Observations And Studies On K-type Close Binaries

The K-type close binaries is a category of close binaries with K-type spectra.Broadly speaking, they can be made of dwarf component stars or giant stars. Thelatter have already fnished their main-sequence evolution. This paper mainlydiscuss the objects with very short periods whose components are mostly similarto dwarf stars. They are divided into the contact type and the near contact typeand then discussed. The near contact type consists of EA type and EB typevariables in our classifcation. In this paper, the author presents an overviewof the current progress in the research of the K-type close binaries; gives anintroduction of the photometric and spectroscopic methods; presents the analysisof a group of observed K-type close binary samples, including six contact samplesand one near contact sample; makes a statistic study of the collected K-type closebinary samples. The main contents and results are shown as follows.1. Through photometric solutions of the contact samples, the light curvesand the orbital parameters (e.g. inclination and mass ratio) are derived. Thedegrees of contact, confgurations and temperature relationship are also derived.It is discovered that all of them are shallow contact, W-type systems. Thetemperature diferences between the components of them are about0to morethan200K. The lowest degree of contact is3.5%which belongs V1799Ori, andthe highest is17%that belongs to J0529. The LCs (light curves) of J0529aredistinctively diferent from that of other samples. The secondary eclipse of theformer is fat, which is similar to that of A-type. Its temperature diference is thesmallest, the secondary is only slight hotter than the primary. Its mass ratio issmaller than0.4, which is the smallest among the analyzed samples. Its period isthe shortest, only0.227days, which is very close to the cut-of period of contactbinaries. The mass ratio of BI Vul is close to the unit1, and its degree of contactis also low, only8.7%, which implies that the object may just evolve into thecontact stage.2. Through period analysis of the contact samples, RV CVn is discovered to keep a long-term stable period. Its period is found to have no obvious changeduring the past almost90years. Only periodic oscillation with period of about60years is presented. Three of the rest samples show decreasing periods andonly V1799Ori has a weak increasing period. Four contact binaries are found toshow periodic oscillation in their orbital period. Among them,V1104Her seemsto have two periodic components, which meets the near-integer-ratio. Thereforethe most plausible explanation is two stellar companion with small masses in theouter orbits, the orbital resonance helps the outer triple system to keep stable.3. It is discovered that the near contact binary DV Psc is a highly activesystem. The photometric mass ratio is locally in agree with the spectroscopicresult. Both of them are around0.7. The fll factors of the two componentsstars are calculated to be86%(primary) and88%(secondary). The fll factorsof the secondary is a little higher than that of the primary, which indicates thatthe volume of the secondary may be oversized. The LCs of DV Psc show strongasymmetry, as well as distortion and fast changes, which makes the spot solutionswith large uncertainties. The surface activities of the system strongly afect theLCs, thus infuence the determination of times of minimum light. Therefore thedata which is strongly afected is not used and then a periodic oscillation witha period of close to2.5years and a amplitude of close to0.001days is found, aswell as a uncertain long-term increase. In consideration of a large fraction of athird light in the system, the periodic oscillation may be caused by an additionalcompanion, which may revolve around the binary with a high inclination relativeto the binary orbit.4. From the statistic study of40K-type contact binary samples, it is foundthat almost all of them are W-type shallow contact systems. However, foursamples have LCs similar to that of A-type samples. But their photometricsolutions reveal that they are indeed W-type systems. This is discovered the frsttime that the W-type confguration prevails in a whole spectral type of contactbinaries, which demonstrate the specialty of K-type (low mass) contact binaries.It is also discovered that the degree of contact is correlated to the mass ratio.The lower the mass ratio, the higher the degree of contact. This seems to implythat in the evolution of the contact binaries, the mass ratio may get smaller, and the degree of contact will increase, and they may coalesce into a single time whenthey arrive at a critical point.5.16near contact binaries are collected, among which the EB type samplesare rare. In the only three EB type samples, even one sample may have LCsclose to EA type. This result may strong restrict the TRO theory. In the nearcontact binaries,5are semi-detached systems, among which3are secondary flled(algols) and2are primary flled. The periods of the primary flled systems areshorter than that will algols.6. Through statistic analysis, it is discovered that majority of K-type contactbinaries show O’Connell efect. Among them most are negative efects, a few arechangeable and several are positive efects. LCs with this efect can be wellmodeled with spot models. The LCs of near contact binaries also show strongasymmetry, and usually remarkably larger than contact binaries. According tothe available samples, they are mostly positive O’Connell efects.