Qiu-he Peng

The role of the outer boundary condition in accretion disk models: Theory and application

In a previous paper, we find that the outer boundary conditions (OBCs) of an optically thin accretion flow play an important role in determining the structure of the flow. Here in this paper, we further investigate the influence of OBCs on the dynamics and radiation of the accretion how on a more detailed level. Bremsstrahlung and synchrotron radiations amplified by Comptonization are taken into account, and two-temperature plasma assumption is adopted. The three OBCs we adopted are the temperatures of the electrons and ions and the specific angular momentum of the accretion flow at a certain outer boundary. We investigate the individual role of each of the three OBCs on the dynamical structure and the emergent spectrum. We find that when the general parameters such as the mass accretion rate M and the viscous parameter alpha are fixed the peak flux at various bands such as radio, IR, and X-ray can differ by as much as several orders of magnitude under different OBCs in our example. Our results indicate that the OBC is both dynamically and radiatively important and therefore should be regarded as a new parameter in accretion disk models. As an illustrative example, we further apply the above results to the compact radio source Sgr A* located at the center of our Galaxy. The advection-dominated accretion flow (ADAF) model has turned out to be a great success in explaining its luminosity and spectrum. However, there exists a discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three-dimensional hydrodynamical simulation, with the former being 10-20 times smaller than the latter. By seriously considering the outer boundary condition of the accretion flow, we find that because of the low specific angular momentum of the accretion gas the accretion in Sgr A* should belong to a new accretion pattern, which is characterized by the possession of a very large sonic radius. This accretion pattern can significantly reduce the discrepancy between the mass accretion rates. We argue that the accretion occurred in some detached binary systems; the core of nearby elliptical galaxies and active galactic nuclei very possibly belongs to this accretion pattern.

Nucleosynthesis in Accretion and Outflow Regions around Black Holes

Nucleosynthesis could take place in the hot regions of accretion and outflow around a black hole. We here investigate possible composition changes due to nuclear reaction processes in an accretion disk by calculating a set of self-similar solutions of an advection-dominated accretion flow. We find that 26Al,51Cr,53Mn, and 55Fe produced in the accretion flow with a heating factor f = 0.001 are significantly overabundant compared to solar abundances, and could contribute to metal-contaminated galaxies. We find a richness of protons and neutrons due to photodisintegration in very hot flows. We also discuss nucleosynthesis in outflow regions via an expansion burning reaction process; the abundances of some species could increase further due to such nuclear reactions. A possible new origin of 26Al, which depends on the quantity of stellar-mass black holes and the mass of the host galaxy, is also discussed.

High-energy radiation from a model of quasars, active galactic nuclei, and the Galactic center with magnetic monopoles

The fact that magnetic monopoles may catalyze nucleon decay (the Rubakov-Callan [RC] effect) as predicated by the grand unified theory of particle physics is invoked as the energy source of quasars and active galactic nuclei. Recent study of this model revealed that the radius of the supermassive object (SMO) located at the Galactic center is much larger than its Schwarzschild radius. We propose that this SMOs could be the source of high-energy gamma-ray radiation, although the emitted radiation may be mainly concentrated in the infrared. The surface temperature of the SMO at the Galactic center is taken as 121 K, inferred from the observed maximum of the flux spectrum of Sgr A* at the near infrared (1 × 1013 Hz); the radius of the SMO is about 8.1 × 1015 cm or 1.1 × 104RS (RS is the Schwarzschild radius). The mass of the SMO is derived from the observed total luminosity of Sgr A* (1 × 1037 ergs s-1) as 2.5 × 106 M☉. Strong gamma-ray radiation with energy higher than 0.5 MeV may be emitted from the SMO. The flux of positrons emitted from the SMO is estimated to be 6.5 × 1042e+ s-1. The content parameter of magnetic monopoles ξ ≡ [(Nm/NB)/1.9 × 10-25](σβ/10-27) also may be deduced from observations to be 230. Taking the cross section of the RC effect as 1 × 10-27 cm2, the strength of the radial magnetic field at the surface of the SMO is estimated to be 20-100 G. Our model also can predict the production of extreme ultra-high-energy cosmic rays.

Some statistical properties of spiral galaxies along the Hubble sequence

Abstract A statistical study has been made for the variations along the Hubble sequence, os such parameters as the degree of tightness of winding of spiral arm λ, the pitch angle μ, the flatness of the disk H/D25 and the thickness H along the Hubble sequence for 365 spiral galaxies published in A&Ap Supplement Series. The mean values of these quantities for the various Hubble types have been obtained for the first time. The results of the statistics show clearly 1) that the Hubble classification of spiral galaxies is one which has only a qualitative and statistical significance, and 2) that the dispersion relation in the density wave theory is valid for most spiral galaxies, i.e., the arms of most spiral galaxies satisfy the requirements of being tightly wound.

A Model for Calculating the Abundances of Neutron-Capture Elements in Metal-poor Stars

Several previous studies on the abundances of neutron-capture elements have indicated that, for most metal-poor stars, the observed abundances of the heavy elements cannot be matched by only one neutron-capture process, either the solar system r-process or the solar s-process abundances. However, the observed abundances can be well matched by the combined contributions from both of these solar system neutron-capture processes in certain proportions. So it is necessary to determine the relative contributions from the individual neutron-capture processes to the abundances of the heavy elements in metal-poor stars. In this paper we suggest a new concept of component coefficients to describe the relative contributions of the individual n-processes to the synthesis of the heavy elements and we set up a model to calculate the component coefficients and the abundances of heavy elements in metal-poor stars with different metallicities. With this model, we then calculate the component coefficients and the abundances of the heavy elements in 18 metal-poor stars. We find that, for most sample stars, the model calculations are basically in agreement with the observations of the heavy elemental abundances within the error limits and the fits of the model predictions are much better for the heaviest elements than for the lighter elements, specifically for Sr, Y, and Zr. We discuss this result and give a possible explanation for it. Moreover, we also discuss the physical meanings of the component coefficients.

Stability of stellar structure in non-equilibrium thermodynamics. I. the PPI reaction

Abstract Using Prigogines stability criterion of excess entropy production in non-linear and non-equilibrium thermodynamics, we re-examined the stability of stellar structure with respect to the PPI reaction in solar type stars. We show that the star is stable if the relative mass abundence of Hydrogen to Helium is less than 8.9, a condition which is amply satisfied by the normal stars.

Model of wind accretion in extrinsic AGB stars under conservation of total angular momentum

Abstract The equations of wind accretion and variation of orbital parameters are rederived under conservation of total (rather than tangential) angular momentum, and applied to extrinsic AGB stars. These equations, together with nucleosynthesis in intrinsic AGB stars, are used to calculate heavy element overabundance for the scenario of successive pulses and mixing. The results show that wind accretion pertains when the orbital period is longer than 1300 d or 1600 d according as the initial mass of the Ba star is 2.5 M⊙ or 1.3 M⊙, while cataclysmic variables will result if the period is shorter than 600 d in either case. The results are advantageous for interpreting a) the observations on the overabundance and orbital parameters, b) the observed lower limit of 600 d in the period of extrinsic S stars, and c) the observed fact that the mean eccentricity of Ba stars is greater than the mean eccentricities of extrinsic S stars and CH stars.

The effect of a strong magnetic field on the thermonuclear reactions in the shells of neutron stars

Abstract We analyse the Fermi energy of an electron gas in a strong magnetic field, taking into account the quantum effects and discuss the effect of the field on the screening potential of the nucleus and hence calculate the effect on several important thermonuclcar reactions. The results show that, a sufficiently strong field will greatly increase the screening potential when the density is low, but for densities in excess of 10 5 mol cm −3 and for fields in the range of 10 5 ∼ 10 9 T , such as are present on the surface of neutron stars, the effect of the field on nuclear reactions is almost nil.

The role of the outer boundary condition in accretion disk models

Taking optically thin accretion flows as an example, we investigate the effects of the outer boundary condition (OBC) on the dynamics and the emergent spectra of accretion flows. We find that OBC plays an important role. This is because the accretion equations describing the behavior of accretion flows are a set of differential equations, therefore, accretion is intrinsically an initial-value problem. The result means that we should seriously consider the initial physical state of the accretion flow such as its angular momentum and its temperature. An application example to Sgr A* is presented.

The nucleosynthesis of heavy elements in low-mass AGB stars

Abstract We take 13C(α,n)16O as netron source, allow for variation of the mass of the nucleus with thermal pulse number and adopt the unbranched s process path from 56Fe to 210Bi in an attempt to explain the overabundace of heavy elements in MS and S stars. We found stars with larger masses have larger dilution factors because of a stronger convection and that these stars fall in four different regions in the abundance diagram, thus enabling a rough estimate of the mass to be made. We show that, for a 2.5 M⊙ star, the third dredge-up stops after a certain number of pulses because of the formation of a hollow shell structure, so the star will show no Tc and this can account for the existence of binary systems consisting of a “Tc-no” AGB star with a main-sequence star companion.