arXiv subjects
Publications and source records attributed to Xin Ma.
A Hermitian quantum phase operator is formulated that mirrors the classical phase variable with proper time dependence and satisfies trigonometric identities. The eigenstates of the phase operator are solved in terms of Gegenbauer ultraspherical polynomials in the number state representation.
We demonstrate that in-plane charge current can effectively control the spin precession resonance in an Al2O3/CoFeB/Ta heterostructure. Brillouin Light Scattering (BLS) was used to detect the ferromagnetic resonance field under microwave excitation of spin waves at fixed frequencies. The current control of spin precession resonance originates from modification of the in-plane uniaxial magnetic anisotropy field H_k, which changes symmetrically with respect to the current direction. Numerical simulation suggests that the anisotropic stress introduced by Joule heating plays an important role in controlling H_k. These results provide new insights into current manipulation of magnetic properties and have broad implications for spintronic devices.
Supernovae of type Ia (SNe Ia) are believed to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs). However, the mass accretion process onto CO WDs is still not completely understood. In this paper, we study the accretion of He-rich matter onto CO WDs and explore a scenario in which a strong wind forms on the surface of the WD if the total luminosity exceeds the Eddington limit. Using a stellar evolution code called modules for experiments in stellar astrophysics (MESA), we simulated the He accretion process onto CO WDs for WDs with masses of 0.6-1.35Msun and various accretion rates of 10^{-8}-10^{-5}Msun/yr. If the contribution of the total luminosity is included when determining the Eddington accretion rate, then a super-Eddington wind could be triggered at relatively lower accretion rates than those of previous studies based on steady-state models. The super-Eddington wind can prevent the WDs with high accretion rates from evolving into red-giant-like He stars. We found that the contributions from thermal energy of the WD are non-negligible, judging by our simulations, even though the nuclear burning energy is the dominating source of luminosity. We also provide the limits of the steady He-burning regime in which the WDs do not lose any accreted matter and increase their mass steadily, and calculated the mass retention efficiency during He layer flashes for various WD masses and accretion rates. These obtained results can be used in future binary population synthesis computations.
The super-Eddington wind scenario has been proposed as an alternative way for producing type Ia supernovae (SNe Ia). The super-Eddington wind can naturally prevent the carbon--oxygen white dwarfs (CO WDs) with high mass-accretion rates from becoming red-giant-like stars. Furthermore, it works in low-metallicity environments, which may explain SNe Ia observed at high redshifts. In this article, we systematically investigated the most prominent single-degenerate WD+MS channel based on the super-Eddington wind scenario. We combined the Eggleton stellar evolution code with a rapid binary population synthesis (BPS) approach to predict SN Ia birthrates for the WD+MS channel by adopting the super-Eddington wind scenario and detailed mass-accumulation efficiencies of H-shell flashes on the WDs. Our BPS calculations found that the estimated SN Ia birthrates for the WD+MS channel are ~0.009-0.315*10^{-3}{yr}^{-1} if we adopt the Eddington accretion rate as the critical accretion rate, which are much lower than that of the observations (<10% of the observed SN Ia birthrates). This indicates that the WD+MS channel only contributes a small proportion of all SNe Ia. The birthrates in this simulation are lower than previous studies, the main reason of which is that new mass-accumulation efficiencies of H-shell flashes are adopted. We also found that the critical mass-accretion rate has a significant influence on the birthrates of SNe Ia. Meanwhile, the results of our BPS calculations are sensitive to the values of the common-envelope ejection efficiency.
In this paper, we study Schauder equivalence relations, which are Borel equivalence relations generated by Banach spaces with basic sequences. We prove that the set of equivalence relations generated by basic sequences has boundaries. Then we show that equivalence relations generated by the basis in Tsirelson spaces has similar properties of Tsirelson spaces in the Banach space theory. In particular, we prove that both l_p and c_0 are not reducible to the equivalence relation generated by Tsirelson space T with the unit vector basis \{t_n\}. We also show that Borel equivalence relation generated by α-Tsirelson spaces are mutually incompatible. Based on this argument, we show that any basis of Schauder equivalence relations must be of cardinal 2^ω.
The accretion of hydrogen-rich material onto carbon-oxygen white dwarfs (CO WDs) is crucial for understanding type Ia supernova (SN Ia) from the single-degenerate model, but this process has not been well understood due to the numerical difficulties in treating H and He flashes during the accretion. For the CO WD masses from 0.5 to $1.378\,{M}_\odot$ and accretion rates in the range from $10^{-8}$ to $10^{-5}\,{M}_\odot\,\mbox{yr}^{-1}$, we simulated the accretion of solar-composition material onto CO WDs using the state-of-the-art stellar evolution code of {\sc MESA}. For comparison with the steady-state models (e.g \citet{nskh07}), we firstly ignored the contribution from nuclear burning to the luminosity when determining the Eddington accretion rate and found that the properties of H burning in our accreting CO WD models are similar to those from the steady-state models, except that the critical accretion rates at which the WDs turn into red giants or H-shell flashes occur on their surfaces are slightly higher than those from the steady-state models. However, the super-Eddington wind is triggered at much lower accretion rates, than previously thought, when the contribution of nuclear burning to the total luminosity is included. This super-Eddington wind naturally prevents the CO WDs with high accretion rates from becoming red giants, thus presenting an alternative to the optically thick wind proposed by \cite{hkn96}. Furthermore, the super-Eddington wind works in low-metallicity environments, which may explain SNe Ia observed at high redshifts.