Evolution Of Contact Binaries Research Articles

The Energy Transport Induced by Horizontal Turbulence in Rotating W-type W UMa Contact Binaries

Abstract The aim of this paper is to investigate the impact of energy transfer on the evolution of contact binaries. Horizontal turbulence is triggered by the differential rotation induced by meridional circulation, which is a direct consequence of nonuniform heating at the inner critical Roche lobes because of strong rotational and tidal distortions. Thermal energy is transferred by the horizontal turbulence from the more massive star to the less massive one, and horizontal turbulence can be responsible for the redistribution of what is a significant fraction of the total core luminosity. The secondary becomes overluminous and oversized owing to energy transfer from the companion star, whereas the primary shifts toward smaller luminosity and is undersized. The convective regions for primaries are enlarged by the improved radiative temperature gradient. The main region for energy transport is located at the bottom of the common envelope because of a higher local density and enthalpy difference. One can find that thermal structure can be disturbed and display periodic thermal relaxation oscillations between the semidetached stage and the contact stage. W-type W UMa contact binaries acquire efficient energy transfer, which can cause the temperature of secondaries to exceed that of the primaries. However, angular momentum loss owing to nonconservative mass transfer can make the system maintain shallow contact and not evolve from overcontact to semidetached configurations, and the system may appear as an A-type W UMa contact binary.

Very wide companion fraction from Gaia DR2: A weak or no enhancement for hot Jupiter hosts, and a strong enhancement for contact binaries

ABSTRACT There is an ongoing debate on whether hot Jupiter hosts are more likely to be found in wide binaries with separations of ≳100 AU. In this paper, we search for comoving, very wide companions with separations of 103–104 AU for hot Jupiter hosts and main-sequence contact binaries in Gaia DR2, and compare the very wide companion fractions with their object-by-object-matched field star samples. We find that 11.9 ± 2.5 per cent of hot Jupiter hosts and 14.1 ± 1.0 per cent of contact binaries have companions at separations of 103–104 AU. While the very wide companion fraction of hot Jupiter hosts is a factor of 1.9 ± 0.5 larger than their matched field star sample, it is consistent, within ∼1σ, with that of matched field stars if the matching is only with field stars without close companions (within ∼50 AU) as is the case for hot Jupiter hosts. The very wide companion fraction of contact binaries is a factor of 3.1 ± 0.5 larger than their matched field star sample, suggesting that the formation and evolution of contact binaries are either tied to or correlated with the presence of wide companions. In contrast, the weak enhancement of very wide companion fraction for hot Jupiter hosts implies that the formation of hot Jupiters is not as sensitive to those environment properties. Our results also hint that the occurrence rates of dual hot Jupiter hosts and dual contact binaries may be higher than the expected values from random pairing of field stars, which may be due to their underlying metallicity and age dependence.

Investigations into the thermal non-equilibrium of W UMa-type contact binaries

Traditionally, some physical details (e.g., magnetic braking, energy transfer, angular momentum loss, etc.) have to be taken into consideration during investigations into the evolution of contact binaries. However, the real evolutionary processes which usually contain several of these physical mechanisms are very complicated as a result of strong interaction between components. To avoid dealing with these factors, a linear relationship is applied to the temperatures of components. It is found that the higher the mass ratio (M2/M1) of a contact system, the weaker the deviation from thermal equilibrium. On this basis, a variation trend of fill-out factor (f) changing with mass ratio can be inferred, which is consistent with observations. Moreover, if we stick to this point of view, it should be natural that the number of semi-detached binaries in the predicted broken-contact phase of relaxation oscillations is less than the number in the contact phase.

Close-in companions to contact binary stars

W UMa-type contact binaries are composed of two late-type main-sequence stars, where both components are filling their critical Roche Lobes and sharing a common convective envelope. Their formation and evolution are unsolved problems in stellar astrophysics. This kind of binary systems have the lowest angular momentum and shortest orbital periods among main-sequence binaries. One of the possibilities for their origin is that the angular momentums of binary stars are transferred from the central binaries to close-in companions. In this paper, we will summarize some of our recent progresses on searching for close-in companions to contact binary stars, including the closest stellar companion to a contact binary at an orbital separation of about 0.8 AU. Then, based on the observational properties of those close-in companions, the formation and the evolution of contact binaries will be discussed.

The lifetime of contact binaries

AbstractThe lifetime of contact binaries is an important parameter for studying the evolution of contact binaries. By reproducing the relative frequency of occurrence for contact binaries, we found that the lifetime of contact binaries decreases with increasing mass of the primaries. The lifetime of contact binaries is about 4%–10% of the main sequence lifetime of the primaries, and is about 0.006–0.65 Gyr. This suggests that the lifetime of contact binaries strongly depends on the mass of the primaries.

Formation and Evolution of Contact Binaries

I describe a series of processes, including hierarchical fragmentation, gravitational scattering, Kozai cycles within triple systems, tidal friction and magnetic braking, that I believe are responsible for producing the modest but significant fraction of stars that are observed as contact binaries. I also discuss further processes, namely heat transport, mass transport, nuclear evolution, thermal relaxation oscillations, and further magnetic braking with tidal friction, that influence the evolution during contact. The endpoint, for contact, is that the two components merge into a single star, as recently was observed in the remarkable system V1309 Sco. The single star probably throws off some mass and rotates rapidly at first, and then slows by magnetic braking to become a rather inconspicuous but normal dwarf or subgiant. If however the contact binary was part of a triple system originally-as I suggested above was rather likely-then the result could be a binary with apparently non-coeval components. There are several such known.

Contact binaries: II. The importance of deformation caused by rotation and tides to the light curve of a contact binary

The theoretical light curves of contact binaries are calculated with and without putting in the contact binary evolution model. Firstly, we do not use the contact binary evolution model. A comparison of the light curve is performed with and without the deformation caused by rotation and tides. It shows that the light curve presents many differences, especially on the bottom and top. Secondly, we adopt the contact binary model [Huang R Q, et al. Chin J Astron Astrophys, 2007, 7: 235–244; Song H F, et al. Chin J Astron Astrophys, 2007, 7: 539–550] and compute the theoretical light curve with and without rotational and tidal effects by studying three binary systems (with low-, intermediate- and high-mass components). The bottom and top of the theoretical light curves are discussed and compared to observations. The results show that taking into account the rotational effect has a better agreement with observations than without it. Therefore, the deformation of the light curve of contact binaries caused by rotation and tides is very important. Meanwhile, the rotational and tidal effect can advance the start of the semi-detached, contact phase and the time of mass-reversal.

Photometric investigation of three W UMa-type binary stars in the old open cluster NGC 188

In a programme to understand the evolution of contact binaries, we have re-observed EQ Cep, ER Cep and V371 Cep, which are members of the old open cluster NGC 188, photometrically. Combining our photometric solutions with some known parameters of the cluster, we derived the mass and the radius for each component. Meanwhile, we investigated the period changes of the three systems, finding a long-term increase of period in every system and discovering two cyclic variations in ER Cep besides the long-term changes (T(3) = 5.40 +/- 0.01 yr, A(3) = 0.0135 +/- 0.0016 d; T(4) = 17.6 +/- 0.1 yr). From the derived physical parameters and analysis of the orbital period, we conclude that (i) both components of V371 Cep have very possibly evolved off the main sequence, as a result of the derived accelerations of gravity log g(1) = 3.93 and log g(2) = 3.98, and (ii) ER Cep is at least a triple, or even quadruple, system, something suggested by the overmassive derived total mass (similar to 3.51 M(circle dot)) and the 5.4-yr-cycle periodic oscillation.

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.

Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation

We study the kinematics of 129 W UMa binaries and we discuss its implications on the contact binary evolution. The sample is found to be heterogeneous in the velocity space. That is, kinematically younger and older contact binaries exist in the sample. A kinematically young (0.5 Gyr) subsample (moving group) is formed by selecting the systems that satisfy the kinematical criteria of moving groups. After removing the possible moving group members and the systems that are known to be members of open clusters, the rest of the sample is called the field contact binary (FCB) group. The FCB group is further divided into four groups according to the orbital period ranges. Then, a correlation is found in the sense that shorter-period less-massive systems have larger velocity dispersions than the longer-period more-massive systems. Dispersions in the velocity space indicate a 5.47-Gyr kinematical age for the FCB group. Compared with the field chromospherically active binaries (CABs), presumably detached binary progenitors of the contact systems, the FCB group appears to be 1.61 Gyr older. Assuming an equilibrium in the formation and destruction of CAB and W UMa systems in the Galaxy, this age difference is treated as an empirically deduced lifetime of the contact stage. Because the kinematical ages (3.21, 3.51, 7.14 and 8.89 Gyr) of the four subgroups of the FCB group are much longer than the 1.61-Gyr lifetime of the contact stage, the pre-contact stages of the FCB group must dominantly be producing the large dispersions. The kinematically young (0.5 Gyr) moving group covers the same total mass, period and spectral ranges as the FCB group. However, the very young age of this group does not leave enough room for pre-contact stages, and thus it is most likely that these systems were formed in the beginning of the main sequence or during the pre-main-sequence contraction phase, either by a fission process or most probably by fast spiralling in of two components in a common envelope.

Structure and evolution of low-mass W Ursae Majoris type systems - II. With angular momentum loss

In a preceding paper, using Eggleton's evolution code we have discussed the structure and evolution of low-mass W UMa type contact binaries without angular momentum loss (AML). The models exhibit cyclic behavior about a state of marginal contact on a thermal time-scale. Part of the time of each cycle is spent in contact and part in a semi-detached state. According to observations, W UMa systems suffer AML. We present the models of low-mass contact binaries with AML due to gravitational wave radiation (GR) or magnetic stellar wind (MSW) are presented. We find that gravitational radiation cannot prevent the cyclic evolution of W UMa systems, and the effect of gravitational radiation on the cyclic behavior of contact binary evolution is almost negligible. We also find that the most likely AML mechanism for W UMa systems is magnetic braking, and that magnetic braking effects can increase the period of the cyclic evolution, and shorten the fraction of the time spent in the poor thermal contact state. If W UMa stars do not undergo cyclic evolution, and their angular momentum loss is caused simultaneously by MSW of both components, we find that the value of the parameter, $\lambda$, should be taken a larger value in comparison with those derived from observations of single stars. This indicates that the AML efficiency in W UMa systems may be lowered in comparison with non-contact stars because of the less mass contained in the convective envelopes of the components in W UMa systems. If W UMa systems lose their angular momentum at a constant rate. An angular momentum rate of $\frac{{\rm dln}J}{{\rm d}t}\approx 1.6\times 10^{-9} {\rm yr^{-1}}$ can prevent the cyclic behaviour of the model, and the model can keep in good contact with an essentially constant depth of contact.

Structure and evolution of low-mass W UMa-type systems

The structure and evolution of low-mass W UMa-type contact binaries are discussed by employing Eggleton's stellar evolution code. Assuming that these systems completely satisfy Roche geometry for contact binaries with every kind of mass ratio (0.02–1.0) we calculate the relative radii (R1,2/A, where R1,2 are the radii of both stars and A is the orbital separation) of both components of contact binaries with different contact depths between inner and outer Roche lobes. We obtain a radius grid of contact binaries and can ensure the surfaces of two components lying on an equipotential surface by interpolation using this radius grid when we follow the evolution of the contact binaries. There are serious uncertainties concerning mainly the transfer of energy in these systems, i.e. it is unclear how and where the energy is transferred. We assume that the energy transfer takes place in different regions of the common envelope to investigate the effects of the region of energy transfer on the structure and evolution of contact binaries. We find that the region of energy transfer has a significant influence on the structure and evolution of contact binaries, and conclude that the energy transfer may occur in the outermost layers of the common convective envelope for W-type systems, and that this transfer takes place in the deeper layers of the common envelope for A-type systems. Meanwhile, if we assume that the energy transfer takes place in the outermost layers for our model with low total mass, and find that our model steadily evolves towards a system with a smaller mass ratio and a deeper envelope, suggesting that some A-type W UMa systems with low total mass could be considered as the later evolutionary stages of W-subtype systems, and that the surface temperature of the secondary exceeds that of the primary during the time when the primary expands rapidly, or the secondary contracts rapidly, suggesting that W-subtype systems may be caused by expansion of the primary, or by the contraction of the secondary.

The WR+OB Progenitor RY Scuti: Intensive Spectroscopy of Its Compact Double‐Ring Nebula

We present a detailed spectroscopic analysis of the peculiar nebula around RY Scuti using data from the Space Telescope Imaging Spectrograph (STIS) and various ground-based observatories. This massive contact binary may represent a rapid evolutionary phase in transition to a short-period WR+OB system, and it is surrounded by a young nebula with unusual geometry. The ionized nebula is only about 1'' across, so STIS has allowed us to spatially resolve the spectrum of the star from its circumstellar nebula for the first time. Combining STIS data with ground-based spectra at optical and infrared wavelengths solves some geometric ambiguities posed by previous images and reveals the kinematic relationship between the nebular geometry and RY Scuti's complex emission-line profiles. We offer a new determination of the systemic velocity of 20 ± 3 km s-1, and we estimate the distance to RY Scuti as 1.8 ± 0.1 kpc. Nebular lines show subtle phase-dependent variability in addition to expected changes in equivalent width due to eclipses of stellar continuum. These changes in equivalent width yield an updated ephemeris, but a previously suggested period change is still uncertain after considering these new data. The nebula contains roughly 0.003 M☉ of material that is He- and N-rich and probably O-deficient compared to solar abundances, indicating that CNO-processed material has reached the surface in at least one component of the binary system. Diagnostic line ratios suggest that the electron density and temperature in the nebula are 2 × 105 cm-3 and ~9500 K, respectively. There are concentrated regions of higher density as well; emission knots seen around the rings suggest that common envelope mass loss during massive contact binary evolution may be characterized by strong azimuthal asymmetry and sporadic mass ejection events.

The Photometric Amplitude and Mass Ratio Distributions of Contact Binary Stars

The distribution of the light-variation amplitudes, A(a), in addition to determining the number of undiscovered contact binary systems falling below photometric detection thresholds and thus lost to statistics, can serve as a tool in determination of the mass-ratio distribution, Q(q), which is very important for understanding of the evolution of contact binaries. Calculations of the expected A(a) show that it tends to converge to a mass-ratio dependent constant value for a->0. Strong dependence of A(a) on Q(q) can be used to determine the latter distribution, but the technique is limited by the presence of unresolved visual companions and by blending in crowded areas of the sky. The bright-star sample to 7.5 magnitude is too small for an application of the technique while the the Baade's Window sample from the OGLE project may suffer stronger blending; thus the present results are preliminary and illustrative only. Estimates based on the Baade's Window data from the OGLE project, for amplitudes a>0.3 mag. where the statistics appear to be complete allowing determination of Q(q) over 0.12 0. The mass-ratio distribution can be approximated by a power law, either Q(q)~(1-q)^a1 with a1=6+/-2 or Q(q)~q^b1, with b1=-2+/-0.5, with a slight preference for the former form. Both forms must be modified by the theoretically expected cut-off caused by a tidal instability at about q_min 0.07-0.1. An expected maximum in Q(q), is expected to be mapped into a local maximum in A(a) around 0.2-0.25 mag.

An Ultraviolet Study of the Short-Period Binary OO Aquilae

OO Aql is a rare W UMa-type eclipsing binary in which the two solar-type stars may have only recently evolved into contact. The binary has an unusually high mass ratio (0.84), and a relatively long orbital period (0.506 d) for it spectral type (mid-G). Twelve ultraviolet spectra of OO Aql were obtained in 1988 with the IUE satellite, including a series of consecutive observations that cover nearly a complete orbital cycle. Chromospheric activity is studied by means of the Mg II h+k emission at 2800 A. The Mg II emission is found to vary, even when the emission is normalized to the adjacent continuum flux. This variation may be correlated with orbital phase in the 1988 observations. It also appears that the normalized Mg II emission varies with time, as seen in spectra obtained at two different epochs in 1988 and when compared with two spectra obtained several years earlier. The level of chromospheric activity in OO Aql is less than that of other W UMa-type binaries of similar colors, but this is attributed to its early stage of contact binary evolution. Ultraviolet light curves were composed from measurements of the ultraviolet continuum in the spectra. These were analyzed along with visible light curves of OO Aql to determine the system parameters. The large wavelength range in the light curves enabled a well-constrained fit to a cool spot in the system.

High mass ratio contact binaries: Recent evolution into contact?

Recent theories of the origin and evolution of contact binaries suggest that the two stars evolve into contact through angular momentum loss (AML; Mochnacki 1981, Vilhu 1982). When in contact, the system then evolves toward smaller mass ratio through mass transfer from the secondary to the primary component (Webbink 1976, Rahunen and Vilhu 1982). Most contact binaries have mass ratios of 0.3 to 0.5.

Radial velocity studies and absolute parameters of contact binaries. II - OO Aquilae

New high-precision radial velocities of the contact binary OO Aql have been obtained using the cross-correlation technique. The orbital elements have been corrected for proximity effects, using an analysis of published light curves of the system. The spectroscopically determined mass ratio of 0.843 is in excellent agreement with the photometrically determined value. OO Aql thus has one of the largest mass ratios observed for a contact binary. In contrast to almost all other contact binaries of G spectral type, the primary minimum is due to a transit by the less massive component, and thus the system is classified as an A-type contact binary. Absolute parameters are determined for OO Aql, which indicate that the primary component, although similar to the Sun in mass, is significantly more evolved. An age of about 8 Gyr and a metal abundance of one-half that of the Sun are determined. It seems that the system may have only recently evolved into contact, as suggested by Mochnacki, and that it is an important object for studies of the structure and evolution of contact binaries.

High Mass Ratio Contact Binaries: Recent Evolution into Contact?

Recent theories of the origin and evolution of contact binaries suggest that the two stars evolve into contact through angular momentum loss (AML; Mochnacki 1981, Vilhu 1982). When in contact, the system then evolves toward smaller mass ratio through mass transfer from the secondary to the primary component (Webbink 1976, Rahunen and Vilhu 1982). Most contact binaries have mass ratios of 0.3 to 0.5.

Evolutionary Effects in Contact Binaries

The structure and evolution of contact binaries is an extremely difficult problem in theoretical astrophysics.Several attempts have been made to set up a satisfactory model, see e.g. Lucy,1976, Flannery, 1976, Shu, Lubow and Anderson, 1976, Robertson and Eggleton, 1977, Rahunen and Vilhu, 1977 and others. There are, roughly, two lines of sight: Shu, Lubow and Anderson postulate the existence of a contact discontinuity in one of the two components of a contact binary (DSC Theory); in this case age zero models in thermal equilibrium are possible. The other authors find that unevolved contact binaries cannot achieve thermal equilibrium; therefore, relaxation oscillations on a thermal time scale are expected (TRO Theory). For a detailed discussion of the different effects see Lucy and Wilson,1979.

Origin and Evolution of Contact Binaries of W UMa Type

AbstractUsing angular momentum loss estimates from single star studies, it is shown that detached binaries are good candidates as progenitors of contact binaries. Three theories constructed for contact binary evolution (DSC, TRO and AML, see Fig. 1) are discussed. The DSC and TRO theories require a contact binary formation mechanism which produces unequal components (fission) while the AML theory can start from equal components (initially detached binaries). In all theories the end-product is a single star.The stability of unequal entropy models was studied using a formula (2) which couples the energy transfer with the depth of contact and with the entropy difference. The models experience cyclic behaviour on a time scale of 106-107 years (Fig. 2) and the contact never breaks even on the nuclear time scale. This is the important consequence of a formula of type (2). Similar behaviour (with similar formula) is expected also for TRO models and even for DSC models if the discontinuity can be preserved during one cycle period.The DSC and TRO theories, which at first sight look quite different, are in fact complementary. The most probable contact binary theory is perhaps a suitable combination of all three. In this theory angular momentum loss is the new and important factor which may manifest itself in the UV- and X-ray activity observed in W UMa stars.