Contact Binary Research Articles

DM Delphini: A Progenitor of a Contact Binary with a Compact Tertiary Companion

Abstract The orbital period of the short-period eclipsing binary DM Del was analyzed by using two newly determined eclipse times together with the others compiled from the literature. It has been discovered that the period shows a long-time continuous decrease at a rate of $dP/dt = -$2.27 $\times$ 10$^{-7}$ d yr$^{-1}$, while it undergoes a cyclic variation with a period of 31.73 years. Meanwhile, photometric light curves of a close binary in the $B$ and $V$ bands, published by Güdür et al., were analyzed with the 2003 version of the W–D code. It was found that the system is a semi-detached binary with a lobe-filling primary and a mass ratio of $q =$ 0.55. The long-time period decrease can be explained by mass transfer from the primary to the secondary. This is consistent with the semi-detached configuration. The geometrical structure of the binary and the long-term period decrease suggest that DM Del may be a progenitor of an overcontact binary or just in a broken-contact stage predicted by thermal relaxation oscillations. A cyclic period change reveals the presence of a tertiary unseen companion that may play an important role in the formation of the close binary by removing angular momentum from the central system.

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

AbstractWe look for high‐amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS‐3 Photometric V ‐band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (ε Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with Hα emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known σ Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the onset of runaway stellar collisions in dense star clusters - II. Hydrodynamics of three-body interactions

The onset of runaway stellar collisions in young star clusters is more likely to initiate with an encounter between a binary and a third star than between two single stars. Using the initial conditions of such three-star encounters from direct N-body simulations, we model the resulting interaction by means of Smoothed Particle Hydrodynamics (SPH). Our code implements new equations of motion that allow for ecient use of non-equal mass particles and is capable of evolving contact binaries for thousands of orbits. We nd that, in the majority of the cases considered, all three stars merge together. In addition, we compare our SPH calculations against those of the sticky-sphere approximation. If one is not concerned with mass loss, then the sticky sphere approach gives the correct qualitative outcome in approximately 75% of the cases considered. Among those cases in which the sticky-sphere algorithm identies only two particular stars to collide, the hydrodynamic calculations nd the same

The Spectroscopic Orbit of SAO 167450, Visual Companion of AA Ceti

ABSTRACTWe have determined a spectroscopic orbit for SAO 167450, which is the visual secondary of the W UMa-type eclipsing binary AA Cet, and so the system is quadruple. Radial velocities from the coudé feed and 4 m telescopes at Kitt Peak National Observatory produce an orbit with a period of 25.68 days and an eccentricity of 0.50. We classify both the primary and secondary as G0 subgiant/dwarf stars and find that they are somewhat metal rich relative to the Sun. The high lithium abundances of both components argue that the stars are less than 1 Gyr old. If the components are dwarfs, they are pseudosynchronously rotating, while if they are subgiants, they are rotating more slowly than pseudosynchronous. We very roughly estimate a period of 5,000–15,000 yr for the visual pair. Although it has been suggested that in a multiple system, the combination of Kozai cycles and tidal friction may produce contact binaries, the separation between the visual components in this system appears to be much too large to accomplish that result.

An Orbital Period Investigation of the Solar-Type Overcontact Binary V700 Cygni

Orbital period changes of the solar-type contact binary V700 Cyg were investigated based on one new eclipse time combined with the others collected from the literature. A cyclic oscillation, with a period of 39.1 yr and an amplitude of 0.$^{\rm d}$0197, was found to be superimposed on a secular period increase at a rate of $dP/dt$$=$$+$2.8 $\times$ 10$^{-8}{\rm d}$ yr$^{-1}$. Weak evidence indicates that there exists another small-amplitude period oscillation. The long-term period increase is in agreement with the conclusion that high-mass ratio W-type overcontact binaries usually show period increasing. This suggests that it may be in the stage controlled by thermal relaxation oscillations. Cyclic period oscillation can be explained as being the result of a light-travel time effect via the presence of an additional body. On the other hand, since both components of V700 Cyg are late-type stars, the period oscillation may be evidence of magnetic activity cycles of the two component stars.

Absolute dimensions of the close binary systems V453 Monocerotis and V523 Cassiopeiae

We present multi-colour CCD observations of the low-temperature contact binaries, V453 Mon and V523 Cas. Their light curves are modelled to determine a new set of stellar and orbital parameters. Analysis of mid-eclipse times yields a new linear ephemeris for both systems. A period decrease (dP/dt=2.3×10−7 days/yr) in V453 Mon is discovered. V523 Cas, however, is detected to show a period increase (dP/dt=9.8×10−8 days/yr) because of the mass transfer of a rate of 1.1×10−7 M⊙ yr−1, from a less massive donor. Using these findings we can determine the physical parameters of the components of V523 Cas to be M 1=0.76 (3)M ⊙, M 2=0.39 (2)M ⊙, R 1=0.74 (2)R ⊙, R 2=0.55 (2)R ⊙, L 1=0.19 (3)L ⊙, L 2=0.14 (3)L ⊙, and the distance of system as 46(9) pc.

Energy transfer and its effects on the secondaries in W Ursae Majoris type contact binaries

The energy transfer of W UMa contact binaries and its effects on the secondary in W UMa contact binaries are investigated. Relations are given between the mass ratio (q) and the relative energy transfer rates, i.e. U1, the ratio of the transferred luminosity to the surface luminosity of the primary and U2, the ratio of the transferred luminosity to the nuclear luminosity of the secondary. The theoretical curves(U1 vs q and U2 vs q) are derived. Although these curves can reflect the distribution of U1 vs q and U2 vs q, some observational systems are significantly deviated from these curves. It is mainly resulted from the difference in the effective temperatures of the components in W UMa systems.The radius and the density of the secondary are related to the relative energy tran-sfer rate U2: the higher is U2, the greater is the expansion and the lower is the density of the secondaries in W UMa systems. The temperature difference of W UMa binary components is correlated with the relative energy transfer rate U1 and decreases with increasing U1. This might suggest that there is a thermal coupling between two components in W UMa contact binaries, and that the classif-ication of W UMa contact binaries into A- or W-types depends on the energy tran-sfer from the primary to the secondary. The temperature difference of W UMa bin-ary components is poorly correlated with the mass of the primary. This suggests that the properties of the common envelope of W UMa contact binaries might not have a significant effect on the energy transfer between two components.

DISTANCES TO FOUR SOLAR NEIGHBORHOOD ECLIPSING BINARIES FROM ABSOLUTE FLUXES

Eclipsing binary (EB)-based distances are estimated for four solar neighborhood EBs by means of the Direct Distance Estimation (DDE) algorithm. Results are part of a project to map the solar neighborhood EBs in three dimensions, independently of parallaxes, and provide statistical comparisons between EB and parallax distances. Apart from judgments on adopted temperature and interstellar extinction, DDE's simultaneous light-velocity solutions are essentially objective and work as well for semidetached (SD) and overcontact binaries as for detached systems. Here, we analyze two detached and two SD binaries, all double lined. RS Chamaeleontis is a pre-main-sequence (MS), detached EB with weak ? Scuti variations. WW Aurigae is detached and uncomplicated, except for having high metallicity. RZ Cassiopeiae is SD and has very clear ? Scuti variations and several peculiarities. R Canis Majoris (R CMa) is an apparently simple but historically problematic SD system, also with weak ? Scuti variations. Discussions include solution rules and strategies, weighting, convergence, and third light problems. So far there is no indication of systematic band dependence among the derived distances, so the adopted band-calibration ratios seem consistent. Agreement of EB-based and parallax distances is typically within the overlapped uncertainties, with minor exceptions. We also suggest an explanation for the long-standing undermassiveness problem of R CMa's hotter component, in terms of a fortuitous combination of low metallicity and evolution slightly beyond the MS.

Large-scale circulations and energy transport in contact binaries

A hydrodynamic model for the energy transport between the components of a contact binary is presented. Energy is transported by a large-scale, steady circulation carrying high entropy matter from the primary to secondary component. The circulation is driven by the baroclinic structure of the common envelope, which is a direct consequence of the nonuniform heating at the inner critical Roche lobes due to unequal emergent energy fluxes of the components. The mass stream flowing around the secondary is bound to the equatorial region by the Coriolis force and its width is determined primarily by the flow velocity. Its bottom is separated from the underlying secondary's convection zone by a radiative transition layer acting as an insulator. For a typically observed degree of contact the heat capacity of the stream matter is much larger than radiative losses during its flow around the secondary. As a result, its effective temperature and entropy decrease very little before it returns to the primary. The existence of the stream changes insignificantly specific entropies of both convective envelopes and sizes of the components. Substantial oversize of the secondaries, required by the Roche geometry, cannot be explained in this way. The situation can, however, be explained by assuming that the primary is a main sequence star whereas the secondary is in an advanced evolutionary stage with hydrogen depleted in its core. Such a configuration is reached past mass transfer with mass ratio reversal. Good agreement with observations is demonstrated by model calculations applied to actual W UMa-type binaries. In particular, a presence of the equatorial bulge moving with a relative velocity of 10-30 km/s around both components of AW UMa is accounted for.

Possible solution to the problem of the extreme mass ratio W UMa-type binaries

When the total angular momentum of a binary system is at a critical (minimum) value, a tidal instability occurs (Darwin’s instability), eventually forcing the stars to merge into a single, rapidly rotating object. The instability sets in at some critical separation which in the case of contact binaries corresponds to a minimum mass ratio depending on dimensionless gyration radius k1. If one considers n = 3 polytrope (fully radiative primary with � 1 = 4/3), k 2 = 0.075 and qmin ≈ 0.085–0.095. There appears to be, however, some W UMa-type binaries with q values very close, if not below these theoretical limits, implying that primary in these systems is probably more centrally condensed. We try to solve the discrepancy between theory and observations by considering rotating polytropes. We show by deriving and solving a modified Lane–Emden equation for n = 3 polytrope that including the effects of rotation does increase the central concentration and could reduce qmin to as low as 0.070–0.074, more consistent with the observed population.

RADIAL VELOCITY STUDIES OF CLOSE BINARY STARS. XIV

Radial-velocity measurements and sine-curve fits to the orbital radial velocity variations are presented for ten close binary systems: TZ Boo, VW Boo, EL Boo, VZ CVn, GK Cep, RW Com, V2610 Oph, V1387 Ori, AU Ser, and FT UMa. Our spectroscopy revealed two quadruple systems, TZ Boo and V2610 Oph, while three stars showing small photometric amplitudes, EL Boo, V1387 Ori, and FT UMa, were found to be triple systems. GK Cep is close binary with a faint third component. While most of the studied eclipsing systems are contact binaries, VZ CVn and GK Cep are detached or semi-detached double-lined binaries, and EL Boo, V1387 Ori and FT UMa are close binaries of uncertain binary type. The large fraction of triple and quadruple systems found in this sample supports the hypothesis of formation of close binaries in multiple stellar systems; it also demonstrates that low photometric amplitude binaries are a fertile ground for further discoveries of multiple systems.

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.

DIVISION V: VARIABLE STARS

Division V deals with all aspects of stellar variability, either intrinsic or due to eclipses by its companion in a binary system. In the case of intrinsic stellar variability the analysis of pulsating stars, surface inhomogeneities, stellar activity and oscillations are considered. For close binaries, classical detached eclipsing binaries are studied as well as more interacting systems, like contact and semi-detached binaries, or those with compact components, like cataclysmic variables and X-ray binaries, including the physics of accretion processes.

A NEW PHOTOMETRIC INVESTIGATION OF THE W UMA-TYPE BINARY BI CVn

New photometric observations and their investigation of the W UMa-type binary, BI CVn, are presented. The variations of the orbital period were analyzed based on 12 new determined times of light minimum together with the others compiled from the literature. It is discovered that the period of BI CVn shows a long-term period decrease at a rate of. (P) over dot = -1.51(+/- 0.12) x 10(-7) days year(-1) while it undergoes a cyclic variation with a period of 27.0 years and an amplitude of 0(d).0151. Photometric solutions determined with the Wilson-Devinney method suggest that BI CVn is a contact binary with a degree of contact of 18.0(+/- 1.7)%. The asymmetry of the light curves was interpreted by the presence of dark spots on both components, and absolute parameters were determined by combining the photometric elements with the spectroscopic solutions given by Lu. The observed period decrease can be plausibly explained by a combination of the mass transfer from the primary to the secondary and angular momentum loss via magnetic braking. The cyclic period oscillation suggests that BI CVn is a triple system containing a tertiary component with a mass no less than 0.58 M(circle dot) in a 27.0 year orbit. As in the cases of the other contact binaries (e. g., AH Cnc, AP Leo, AD Cnc, and UX Eri), it is possible that this tertiary companion played an important role for the formation and evolution of the contact system by removing angular momentum from the central system via Kozai oscillation or a combination of Kozai cycle and tidal friction, which causes the eclipsing pair to have a short initial orbital period (e. g., P < 5(d)). In that case, can the initially detached system evolve into the present contact configuration via a combination of magnetic torques from stellar winds and a case A mass transfer?

Angular momentum and mass evolution of contact binaries

Various scenarios of contact binary evolution have been proposed in the past, giving hints of (sometimes contradictory) evolutionary sequences connecting A- and W-type systems. As the components of close detached binaries approach each other and contact binaries are formed, following evolutionary paths transforms them into systems of two categories: A-type and W-type. The systems evolve in a similar way but under slightly different circumstances. The mass/energy transfer rate is different, leading to quite different evolutionary results. An alternative scenario of evolution in contact is presented and discussed, based on the observational data of over one hundred low-temperature contact binaries. It results from the observed correlations among contact binary physical and orbital parameters. Theoretical tracks are computed assuming angular momentum loss from a system via stellar wind, accompanied by mass transfer from an advanced evolutionary secondary to the main-sequence primary. A good agreement is seen between the tracks and the observed graphs. Independently of details of the evolution in contact and a relation between A- and W-type systems, the ultimate fate of contact binaries involves the coalescence of both components into a single fast rotating star.

The shortest period field contact binary

Abstract Photometric and spectroscopic results for the contact binary GSC 01387–00475 (ASAS 083128+1953.1) are presented. The existence of this binary with the orbital period of P= 0.2178 d strengthens the argument that the cut-off of the period distribution for contact binaries – until now defined by CC Comae – is very sharp. The only case of a still shorter period is known in a globular cluster where more compact contact configurations are in fact expected. While the spectroscopic orbit of GSC 01387–00475 is well defined, the low orbital inclination of the binary and the presence of a spectroscopic companion contributing about 1/3 of the total light conspire to reduce the photometric variability to ≃0.09 mag. The photometric data are currently inadequate to identify the source of the small amplitude (0.02–0.03 mag) intrinsic variability of the system.

RADIAL VELOCITY STUDIES OF CLOSE BINARY STARS. XIII

Radial-velocity measurements and sine-curve fits to the orbital radial velocity variations are presented for ten close binary systems: EG Cep,V1191 Cyg, V1003 Her, BD+7_3142, V357 Peg, V407 Peg, V1123 Tau, V1128 Tau, HH UMa, and PY Vir. While most of the studied eclipsing systems are contact binaries, EG Cep is a detached or a semi-detached double-lined binary and V1003 Her is a close binary of an uncertain type seen at a very low inclination angle. We discovered two previously unknown triple systems, BD+7_3142 and PY Vir, both with late spectral-type (K2V) binaries. Of interest is the low-mass ratio (q = 0.106) close binary V1191 Cyg showing an extremely fast period increase; the system has a very short period for its spectral type and shows a W-type light curve, a feature rather unexpected for such a low mass-ratio system.

Gravity and effective temperature distribution on the surface of contact binary systems

AbstractThe aim of this work is to study the gravity and the effective temperature distribution on the outer equipotential surface of a contact binary. In particular, the lines of constant g, (the iso‐g curves), are computed and plotted on the outer surface Cs of contact configurations. Since the gravity distribution would also specify the effective temperature distribution on the binary's common photosphere through the gravity darkening effect, these lines correspond to isothermals too. Applications have been made to two contact binaries; namely, to AW UMa and OO Aql, two systems with very different mass ratios. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Tomographic Separation of Composite Spectra. XI. The Physical Properties of the Massive Close Binary HD 100213 (TU Muscae)

We present the results of a Doppler tomographic reconstruction of the UV spectra of the double-lined, O binary HD 100213 based on observations made with the International Ultraviolet Explorer (IUE). We used cross-correlation methods to obtain radial velocities, confirm the orbital elements, estimate the UV flux ratio, and determine projected rotational velocities. The individual component spectra are classified as O7 V + O8 V using UV criteria defined by Penny, Gies, & Bagnuolo. We present a model fit of the eclipsing light curve from observations from the Hipparcos satellite and published observations of Andersen & Gronbech. We derive an orbital inclination, i=77.7deg+/-1.0deg. This analysis indicates that both stars are currently experiencing Roche lobe overflow (RLOF), which confirms earlier results that this is one of only a few massive contact binaries. Our derived masses, Mp/Msolar=16.7+/-0.4 and Ms/Msolar=10.4+/-0.4, are significantly lower than those computed from the Doppler shifts of lines in the optical spectrum. We suggest that the difference occurs because mutual irradiation decreases the upper atmospheric temperature gradient in the inward-facing hemispheres of both stars, which makes lower excitation lines appear weaker there and shifts their center of light away from the center of mass. We compare the current state of HD 100213 with predicted outcomes of massive close binary evolutionary models, and suggest that the system is currently in a very slow case AA mass transfer stage.

Period changes in six contact binaries: WZ And, V803 Aql, DF Hya, PY Lyr, FZ Ori, and AH Tau

Six contact binaries lacking a period analysis have been chosen to search for the presence of a third body. The O – C diagrams of these binaries were analyzed with the least-squares method by using all available times of minima. Ten new minima times, obtained from our observations, were included in the present research. The Light–Time Effect was adopted for the first time as the main cause for the detailed description of the long-term period changes. Third bodies were found with orbital periods from 49 up to 100 years, and eccentricities from 0.0 to 0.56 for the selected binaries. In one case (WZ And), a fourth-body LITE variation was also applied. The mass functions and the minimal masses of such bodies were also calculated and a possible angular separation and magnitude differences were discussed for a prospective interferometric discovery of these bodies.