China
Africa
Explore chapters and articles related to this topic
Introductory Material
Published in Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr, Fundamentals of Radio Astronomy, 2019
Ronald L. Snell, Stanley E. Kurtz, Jonathan M. Marr
Some units in astronomy are set by using the Sun as a standard. The Sun is a typical star and one whose parameters we know exceedingly well, so the units of mass and luminosity are often expressed in comparison to the Sun's mass and luminosity. We call these units solar massesUnits!astronomy units!solar mass (M⊙)Solar mass (M⊙) and solar luminositiesUnits!astronomy units!solar luminosity (L⊙)Solar luminosity (L⊙) and they are symbolized by M⊙ and L⊙, respectively, where ⊙ is the symbol for the Sun. The values of these units are the following SolarMass(M⊙)=1.99×1033g=1.99×1030kg,andSolarLuminosity(L⊙)=3.83×1033ergs−1=3.83×1026watts. The luminosity of an object is a measure of the total energy radiated by the object per unit time and will be discussed further in Section 1.2. These astronomy units are also summarized in Appendix A.Units!astronomy unitsUnits
Experiments and long-term high-performance computations on amplitude modulations of strato-rotational flows
Published in Geophysical & Astrophysical Fluid Dynamics, 2021
G. Meletti, S. Abide, S. Viazzo, A. Krebs, U. Harlander
Understanding the hydrodynamical mechanisms that can result in an outward transport of angular momentum is a central problem regarding stars and planets formation, particularly in the theory of accretion discs (Fromang and Lesur 2017). Accretion disks are astrophysical disk-like shape objects composed of gas and dust that rotate around a central object, as a star or a planet. One example of such astrophysical objects is the one observed by the Atacama Large Millimeter/submillimeter Array (ALMA) collaboration (Brogan et al.2015), that is an ideal system for the study of disk instabilities and early planet formation, since it consists of a young star surrounded by a disk with high mass. The disk mass is estimated between , and its outer radius is estimated to be , with Keplerian velocity profile. At a radius of , . The mass of the HL Tauri star found in the centre of the disk is estimated to be 30% higher than the solar mass (), enclosed in a radius . Central objects in accretion disks, as the HL Tauri star, are formed by the gravitational collapse of the disk matter, but the large mass and sizes values in these systems show that even the slight rotations lead to too much angular momentum (Fromang and Lesur 2017), large enough to overcome gravitational forces that would allow the formation of central massive objects. Since astrophysical observations show these massive bodies in the centre of accretion disks, the gas flow surrounding the objects should be turbulent, as turbulence, unlike viscous diffusion, can efficiently transport these high angular momentum away from the centre of the disk, removing energy from the disk during this process, and allowing gravity to be stronger than the outer radial angular momentum component, collapsing matter to form the observed astrophysical bodies.
The time step constraint in radiation hydrodynamics
Published in Geophysical & Astrophysical Fluid Dynamics, 2020
We adopt here one of the stellar surface layer models of the unpublished work of Brandenburg and Spiegel, who considered a star of solar mass , solar radius and (the solar value), but with a luminosity L that is times the solar value . The radiative flux is given by and the effective temperature . We solve the time-dependent equations (13)–(16) for the surface layers of the above star using the Pencil Code, with initial . We also put , , and . We refer to appendix 1 regarding the boundary conditions imposed on the intensity, and appendix 3 for those on the hydrodynamic variables. Our model has a depth of and uses 256 uniformly spaced mesh points. The surface is roughly in the middle of the domain, which we define to be at z = 0. The temperature then varies between at and at . The sound speed varies between at the bottom and at the top, while at the bottom and about at the top. The domain has a density contrast of , so the number of density scale heights is . Note that, for this case, we have used (i.e. in figure 3 only) instead of expression (27), but the difference would be minor.
Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection
Published in Geophysical & Astrophysical Fluid Dynamics, 2020
P. J. Käpylä, F. A. Gent, N. Olspert, M. J. Käpylä, A. Brandenburg
Our simulation setup is similar to that used in Käpylä et al. (2019) with a few variations that will be explained in detail. We solve a set of fully compressible hydromagnetics equations where is the magnetic vector potential, is the velocity, is the magnetic field, η is the magnetic diffusivity, is the permeability of vacuum, is the current density, is the advective time derivative, ρ is the density, ν is the kinematic viscosity, p is the pressure, and s is the specific entropy with , where and are the specific heats at constant volume and pressure, respectively. The gas is assumed to obey the ideal gas law, , where is the gas constant. The rate of strain tensor is given by where the semicolons refer to covariant derivatives (Mitra et al.2009). The acceleration due to gravity, and the Coriolis and centrifugal forces are given by where N m2 kg−2 is the universal gravitational constant, kg is the solar mass, is the angular velocity vector, where is the rotation rate of the frame of reference, is the radial coordinate, and the corresponding radial unit vector. The parameter is used to control the magnitude of the centrifugal force.