18 research outputs found
An equation of state from cool-dense fluids to hot gases for mixed elements
An equation of state for the domain extending from hot gases to cool-dense
fluids is formulated for a hydrogen-helium mixture. The physical processes take
account of temperature ionization and dissociation, electron degeneracy,
Coulomb coupling and pressure ionization. Pressure ionization and Coulomb
coupling are studied with simple and comprehensive modeling. A single and
complete algorithm is achieved with explicit expressions available for the
whole domain from hot gases to cool dense fluids (). Pressure
ionization and Coulomb coupling have been examined for their contributions to
the pressure and internal energy. The result reveals that their contributions
smooth the variation of the pressure and internal energy in the region of
pressure ionization even at very low temperatures.Comment: 10 pages, 8 figures, ApJ, accepted, E-mail: [email protected]
Accretion Disks around Young Objects. I. The Detailed Vertical Structure
We discuss the properties of an accretion disk around a star with parameters
typical of classical T Tauri stars (CTTS), and with the average accretion rate
for these disks. The disk is assumed steady and geometrically thin. The
turbulent viscosity coefficient is expressed using the alpha prescription and
the main heating mechanisms considered are viscous dissipation and irradiation
by the central star. The energy is transported by radiation, turbulent
conduction and convection.
We find that irradiation from the central star is the main heating agent of
the disk, except in the innermost regions, R less than 2 AU. The irradiation
increases the temperature of the outer disk relative to the purely viscous
case. As a consequence, the outer disk (R larger than 5 AU) becomes less dense,
optically thin and almost vertically isothermal, with a temperature
distribution T proportional to R^{-1/2}. The decrease in surface density at the
outer disk, decreases the disk mass by a factor of 4 respect to a purely
viscous case. In addition, irradiation tends to make the outer disk regions
stable against gravitational instabilities.Comment: 41 pages, 14 postscript figures, LaTeX, accepted by Ap
Tonic excitation or inhibition is set by GABAA conductance in hippocampal interneurons
Inhibition is a physiological process that decreases the probability of a neuron generating an action potential. The two main mechanisms that have been proposed for inhibition are hyperpolarization and shunting. Shunting results from increased membrane conductance, and it reduces the neuron-firing probability. Here we show that ambient GABA, the main inhibitory neurotransmitter in the brain, can excite adult hippocampal interneurons. In these cells, the GABAA current reversal potential is depolarizing, making baseline tonic GABAA conductance excitatory. Increasing the tonic conductance enhances shunting-mediated inhibition, which eventually overpowers the excitation. Such a biphasic change in interneuron firing leads to corresponding changes in the GABAA-mediated synaptic signalling. The described phenomenon suggests that the excitatory or inhibitory actions of the current are set not only by the reversal potential, but also by the conductance
Spectroscopic Survey of the Galaxy with Gaia II. The expected science yield from the Radial Velocity Spectrometer
The Gaia mission is designed as a Galaxy explorer, and will measure
simultaneously, in a survey mode, the five or six phase space parameters of all
stars brighter than 20th magnitude, as well as providing a description of their
astrophysical characteristics. These measurements are obtained by combining an
astrometric instrument with micro-arcsecond capabilities, a photometric system
giving the magnitudes and colours in 15 bands and a medium resolution
spectrograph named the Radial Velocity Spectrometer (RVS). The latter
instrument will produce spectra in the 848 to 874 nm wavelength range, with a
resolving power R = 11 500, from which radial velocities, rotational
velocities, atmospheric parameters and abundances can be derived. A companion
paper (Katz et al. 2004) presents the characteristics of the RVS and its
performance. This paper details the outstanding scientific impact of this
important part of the Gaia satellite on some key open questions in present day
astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial
velocities and individual abundances of key elements in parallel with the
astrometric parameters is essential for the determination of the dynamical
state and formation history of our Galaxy. Moreover, for stars brighter than
V=15, the resolving power of the RVS will give information about most of the
effects which influence the position of a star in the Hertzsprung-Russell
diagram, placing unprecedented constraints on the age, internal structure and
evolution of stars of all types. Finally, the RVS multi-epoch observations are
ideally suited to the identification, classification and characterisation of
the many types of double, multiple and variable stars.Comment: 33 pages, 11 figures, in press at MNRAS. Figs 1, 3 and 9 included at
reduced resolution; available in full resolution at
http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1365-2966.2005.09012.