. Using this photometry we estimate the cluster age to be equal to 6±2 Myr, and the distance modulus, 11.0±0.3 mag. The reddening is high and variable across the cluster. In terms of the E(V-IC) color-excess it amounts to 1.25-1.7 mag, corresponding to E(B-V) between 1.0 mag and 1.4 mag. The average reddening in the central field is E(V-IC)=1.47±0.04 mag.

13

Reanalysis of the OGLE-I Observations with the Image Subtraction Method. I. Galactic Bar Fields MM1-A, MM1-B, MM7-A, and MM7-B

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2003

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Vol. 53, No. 1

27--50

EN

As a result of the reanalysis of the OGLE-I observations by means of the image subtraction method, we present the first part of a catalog, consisting of data for variable stars in four Galactic fields observed by OGLE-I, viz. MM1-A, MM1-B, MM7-A, and MM7-B. In total, 2016 variable stars have been found. This increased the number of known variable stars in these fields more than twofold. We comment on the detectability of the variable stars in previous studies. Some interesting findings are also discussed. Among others, we found 45 δ Sct stars (38 are new) including several multiperiodic objects. Detailed analysis of the light curves of 47 RR Lyr stars (24 are new detections) allowed us to find five stars which exhibit nonradial pulsations and one RRd star. Three RRab stars are members of the Sagittarius dwarf galaxy (Sgr dSph). We also find four objects which are probably Galactic RV Tau stars and one W Vir star which seems to belong to Sgr dSph. This is the first Population II Cepheid found in this satellite galaxy and dSph other than the Fornax one. For eclipsing EW-type binaries, which are the most abundant variables in our catalog, we investigate the amplitude and period distributions. A comparison with the previous OGLE-I catalogs indicates that we found more stars with smaller amplitudes. Finally, in addition to the two microlensing events discovered previously in these fields, we find five more.

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A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. V. NGC 2169

Jerzykiewicz M., Kopacki G., Molenda-Żakowicz J., Kołaczkowski Z.

Acta Astronomica

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2003

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Vol. 56, No. 2

151--165

EN

We present results of a search for variable stars in the field of the young open cluster NGC 2169. The General Catalogue of Variable Stars (GCVS, http://www.sai.msu.su/groups/cluster/gcvs/) lists four variable stars in the field we observed, viz., two β Cep stars, V 916 and V 917 Ori, an α2 CVn variable, V 1356 Ori, and an RRc variable, V 1154 Ori. We find V 916 and V 1154 Ori to be constant in light. We confirm the variability of V 917 Ori, but not the period given in GCVS. For the chemically peculiar A0 V Si star V 1356 Ori we definitely establish the period of 1.565 d, thus settling the uncertainty persisting in the literature since the star was discovered to be variable. In addition, we find two other stars to be variable in light. Both show irregular variations. For V 917 Ori, one of the two GCVS &beta Cep variables, we determine a period of 0.267 d (frequency 3.7477 d-1). However, prewhitening with this period leaves a significant amount of the star's light-variation unaccounted for. Since the star shows emission at Hα, we hypothesize that the unaccounted for variation is caused by an erratic, Be-type activity. As to the periodic term, we consider three hypotheses: (1) β Cep-type pulsation, (2) rotational modulation of the λ Eri type, and (3) ellipsoidal variation due to distorted primary component in a close binary system. After deriving the star's effective temperature from Strömgren indices and the luminosity from the distance modulus of the cluster, we show that while the third hypothesis is untenable, the first two should be retained. However, neither is entirely satisfactory. For a number of stars we provide the V magnitudes. For 14 brightest stars in our field we also obtain the photometric α-index, a measure of the Hα equivalent width. From the α index, we detect mild emission at Hα in two stars, V 917 Ori and NGC 2169-8.

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A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. IV. NGC 663

Pigulski A., Kopacki G., Kołaczkowski Z.

Acta Astronomica

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2001

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Vol. 51, No. 2

159--174

EN

We present results of the variability search in the field of the young open cluster NGC 663. In addition to the one β Cep-type variable known in this cluster, we found another one. It is a mono-periodic pulsator changing brightness with a period of 0.27640 d. In total, 19 new variables were discovered and the variability of 5 other ones was confirmed. Out of all 24 variables in the observed field, 21 are probable cluster members. One SPB candidate and three eclipsing or ellipsoidal variables could be classified. Moreover, ten out of fourteen Be stars we observed vary in brightness. Only one of them shows periodic variations of the λ Eri-type, while the remaining ones exhibit irregular changes with the range up to 0.4 mag in the IC band. We also provide new VIC photometry of 477 stars in the field of the cluster and check the consistence of the present photometry with cluster parameters derived earlier. The average cluster E(V-IC) color excess amounts to about 0.92 mag.

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Erratum: A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2000

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Vol. 50, No. 2

278

EN

In the paper ”A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823” (Acta Astron., 50, p. 113) the entries in the last column of Table 2 (times of maximum light) are too large by 1000 days. The correct Table 2 follows:

17

A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. III. NGC 6823

Pigulski A., Kołaczkowski Z., Kopacki G.

Acta Astronomica

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2000

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Vol. 50, No. 1

113--132

EN

We present the results of variability search in the field of a very young open cluster NGC 6823. Two δ Sct stars, still at the pre-main sequence (PMS) stage of evolution, were found. These objects could be used for future testing of the evolutionary period changes in this class of variable stars. In addition, we found 13 other variables including a bright cluster eclipsing binary and an SPB candidate. A few other variables could be PMS stars of the UX Ori type. The Hα photometry, which we made for 69 brightest stars, revealed only one object with strong emission, discovered recently by the photographic methods. Two other stars announced to have Hα emission, do not show any evidence for its presence. For a number of stars in the search field, we also provide the BV(RI)C photometry. We explain how the distribution of absorbing matter along the line of sight results in the unusual morphology of the cluster color-magnitude (CM) diagram. The dereddened CM diagram is used in the calculation of the extinction map for the observed field, in which the E(R-I)C color excess varies from 0.54 to 0.72 mag, with the average value equal to 0.62 mag. It is also shown that all cluster stars with spectral types later than A0 are PMS objects. We use these stars to estimate the cluster age: 3±1 Myr. Using the cluster CM diagram, we compare and discuss the position of the two discovered δ Sct stars with respect to the theoretical instability strip for PMS stars of this type.

18

V879 Aql: a W UMa-type Eclipsing Binary System

Kopacki G., Pigulski A.

Acta Astronomica

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1998

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Vol. 48, No. 4

747--755

EN

We present new CCD observations of V879 Aql. Our data clearly show that this star, hitherto classified as a δ Scuti variable, is in fact an eclipsing binary system of the W UMa type. From the photometric point of view it belongs to subclass A. Analysis of the light curves with the Wilson-Devinney program indicates contact geometry, total eclipse, and mass ratio equal to 1.97. The smaller secondary is slightly hotter than the primary and the temperature difference amounts to about 90 K. As a result of a transformation to the standard system, we obtained V magnitudes and (V-I) color indices for 211 stars in the field of V879 Aql. Among them we found two new red variables.