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The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. IV. Long-Period Variables in the Large Magellanic Cloud

Soszyński I., Udalski A., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K., Poleski R.

Acta Astronomica

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2009

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Vol. 59, No. 3

239--253

EN

The fourth part of the OGLE-III Catalog of Variable Stars presents 91 995 long-period variables (LPVs) in the Large Magellanic Cloud (LMC). This sample consists of 79 200 OGLE Small Amplitude Red Giants (OSARGs), 11 128 semiregular variables (SRVs) and 1667 Mira stars. The catalog data include basic photometric and astrometric properties of these stars, long-term multi-epoch VI photometry and finding charts. We describe the methods used for the identification and classification of LPVs. The distribution of I-band amplitudes for carbon-rich stars shows two maxima, corresponding to Miras and SRVs. Such a distinction between Miras and SRVs is not obvious for oxygen-rich stars. We notice additional period-luminosity sequence located between Wood's sequences C and C' and populated by SRVs.

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The Optical Gravitational Lensing Experiment. Period-Luminosity Relations of Variable Red Giant Stars

Soszyński I., Dziembowski W. A., Udalski A., Kubiak M., Szymański M. K., Pietrzyński G., Wyrzykowski Ł., Szewczyk O., Ulaczyk K.

Acta Astronomica

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2007

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Vol. 57, No. 3

201--225

EN

Period-luminosity (PL) relations of variable red giants in the Large (LMC) and Small Magellanic Clouds (SMC) are presented. The PL diagrams are plotted in three planes: log P-KS, log P-WJK, and log P-WI, where WJK and WI are reddening free Wesenheit indices. Fourteen PL sequences are distinguishable, and some of them consist of three closely spaced ridges. Each of the sequences is fitted with a linear or quadratic function. The similarities and differences between the PL relations in both galaxies are discussed for four types of red giant variability: OGLE Small Amplitude Red Giants (OSARGs), Miras and Semiregular Variables (SRVs), Long Secondary Periods (LSPs) and ellipsoidal variables. We propose a new method of separating OSARGs from non-variable stars and SRVs. The method employs the position in the reddening-free PL diagrams and the characteristic period ratios of these multiperiodic variables. The PL relations for the LMC OSARG are compared with the calculated relations for RGB models along isochrones of relevant ages and metallicities. We also compare measured periods and amplitudes of the OSARGs with predictions based on the relations valid for less luminous solar-like pulsators. Miras and SRVs seem to follow PL relation of the same slopes in the LMC and SMC, while for LSP and ellipsoidal variables slopes in both galaxies are different. The PL sequences defined by LSP variables and binary systems overlap in the whole range of analyzed wavebands. We put forward new arguments for the binary star scenario as an explanation of the LSP variability and elaborate on it further. The measured pulsation to orbital period ratio implies nearly constant ratio of the star radius to orbital distance, R/A≈0.4, as we find. Combined effect of tidal friction and mass loss enhanced by the low-mass companion may explain why such a value is preferred.

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Evolutionary Status of Late-Type Contact Binaries

Stępień K.

Acta Astronomica

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2006

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Vol. 56, No. 2

199--218

EN

The old model of an unevolved, cool contact binary, in which the secondary component is strongly oversized due to energy transfer from the primary, and the whole system is out of thermal equilibrium, encounters serious problems. I present a new scenario for evolution of contact binaries, which solves the problem of thermal nonequilibrium by assuming that contact binaries are past mass exchange with a mass ratio reversal. The scenario is divided into three phases. In Phase I loss of angular momentum (AM) due to magnetized wind of a detached binary is followed until the primary component fills its critical Roche lobe. In Phase II mass transfer takes place until mass ratio reversal. Arguments are given in favor of such a process in pre-contact binaries. In Phase III an approximate evolutionary path of the contact binary is followed until a possible coalescence. AM loss, evolutionary effects of the components and mass transfer to the primary are taken into account. It is concluded that W UMa type binaries are old objects with secondaries in an advanced evolutionary stage, possibly with small helium cores. Both components fulfill the mass-radius relation for contact binaries while being in thermal equilibrium.