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1

Double-Overtone Cepheids in the Large Magellanic Cloud

Dziembowski W. A., Smolec R.

Acta Astronomica

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2009

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Vol. 59, No. 1

19--31

EN

One of the most interesting results from the OGLE-III study of the LMC Cepheids is the large number of objects that pulsate simultaneously in the first and second overtone (denoted 1O/2O). Double-mode Cepheids yield important constraint on stellar evolution models. We show that great majority of the LMC 1O/2O Cepheids have masses M=3.0±0.5 Msolar. According to current stellar evolution calculations these masses are lower than needed for the blue loop in the helium burning phase to reach the instability strip. On the other hand, we found most of these stars significantly overluminous if they are crossing the instability strip before helium ignition. A possible solution of this discrepancy is to allow for a large overshooting from the convective core in the main sequence phase. We also discuss origin of double-mode pulsation. At the short period range we find two types of resonances that are conducive to this form of pulsation. However, at longer periods, it has a different (non-resonant) origin.

2

The Optical Gravitational Lensing Experiment. Triple-Mode and 1O/3O Double-Mode Cepheids in the Large Magellanic Cloud

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

Acta Astronomica

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2008

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Vol. 58, No. 3

153--162

EN

We report the discovery of three new triple-mode classical Cepheids in the Large Magellanic Cloud, two of them with the fundamental, first overtone and second overtone excited, and one pulsating simultaneously in the first three overtones. Thus, the number of triple-mode Cepheids in the LMC is increased to five. We also present two objects belonging probably to a new type of double-mode Cepheids having the first and third overtones excited. We measure the rates of period change in these stars and detect decrease of periods in two of them, what is in conflict with theoretical predictions.

3

Coronal Activity from the ASAS Eclipsing Binaries

Szczygieł D. M., Socrates A., Paczyński B., Pojmański G., Pilecki B.

Acta Astronomica

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2008

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Vol. 58, No. 4

405--418

EN

We combine the catalog of eclipsing binaries from the All Sky Automated Survey (ASAS) with the ROSAT All Sky Survey (RASS). The combination results in 836 eclipsing binaries that display coronal activity and is the largest sample of active binary stars assembled to date. By using the (V-I) colors of the ASAS eclipsing binary catalog, we are able to determine the distances and thus bolometric luminosities for the majority of eclipsing binaries that display significant stellar activity. A typical value for the ratio of soft X-ray to bolometric luminosity is L_X/L_bol≈ a few ×10^-4, similar to the ratio of soft X-ray to bolometric flux F_X/F in the most active regions of the Sun. Unlike rapidly rotating isolated late-type dwarfs - stars with significant outer convection zones - a tight correlation between Rossby number and activity of eclipsing binaries is absent. We find evidence for the saturation effect and marginal evidence for the so-called ``super-saturation'' phenomena. Our work shows that wide-field stellar variability searches can produce a high yield of binary stars with strong coronal activity.

4

The Low-Mass Limit for Total Mass of W UMa-type Binaries

Stępień K.

Acta Astronomica

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2006

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Vol. 56, No. 4

347--364

EN

The observations of W UMa type stars show a well-defined short-period limit of 0.22 d, which is equivalent to a lower mass limit of approximately 1 Msolar for the total binary mass. It is currently believed that cool contact binaries are formed from detached binaries losing angular momentum (AM) via a magnetized wind. Orbital evolution of detached binaries with various component masses was followed until the primary component reached the critical Roche surface and the Roche lobe overflow (RLOF) began. It was assumed that the minimum initial, i.e., ZAMS, orbital period of such binaries is equal to 2 d and that the components lose AM just as single stars. According to the mass-dependent formula for AM loss rate of single stars, derived in this paper, the AM loss time scale increases substantially with decreasing stellar mass. The formula was applied to binaries with the initial primary component masses between 1.0 Msolar and 0.6 Msolar and two values of mass ratio q=1 and 0.5. Detailed calculations show that the time needed to reach RLOF by a 1 Msolar primary is of the order of 7.5 Gyr, but it increases to more than 13 Gyr for a binary with an initial primary mass of 0.7 Msolar Binaries with less massive primaries have not yet had time to reach RLOF even within the age of the Universe. This sets a lower mass limit for the presently existing contact binaries at about 1.0 Msolar-1.2 Msolar, in a good agreement with observations.

5

On the Peculiarities of Evolutionary History of EHB Objects in Binary Systems with Hot Subdwarf Companions

Pustynski V.-V., Pustylnik I.

Acta Astronomica

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2006

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Vol. 56, No. 1

83--95

EN

It has been shown quite recently (Maxted et al. 2001, Morales-Rueda et al. 2003) that dB stars, extreme horizontal branch (EHB) objects, likely all belong to binary systems. We study in detail the mass and angular momentum loss in the giant progenitors of sdB stars in an attempt to clarify why binarity must be a crucial factor in producing EHB objects. Assuming that the progenitors of EHB objects belong to the binaries with initial separations of 100-150 Rsolar and fill in their critical Roche lobes while being close to the RGB tip we have found that considerable shrinkage of the orbit can be achieved due to the combined effect of angular momentum loss from the red giant and appreciable accretion on its low mass companion on the hydrodynamical time scale of the donor resulting in formation of helium white dwarfs with masses about 0.5 Msolar and thus evading the common envelope stage.

6

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.