University of Warsaw

About: University of Warsaw is a education organization based out in Warsaw, Poland. It is known for research contribution in the topics: Population & Large Hadron Collider. The organization has 20832 authors who have published 56617 publications receiving 1185084 citations. The organization is also known as: Uniwersytet Warszawski & Warsaw University.

Eclipsing binaries in the All Sky Automated Survey catalogue

Abstract: The All Sky Automated Survey (ASAS) is a long-term project to monitor bright variable stars over the whole sky. It has discovered 50 099 variables brighter than V < 14 mag south of declination +28 ◦ , and among them 11 076 eclipsing binaries. We present a preliminary analysis of 5384 contact, 2949 semi-detached, and 2743 detached systems. The statistics of the distribution provides a qualitative confirmation of decades old idea of Flannery and Lucy that the W UMa-type binaries evolve through a series of relaxation oscillations: the ASAS finds comparable number of contact and semi-detached systems. The most surprising result is a very small number of detached eclipsing binaries with periods P < 1 d, the systems believed to be the progenitors of the W UMa stars. As many (perhaps all) contact binaries have companions, there is a possibility that some were formed in a Kozai cycle, as suggested by Eggleton and his associates.

Classical Cepheid Pulsation Models. X. The Period-Age Relation

Abstract: We present new period-age (PA) and period-age-color (PAC) relations for fundamental and first-overtone classical Cepheids. Current predictions rely on homogeneous sets of evolutionary and pulsation models covering a broad range of stellar masses and chemical compositions. We found that PA and PAC relations present a mild dependence on metal content. Moreover, the use of different PA and PAC relations for fundamental and first-overtone Cepheids improves the accuracy of age estimates in the short-period (log P < 1) range (old Cepheids), because they present smaller intrinsic dispersions. At the same time, the use of the PAC relations improves the accuracy in the long-period (log P ≥ 1) range (young Cepheids), since they account for the position of individual objects inside the instability strip. We performed a detailed comparison between evolutionary and pulsation ages for a sizable sample of LMC (15) and SMC (12) clusters which host at least two Cepheids. In order to avoid deceptive uncertainties in the photometric absolute zero point, we adopted the homogeneous set of B, V, and I data for clusters and Cepheids collected by OGLE. We also adopted the same reddening scale. The different age estimates agree at the level of 20% for LMC clusters and of 10% for SMC clusters. We also performed the same comparison for two Galactic clusters (NGC 6067, NGC 7790), and the difference in age is smaller than 20%. These findings support the use of PA and PAC relations to supply accurate estimates of individual stellar ages in the Galaxy and in external Galaxies. The main advantage of this approach is its independence from the distance.

In-plane uniaxial anisotropy rotations in (Ga,Mn)As thin films

Abstract: We show, by superconducting quantum interference device magnetometry, that in (Ga,Mn)As films the in-plane uniaxial magnetic easy axis is consistently associated with particular crystallographic directions and that it can be rotated from the $[\overline{1}10]$ direction to the [110] direction by low-temperature annealing. We show that this behavior is hole-density dependent and does not originate from surface anisotropy. The presence of uniaxial anisotropy as well its dependence on the hole concentration and temperature can be explained in terms of the $p\text{\penalty1000-\hskip0pt}d$ Zener model of the ferromagnetism assuming a small trigonal distortion.