Analysis And Investigation On Close Binaries In Globular Clusters

Globular Clusters are composed with thousands of stars by gravity. Asglobular clusters are very dense, metal poor and their member stars have similardistance modulus, they provide us natural laboratories for analysis. Because ofthe limitation on observation, the study of close binaries in globular clusters wasstarted a few years ago. In this paper, we analyzed the light curves of closebinaries in globular clusters by compiling photometric data. In order to comparewith close binaries in the feld, we also analyzed three feld close binaries, theresults are obtained as follows:1. Based on the analysis of light curves of four EA type binaries （V211, V239,NV358and NV364） in the globular cluster ω Centauri, we found that V211andV239are typical Algol type binaries, they are also blue stragglers and shouldbe formed by mass transfer from the secondary component to the primary. Asthe two binaries are located in the outer region of the cluster, they can improvethe scenario that blue stragglers in the low density cluster outskirts were formedby mass transfer in close binaries. NV358and NV364are detached binaries andalso second generation binaries, this is an evidence of multiple populations of thiscluster. The two binaries are not located in the center of ω Centauri but in theoutskirts, improving the result that ω Centauri was previously the nucleus of anucleated dwarf galaxy and later it has been completely destroyed by galactictidal feld so that only its nucleus is now observed.2. Using the WD program, we analyzed the light curves of two binaries,V7and V8, in the globular cluster NGC6397, it is found that the two binariesare contact binaries. V7is a W-type shallow contact binary with a contactdegree of f=2.7%, while V8is an extremely low mass ratio （～0.159） A-typeW UMa system with a contact degree of f=46.1%. We also analyzed theorbital period changes of the two binaries, this is the frst time to analyze periodchanges for contact binaries in globular clusters, long-term period increase hasbeen detected for both binaries. We derived a long-term period increase rate at dP/dt=4.071（14）×10^-8days yr^-1for V7and dP/dt=2.569（2）×10^-7daysyr1for V8. With the increasing period, V7is evolving from the present shallowcontact phase to the more shallow overcontact phase and fnally move to contactconfguration broken stage predicted by the thermal relaxation oscillation. Asthe period increase, the mass ratio of V8will decrease, which will cause it fnallyto merge into a single rapidly rotating star after the orbital angular momentumis less than three times the total spin angular momentum. V8is not only aneclipsing binary but also a blue straggler candidate. This double nature makes itan important object to strengthen the hypothesis that blue stragglers are formedby mass transfer in close binaries.3. According to the study of the light curves of two binaries, V134andV144, in the globular cluster M54, we found that V134is a middle mass ratio（q=0.530） deep contact binary with a contact degree of f=50.6%and V144isan extremely low mass ratio （q=0.160） shallow contact binary with a contactdegree of f=19.5%，their light curves exhibit very strong O’Connell efect. Thetwo contact binaries are very special, the values for the acceleration of gravityat the surface of the components of them are very diferent from the values formain sequence stars but consist with the values for subgiant stars. This type ofcontact binary is quite rare. On the color-magnitude diagram of M54, both ofthe two binaries are located in the blue straggler region and they are probablemember stars, so they are blue straggler candidates and may be formed by masstransfer.4. The investigation of three close binaries in the feld.1) By analyzing VIRlight curves and orbital period of HV Aqr, we found that HV Aqr is a low massratio deep contact binary and that the period of HV Aqr is secular decrease,the decrease rate is dP/dt=8.84（±0.18）×10^-8days yr^-1. The secular perioddecrease is caused by mass transfer from the primary component to the secondaryand angular momentum loss.2) EQ Tau is a G-tpye binary, we derived that it is ashallow contact binary with a mass ratio of0.442according to the analysis of fvesets light curves, the primary component of EQ Tau shows very strong magneticactity. The orbital period of EQ Tau is also analyzed by us, we found thatits orbital period includes a secular decrease and a cyclic oscillation. The cyclic magnetic activity of EQ Tau can not explain the cyclic orbital period change verywell. A probable transit-like event was observed in the2010observing season.Therefore, the cyclic orbital period change of EQ Tau may be due to the lighttime efect of a third body.3) The W UMa type binary UZ CMi is a neglectedobject all the time, we carried out the frst photometric investigation and orbitalperiod analysis. It is found that UZ CMi is a contact binary, the contact degreeis38.4%, the mass ratio is0.452. The period of it shows a cyclic oscillationsuperimposed on long term increase. We think the cyclic oscillation is caused bythe light time efect of the third companion.