Search

Abstract Considering accretion onto a charged dilaton black hole, the fundamental equations governing accretion, general analytic expressions for critical points, critical velocity, critical speed of sound, and ultimately the mass accretion rate are obtained. A new constraint on the dilation parameter coming from string theory is found and the case for polytropic gas is delved into a detailed discussion. It is found that the dialtion and the adiabatic index of accreted material have deep effects on the accretion process.

We consider geodesic motions in Kerr–Sen–AdS4 spacetime. We obtain equations of motion for light rays and test particles. Using parametric diagrams, we show some regions where radial and latitudinal geodesic motions are allowed. We analyze the impact of parameters related to the dilatonic scalar on the orbit and find that it will result in more rich and complex orbital types.

Hierarchical massive quadruple systems are ideal laboratories for examining the theories of star formation, dynamical evolution, and stellar evolution. The successive mergers of hierarchical quadruple systems might explain the mass gap between neutron stars and black holes. Looking for light curves of O-type binaries identified by LAMOST, we find a (2+2) quadruple system: TYC 3340-2437-1, located in the stellar bow-shock nebula (SBN). It has a probability of over 99.99% being a quadruple system derived from the surface density of the vicinity stars. Its inner orbital periods are 3.390602(89) days and 2.4378(16) days, respectively, and the total mass is about (11.47 + 5.79) + (5.2 + 2.02) = 24.48 M _⊙ . The line-of-sight inclinations of the inner binaries, B _1 and B _2 , are 55.°94 and 78.°2, respectively, indicating that they are not coplanar. Based on observations spanning 34 months and the significance of the astrometric excess noise ( D > 2) in Gaia Data Release 3 data, we guess that its outer orbital period might be a few years. If it were true, the quadruple system might form through the disk fragmentation mechanism with outer eccentric greater than zero. This eccentricity could be the cause of both the arc-like feature of the SBN and the noncoplanarity of the inner orbit. The outer orbital period and outer eccentric could be determined with the release of future epoch astrometric data of Gaia.

Mixing in the convective core is quite uncertain in core helium-burning stars. To explore the overshooting mixing beyond the convective core, we incorporated the k–ω proposed by Li into the Modules of Experiments in Stellar Astrophysics (MESA), and investigated the overshooting mixing in evolution of subdwarf B (sdB) models. We found that the development of the convective core can be divided into three stages. When the radiative temperature gradient, , monotonically decreases outwardly, the overshooting mixing presents exponential decay similar with Herwig, and the overshooting distance is to make at the boundary of the convective core, in agreement with the prediction of the self-driving mechanism Castellani et al. When the radiative temperature gradient, , shows a minimum value near the convective boundary, the convective core may be divided into two if the minimum value of is smaller than the adiabatic temperature gradient . For the single-zone case, the overshooting mixing shows exponential decay, but the overshooting distance is much smaller than in the initial stage. For the double-zone case, the overshooting mixing is similar to that of the single case beyond the convective core, while it almost stops on both sides of the above convective shell. Our overshooting mixing scheme is similar to the maximal overshoot scheme of Constantino et al. In the final stage, helium injections into the convective core by the overshooting mixing happens, similar to the “core breathing pulses.” [ABSTRACT FROM AUTHOR]

We present a catalog of 166 spectroscopically identified hot subdwarf stars from LAMOST DR1, 44 of which show the characteristics of cool companions in their optical spectra. Atmospheric parameters of 122 subdwarf stars with non-composite spectra were measured by fitting the profiles of hydrogen (H) and helium (He) lines with synthetic spectra from non-LTE model atmospheres. Most of the sdB stars scatter near the Extreme Horizontal Branch in the Teff– diagram and two well defined groups can be outlined. A clustering of He-enriched sdO stars appears near Teff = 45,000 K and log g = 5.8. The sdB population separates into several nearly parallel sequences in the Teff–He abundance diagram with clumps corresponding to those in the Teff– diagram. Over 38,000 K (sdO) stars show abundance extremes; they are either He-rich or He-deficient and we observe only a few stars in the abundance range . With increasing temperature these extremes become less prominent and the He abundance approaches log y ∼ −0.5. A unique property of our sample is that it covers a large range in apparent magnitude and galactic latitude, therefore it contains a mix of stars from different populations and galactic environments. Our results are consistent with the findings of Hirsch and we conclude that He-rich and He-deficient sdB stars (log y < 1) probably originate from different populations. We also find that most sdO and sdB stars lie in a narrow strip in the plane of luminosity and helium abundance, which suggests that these atmospheric parameters are correlated. [ABSTRACT FROM AUTHOR]