arXiv subjects

Eliot Quataert

Publications and source records attributed to Eliot Quataert.

435 records · Page 25Linked to original sources

arXiv1997

Are Particles in Advection-Dominated Accretion Flows Thermal?

We investigate the form of the momentum distribution function for protons and electrons in an advection-dominated accretion flow (ADAF). We show that for all accretion rates, Coulomb collisions are too inefficient to thermalize the protons. The proton distribution function is therefore determined by the viscous heating mechanism, which is unknown. The electrons, however, can exchange energy quite efficiently through Coulomb collisions and the emission and absorption of synchrotron photons. We find that for accretion rates greater than \sim 10^{-3} of the Eddington accretion rate, the electrons have a thermal distribution throughout the accretion flow. For lower accretion rates, the electron distribution function is determined by the electron's source of heating, which is primarily adiabatic compression. Using the principle of adiabatic invariance, we show that an adiabatically compressed collisionless gas maintains a thermal distribution until the particle energies become relativistic. We derive a new, non-thermal, distribution function which arises for relativistic energies and provide analytic formulae for the synchrotron radiation from this distribution. Finally, we discuss its implications for the emission spectra from ADAFs.

astro-ph↗

arXiv1995

Observational Searches for Solar g-modes: Some Theoretical Considerations

We argue that the solar g-modes are unlikely to have caused the discrete peaks in the power spectrum of the solar wind flux observed by Thomson et al. (1995). The lower limit to the energy of individual g-modes, using the amplitudes given by Thomson et al., is estimated to be at least 10$^{36}$ erg for low order g-modes; the resulting surface velocity amplitude is at least 50 cm s$^{-1}$, larger than the observational upper limit (5 cm s$^{-1}$). We suggest that the most likely source for the excitation of solar g-modes is turbulent stresses in the convection zone. The surface velocity amplitude of low degree and low order g-modes resulting from this process is estimated to be of order 10$^{-2}$ cm s$^{-1}$. This amplitude is interestingly close to the detection threshold of the SOHO satellite. The long lifetime of g-modes ($\sim 10^6$ years for low order modes) should be helpful in detecting these small amplitude pulsations.

astro-ph↗

arXiv1995

On the Validity of the Classical Apsidal Motion Formula for Tidal Distortion

We check the validity of the widely used classical apsidal motion formula as a function of orbital parameters, stellar structure, and stellar rotation rate by comparing dynamical calculations of the periastron advance with the static tidal formula. We find that the classical formula gives very accurate results when the periods of the low order quadrupole g, f and p modes are smaller than the periastron passage time by a factor of about 7 or more. However, when this condition is not satisfied, the difference between the classical formula and the exact result can be quite large, and even periastron recession can result. The largest difference arises when one of the low order modes of the star is nearly resonant with an integer multiple of the orbital frequency minus twice the rotation rate of the star. The resonance of higher order g-modes (number of radial nodes $\gta 4$) with the orbit is very unlikely to cause significant deviation from the classical result because of their weak coupling to the tidal force and thus their small contribution to the apsidal motion. Resonances involving rotational modes of the star are also unlikely to make much contribution to the apsidal motion because of their small overlap with the tidal force, even though they have periods comparable to the periastron passage time. We apply our work to two famous binary systems (AS Cam and DI Her) which show abnormally small apsidal motion, and conclude that dynamical effects are unimportant for these systems, i.e. the static tide assumption is an excellent approximation.

astro-ph↗