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The hot subdwarf O/B stars (sdO/Bs) are known as extreme horizontal branch stars, which is of great importance in stellar evolution theory. The sdO/Bs are generally thought to have a helium-burning core and a thin hydrogen envelope (M env < 0.02 M ⊙). In the canonical binary evolution scenario, sdO/Bs are considered to be the stripped cores of red giants. However, such a scenario cannot explain the recently discovered sdO/B binary SMSS J1920, where the strong Ca H and K lines in the spectrum are found. It suggests that this binary likely originated from the recent ejection of the common envelope (CE). In this work, we propose a new formation channel of massive sdO/Bs, namely, sdO/Bs produced from a CE ejection process with an asymptotic giant branch (AGB) star (hereafter the AGB CE channel). We constructed the evolutionary model of sdO/Bs and successfully explained most of the important observed parameters of the sdO/B star in SMSS J1920, including the evolutionary age, sdO/B mass, effective temperature, surface gravity, and surface helium abundance. The minimum sdO/B mass produced from the AGB CE channel is about 0.48 M ⊙. The evolutionary tracks in the log T eff – log g plane may explain a fraction of the observational samples with high log T eff and low log g . Considering the wind mass loss of sdO/Bs, the model could produce helium-rich hot subdwarfs with log (n He / n H) ≳ − 1 . [ABSTRACT FROM AUTHOR]

The origin of chemically peculiar stars and nonzero eccentricity in evolved close binaries have been long-standing problems in binary stellar evolution. Answers to these questions may trace back to an intense mass transfer during the asymptotic-giant-branch (AGB) binary phase. We use AstroBEAR to solve the 3D radiation hydrodynamic equations and calculate the mass transfer rate in AGB binaries that undergo the wind-Roche-lobe overflow or Bondi-Hoyle-Lyttleton (BHL) accretion. One of the goals of this work is to illustrate the transition from the wind- Roche-lobe overflow to BHL accretion. Both circumbinary disks and spiral structure outflows can appear in the simulations. As a result of enhanced mass transfer and angular momentum transfer, some AGB binaries may undergo orbit shrinkage, and some will expand. The high mass transfer efficiency is closely related to the presence of the circumbinary disks. [ABSTRACT FROM AUTHOR]

The number ratio of carbon-rich to oxygen-rich asymptotic giant branch (AGB) stars (the so-called C/M ratio) is closely related to the evolution environment of the host galaxy. This work studies the C/M ratio in 14 galaxies within the Local Group with the most complete and clean sample of member stars identified in our previous works. The borderlines between carbon-rich AGB and oxygen-rich AGB stars as well as red supergiants are defined by Gaussian mixture model fitting to the number density in the (J − K) / K diagram for the member stars of the LMC and M33, and then applied to the other galaxies by shifting the difference in the position of tip red giant branch (TRGB). The C/M ratios are obtained after precise and consistent categorization. Although for galaxies with larger distance modulo there is greater uncertainty, the C/M ratio is clearly found to decrease with the color index (J − K) 0 of TRGB as the indicator of metallicity, which agrees with previous studies and can be explained by the fact that carbon stars are more easily formed in a metal-poor environment. Furthermore, the C/M ratio within M33 is found to increase with galactocentric distance, which coincides with this scenario and the galactic chemical evolution model. On the other hand, the C/M ratio within M31 is found to decrease with galactocentric radius, which deserves further study. [ABSTRACT FROM AUTHOR]

The mechanisms and kinetics of the formation of two-ring polycyclic aromatic hydrocarbons via the recombination of benzyl (C7H7) and propargyl (C3H3) radicals under the circumstellar envelopes' conditions of carbon-rich stars of the asymptotic giant branch, as well as their combustion, were refined on the basis of high-level quantum chemistry methods and the advanced transition state theory. Based on the constructed diagrams of potential energy surfaces and the calculated temperature- and pressure-dependence rate constants of the processes, the reaction pathways and their relative contributions to the composition of the final products are revealed. It is shown that under the conditions of the circumstellar envelopes of the asymptotic giant branch stars, stabilization of the initial complexes C10H10, in contrast to the combustion flame, does not occur, which contributes to an increase in the yield of two-cyclic reaction products. [ABSTRACT FROM AUTHOR]

The mass-loss mechanism in asymptotic giant branch (AGB) stars is not yet fully understood. We present 20-milliarcsecond resolution ALMA imaging of the well-studied AGB star W Hya in multiple molecular lines at 250–269 GHz, including masers from SiO, H2O, and SO2. The images show complex plumes, arcs, and clumps over the stellar disk and in the atmosphere extending to several stellar radii. We detected prominent emission components over the stellar disk—instead of pure absorption as expected—in some Si17O, 30SiO, H2O, and SO2 lines. The surface emission seen in the Si17O and vibrationally excited H2O lines is particularly strong, indicating maser actions. The masers seen over the stellar disk indicate radial amplification. [ABSTRACT FROM AUTHOR]

Studies of semiregular variable stars by different authors are examined and the main theoretical and observational problems associated with these stars are reviewed. Their evolutionary status and a possible connection with long-period variables such as Mira Ceti are discussed. Individual objects belonging to different types of semiregular variables are described in detail. The periods of all the stars considered here have been studied by periodogram and/or wavelet analysis. The previously developed additional photometric classification criteria are supplemented by the study of the phase portraits of the smoothed brightness oscillations. It is also noted that all the methods for analyzing photometric behavior applied to stars such as Mira Ceti can also be applied to semiregular variables. On leaving the Main sequence the stars pass through a Cepheid instability stage, turning into radially pulsating variables of a δ Cepheus type. Semiregular variable giants and supergiants of spectral classes F-K, which are customarily denoted by the symbol SRd, may be related to these stars. As they evolve further, the high luminosity variables fall into the region of red supergiants and are converted into SRc (or Lc) variables, while the variables with lower luminosity are converted into SRab semiregular (or irregular Lb) variables of later spectral classes. The RV Tau variables are a class of low-mass (with masses on the order of one solar mass) pulsating F-K supergiants (Mv=-3m÷5m) which may lie on the short-time evolutionary stage of a transition from red giant to protoplanetary nebula, which explains the scarcity of this type of variability. [ABSTRACT FROM AUTHOR]

The pumping of 22.2-GHz HO masers in the circumstellar envelopes of asymptotic giant branch stars has been simulated numerically. The physical parameters adopted in the calculations correspond to those of the circumstellar envelope around IK Tau. The one-dimensional plane-parallel structure of the gas-dust cloud is considered. The statistical equilibrium equations for the HO level populations and the thermal balance equations for the gas-dust cloud are solved self-consistently. The calculations take into account 410 rotational levels belonging to the five lowest vibrational levels of HO. The stellar radiation field is shown to play an important role in the thermal balance of the gas-dust cloud due to the absorption of emission in rotational-vibrational HO lines. The dependence of the gain in the 22.2-GHz maser line on the gas density and HO number density in the gas-dust cloud is investigated. Gas densities close to the mean density of the stellar wind, 10−10 cm, and a high relative HO abundance, more than 10, have been found to be the most likely physical conditions in maser sources. [ABSTRACT FROM AUTHOR]