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Photometric Study of the Star Cluster NGC 2155 in the Large Magellanic Cloud: Age Estimation and Variable Stars

Otulakowska M., Olech A., Pych W., Pamyatnykh A. A., Zdravkov T., Rucinski S. M.

Acta Astronomica

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2011

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Vol. 61, No. 2

161--177

EN

We present results of new photometry for the globular star cluster NGC 2155 in the Large Magellanic Cloud (LMC). Our I- and V-band observations were obtained with the 6.5-m Magellan-1 Baade Telescope at Las Campanas Observatory resulting in deep photometry down to V≈24 mag. By analyzing the color-magnitude diagram for the cluster and utilizing the Victoria-Regina grid of isochrones models we estimated the age of the cluster at ≅2.25 Gyr and [Fe/H]=-0.71, the numbers which place NGC 2155 outside the age-gap in the age-metallicity relation for LMC clusters. Using the Difference Image Analysis Package (DIAPL), we detected 7 variable stars in the cluster field with variability at the level of 0.01 mag in the I-band. Three variable stars are particularly interesting: two SX Phe stars pulsating in the fundamental mode, and a detached eclipsing binary which is a prime candidate to estimate the distance to the cluster.

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On the Prospects for Detection and Identification of Low-Frequency Oscillation Modes in Rotating B Type Stars

Daszyńska-Daszkiewicz J., Dziembowski W. A., Pamyatnykh A. A.

Acta Astronomica

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2007

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Vol. 57, No. 1

11--32

EN

We study how rotation affects observable amplitudes of high-order g- and mixed r/g-modes and examine prospects for their detection and identification. Our formalism, which is described in some detail, relies on a nonadiabatic generalization of the traditional approximation. Numerical results are presented for a number of unstable modes in a model of SPB star, at rotation rates up to 250 km/s. It is shown that rotation has a large effect on mode visibility in light and in mean radial velocity variations. In most cases, fast rotation impairs mode detectability of g-modes in light variation, as Townsend (2003b) has already noted, but it helps detection in radial velocity variation. The mixed modes, which exist only at sufficiently fast rotation, are also more easily seen in radial velocity. The amplitude ratios and phase differences are strongly dependent on the aspect, the rotational velocity and on the mode. The latter dependence is essential for mode identification.

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Pulsational Instability Domains in the Upper Main Sequence

Pamyatnykh A. A.

Acta Astronomica

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1999

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Vol. 49, No. 2

119--148

EN

Updated theoretical instability domains in the Hertzsprung-Russell and in the log g-log Teff diagrams for the β Cep, the SPB and the δ Sct star models are presented. The position of the instability domains is very sensitive to the chemical composition parameters (X,Z) and to the extent (dover) of the convective overshooting, due to influence of these effects on the evolution, in particular, on the location of the Zero-Age Main Sequence (ZAMS) and the Terminal-Age Main Sequence (TAMS). Also, the heavy element abundance, Z, is of critical importance on the instability itself for the β Cep and the SPB models. The comparison of theoretical and observational domains may be used to constrain these parameters.