Carbon-enhanced metal-poor stars (CEMP) are observed as a substantial fraction of the very metal-poor stars in the Galactic halo. Most CEMP stars are also enriched in s-process elements and these are often found in binary systems. This suggests that the carbon enrichment is due to mass transfer in the past from an asymptotic giant branch (AGB) star on to a low-mass companion. Models of binary population synthesis are not able to reproduce the observed fraction of CEMP stars without invoking non-standard nucleosynthesis or a substantial change in the initial mass function. This is interpreted as evidence of missing physical ingredients in the models. Recent hydrodynamical simulations show that efficient wind mass transfer is possible in the case of the slow and dense winds typical of AGB stars through a mechanism called wind Roche-lobe overflow (WRLOF), which lies in between the canonical Bondi-Hoyle-Lyttleton (BHL) accretion and Roche-lobe overflow. WRLOF has an effect on the accretion efficiency of mass transfer and on the angular momentum lost by the binary system. The aim of this work is to understand the overall effect of WRLOF on the population of CEMP stars. To simulate populations of low-metallicity binaries we combined a synthetic nucleosynthesis model with a binary population synthesis code. In this code we implemented the WRLOF mechanism. We used the results of hydrodynamical simulations to model the effect of WRLOF on the accretion efficiency and we took the effect on the angular momentum loss into account by assuming a simple prescription. As a result the number of CEMP stars predicted by our model increases by a factor 1.2-1.8 compared to earlier results that consider the BHL prescription. Moreover, higher enrichments of carbon are produced and the final orbital period distribution is shifted towards shorter periods.
We have simulated a common envelope interaction of a 0.88-M$_{\odot}$, 90-R$_{\odot}$, red giant branch star and a 0.6-M$_{\odot}$, compact companion with the smoothed particle hydrodynamics code, Phantom, from the beginning of the Roche lobe overflow phase to the beginning of the self-regulated inspiral, using three different resolutions. The duration of the Roche lobe overflow phase is resolution dependent and would lengthen with increased resolution beyond the $\sim$20 years observed, while the inspiral phase and the post-common envelope separation are largely independent of resolution. Mas…
One of the principal mechanisms that is responsible for the origin of blue stragglers is mass transfer that takes place while one of the binary companions overfills its Roche lobe. In this Chapter, we overview the theoretical understanding of mass transfer via Roche lobe overflow: classification, how both the donor and of the accretor respond to the mass transfer on different timescales (adiabatic response, equilibrium response, superadiabatic response, time-dependent response) for different types of their envelopes (convective and radiative). These responses, as well as the assumption on how …
In this study, we concentrate on the formation and evolution of hot subdwarfs binaries through the stable Roche lobe overflow (RLOF) channel of intermediate-mass binaries. We aim at setting out the properties of hot subdwarfs and their progenitors, so that we can understand the formation and evolution of hot subdwarfs comprehensively. We have obtained the ranges of the initial parameters of progenitor binaries and the properties of hot subdwarfs through the stable RLOF channel of intermediate-mass binaries, e.g. mass, envelope mass and age of hot subdwarfs. We have found that hot subdwarfs cou…
Several observational pieces of evidence indicate that specific evolutionary channels which involve Roche lobe overflow are not correctly accounted for by the classical Roche model. We generalize the concept of Roche lobe in the presence of extra forces (caused by radiation pressure or pulsations). By computing the distortion of the equipotential surfaces, we are able to evaluate the impact of these perturbing forces on the stability of Roche-lobe overflow (RLOF). Radiative forces are parametrized through the constant reduction factor that they impose on the gravitational force from the radiat…
One of the important issues regarding the final evolution of stars is the impact of binarity. A rich zoo of peculiar, evolved objects are born from the interaction between the loosely bound envelope of a giant, and the gravitational pull of a companion. However, binary interactions are not understood from first principles, and the theoretical models are subject to many assumptions. It is currently agreed upon that hot subdwarf stars can only be formed through binary interaction, either through common envelope ejection or stable Roche-lobe overflow (RLOF) near the tip of the red giant branch (R…
Many giant exoplanets are found near their Roche limit and in mildly eccentric orbits. In this study we examine the fate of such planets through Roche-lobe overflow as a function of the physical properties of the binary components, including the eccentricity and the asynchronicity of the rotating planet. We use a direct three-body integrator to compute the trajectories of the lost mass in the ballistic limit and investigate the possible outcomes. We find three different outcomes for the mass transferred through the Lagrangian point $L_{1}$: (i) self-accretion by the planet, (ii) direct impact …
Long-orbital-period subdwarf B (sdB) stars with main-sequence companions are believed to be the product of stable Roche Lobe overflow (RLOF), a scenario challenged by recent observations. Here we represent the results of a systematic study of the orbital-period distribution of sdB binaries in this channel using detailed binary evolution calculations. We show that the observed orbital-period distribution of long-period sdB binaries can be well explained by this scenario. Furthermore, we find that, if the progenitors of the sdB stars have initial masses below the helium flash mass, the sdB binar…
We find that applying a theoretical wind mass-loss rate from Monte Carlo radiative transfer models for hydrogen-deficient stars results in significantly more leftover hydrogen following stable mass transfer through Roche-lobe overflow than when we use an extrapolation of an empirical fit for Galactic Wolf-Rayet stars, for which a negligible amount of hydrogen remains in a large set of binary stellar evolution computations. These findings have implications for modelling progenitors of Type Ib and Type IIb supernovae. Most importantly, our study stresses the sensitivity of the stellar evolution …
The evolution of triples has not attracted much attention in the literature, although their evolution can be dramatically different from binaries and single stars. Triples are quite common, and we find that for about 1% of the triples in the Tokovinin catalogue of multiple stellar systems in the solar neighbourhood, the tertiary star will overflow its Roche lobe at some time in its evolution, before any of the inner stars leave the main sequence. For two of these systems, Xi Tauri and HD97131 we simulate in detail this phase of mass transfer, during which stellar evolution, gravitational dynam…
Through tidal dissipation in a slowly spinning host star the orbits of many hot Jupiters may decay down to the Roche limit. We expect that in most cases the ensuing mass transfer will be stable. Using detailed numerical calculations we find that this evolution is quite rapid, potentially leading to complete removal of the gaseous envelope in a few Gyr, and leaving behind an exposed rocky core ("hot super-Earth"). Final orbital periods are quite sensitive to the details of the planet's mass-radius relation, and to the effects of irradiation and photo-evaporation, but could be as short as a few …
Symbiotic stars are interacting binaries with one of the longest orbital periods. Since they can contain a massive white dwarf with a high accretion rate they are considered a possible type Ia supernovae (SNe Ia) progenitors. Among symbiotic binaries there are systems containing a Mira donor, which can have orbital periods of a few tens of years and more. This subclass of symbiotic stars due to their very large separation usually was not considered promising SNe Ia progenitors. We analysed evolution of one of the well studied symbiotic star with a Mira donor, V407 Cyg. We showed that the stand…
A very large lobe overflow event is suggested to explain the $0.^m4$ brightening observed in K band at pericenter passage of the star known as S2 that orbits the Galaxy's supermassive black hole (SMBH). Known observed properties of S2 that contribute to lobe filling are 1) the enormous mass ratio, $M_{SMBH}/M_{S2}$, 2) S2's fast rotation, and 3) S2's large orbital eccentricity. Published estimates have given limiting lobe sizes of order 100 to 300 $R_\odot$ but, with S2's fast rotation taken into account, the computed lobe size is much smaller, being compatible with either a main sequence OB s…
We present phase-resolved spectroscopy and photometry of the optical counterpart to the X-ray binary IGR J19308+0530. Ellipsoidal modulations in the light curve show that the F-type companion star in the system is Roche lobe filling. The optical spectra are dominated by absorption features from the donor star, with ~10-20 per cent disc contribution to the optical continuum. We measure an orbital period of 14.662+-0.001 h, a radial velocity semiamplitude for the companion star of K2 = 91.4+-1.4 kms-1 and a rotational broadening of vsini = 108.9+-0.6 kms-1. From K2 and vsini, given that the dono…
We discuss a method for determination of the size of the emitting region close to the compact star in a binary system with eclipses by a secondary, which fills its Roche lobe. The often used approach is to model the Roche lobe by a sphere with the "effective radius" corresponding to the volume of the Roche lobe. This approach leads to a 4-6% overestimate of the radius, if taking into account the angular dimensions of the Roche lobe seen form the compact star. Andronov (1992) had shown that the projection of the Roche lobe onto the celestial sphere is close to an ellipse and had tabulated these…
In compact stellar triple systems, an evolved tertiary star can overflow its Roche lobe around the inner binary. Subsequently, the tertiary star can transfer mass to the inner binary in a stable manner, or Roche lobe overflow (RLOF) can be unstable and lead to common-envelope (CE) evolution. In the latter case, the inner binary enters the extended envelope of the tertiary star and spirals in towards the donor's core, potentially leading to mergers or ejections. Although studied in detail for individual systems, a comprehensive statistical view on the various outcomes of triple RLOF is lacking.…
Common-envelope (CE) evolution is an outstanding open problem in stellar evolution, critical to the formation of compact binaries including gravitational-wave sources. In the ``classical'' isolated binary evolution scenario for double compact objects, the CE is usually the second mass transfer phase. Thus, the donor star of the CE is the product of a previous binary interaction, often stable Roche-lobe overflow (RLOF). Because of the accretion of mass during the first RLOF, the main-sequence core of the accretor star grows and is ``rejuvenated''. This modifies the core-envelope boundary region…
Revisiting atmospheric Roche lobe overflow in symbiotic binaries
Yunnan Observatories, Chinese Academy of Sciences (CAS)（中国科学院云南天文台）；International Centre of Supernovae (ICESUN), Yunnan Key Laboratory of Supernova Research（超新星国际中心（ICESUN），云南省超新星研究重点实验室）；University of Chinese Academy of Sciences（中国科学院大学）；Department of Physics, University of Alberta（阿尔伯塔大学物理系）；London Institute of Stellar Astrophysics（伦敦恒星天体物理学研究所）；University of Oxford, St Edmund Hall（牛津大学圣埃德蒙学堂）；Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences（波兰科学院尼古拉·哥白尼天文中心）
Some close-in gaseous exoplanets are nearly in Roche-lobe contact, and previous studies show tidal decay can drive hot Jupiters into contact during the main sequence of their host stars. Improving upon a previous model, we present a revised model for mass transfer in a semi-detached binary system that incorporates an extended atmosphere around the donor and allows for an arbitrary mass ratio. We apply this new formalism to hypothetical, confirmed, and candidate planetary systems to estimate mass loss rates and compare with models of evaporative mass loss. Overflow may be significant for hot Ne…
We study the evolution of close binary systems formed by a normal (solar composition), intermediate mass donor star together with a neutron star. We consider models including irradiation feedback and evaporation. These non-standard ingredients deeply modify the mass transfer stages of these binaries. While models that neglect irradiation feedback undergo continuous, long standing mass transfer episodes, models including these effect suffer a number cycles of mass transfer and detachment. During mass transfer the systems should reveal themselves as low-mass X-ray binaries (LMXBs), whereas when …
FQ Circini: An Ordinary Nova with a High-mass B1 V(n)(e) Companion Whose Decretion Disk Transfers Mass to the White Dwarf via Roche-Lobe Overflow
Louisiana State University（路易斯安那州立大学）；American Association of Variable Star Observers（变星观测者协会）；Variable Stars South, Royal Astronomical Society of New Zealand（南半球变星，新西兰皇家天文学会）；Centre for Astrophysics, University of Southern Queensland（南昆士兰大学天体物理中心）；Department of Astrophysics, American Museum of Natural History（美国自然历史博物馆天体物理学系）；South African Astronomical Observatory（南非天文台）；Southern African Large Telescope（南非大型望远镜）；University of Southern Queensland（南昆士兰大学）；Vatican Observatory Research Group, University of Arizona（亚利桑那大学梵蒂冈天文台研究组）
Using the PIONIER visitor instrument that combines the light of the four Auxiliary Telescopes of ESO's Very Large Telescope Interferometer, we measure precisely the diameters of several symbiotic and related stars: HD 352, HD 190658, V1261 Ori, ER Del, FG Ser, and AG Peg. These diameters - in the range of 0.6 to 2.3 milli-arcseconds - are used to assess the filling factor of the Roche lobe of the mass-losing giants and provide indications on the nature of the ongoing mass transfer. We also provide the first spectroscopic orbit of ER Del, based on CORAVEL and HERMES/Mercator observations. The s…
We have run 600 N-body simulations of intermediate-mass (~3500 Msun) young star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1 Zsun). The simulations include the dependence of stellar properties and stellar winds on metallicity. Massive stellar black holes (MSBHs) with mass >25 Msun are allowed to form through direct collapse of very massive metal-poor stars (Z<0.3 Zsun). We focus on the demographics of black hole (BH) binaries that undergo mass transfer via Roche lobe overflow (RLO). We find that 44 per cent of all binaries that undergo an RLO phase (RLO binaries) formed…
The inflated transiting hot Jupiter HD 209458b is one of the best studied objects since the beginning of exoplanet characterization. Transmission observations of this system between the mid infrared and the far ultraviolet have revealed the signature of atomic, molecular, and possibly aerosol species in the lower atmosphere of the planet, as well as escaping hydrogen and metals in the upper atmosphere. From a re-analysis of near-ultraviolet (NUV) transmission observations of HD 209458b, we detect ionized iron (Fe II) absorption in a 100 A-wide range around 2370 A, lying beyond the planetary Ro…
We report the discovery of the first short period binary in which a hot subdwarf star (sdOB) fills its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic Plane named the Zwicky Transient Facility and exhibits a period of $P_{\rm orb}=39.3401(1)$ min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{\rm eff}=42,400\pm300$ K and a surface gravity of $\log(g)=5.77\pm0.05$. A high-sig…
We investigate the effects of mass transfer and gravitational wave (GW) radiation on the orbital evolution of contact neutron-star-white-dwarf (NS-WD) binaries, and the detectability of these binaries by space GW detectors (e.g., Laser Interferometer Space Antenna, LISA; Taiji; Tianqin). A NS-WD binary becomes contact when the WD component fills its Roche lobe, at which the GW frequency ranges from ~0.0023 to 0.72 Hz for WD with masses ~0.05-1.4 Msun. We find that some high-mass NS-WD binaries may undergo direct coalescence after unstable mass transfer. However, the majority of NS-WD binaries …
There is a mismatch between modelled and observed distributions of optical stars masses in BH LMXB. Companion masses in BH LMXB are found in the mass range 0.1 - 1.6 $M_{\odot}$ with the peak at 0.6 $M_{\odot}$. The standard evolutionary scenarios require the donor mass distribution peaks to be at least $\sim 1\, M_{\odot}$ to eject a massive envelope of the black hole progenitor. Imperfect of the methods of optical stars masses determination may cause this difference. Two common used approximations of real Roche lobe filling star as distortion sources have been tested. On the one hand, there …
Eta Carinae's historical light curve: evidence for cyclic Roche lobe overflow from the primary star
Instituto de Astr., Geofísica e Ciências Atmosf. da USP（圣保罗大学天文、地球物理和大气科学研究所）；Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte（北里奥格兰德联邦大学科学与技术学院）；Instituto Nac. de Pesq. Espaciais/MCTI（国家空间研究院/科技部）；Instituto de Astrofísica de La Plata (CCT La Plata - CONICET/UNLP)（拉普拉塔天体物理研究所（拉普拉塔中心-阿根廷国家科学研究委员会/拉普拉塔国立大学））；Laboratório Nacional de Astrofísica（国家天体物理实验室）；Max Planck Institute for Radio Astronomy（马克斯·普朗克射电天文研究所）
We present ULTRACAM multiband optical photometry of two transitional millisecond pulsars, PSR J1023+0038 and PSR J1227$-$4853, taken while both were in their radio pulsar states. The light curves show significant asymmetry about the flux maxima in all observed bands, suggesting an asymmetric source of heating in the system. We model the light curves using the Icarus binary code, using models with an additional "hot spot" heating contribution and an anisotropic heat redistribution model to treat the asymmetry. Our modelling reveals companion stars with under-filled Roche lobes in both PSRs J102…
WOCS 4540 is the longest orbital period ($P_{\rm orb}=3030$ days) blue straggler (BSS) - white dwarf (WD) pair in the old open cluster NGC 188 . It also contains one of the most luminous BSS in the cluster. Prior \textit{Hubble Space Telescope} COS spectroscopy measured a WD mass of 0.53 $M_{\odot}$, indicative of a carbon-oxygen WD and suggesting previous mass transfer from an asymptotic branch giant (AGB). Detailed modeling of the system evolution, including red giant branch phase wind mass transfer, AGB wind Roche-lobe overflow and regular Roche-lobe overflow, is done with Modules for Exper…
Roche tomography is a technique designed for mapping the intensity distribution over the surface of binary star components that fill (or nearly fill) their Roche lobes and so are both rotationally and tidally distorted. It builds on and develops the successful techniques used for mapping rapidly rotating single stars, which have revealed the presence of starspots. It has so far been applied to only a small number of cataclysmic variables, and a few other related systems. This paper reviews the technique and presents maps of spots on most of these systems. The review has been written with the h…
An XMM-Newton observation of the nearby "pre-cataclysmic" short-period (P_orb = 3.62 hr) binary QS Vir (EC 13471-1258) revealed regular narrow X-ray eclipses when the white dwarf passed behind its M2-4 dwarf companion. The X-ray emission provides a clear signature of mass transfer and accretion onto the white dwarf. The low-resolution XMM-Newton EPIC spectra are consistent with a cooling flow model and indicate an accretion rate of Mdot= 1.7\times10^-13M\odot/yr. At 48 pc distant, QS Vir is then the second nearest accreting cataclysmic variable known, with one of the lowest accretion rates fou…
We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\r…
Binaries consisting of a hot subdwarf star and an accreting white dwarf (WD) are sources of gravitational wave radiation at low frequencies and possible progenitors of type Ia supernovae if the WD mass is large enough. Here, we report the discovery of the third binary known of this kind: it consists of a hot subdwarf O (sdO) star and a WD with an orbital period of 3.495 hours and an orbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe and likely transfers mass to the WD via an accretion disk. From spectroscopy, we obtain an effective temperature of $T_{\mathrm{eff}}=54\,24…
Close double neutron stars have been observed as Galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational-wave sources. They are believed to have experienced at least one common-envelope episode during their evolution prior to double neutron star formation. In the last decades there have been numerous efforts to understand the details of the common-envelope phase, but its computational modelling remains challenging. We present and discuss the properties of the donor and the binary at the onset of the Roche-lobe overflow leading to these common-envelo…
We present an improved method for determining the mass of neutron stars in eclipsing X-ray pulsar binaries and apply the method to six systems, namely Vela X-1, 4U 1538-52, SMC X-1, LMC X-4, Cen X-3, and Her X-1. In previous studies to determine neutron star mass, the X-ray eclipse duration has been approximated analytically by assuming the companion star is spherical with an effective Roche lobe radius. We use a numerical code based on Roche geometry with various optimizers to analyze the published data for these systems, which we supplement with new spectroscopic and photometric data for 4U …
Barium stars have been studied extensively over the past few decades, yet our current understanding of how these intriguing objects formed leaves much to be desired. Many trends observed in systems containing barium stars cannot be satisfactorily explained by classical binary evolution models, naturally raising the question of whether triples and other higher-order multiples can give rise to such exotic objects. In this paper, we study the possibility that a Roche Lobe overflow from a tertiary in a hierarchical triple system can potentially lead to surface barium enrichment within the inner bi…
In our earlier Paper I we showed that the accretion disc radius of the dwarf nova HT Cas in its quiescent state has not changed significantly during many years of observations. It has remained consistently large, close to the tidal truncation radius. This result is inconsistent with the modern understanding of the evolution of the disc radius through an outburst cycle. Here we present a new set of time-resolved spectra of HT Cas obtained in the middle of its 2017 superoutburst. We used Doppler tomography to map emission structures in the system, which we compared with those detected in quiesce…
Symbiotic stars are eccellent laboratories to study a broad range of poorly understood physical processes, such as mass loss of red giants, accretion onto compact objects, and evolution of nova-like outbursts. As their evolution is strongly influenced by the mass transfer episodes, understanding the history of these systems requires foremost to determine which process is at play: Roche lobe overflow, stellar wind accretion, or some more complex mixture of both. We report here an interferometric study of the symbiotic system SS Leporis, performed with the unique PIONIER instrument. By determini…
We report analysis of VRIJH photometry, and phase-resolved optical spectroscopy of the eclipsing DA white dwarf plus dMe dwarf binary QS Vir. Modeling of the photometric data yields an inclination of $i = 74.9\pm0.6$ and a mass ratio of $q = M_2/M_1 = 0.50\pm0.05$. Our Doppler maps indicate the presence of material in the Roche lobe of the white dwarf, at a location near the M star, likely due to accretion from the stellar wind of the M star (as opposed to Roche-lobe overflow accretion). We also constructed images of the brightness distribution of the M star at different epochs which reveal th…
HD149404 is an evolved non-eclipsing O-star binary that has previously undergone a Roche lobe overflow interaction. Understanding some key properties of the system requires a determination of the orbital inclination and of the dimensions of the components. The BRITE-Heweliusz satellite was used to collect photometric data of HD149404. Additional photometry was retrieved from the SMEI archive. These data were analysed using a suite of period search tools. The orbital part of the lightcurve was modelled with the nightfall binary star code. The Gaia-DR2 parallax of HD149404 was used to provide ad…
We present the discovery of a 3h5m orbital-period binary star at the heart of the planetary nebula M 3-1 - the shortest period photometrically-variable central star known and second only to V458 Vul, in general. Combined modelling of light and radial velocity curves reveals both components to be close to Roche-lobe-filling, strongly indicating that the central star will rapidly evolve to become a cataclysmic variable, perhaps experiencing a similar evolution to V458 Vul resulting in a nova eruption before the planetary nebula has fully dissipated. While the short orbital period and near Roche-…
We present the physical parameters of three short period close binaries using the observed data from Kepler Space Telescope. All of these observations were taken in a single bandpass (which approximates the Johnson V-band). Our three systems are KIC 2715417, KIC 6050116 and KIC 6287172. The first system KIC 2715417 is considered a semi-detached system with the secondary component filling the Roche lobe. The second system KIC 6050116 is an overcontact system, while the third system KIC 6287172 belongs to ellipsoidal variables (ELV) as deduced from the Roche lobe geometry. For photometric analys…
V1082 Sgr is a cataclysmic variable with accretion luminosity above $10^{34}$erg s$^{-1}$, indicating a mass transfer rate above $10^{-9}M_{\odot}$yr$^{-1}$. However, its K type companion was suggested to be under-filling its Roche lobe (RL), making the high mass transfer rate a mystery. In this work we propose a possible model to explain this discrepancy. The system is proposed to be an intermediate polar, with its K type companion filling its RL. The mass of the white dwarf star is evaluated to be $0.77\pm0.11M_{\odot}$ from both X-ray continuum fitting and Fe line flux ratio measurements. W…
In this study, we present long term photometric variations of the close binary system \astrobj{GO Cyg}. Modelling of the system shows that the primary is filling Roche lobe and the secondary of the system is almost filling its Roche lobe. The physical parameters of the system are $M_1 = 3.0\pm0.2 M_{\odot}$, $M_2 = 1.3 \pm 0.1 M_{\odot}$, $R_1 = 2.50\pm 0.12 R_{\odot}$, $R_2 = 1.75 \pm 0.09 R_{\odot}$, $L_1 = 64\pm 9 L_{\odot}$, $L_2 = 4.9 \pm 0.7 L_{\odot}$, and $a = 5.5 \pm 0.3 R_{\odot}$. Our results show that \astrobj{GO Cyg} is the most massive system near contact binary (NCB). Analysis o…
The stability of mass transfer in binaries with convective giant donors remains an open question in modern astrophysics. There is a significant discrepancy between what the existing methods predict for a response to mass loss of the giant itself, as well as for the mass transfer rate during the Roche lobe overflow. Here we show that the recombination energy in the superadiabatic layer plays an important and hitherto unaccounted-for role in he donor's response to mass loss, in particular on its luminosity and effective temperature. Our improved optically thick nozzle method to calculate the mas…
