Hydrostatic equilibrium

About: Hydrostatic equilibrium is a research topic. Over the lifetime, 2451 publications have been published within this topic receiving 62172 citations.

Late stages of solar type protostars

Abstract: A consistent hydrodynamical and radiative transfer calculation in spherical symmetry for a 1M ⊙ protostar is presented. The calculation starts with Larson's initial conditions and continues until almost all the material has fallen onto a hydrostatic core with a large outer convection zone. The innermost percent of the mass is partially degenerate. Due to the numerical technique used, the radius of the hydrostatic core is determined with a high degree of accuracy.

Surface forces between colloidal particles at high hydrostatic pressure

Abstract: It was recently suggested that the electrostatic double-layer force between colloidal particles might weaken at high hydrostatic pressure encountered, for example, in deep seas or during oil recovery. We have addressed this issue by means of a specially designed optical trapping setup that allowed us to explore the interaction of a micrometer-sized glass bead and a solid glass wall in water at hydrostatic pressures of up to 1 kbar. The setup allowed us to measure the distance between bead and wall with a subnanometer resolution. We have determined the Debye lengths in water for salt concentrations of 0.1 and 1 mM. We found that in the pressure range from 1 bar to 1 kbar the maximum variation of the Debye lengths was <1 nm for both salt concentrations. Furthermore, the magnitude of the zeta potentials of the glass surfaces in water showed no dependency on pressure.

Power spectrum of turbulent convection in the solar photosphere

Abstract: The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two balloon-borne SUNRISE missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the photospheric overshooting flow with the help of its governing equations and simulations. We find that quiet-Sun observations and smeared simulations are consistent with each other and exhibit a power-law behavior in the subgranular range of their Doppler velocity power spectra with a power-law index of ≈ − 2. The unsmeared simulations exhibit a power law that extends over the full range between the integral and Taylor scales with a power-law index of ≈ − 2.25. The smearing, reminiscent of observational conditions, considerably reduces the extent of the power-law-like portion of the power spectra. This suggests that the limited spatial resolution in some observations might eventually result in larger uncertainties in the estimation of the power-law indices. The simulated vertical velocity power spectra as a function of height show a rapid change in the power-law index (at the subgranular range) from roughly the optical depth unity layer, that is, the solar surface, to 300 km above it. We propose that the cause of the steepening of the power-law index is the transition from a super- to a subadiabatic region, in which the dominant source of motions is overshooting convection. A scale-dependent transport of the vertical momentum occurs. At smaller scales, the vertical momentum is more efficiently transported sideways than at larger scales. This results in less vertical velocity power transported upward at small scales than at larger scales and produces a progressively steeper vertical velocity power law below 180 km. Above this height, the gravity work progressively gains importance at all relevant scales, making the atmosphere progressively more hydrostatic and resulting in a gradually less steep power law. Radiative heating and cooling of the plasma is shown to play a dominant role in the plasma energetics in this region, which is important in terms of nonadiabatic damping of the convective motions.

Helium-Star Models with Optically Thick Winds: Implications for the Internal Structures and Mass-Loss Rates of Wolf-Rayet Stars

Abstract: We construct helium (He) star models with optically thick winds and compare them with the properties of Galactic Wolf-Rayet (WR) stars. Hydrostatic He-core solutions are connected smoothly to trans-sonic wind solutions that satisfy the regularity conditions at the sonic point. Velocity structures in the supersonic parts are assumed by a simple beta-type law. By constructing a center-to-surface structure, a mass-loss rate can be obtained as an eigenvalue of the equations. Sonic points appear at temperatures ~ 1.8e5 - 2.8e5 K below the Fe-group opacity peak, where the radiation force becomes comparable to the local gravity. Photospheres are located at radii 3-10 times larger than sonic points. The obtained mass-loss rates are comparable to those of WR stars. Our mass-loss rate - luminosity relation agrees well with the relation recently obtained by Graefener et al. (2017). Photospheric temperatures of WR stars tend to be cooler than our predictions. We discuss the effects of stellar evolution, detailed radiation transfer, and wind clumping, which are ignored in this paper.