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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 could be formed through the stable Roche lobe overflow at main sequence and Hertzsprung gap. We have also found that some subdwarf B or OB stars have anomalous high mass (around 1 solar mass) with thick envelope (0.07 solar mass to 0.16 solar mass) in our models. By comparing our theoretical results with observations on the hot subdwarfs in open clusters, we suppose a quantity of hot subdwarfs in binary systems might be found in open clusters in the future.
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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 (RGB). These systems are therefore an ideal testing ground for binary interaction models. With our long term study of wide hot subdwarf (sdB) binaries we aim to improve our current understanding of stable RLOF on the RGB by comparing the results of binary population synthesis studies with the observed population. In this article we describe the current model and possible improvements, and which observables can be used to test different parts of the interaction model.
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 $T_{rm eff}=42,400pm300$ K and a surface gravity of $log(g)=5.77pm0.05$. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass $M_{rm sdOB}=0.337pm0.015$ M$_odot$ and a white dwarf accretor with a mass $M_{rm WD}=0.545pm0.020$ M$_odot$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M$_odot$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{rm orb}$, $T_{rm eff}$, $log(g)$, and masses, we estimate a post-common envelope period of $P_{rm orb}approx150$ min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of $approx0.1$ M$_odot$ and will merge with its carbon/oxygen white dwarf companion after $approx17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star.
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 models to the assumed mass-loss rates and the need to develop a better theoretical understanding of stellar winds.
The destruction of planets by migration into the star will release significant amounts of energy and material, which will present opportunities to observational study planets in new ways. To observe planet destruction, it is important to understand the processes of how this energy and material is released as planets are destroyed. It is not known how fast the large amounts of energy and material are released, making it difficult to predict how observable planet destruction will be. There is a huge amount of energy made available by falling deep into the stars potential well: Simple calculations show that many of the currently known hot Jupiters can potentially produce events as luminous as a small nova if the energy is released fast enough. To observe these events, the important questions are how will this energy be released, and whether the energy will be released rapidly enough to create an event luminous enough to be found by transient surveys. Alternatively, if planet destruction is slowed by the inward migration alternating with periods of outward migration caused by Roche lobe overflow (RLOF), then the primary signature may be the effects of the release of large amounts of gas. The infall of this gas also may significantly contribute to the systems luminosity. The release of planetary gas may be a searchable signature of planet destruction. Signs of runaway RLOF and outward or alternating RLOF should be searched for. Observing planet destruction will provide a new window for study of exoplanets.
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 standard evolution model predicts that the system will not become a SN Ia. However, by simply adding a Wind Roche Lobe Overflow as one of the mass transfer modes we predict that the white dwarf in V407 Cyg will reach the Chandrasekhar limit in 40-200 Myr.
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