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Post Info TOPIC: W UMa systems


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Posts: 131433
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RE: W UMa systems
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Title: Observations and light curve solutions of the W UMa binaries V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195
Author: D. P. Kjurkchieva, V. A. Popov, S. I. Ibryamov, D. L. Vasileva, N. I. Petrov

Photometric observations in Sloan g' and i' bands of four W UMa binaries, V796 Cep, V797 Cep, CSS J015341.9+381641 and NSVS 3853195, are presented. Our observations showed that CSS J015404.1+382805 and NSVS 3853195 are the same star. We determined the initial epochs T_{0} of all targets and improved the period of NSVS 3853195. The light curve solutions of our data revealed that the components of each target are almost the same in mass, temperature, radius and luminosity. The stellar components are of G and K spectral types and undergo partial eclipses. All systems have barely-overcontact configurations and belong to H subtype W UMa binaries. We established that the relation between the luminosity ratio l_{2}/l_{1} and mass ratio q of our targets is approximately l_{2}/l_{1} = q^{1.5}.

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Posts: 131433
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W UMa stars
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Title: Light curve solutions of six eclipsing binaries at the lower limit of periods of the W UMa stars
Author: Diana P. Kjurkchieva, Dinko P. Dimitrov, Sunay I. Ibryamov

Photometric observations in V and I bands of six eclipsing binaries at the lower limit of the orbital periods of W UMa stars are presented. Three of them are newly discovered eclipsing systems. The light curve solutions revealed that all short-period targets were contact or overcontact binaries and added new six binaries to the family of short-period systems with estimated parameters. Four binaries have equal in size components and mass ratio near 1. The phase variability of the V-I colours of all targets may be explained by lower temperatures of their back surfaces than those of their side surfaces. Five systems revealed O'Connell effect that was reproduced by cool spots on the side surfaces of their primary components. The light curves of V1067 Her in 2011 and 2012 were fitted by diametrically opposite spots. The applying of the criteria for subdivision of the W UMa stars to our targets led to ambiguous results.

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Posts: 131433
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W UMa binaries
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Title: On the minimum mass ratio of W UMa binaries
Authors: Dengkai Jiang, Zhanwen Han, Jiancheng Wang, Tianyu Jiang, Lifang

Using Eggleton's stellar evolution code, we study the minimum mass ratio (q_{min}) of W Ursae Majoris (W UMa) binaries that have different primary masses. It is found that the minimum mass ratio of W UMa binaries decreases with increasing mass of the primary if the primary's mass is less than about 1.3 solar masses, and above this mass the ratio is roughly constant. By comparing the theoretical minimum mass ratio with the observational data, it is found that the existence of low-q systems can be explained by the different structure of the primaries with different masses. This suggests that the dimensionless gyration radius (k_1) and thus the structure of the primary is very important in determining the minimum mass ratio. In addition, we investigate the mass loss during the merging process of W UMa systems and calculate the rotation velocities of the single stars formed by the merger of W UMa binaries due to tidal instability. It is found that in the case of the conservation of mass and angular momentum, the merged single stars rotate with a equatorial velocity of about 588 ~819 km s^{-1}, which is much larger than their break-up velocities (v_{b}). This suggests that the merged stars should extend to a very large radius (3.7 ~5.3 times the radii of the primaries) or W UMa systems would lose a large amount of mass (21 ~33 per cent of the total mass) during the merging process. If the effect of magnetic braking is considered, the mass loss decreases to be 12 ~18 per cent of their total masses. This implies that the significant angular momentum and mass might be lost from W UMa systems in the course of the merging process, and this kind of mass and angular momentum loss might be driven by the release of orbital energy of the secondaries, which is similar to common-envelope evolution.

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Posts: 131433
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W UMa-type eclipsing binaries
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Title: Period decrease in three SuperWASP eclipsing binary candidates near the short-period limit
Authors: M. E. Lohr (1), A. J. Norton (1), U. C. Kolb (1), D. R. Anderson (2), F. Faedi (3), R. G. West (4) ((1) The Open University, Milton Keynes, UK, (2) Keele University, Staffordshire, UK, (3) Queen's University, Belfast, UK, (4) University of Leicester, Leicester, UK)

SuperWASP light curves for 53 W UMa-type eclipsing binary (EB) candidates, identified in previous work as being close to the contact binary short-period limit, were studied for evidence of period change. The orbital periods of most of the stars were confirmed, and period decrease, significant at more than 5 sigma, was observed in three objects: 1SWASP J174310.98+432709.6 (-0.055 0.003 s/yr), 1SWASP J133105.91+121538.0 (-0.075 0.013 s/yr) and 1SWASP J234401.81-212229.1 (-0.313 0.019 s/yr). The magnitudes of the observed period changes cannot be explained by magnetic braking or gravitational radiation effects, and are most likely primarily due to unstable mass transfer from primary to secondary components, possibly accompanied by unstable mass and angular momentum loss from the systems. If these period decreases persist, the systems could merge on a relatively short timescale.

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Posts: 131433
Date:
W Ursae Majoris binary stars
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Title: A BVRcIc Survey of W Ursae Majoris Binaries
Authors: Dirk Terrell, John Gross, Walter R. Cooney Jr

We report on a BVRcIc survey of field W Ursae Majoris binary stars and present accurate colours for 606 systems that have been observed on at least three photometric nights from a robotic observatory in southern Arizona. Comparison with earlier photometry for a subset of the systems shows good agreement. We investigate two independent methods of determining the interstellar reddening, although both have limitations that can render them less effective than desired. A subset of 101 systems shows good agreement between the two reddening methods.

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Posts: 131433
Date:
W Ursa Majoris type binaries
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Title: New absolute magnitude calibrations for W Ursa Majoris type binaries
Authors: Z. Eker, S. Bilir, E. Yaz, O. Demircan, M. Helvaci

Parallaxes of W UMa stars in the Hipparcos catalogue have been analysed. 31 W UMa stars, which have the most accurate parallaxes (\sigma_{\pi}/\pi<0.15) which are neither associated with a photometric tertiary nor with evidence of a visual companion, were selected for re-calibrating the Period--Luminosity--Colour (PLC) relation of W UMa stars. Using the Lutz--Kelker (LK) bias corrected (most probable) parallaxes, periods (0.26< P(day)< 0.87), and colours (0.04< (B-V)_{0} <1.28) of the 31 selected W UMa, the PLC relation have been revised and re-calibrated. The difference between the old (revised but not bias corrected) and the new (LK bias corrected) relations are almost negligible in predicting the distances of W UMa stars up to about 100 parsecs. But, it increases and may become intolerable as distances of stars increase. Additionally, using (J-H)_{0} and (H-K_{s})_{0} colours from 2MASS (Two Micron All Sky Survey) data, a PLC relation working with infrared data was derived. It can be used with infrared colours in the range -0.01<(J-H)_{0}<0.58, and -0.10<(H-K_{s})_{0}<0.18. Despite 2MASS data are single epoch observations, which are not guaranteed at maximum brightness of the W UMa stars, the established relation has been found surprisingly consistent and reliable in predicting LK corrected distances of W UMa stars.

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Posts: 131433
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RE: W UMa systems
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Title: The evolutionary status of W Ursae Majoris-type systems
Authors: Lifang Li, Fenghui Zhang, Zhanwen Han, Dengkai Jiang, Tianyu Jiang

Well-determined physical parameters of 130 W UMa systems have been collected from the literature. Based on these data, the evolutionary status and dynamical evolution of W UMa systems are investigated. It is found that there is no evolutionary difference between W- and A-type systems in M-J diagram which is consistent with the results derived from the analysis of observed spectral type, M-R and M-L diagrams of W UMa systems. M-R and M-L diagrams of W- and A-type systems indicate that a large amount of energy should be transferred from the more massive to the less massive component so that they are not in thermal equilibrium and undergo thermal relaxation oscillation (TRO). Meanwhile, the distribution of angular momentum, together with the distribution of mass ratio, suggests that the mass ratio of the observed W UMa systems is decreased with the decrease of their total mass. This could be the result of the dynamical evolution of W UMa systems which suffer angular momentum loss (AML) and mass loss due to magnetic stellar wind (MSW). Consequently, the tidal instability forces these systems towards the lower q values and finally to fast rotating single stars.

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Title: The dynamical stability of W Ursae Majoris-type systems
Authors: Lifang Li, Fenghui Zhang

Theoretical study indicates that a contact binary system would merge into a rapidly rotating single star due to tidal instability when the spin angular momentum of the system is more than a third of its orbital angular momentum. Assuming that W UMa contact binary systems rigorously comply with the Roche geometry and the dynamical stability limit is at a contact degree of about 70%, we obtain that W UMa systems might suffer Darwin's instability when their mass ratios are in a region of about 0.076--0.078 and merge into the fast-rotating stars. This suggests that the W UMa systems with mass ratio q <= 0.076 can not be observed. Meanwhile, we find that the observed W UMa systems with a mass ratio of about 0.077, corresponding to a contact degree of about 86% would suffer tidal instability and merge into the single fast-rotating stars. This suggests that the dynamical stability limit for the observed W UMa systems is higher than the theoretical value, implying that the observed systems have probably suffered the loss of angular momentum due to gravitational wave radiation (GR) or magnetic stellar wind (MSW).

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