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Register a new userThe effects of diffusion in hot subdwarf progenitors from the common envelope channel
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Diffusion of elements in the atmosphere and envelope of a star can drastically alter its surface composition, leading to extreme chemical peculiarities. We consider the case of hot subdwarfs, where surface helium abundances range from practically zero to almost 100 percent. Since hot subdwarfs can form via a number of different evolution channels, a key question concerns how the formation mechanism is connected to the present surface chemistry. A sequence of extreme horizontal branch star models was generated by producing post-common envelope stars from red giants. Evolution was computed with MESA from envelope ejection up to core-helium ignition. Surface abundances were calculated at the zero-age horizontal branch for models with and without diffusion. A number of simulations also included radiative levitation. The goal was to study surface chemistry during evolution from cool giant to hot subdwarf and determine when the characteristic subdwarf surface is established. Only stars leaving the giant branch close to core-helium ignition become hydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, including radiative levitation, depletes the initial surface helium in all cases. All subdwarf models rapidly become more depleted than observations allow. Surface abundances of other elements follow observed trends in general, but not in detail. Additional physics is required.
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I construct the class of supernovae and supernova progenitors that result from fatal common envelope evolution (CEE). The fatal CEE progenitors are stellar binary systems where a companion spirals-in inside the envelope of a giant star and merges with the core. The companion can be a neutron star (NS; or a black hole) that destroys the core and by that forms a common envelope jets supernova (CEJSN), a white dwarf (WD) that merges with the core to form a massive WD that later might explode as a Type Ia supernova (the core degenerate scenario), or a main sequence companion. In the latter case the outcome might be a core collapse supernova (CCSN) of a blue giant, a CCSN of type IIb or of type Ib. In another member of this class two giant stars merge and the two cores spiral-in toward each other to form a massive core that later explodes as a CCSN with a massive circumstellar matter (CSM). I discuss the members of this class, their characteristics, and their common properties. I find that fatal CEE events account for $approx 6-10 %$ of all CCSNe, and raise the possibility that a large fraction of peculiar and rare supernovae result from the fatal CEE. The study of these supernova progenitors as a class will bring insights on other types of supernova progenitors, as well as on the outcome of the CEE.
Hot subdwarf B stars (sdBs) are evolved, core helium-burning objects located on the extreme horizontal branch. Their formation history is still puzzling as the sdB progenitors must lose nearly all of their hydrogen envelope during the red-giant phase. About half of the known sdBs are in close binaries with periods from 1.2 h to a few days, a fact that implies they experienced a common-envelope phase. Eclipsing hot subdwarf binaries (also called HW Virginis systems) are rare but important objects for determining fundamental stellar parameters. Even more significant and uncommon are those binaries containing a pulsating sdB, as the mass can be determined independently by asteroseismology. Here we present a first analysis of the eclipsing hot subdwarf binary V2008-1753. The light curve shows a total eclipse, a prominent reflection effect, and low--amplitude pulsations with periods from 150 to 180 s. An analysis of the light-- and radial velocity (RV) curves indicates a mass ratio close to $ q = 0.146$, an RV semi-amplitude of $K=54.6 ,rm kms^{-1}$, and an inclination of $i=86.8^circ$. Combining these results with our spectroscopic determination of the surface gravity, $log ,g = 5.83$, the best--fitting model yields an sdB mass of 0.47$M_{rm odot}$ and a companion mass of $69 M_{rm Jup}$. As the latter mass is below the hydrogen-burning limit, V2008-1753 represents the first HW Vir system known consisting of a pulsating sdB and a brown dwarf companion. Consequently, it holds great potential for better constraining models of sdB binary evolution and asteroseismology.
Context. An important ingredient in binary evolution is the common-envelope (CE) phase. Although this phase is believed to be responsible for the formation of many close binaries, the process is not well understood. Aims. We investigate the characteristics of the population of post-common-envelope binaries (PCEB). As the evolution of these binaries and their stellar components are relatively simple, this population can be directly used to constraint CE evolution. Methods. We use the binary population synthesis code SeBa to simulate the current-day population of PCEBs in the Galaxy. We incorporate the selection effects in our model that are inherent to the general PCEB population and that are specific to the SDSS survey, which enables a direct comparison for the first time between the synthetic and observed population of visible PCEBs. Results. We find that selection effects do not play a significant role on the period distribution of visible PCEBs. To explain the observed dearth of long-period systems, the {alpha}-CE efficiency of the main evolutionary channel must be low. In the main channel, the CE is initiated by a red giant as it fills its Roche lobe in a dynamically unstable way. Other evolutionary paths cannot be constrained more. Additionally our model reproduces well the observed space density, the fraction of visible PCEBs amongst white dwarf (WD)- main sequence (MS) binaries, and the WD mass versus MS mass distribution, but overestimates the fraction of PCEBs with helium WD companions.
Common-envelope (CE) evolution in massive binary systems is thought to be one of the most promising channels for the formation of compact binary mergers. In the case of merging binary black holes (BBHs), the essential CE phase takes place at a stage when the first BH is already formed and the companion star expands as a supergiant. We study which BH binaries with supergiant companions will evolve through and potentially survive a CE phase. To this end, we compute envelope binding energies from detailed massive stellar models at different evolutionary stages and metallicities. We make multiple physically extreme choices of assumptions that favor easier CE ejection as well as account for recent advancements in mass transfer stability criteria. We find that even with the most optimistic assumptions, a successful CE ejection in BH (and also NS) binaries is only possible if the donor is a massive convective-envelope giant, a red supergiant (RSG). In other words, pre-CE progenitors of BBH mergers are BH binaries with RSG companions. We find that due to its influence on the radial expansion of massive giants, metallicity has an indirect but a very strong effect on the envelope structure and binding energies of RSGs. Our results suggest that merger rates from population synthesis models could be severely overestimated, especially at low metallicity. Additionally, the lack of observed RSGs with luminosities above log($L/L_{odot}$) = 5.6-5.8, corresponding to stars with $M > 40 M_{odot}$, puts into question the viability of the CE channel for the formation of the most massive BBH mergers. Either such RSGs elude detection due to very short lifetimes, or they do not exist and the CE channel can only produce BBH systems with total mass $< 50 M_{odot}$. We discuss an alternative CE scenario, in which a partial envelope ejection is followed by a phase of possibly long and stable mass transfer.
We present an analysis of the binary and physical parameters of a unique pulsating white dwarf with a main-sequence companion, SDSS J1136+0409, observed for more than 77 d during the first pointing of the extended Kepler mission: K2 Campaign 1. Using new ground-based spectroscopy, we show that this post-common-envelope binary has an orbital period of 6.89760103(60) hr, which is also seen in the photometry as a result of Doppler beaming and ellipsoidal variations of the secondary. We spectroscopically refine the temperature of the white dwarf to 12330(260) K and its mass to 0.601(36) Msun. We detect seven independent pulsation modes in the K2 light curve. A preliminary asteroseismic solution is in reasonable agreement with the spectroscopic atmospheric parameters. Three of the pulsation modes are clearly rotationally split multiplets, which we use to demonstrate that the white dwarf is not synchronously rotating with the orbital period but has a rotation period of 2.49(53) hr. This is faster than any known isolated white dwarf, but slower than almost all white dwarfs measured in non-magnetic cataclysmic variables, the likely future state of this binary.
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