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Hα Imaging of X-ray Sources in Selected Globular Clusters with the SOAR Telescope

Pietrukowicz P.

Acta Astronomica

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2009

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Vol. 59, No. 3

291--305

EN

We present results of a search for objects with Hα excess, such as cataclysmic variables (CVs) and chromospherically active binaries (ABs), as counterparts to X-ray sources detected with Chandra satellite observatory in six Galactic globular clusters (GCs): M4, M28, M30, M71, M80, NGC 6752. Binary systems play a critical role in the evolution of GCs, serving as an internal energy source countering the tendency of GC cores to collapse. Theoretical studies predict dozens of CVs in the cores of some GCs (eg., 130 for M28, 40 for M30). A number of such binaries is also expected outside the core radius. However, few CVs are known so far in GCs. Using subtraction technique applied to images taken with the 4.1-m SOAR telescope we have found 27 objects with Hα excess in the field of the observed clusters, of which nine are likely associated with the clusters. Four are candidate CVs, four are candidate ABs, one could be either a CV or an AB. One Hα object seems to be a background galaxy, while other 17 detected objects are probably foreground or background stars.

2

Age-Rotation-Activity Relations for M Dwarf Stars Based on ASAS Photometric Data

Kiraga M., Stępień K.

Acta Astronomica

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2007

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Vol. 57, No. 2

149--172

EN

Based on analysis of photometric observations of nearby M type stars obtained with ASAS, 31 periodic variables were detected. The determined periods are assumed to be related to rotation periods of the investigated stars. Among them 10 new variables with periods longer than 10 days were found, which brings the total number of slowly rotating M stars with known rotation periods to 12 objects. X-ray activity and rotation evolution of M stars follows the trends observed in G-K type stars. Rapidly rotating stars are very active and activity decreases with increasing rotation period but the period-activity relation is mass-dependent which suggests that the rotation period alone is not a proper measure of activity. The investigated stars were grouped according to their mass and the empirical turnover time was determined for each group. It increases with decreasing mass more steeply than for K type stars for which a flat dependence had been found. The resulting Rossby number-activity relation shows an exponential decrease of activity with increasing Rossby number. The analysis of space motions of 27 single stars showed that all rapidly rotating and a few slowly rotating stars belong to young disk (YD) whereas all old disk (OD) stars are slowly rotating. The median rotation period of YD stars is about 2 days and that of OD stars is equal to 47 days, i.e., nearly 25 times longer. The average X-ray flux of OD stars is about 1.7 dex lower than YD stars in a good agreement with the derived Rossby number-activity formula supplemented with rotation-age relation and in a fair agreement with recent observations but in a disagreement with the Skumanich formula supplemented with the activity-rotation relation.

3

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.