The Experts below are selected from a list of 36276 Experts worldwide ranked by ideXlab platform
Mikhail V Medvedev - One of the best experts on this subject based on the ideXlab platform.
-
Self-similar hot accretion on to a spinning neutron star: matching the Outer Boundary conditions
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Ramesh Narayan, Mikhail V MedvedevAbstract:Medvedev & Narayan have described a hot accretion flow on to a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the Outer Boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general Outer Boundary conditions. We confirm the main features of the analytical results with a full numerical solution.
-
self similar hot accretion onto a spinning neutron star matching the Outer Boundary conditions
arXiv: Astrophysics, 2003Co-Authors: Ramesh Narayan, Mikhail V MedvedevAbstract:Medvedev & Narayan have described a hot accretion flow onto a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the Outer Boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general Outer Boundary conditions. We confirm the main features of the analytical results with a full numerical solution.
Ramesh Narayan - One of the best experts on this subject based on the ideXlab platform.
-
Self-similar hot accretion on to a spinning neutron star: matching the Outer Boundary conditions
Monthly Notices of the Royal Astronomical Society, 2003Co-Authors: Ramesh Narayan, Mikhail V MedvedevAbstract:Medvedev & Narayan have described a hot accretion flow on to a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the Outer Boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general Outer Boundary conditions. We confirm the main features of the analytical results with a full numerical solution.
-
self similar hot accretion onto a spinning neutron star matching the Outer Boundary conditions
arXiv: Astrophysics, 2003Co-Authors: Ramesh Narayan, Mikhail V MedvedevAbstract:Medvedev & Narayan have described a hot accretion flow onto a spinning neutron star in which the gas viscously brakes the spin of the star. Their self-similar solution has the surprising property that the density, temperature and angular velocity of the gas at any radius are completely independent of the Outer Boundary conditions. Hence, the solution cannot be matched to a general external medium. We resolve this paradoxical situation by showing that there is a second self-similar solution which bridges the gap between the original solution and the external medium. This new solution has an extra degree of freedom which permits it to match general Outer Boundary conditions. We confirm the main features of the analytical results with a full numerical solution.
M Miller M Bertolami - One of the best experts on this subject based on the ideXlab platform.
-
Outer Boundary conditions for evolving cool white dwarfs
Astronomy & Astrophysics, 2012Co-Authors: R D Rohrmann, L G Althaus, Alejandro H Corsico, Enrique García-berro, M Miller M BertolamiAbstract:White dwarf evolution is essentially a gravothermal cooling process, which,for cool white dwarfs, sensitively depends on the treatment of the Outer Boundary conditions. We provide detailed Outer Boundary conditions appropriate for computing the evolution of cool white dwarfs employing detailed non-gray model atmospheres for pure H composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are computed taken into account non-ideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman alpha quasi-molecular opacity. Our results show that the use of detailed Outer Boundary conditions becomes relevant for effective temperatures lower than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively. Detailed model atmospheres predict ages that are up to approx 10% shorter at log L/L_sun=-4 when compared with the ages derived using Eddington-like approximations at tau_Ross=2/3. We also analyze the effects of various assumptions and physical processes of relevance in the calculation of Outer Boundary conditions. In particular, we find that the Ly_alpha red wing absorption does not affect substantially the evolution of white dwarfs. White dwarf cooling timescales are sensitive to the surface Boundary conditions for T_eff < 6000K. Interestingly enough, non-gray effects have little consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in their cooling rates, except throughout the Rosseland mean opacity.
-
Outer Boundary conditions for evolving cool white dwarfs
Astronomy and Astrophysics, 2012Co-Authors: R D Rohrmann, L G Althaus, E Garciaberro, Alejandro H Corsico, M Miller M BertolamiAbstract:Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the Outer Boundary conditions. Aims. We provide detailed Outer Boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of Outer Boundary conditions provided by updated model atmospheres on the cooling times of 0.60 and 0.90 M white dwarf sequences. Results. Our results show that the use of detailed Outer Boundary conditions becomes relevant for effective temperatures lower than 5800 K for sequences with 0.60 M and 6100 K with 0.90 M . Detailed model atmospheres predict ages that are up to ≈10% shorter at log(L/L ) = −4 when compared with the ages derived using Eddington-like approximations at τRoss = 2/3. We also analyze the effects of various assumptions and physical processes that are relevant in the calculation of Outer Boundary conditions. In particular, we find that the Lyα red wing absorption does not substantially affect the evolution of white dwarfs. Conclusions. White dwarf cooling timescales are sensitive to the surface Boundary conditions for Teff <∼ 6000 K. Interestingly enough, nongray effects have few consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have no noticeable effects on their cooling rates, except throughout the Rosseland mean opacity.
-
Outer Boundary conditions for evolving cool white dwarfs (Research Note)
2012Co-Authors: R D Rohrmann, L G Althaus, M Miller M Bertolami, Enrique García-berro, Gran CapitàAbstract:Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the Outer Boundary conditions. Aims. We provide detailed Outer Boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of
Feng Yuan - One of the best experts on this subject based on the ideXlab platform.
-
The role of the Outer Boundary condition in accretion disk models
AIP Conference Proceedings, 2001Co-Authors: Feng Yuan, Qiuhe Peng, Jianmin WangAbstract: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 role of the Outer Boundary condition in accretion disk models theory and application
arXiv: Astrophysics, 2000Co-Authors: Feng Yuan, Qiuhe Peng, Jianmin WangAbstract:The influence of the Outer Boundary condition (OBC) on the dynamics and radiation of optically thin accretion flow is investigated. 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 find that when the general parameters such as the mass accretion rate and the viscous parameter are fixed, the peak flux at various bands such as radio, IR and X-ray, can differ by as large as several orders of magnitude under different OBCs in our example. Our results indicate that OBC is both dynamically and radiatively important therefore should be regarded as a new ``parameter'' in accretion disk models. We apply the above results to the compact radio source Sgr A* and find that the discrepancy between the mass accretion rate favored by ADAF models in the literature and that favored by the three dimensional hydrodynamical simulation can be naturally resolved by seriously considering the Outer Boundary condition of the accretion flow.
-
The Role of the Outer Boundary Condition in Accretion Disk Models: Theory and Application
The Astrophysical Journal, 2000Co-Authors: Feng Yuan, Qiuhe Peng, Jianmin WangAbstract: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.
-
Accretion Flows: The Role of the Outer Boundary Condition
The Astrophysical Journal, 1999Co-Authors: Feng YuanAbstract:We investigate the influences of the Outer Boundary conditions on the structure of an optically thin accretion flow. We find that the Outer Boundary condition plays an important role in determining the topological structure and the profiles of the density and temperature of the solutions. Therefore, it should be regarded as a new parameter in the accretion model.
-
accretion flows the role of the Outer Boundary condition
arXiv: Astrophysics, 1999Co-Authors: Feng YuanAbstract:We investigate the influences of the Outer Boundary conditions(OBCs) on the structure of an optically thin accretion flow. We find that OBC plays an important role in determining the topological structure and the profiles of the surface density and temperature of the solution, therefore it should be regarded as a new parameter in the accretion disk model.
R D Rohrmann - One of the best experts on this subject based on the ideXlab platform.
-
Outer Boundary conditions for evolving cool white dwarfs
Astronomy & Astrophysics, 2012Co-Authors: R D Rohrmann, L G Althaus, Alejandro H Corsico, Enrique García-berro, M Miller M BertolamiAbstract:White dwarf evolution is essentially a gravothermal cooling process, which,for cool white dwarfs, sensitively depends on the treatment of the Outer Boundary conditions. We provide detailed Outer Boundary conditions appropriate for computing the evolution of cool white dwarfs employing detailed non-gray model atmospheres for pure H composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Detailed non-gray model atmospheres are computed taken into account non-ideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman alpha quasi-molecular opacity. Our results show that the use of detailed Outer Boundary conditions becomes relevant for effective temperatures lower than 5800 and 6100K for sequences with 0.60 and 0.90 M_sun, respectively. Detailed model atmospheres predict ages that are up to approx 10% shorter at log L/L_sun=-4 when compared with the ages derived using Eddington-like approximations at tau_Ross=2/3. We also analyze the effects of various assumptions and physical processes of relevance in the calculation of Outer Boundary conditions. In particular, we find that the Ly_alpha red wing absorption does not affect substantially the evolution of white dwarfs. White dwarf cooling timescales are sensitive to the surface Boundary conditions for T_eff < 6000K. Interestingly enough, non-gray effects have little consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have not noticeable effects in their cooling rates, except throughout the Rosseland mean opacity.
-
Outer Boundary conditions for evolving cool white dwarfs
Astronomy and Astrophysics, 2012Co-Authors: R D Rohrmann, L G Althaus, E Garciaberro, Alejandro H Corsico, M Miller M BertolamiAbstract:Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the Outer Boundary conditions. Aims. We provide detailed Outer Boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of Outer Boundary conditions provided by updated model atmospheres on the cooling times of 0.60 and 0.90 M white dwarf sequences. Results. Our results show that the use of detailed Outer Boundary conditions becomes relevant for effective temperatures lower than 5800 K for sequences with 0.60 M and 6100 K with 0.90 M . Detailed model atmospheres predict ages that are up to ≈10% shorter at log(L/L ) = −4 when compared with the ages derived using Eddington-like approximations at τRoss = 2/3. We also analyze the effects of various assumptions and physical processes that are relevant in the calculation of Outer Boundary conditions. In particular, we find that the Lyα red wing absorption does not substantially affect the evolution of white dwarfs. Conclusions. White dwarf cooling timescales are sensitive to the surface Boundary conditions for Teff <∼ 6000 K. Interestingly enough, nongray effects have few consequences on these cooling times at observable luminosities. In fact, collision-induced absorption processes, which significantly affect the spectra and colors of old white dwarfs with hydrogen-rich atmospheres, have no noticeable effects on their cooling rates, except throughout the Rosseland mean opacity.
-
Outer Boundary conditions for evolving cool white dwarfs (Research Note)
2012Co-Authors: R D Rohrmann, L G Althaus, M Miller M Bertolami, Enrique García-berro, Gran CapitàAbstract:Context. White dwarf evolution is essentially a gravothermal cooling process, which, for cool white dwarfs, depends on the treatment of the Outer Boundary conditions. Aims. We provide detailed Outer Boundary conditions that are appropriate to computing the evolution of cool white dwarfs by employing detailed nongray model atmospheres for pure hydrogen composition. We also explore the impact on the white dwarf cooling times of different assumptions for energy transfer in the atmosphere of cool white dwarfs. Methods. Detailed nongray model atmospheres were computed by considering nonideal effects in the gas equation of state and chemical equilibrium, collision-induced absorption from molecules, and the Lyman α quasi-molecular opacity. We explored the impact of
