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1

Rotation of δ Scuti Stars in the Open Clusters NGC 1817 and NGC 7062

Molenda-Żakowicz J., Arentoft T., Frandsen S., Grundahl F.

Acta Astronomica

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2009

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Vol. 59, No. 1

69--88

EN

We report results of spectroscopic and photometric observations of ten δ Sct stars and one eclipsing binary in the open cluster NGC 1817, and of ten δ Sct stars and two other variables in the open cluster NGC 7062. For all targets in NGC 1817 and for three targets in NGC 7062 the radial velocity and projected rotational velocity are determined. For all stars the effective temperature and surface gravity is measured. Two δ Sct stars, NGC 1817 -V1 and NGC 7062 -V1, and the eclipsing binary, NGC 1817 -V18, are discovered to be single-lined spectroscopic binaries. The eclipsing binary δ Sct star NGC 1817 -V4 is discovered to be a double-lined spectroscopic binary. All δ Sct stars which we observed spectroscopically are found to be moderate or fast rotators.

2

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.

3

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.

4

Multimode Resonant Coupling in Pulsating Stars

Nowakowski R. M.

Acta Astronomica

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2005

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Vol. 55, No. 1

1--41

EN

We consider evolution of an unstable acoustic mode interacting with an ensemble of stable g-modes. We show that the static multimode solution does not exist. We then find the condition for the stability of the statistical equilibrium. Performing numerical integration of amplitude equations for a simplified system we find that the acoustic mode amplitude exhibits a large irregular variability on the timescale given by the inverse of the growth rate. The g-mode pairs are excited in significantly wider range of detuning parameters than it is implied by the parametric instability criterion applied to the average amplitude. However, the number of interacting g-mode pairs is reduced because the pairs differing in the detuning parameter by less than their damping rates are synchronized and effectively act as a single pair. We apply the multimode resonant coupling theory to a realistic stellar model. We choose a seismic model of a &delta Sct star XX Pyx. Although for some l=2 modes we find amplitudes of the order of a few millimagnitudes, the typical amplitudes of low-degree modes are much higher. Taking into account the rotational splitting results in decrease of amplitudes by a factor of few which is not enough to obtain consistency with observations. We conclude that in this star and likely in all evolved &delta Sct stars, the resonant mode coupling cannot be the dominant amplitude limiting effect. The nonresonant saturation of the driving effect must play the role.

5

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.

6

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:

7

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.