X. B. Zhang

ON THE PULSATIONAL-ORBITAL-PERIOD RELATION OF ECLIPSING BINARIES WITH δ-SCT COMPONENTS

We have deduced a theoretical relation between the pulsation and orbital-periods of pulsating stars in close binaries based on their Roche lobe filling. It appears to be of a simple linear form, with the slope as a function of the pulsation constant, the mass ratio, and the filling factor for an individual system. Testing the data of 69 known eclipsing binaries containing δ-Sct-type components yields an empirical slope of 0.020 ± 0.006 for the P{sub pul}-P{sub orb} relation. We have further derived the upper limit of the P{sub pul}/P{sub orb} ratio for the δ-Sct stars in eclipsing binaries with a value of 0.09 ± 0.02. This value could serve as a criterion to distinguish whether or not a pulsator in an eclipsing binary pulsates in the p-mode. Applying the deduced P{sub pul}-P{sub orb} relation, we have computed the dominant pulsation constants for 37 δ-Sct stars in eclipsing systems with definite photometric solutions. These ranged between 0.008 and 0.033 days with a mean value of about 0.014 days, indicating that δ-Sct stars in eclipsing binaries mostly pulsate in the fourth or fifth overtones.

TESTING A PULSATING BINARY MODEL FOR LONG SECONDARY PERIODS IN RED VARIABLES

The origin of the long secondary periods (LSPs) in red variables remains a mystery until now, although many models exist. The light curves of some LSP stars mimic an eclipsing binary with a pulsating red giant component. To test this hypothesis, the observational data of two LSP variable red giants, 77.7795.29 and 77.8031.42, discovered by the MACHO project from the Large Magellanic Cloud, are collected and analyzed. The probable eclipsing features of the light curves are simulated by the Wilson-Devinney method. The simulation yields a contact and a semidetached geometry for the two systems, respectively. In addition, the pulsation constant of the main pulsating component in each binary system is derived. By combining the results of the binary model and the pulsation component, we investigate the feasibility of the pulsating binary model. It is found that the radial velocity curve expected from the binary model has a much larger amplitude than the observed one and a period double the observed one. Furthermore, the masses of the components based on the density derived from the binary orbit solution are too low to be compatible with both the evolutionary stage and the high luminosity. Although the pulsation mode identified by the pulsation constant which is dependent on the density from the binary model is consistent with the first or second overtone radial pulsation, we conclude that the pulsating binary model is a defective model for the LSP.

Photometric Study and Period Analysis of the Contact Binary XZ Leonis

We present multi-color CCD photometry of the neglected contact binary XZ Leo. Completely covered VRI band light curves and four times of minimum light were obtained. Combining the photometric and previously published radial-velocity data, a revised photometric analysis was carried out for the binary system by applying the Wilson?Devinney code. With a hot spot placed on the massive primary component near the neck region of the common envelope, the light curves were satisfactorily modeled. The photometric solution combined with the radial-velocity solution reveals that XZ Leo is an A-type contact binary with a degree of contact of 24(?1)%. The absolute parameters of the components were determined to be M1 = 1.74(?0.06)M?, M2 = 0.61(?0.02)M?, R1 = 1.69(?0.01)R?, R2 = 1.07(?0.01)R?, L1 = 6.73(?0.08) L?, and L2 = 2.40(?0.04)L?. Based on all the available data, the long-term orbital period behavior of the system was investigated. It indicates that the binary system was undergoing a continuous orbital period increase in the past three decades with a rate of , which suggests a probable mass transfer from the secondary to the primary component at a rate of . The binary system is expected to evolve into the broken-contact stage in years. This could be evidence supporting the thermal relaxation oscillation theory.

DISCOVERY OF 14 NEW SLOWLY PULSATING B STARS IN THE OPEN CLUSTER NGC 7654

We carried out time-series BV CCD photometric observations of the open cluster NGC 7654 (Messier 52) to search for variable stars. Eighteen slowly pulsating B (SPB) stars have been detected, among which 14 candidates are newly discovered, three known ones are confirmed, and a previously found delta Scuti star is also identified as an SPB candidate. Twelve SPBs are probable cluster members based on membership analysis. This makes NGC 7654 the richest galactic open cluster in terms of SPB star content. It is also a new discovery that NGC 7654 hosts three gamma Dor star candidates. We found that all these stars (18 SPB and 3 gamma Dor stars) have periods longer than their corresponding fundamental radial mode. With such a big sample of g-mode pulsators in a single cluster, it is clear that multi-mode pulsation is more common in the upper part of the main sequence than in the lower part. All the stars span a narrow strip on the period-luminosity plane, which also includes the gamma Dor stars at the low-luminosity extension. This result implies that there may be a single period-luminosity relation applicable to all g-mode main-sequence pulsators. As a by-product, three EA-type eclipsing binaries and an EW-type eclipsing binary are also discovered.

B-TYPE VARIABLES IN THE YOUNG OPEN CLUSTER NGC 457

We have performed long-term, two-color time-series CCD photometry of the young open cluster NGC 457 to search for B-type variables. The light variations of about 400 stars located in a 165 × 165 field of the cluster were examined. It resulted in the detection of 13 periodic variables, including 8 B-type variables. With the two-color photometric data, we have constructed a color-magnitude diagram for the cluster. A brief isochrone fitting yields a reddening of E(B – V) = 0.46 ± 0.05 and a distance modulus of (m – M) V = 13.30 ± 0.10 with an age of log(Age) = 7.30. With the above results, we identified three probable member stars in addition to six certain ones from the new variable sample. Through a preliminary photometric analysis with the obtained B- and V-band light curves, we have determined the amplitude and period of light variations for each of the variables. Based on this, the physical properties and classification of the detected variables are discussed. As a conclusion, we have identified one β Cep, five candidate SPB stars, and three δ Scuti-type pulsating stars plus three EA-type eclipsing binaries and one EW-type one in NGC 457.

Magnetostatic configuration of a confined magnetic-flux tube

In this paper an isolated magnetic flux tube confined in stratified atmosphere is studied for slender and axisymmetric model. The functions of the pressure, density, and temperature are expanded as a Taylor series of magnetic surface function ψ. Several models of an isolated magnetic flux tube confined in a stratified atmosphere are constructed, and the external pressure of the stratified atmosphere decreases reasonably with increasing height. The distribution of thermal dynamic quantities and the magnetic pressure in the flux tube are also obtained.

Photometric Properties for Selected Algol-type Binaries. V. V1241 Tauri

We present new photometry for the eclipsing binary V1241 Tau, which was obtained on six nights between 2011 December and 2012 January using the 85xa0cm telescope at the Xinglong station of the National Astronomical Observatories of China. By using the updated Wilson-Devinney code, photometric models with third lights were deduced from two sets of light curves. The result implies that V1241 Tau is an Algol-type near-contact binary (NCB), whose mass ratio and filling-out of the primary are q = 0.545 (± 0.003) and f 1 = 82.4% (± 0.2%), respectively. Based on all available times of minimum light spanning over 80 yr, the O – C curve of V1241 Tau appears to show a quasi-sinusoidal oscillation, i.e., a light-time orbit. The modulated period and amplitude are P mod = 47.4 (± 1.7) yr and A = 0.0087 (± 0.0005) days, respectively. This kind of period variation may be more likely attributed to the light-time effect via a presence of an unseen third body. From an analysis of 23 Algol-type NCBs with EB-type light curves, we determine that the fill-out for the primary f 1 will increase as the orbital period P decreases. With angular momentum loss, the orbit of the binary will shrink, which causes f 1 to increase. The primary component finally fills its Roche lobe, and the binary evolves into contact configuration. Therefore, this kind of Algol-type NCB with EB-type light curves, such as V1241 Tau, may be a progenitor of the A-type W UMa binary.

Equilibrium of isolated magnetostatic flux structure

Two-dimensional magnetostatic models of flux structure confined in stratified atmosphere are discussed in the present paper. The magnetic field in the flux structure is assumed to be force-free at the first step. Numerical solutions for this nonlinear free boundary problem are obtained by finite element method. Results show clearly the relation between the inside fields and outside pressure, especially the influence of atmospheric pressure distribution on the flux structure.