Wyniki wyszukiwania - BazTech

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

|

2009

|

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

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

|

2007

|

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.

3

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

Kiraga M., Stępień K.

Acta Astronomica

|

2007

|

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.

4

Evolutionary Status of Late-Type Contact Binaries

Stępień K.

Acta Astronomica

|

2006

|

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.