Li-Ying Zhu

Circumbinary Planets Orbiting the Rapidly Pulsating Subdwarf B-type binary NY Vir

We report here the tentative discovery of a Jovian planet in orbit around the rapidly pulsating subdwarf B-type (sdB-type) eclipsing binary NY Vir. By using newly determined eclipse times together with those collected from the literature, we detect that the observed-calculated (O - C) curve of NY Vir shows a small-amplitude cyclic variation with a period of 7.9 yr and a semiamplitude of 6.1 s, while it undergoes a downward parabolic change (revealing a period decrease at a rate of (P) over dot = -9.2 x 10(-12)). The periodic variation was analyzed for the light-travel-time effect via the presence of a third body. The mass of the tertiary companion was determined to be M-3 sin i = 2.3(+/- 0.3)M-Jupiter when a total mass of 0.60 M-circle dot for NY Vir is adopted. This suggests that it is most probably a giant circumbinary planet orbiting NY Vir at a distance of about 3.3 astronomical units (AU). Since the rate of period decrease cannot be explained by true angular momentum loss caused by gravitational radiation or/and magnetic braking, the observed downward parabolic change in the O - C diagram may be only a part of a long-period (longer than 15 years) cyclic variation, which may reveal the presence of another Jovian planet (similar to 2.5 M-Jupiter) in the system.

DETECTION OF A TERTIARY BROWN DWARF COMPANION IN THE sdB-TYPE ECLIPSING BINARY HS 0705+6700

HS 0705+6700 is a short-period (P = 2.3 hr), close binary containing a hot sdB-type primary and a fully convective secondary. We have monitored this eclipsing binary for more than two years and as a result, 32 times of light minimum were obtained. Based on our new eclipse times together with these compiled from the literature, it is discovered that the observed-calculated curve of HS 0705+6700 shows a cyclic variation with a period of 7.15 years and a semiamplitude of 92.4 s. The periodic change was analyzed for the light-travel time effect that may be due to the presence of a tertiary companion. The mass of the third body is determined to be M3 sin i = 0.0377(+/- 0.0043) M(circle dot) when a total mass of 0.617 M(circle dot) for HS 0705+6700 is adopted. For orbital inclinations i >= 32.degrees 8, the mass of the tertiary component would be below the stable hydrogen-burning limit of M(3) similar to 0.072M(circle dot), and thus it would be a brown dwarf. The third body is orbiting the sdB-type binary at a distance shorter than 3.6 AU. HS 0705+6700 was formed through the evolution of a common envelope after the primary becomes a red giant. The detection of a substellar companion in HS 0705+6700 system at this distance from the binary could give some constraints on stellar evolution in such systems and the interactions between red giants and their companions.

Photometric Study of the Very Short Period Shallow Contact Binary DD Comae Berenices

The first photometric solutions of the very short period (VSP) close binary DD Comae Berenices (P = 0(d).26920811) based on our new complete (IR)(C) light curves are derived by the 2003 version Wilson-Van Hamme code. They show that the system belongs to shallow contact W-type W UMa systems with a degree of overcontact of 8.7%. The observed light curve distortions are explained by employing the spots model due to the late-type nature of both components. We have collected all available photometric data about the system with emphasis on the individual observational data, which we treated simultaneously using our own method based on the usage of computed model light curves as templates. We recalculated published times of light minimum and added new ones of our own to construct an O-C diagram that spans over 70 years. Using a least squares method orthogonal quadratic model function, we found that the orbital period of DD Com is continuously increasing with (P) over dot = 0.00401(22) s yr(-1). The period increase may be caused by the mass transfer from the less-massive component to the more-massive one. With the period increase, the binary is evolving from the present shallow contact phase to the broken stage predicted by the thermal relaxation oscillation (TRO) theory. Compared with other VSP systems, DD Com is a rare system that lies on the expanding phase of the TRO cycle. Until now, only four such systems including DD Com are found in this stage. Thus, this target is another good observational proof of the TRO theory in a very short period region.

DEEP, LOW MASS RATIO OVERCONTACT BINARY SYSTEMS. XI. V1191 CYGNI

Complete CCD photometric light curves in BV(RI)(c) bands obtained on one night in 2009 for the short-period closebinary system V1191 Cygni are presented. A new photometric analysis with the 2003 version of the Wilson-Van Hamme code shows that V1191 Cyg is a W-type overcontact binary system and suggests that it has a high degree of overcontact (f = 68.6%) with very low mass ratio, implying that it is at the late stage of overcontact evolution. The absolute parameters of V1191 Cyg are derived using spectroscopic and photometric solutions. Combining new determined times of light minimum with others published in the literature, the period change of the binary star is investigated. A periodic variation, with a period of 26.7 years and an amplitude of 0.023 days, was discovered to be superimposed on a long-term period increase (dP/dt = +4.5(+/- 0.1) x 10(-7) days yr(-1)). The cyclic period oscillation may be caused by the magnetic activity cycles of either of the components or the light-time effect due to the presence of a third body with a mass of m(3) = 0.77 M-circle dot and an orbital radius of a(3) = 7.6 AU, when this body is coplanar to the orbit of the eclipsing pair. The secular orbital period increase can be interpreted as a mass transfer from the less massive component to the more massive one. With the period increases, V1191 Cyg will evolve from its present low mass ratio, high filled overcontact state to a rapidly rotating single star when its orbital angular momentum is less than three times the total spin angular momentum. V1191 Cyg is too blue for its orbital period and it is an unusual W-type overcontact system with such a low mass ratio and high fill-out overcontact configuration, which is worth monitoring continuously in the future.

Interior structura variations in the secondary components of two Algol-type eclipsing binary systems: SW Cygni and RR Draconis

SW Cyg and DD Dra are two Algol-type eclipsing binary systems. Both contain an A2-type main-sequence primary and a K0-type secondary component with a rather deep primary light minimum in their light curves (2.(m)6 and 3.(m)8, respectively). In the present paper, orbital period changes of these stars are studied based on the analyses of their century-long times of light minimum. Several period jumps are found to superimpose on a rapid secular increase (dP/dt = +4.71x10(-6) and +4.24x10(-6) days/year, respectively). Since the period increases are rather rapid, the mass exchange between the components would be dynamical. SW Cyg is a low-mass ratio system (q=0.20) and it is expected that the mass ratio of RR Dra may also be very low because of the rather deep primary eclipse minimum. The rapid period increases suggest that a rapid mass transfer also occurs in a low-mass ratio Alogl-type close binary which is opposite to the theoretically evolutionary scheme of the binary star. The period jumps superimposed on the long-term increases are caused by the structura variation of the cool mass-loser via instabilities in the convective outer layer (COL) or via cyclic magnetic activity of the K0-type components. As the period jumps do not occur in a short-term alternating way, the structura change caused by instabilities in the COL via a dynamical mass loss from the cool subgiant may be more plausible.

Photometric studies of twelve deep, low-mass ratio overcontact binary systems

The formations of the blue straggler stars and the FK Com-type stars are unsolved problems in stellar astrophysics. One of the possibilities for their formations is from the coalescence of W UMa-type overcontact binary systems. Therefore, deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars are a very important source to understand the phenomena of Blue Straggler/FK Com-type stars. Recently, 12 W UMa-type binary stars, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, V410 Aur, XY Boo, SX CrV, QX And, GSC 619-232, and AH Cnc, were investigated photometrically. Apart from TV Mus, XY boo, and GSC 619-232, new observations of the other 9 binaries were obtained. Complete light curves of the 10 systems, FG Hya, GR Vir, IK Per, TV Mus, CU Tau, V857 Her, GSC 619-232, V410 Aur, XY Boo, and AH Cnc, were analyzed with the 2003 version of the W-D code. It is shown that all of those systems are deep (f > 50%), low-mass ratio (q < 0.25) overcontact binary stars. We found that the system GSC 619-232 has the highest degree of overcontact (f = 93.4%). The derived photometric mass ratio of V857 Her, q = 0.0653, indicates that it is the lowest-mass ratio system among W UMa-type binaries.Of the 12 sample stars, long-term period changes of 11 systems were found. About 58% (seven) of the sample binaries show cyclic period oscillation. No cyclic period changes were discovered for the other 5 systems, which may be caused by the short observational time interval or by insufficient observations. Therefore, we think that all W UMa-type binary stars may contain cyclic period variations. By considering the long-term period changes (both increase and decrease) of those binary stars, we proposed two evolutionary scenarios evolving from deep, low-mass ratio overcontact binaries into Blue Straggler/FK Com-type stars.

THE PHOTOMETRIC STUDY OF A NEGLECTED NEAR CONTACT BINARY: BS VULPECULAE

We present a detailed study of the close eclipsing binary BS Vulpeculae. Although it is relatively bright (V: 10.9-11.6 mag) and belongs to short-periodic variable stars (P = 0.48 days), it is rather neglected. To perform a thorough period analysis, we collected all available photometric observations that span the time interval of 1898-2010. Observations include archive photographic plate measurements and visually determined eclipse minima timings done in 1979-2003, which were later shown to be biased to accommodate the existing linear ephemeris. Applying our own direct period analysis we found a well-defined shortening of the orbital period of dP/dt = -6.70(17) x 10(-11) = -2.11(6) ms yr(-1), which implies a continual mass flow from the primary to the secondary component. Using the 2003 version of the Wilson-Van Hamme code, our new complete BV(IR)(C) light curves were analyzed and the physical parameters of the system were derived. We found that BS Vul is a near contact binary system with the primary component filling its critical Roche lobe. The luminosity enhancement on the left shoulder of the secondary minimum shown in the light curves can be explained as a result of a persistent hot spot on the secondary due to the mass transfer from the primary component to the secondary one and heating the facing hemisphere of the secondary component, which is consistent with our result of period analysis. With the period decrease, BS Vul will evolve toward the contact phase. It is another good observational example as predicted by the theory of thermal relaxation oscillations.

PROPERTIES OF THE CLOSE-IN TERTIARY IN THE QUADRUPLE SYSTEM V401 CYG

V401 Cyg is a quadruple system in which the spectroscopic signature of a close-in tertiary and a distant visual companion star were reported. Orbital properties of the close-in companion should provide valuable information on the formation of close binaries and stellar dynamical interaction. By analyzing new times of minimum light together with those collected from the literature, we discovered that the observed-calculated (O - C) curve of V401 Cyg shows a cyclic change with a short period of 3.5 yr and a semi-amplitude of 0.00436 days while it undergoes an upward parabolic variation. Those photoelectric and CCD data covered more than two cycles and were analyzed for the light-travel time effect via the presence of the tertiary companion. The mass of the third body was determined to be M-3 sin i = 0.65(+/- 0.08) M-circle dot, which is close to the value estimated from the spectroscopic data (M-3 similar to 0.64 M-circle dot). This reveals that the orbital inclination of the tertiary was about i similar to 90 degrees, indicating that the contact components of V401 Cyg have the possibility of being eclipsed by the tertiary at an orbital distance of about 3.0 AU, and it may be a triply eclipsing hierarchical triple system. The upward parabolic change indicates a period increase at a rate of (P) over dot = 1.5 x 10(-7) revealing a mass transfer from the secondary to the primary ((M) over dot(2) = 5.9 x 10(-8) M-circle dot yr(-1)). This is consistent with the predictions of the theory of thermal relaxation oscillation (TRO) suggesting that V401 Cyg is undergoing an expanding-orbit stage in the TRO cycles.

NEAR-CONTACT BINARIES WITH MASS TRANSFER: V473 CASSIOPEIAE AND II PERSEI

Photometric solutions of two short period close binaries, V473 Cas and II Per, are derived with the 2003 version Wilson-Van Hamme code. It is shown that V473 Cas is a semidetached system with the primary component or the secondary component at its critical Roche Lobe, while II Per is a marginal contact binary system with both components filling their critical Roche Lobes, but with a large temperature difference between the two components. Both systems belong to near-contact binaries with the light curves enhanced around the left shoulder of secondary minimum, which can be explained by hot spots on the secondary components due to the mass transfer via a stream hitting the facing surface of the secondary components. The orbital period investigations based on all available times of light minimum show a secular period existing in these two targets decreasing at the rate of dP/dt = -7.61 x 10(-8) days yr(-1) for V473 Cas and dP/dt = -7.54 x 10(-8) days yr(-1) for II Per. The decrease of the orbital period can be the result of mass transfer from the primary component to the secondary one, which is consistent with the asymmetric light curves of V473 Cas and II Per. Therefore, V473 Cas and II Per are other examples displaying evidence of mass transfer between the two components. We have collected NCBs with secular period variation and find that almost all SD1-type NCBs show decreasing periods and enhancing luminosity on the left shoulder of secondary minimum in their light curves. So the semidetached configuration with the lobe filling primary is more plausible for V473 Cas. Furthermore, the detected enhancing amplitude of luminosity for SD2-type NCBs is smaller than other types of NCBs, which implies that the mass transfer from the primary component to the secondary one is stronger than the transfer in opposite direction. With the orbital period decrease, V473 Cas and II Per will evolve into overcontact binaries with true thermal contact.

WZ CEPHEI: A CLOSE BINARY AT THE BEGINNING OF CONTACT PHASE

Photometric photoelectric data of the short-period close binary system, WZ Cephei, are presented. A new photometric analysis with the 2003 version of the Wilson-Devinney method confirmed that it is a shallow contact binary (f similar to 13.3%) with a high level of spot activity on the primary component. Combining new determined times of light minimum with the others published in the literature, the period change of the binary star is investigated. A periodic variation, with a period of 34.2 years and an amplitude of 0(d).013, was discovered to be superimposed on a long-term period decrease (dP/dt = -8.8 x 10(-8) days year(-1)). Both the secular period decrease and the shallow contact configuration suggest that this binary system is at the beginning of contact phase. It is on the way to evolving into a normal overcontact phase via secular angular momentum loss and/or mass transfer from the more massive component to the less massive one. The period oscillation can be explained either by the light-time effect due to the presence of a third body or by magnetic activity. On the one hand, if it is caused by the light-time effect, the mass and the orbital radius of the additional body should be m(3) = 0.17 M(circle dot) and a(3) = 26.6 AU, respectively, when this body is coplanar to the orbit of the eclipsing pair. On the other hand, since the system shows high levels of spot activity, the period oscillation may be explained as a consequence of magnetic cycles. In this case, the required variation of the quadrupole moment is calculated to be 9.2 x 10(49) g cm(2).