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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.

2

Variability Survey in the Open Cluster Stock 14 and the Surrounding Fields

Drobek D., Pigulski A., Shobbrook R. R., Narwid A.

Acta Astronomica

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2013

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Vol. 63, No. 3

339--368

EN

We present the results of a photometric variability survey in the young open cluster Stock 14 and the surrounding fields. In total, we detected 103 variable stars of which 88 are new discoveries. We confirm short-period, low-amplitude light variations in two eclipsing members of the cluster, HD 101838 and HD 101794. In addition, we find two new β Cep stars of which one, HD 101993, is also a member. The sample of pulsating cluster members is supplemented by one multimode slowly pulsating B-type star and several single-mode candidates of this type. The other pulsating stars in our sample are mostly field stars. In particular, we found fourteen δ Sct stars including one γ Dor/δ Sct hybrid pulsator. From our UBV photometry we derived new parameters of Stock 14: the mean reddening E(B-V)=0.21±0.02 mag, the true distance modulus, 11.90±0.05 mag, and the age, 20±10 Myr. Finally, we use the new photometry to analyze changes of the 6.322-d orbital period of the bright eclipsing binary and the member of the cluster, V346 Cen. In addition to the known apsidal motion, we find that another effect, possibly light-time effect in a hierarchical system of a very long orbital period, affects these changes. The updated value of the period of apsidal motion for this system amounts to 306±4 yr. The open cluster Stock 14 was found to be a fairly good candidate for successful ensemble asteroseismology.

3

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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The All Sky Automated Survey. The Catalog of Variable Stars in the Kepler Field of View

Pigulski A., Pojmański G., Pilecki B., Szczygieł D. M.

Acta Astronomica

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2009

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Vol. 59, No. 1

33--46

EN

We present the catalog of 947 variable stars located in the field of view of the Kepler satellite. The catalog is a result of the analysis of VI photometry obtained during the first 17-month observations in the ASAS3-North station. The variable stars we present are divided into eleven groups according to the presented variability; the groups are briefly discussed. The catalog is intended to be a source of information for target selection process and follow-up programs.

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A CCD Search for Variable Stars of Spectral Type B in the Northern Hemisphere Open Clusters. VII. NGC 1502

Michalska G., Pigulski A., Stęślicki M., Narwid A.

Acta Astronomica

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2009

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Vol. 59, No. 4

349--370

EN

We present results of variability search in the field of the young open cluster NGC 1502. Eight variable stars were discovered. Of six other stars in the observed field that were suspected for variability, we confirm variability of two, including one β Cep star, NGC 1502-26. The remaining four suspects were found to be constant in our photometry. In addition, UBVIC photometry of the well-known massive eclipsing binary SZ Cam was obtained. The new variable stars include: two eclipsing binaries of which one is a relatively bright detached system with an EA-type light curve, an α2 CVn-type variable, an SPB candidate, a field RR Lyr star and three other variables showing variability of unknown origin. The variability of two of them is probably related to their emission in Hα, which has been measured by means of the α index obtained for 57 stars brighter than V≈16 mag in the central part of the observed field. Four other non-variable stars with emission in Hα were also found. Additionally, we provide VIC photometry for stars down to V=17 mag and UB photometry for about 50 brightest stars in the observed field. We also show that the 10 Myr isochrone fits very well the observed color-magnitude diagram if a distance of 1 kpc and mean reddening, E(V-IC)=0.9 mag are adopted.

6

Pulsating Components of Eclipsing Binaries in the ASAS-3 Catalog

Pigulski A., Michalska G.

Acta Astronomica

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2007

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Vol. 57, No. 1

61--72

EN

As a result of the search among about 11 000 stars from the public ASAS-3 database, we report detection of pulsating components in eleven eclipsing binaries. In particular, we have found three classical Algols, MX Pav, IZ Tel, and VY Mic, with δ Sct-type primary components. In six other eclipsing binaries, the short-period variability can also be interpreted in terms of δ Sct-type pulsations, but in these systems both components are probably main-sequence stars. In HD 99612, the pulsation mode shows significant amplitude decrease during the time interval covered by observations. In addition, we find variability in one of the components of the eclipsing and double-lined spectroscopic O-type binary ALS 1135 which we interpret as a β Cep-type pulsation. Finally, we find Y Cir to be a good candidate for an SPB star in an eclipsing binary system.

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Physical Parameters of Components in Close Binary Systems: IV

Gazeas K. D., Baran A., Niarchos P., Zola S., Kreiner J. M., Ogloza W., Rucinski S. M., Zakrzewski B., Siwak M., Pigulski A., Dróżdż M.

Acta Astronomica

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2005

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Vol. 55, No. 1

123--140

EN

The paper presents new geometric, photometric and absolute parameters, derived from combined spectroscopic and photometric solutions, for ten contact binary systems. The analysis shows that three systems (EF Boo, GM Dra and SW Lac) are of W-type with shallow to moderate contact. Seven systems (V417 Aql, AH Aur, YY CrB, UX Eri, DZ Psc, GR Vir and NN Vir) are of A-type in a deep contact configuration. For six systems (V417 Aql, YY CrB, GM Dra, UX Eri, SW Lac and GR Vir) a spot model is introduced to explain the O'Connell effect in their light curves. The photometric and geometric elements of the systems are combined with the spectroscopic data taken at David Dunlap Observatory to yield the absolute parameters of the components.

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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.

9

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. 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.

11

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:

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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.

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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.