Mark S. Marley

Thermodynamics of dense molecular hydrogen-helium mixtures at high pressure

Abstract Current equations of state of dense molecular hydrogen-helium mixtures are not experimentally verified beyond densities of ∼0.3 g cm−3. We have used effective intermolecular pair potentials derived from recent shock wave experiments on liquid hydrogen (W.J. Nellis, A.C. Mitchell, M. van Thiel, G.J. Devine, R.J. Trainor, and N. Brown 1983, J. Chem. Phys. 79, 1480–1486) and helium (W.J. Nellis, N.C. Holmes, A.C. Mitchell, R.J. Trainor, G.K. Governo, M. Ross, and D.A. Young 1984, Phys. Rev. Lett. 53, 1248–1251) in Monte Carlo simulations of mixtures of hydrogen and helium at densities up to 1.2 g cm−3. A model interaction Helmholtz free energy has been derived which accurately reproduces the results of the Monte Carlo calculations. This free energy also reproduces the experimental data to densities of ∼0.6 g cm−3. The equation of state derived from the free energy expression will be useful in preparing improved interior models of the Jovian planets, especially Saturn. The pressure of the molecular hydrogen to metallic hydrogen presumed first-order phase transition is also calculated. Uncertainties in the rotational and vibrational frequencies of the compressed hydrogen molecule translate into an uncertain transition pressure of 3–5 Mbar.

The periodicities in the infrared excess of G29-38 - An oscillating brown dwarf?

The oscillatory behavior of brown dwarfs has been investigated. The observed periodicities in the infrared excess of the white dwarf Giclas 29-38 are consistent with low-degree, intermediate radial order p-mode oscillations of a brown dwarf companion to the white dwarf. These oscillation modes have the correct frequencies, act on observable layers of the atmosphere, and may be excited to sufficient amplitudes to explain the observations. 14 refs.

Non-equilibrium Chemistry in the Atmospheres of Brown Dwarfs

Carbon monoxide and ammonia have been detected in the spectrum of Gl 229B at abundances that differ substantially from those obtained from chemical equilibrium. Vertical mixing in the atmosphere is a mechanism that can drive slowly reacting species out of chemical equilibrium. We explore the effects of vertical mixing as a function of mixing efficiency and effective temperature on the chemical abundances in the atmospheres of brown dwarfs and on their spectra. The models compare favorably with the observational evidence and indicate that vertical mixing plays an important role in brown dwarf atmospheres.

Clouds and Clearings in the Atmospheres of the L and T Dwarfs

A sophisticated approach to condensate opacity is required to properly model the atmospheres of L and T dwarfs. Here we review different models for the treatment of condensates in brown dwarf atmospheres. We conclude that models which include both particle sedimentation and upwards transport of condensate (both gas and particles) provide the best fit for the L dwarf colors. While a globally uniform cloud model fits the L dwarf data, it turns to the blue in J-K too slowly to fit the T dwarfs. Models which include local clearings in the global cloud deck, similar to Jupiters prominent five-micron hot spots, better reproduce the available photometric data and also account for the observed resurgence of FeH absorption in early type T dwarfs.

Structure of the Jovian Envelope and the Equation of State of Dense Hydrogen

The interior composition of the planet Jupiter is deduced by comparing models generated from the equation of hydrostatic equilibrium and high-pressure equations of state with the known mass, equatorial radius, and gravitational multipole moments of the planet. The planet is primarily composed of liquid metallic hydrogen, but there appears to be a substantial admixture of denser elements present as well. Jupiter’s hydrogen-rich envelope is substantially enriched in material other than hydrogen and helium, containing approximately 20 to 60 earth masses of such material, in addition to 6 to 4 earth masses of such material in a distinct core. Thus Jupiter’s bulk composition differs from that of the sun. These conclusions are heavily dependent upon accurate pressure-density relations for pure metallic hydrogen in the pressure range from about 3 to 40 megabars, and at temperatures ranging from about 10000 to 20000 K. Experimental results for compression of hydrogen in the nonmetallic pressure range are helpful in constraining models, but accurate theoretical calculations of the thermodynamics of the liquid metallic phase provide the most help in constraining models. We discuss the state of the strongly-coupled plasma in the Jovian interior, and propose a phase diagram for dense liquid hydrogen.

Science capabilities of the WFIRST coronagraph (Conference Presentation)

The WFIRST mission was originally ended as a wide-field survey facility. With the change to a 2.4-m telescope, the mission is capable of carrying an effective coronagraph for exoplanet imaging. The baseline architecture allows use of a hybrid lyot or shaped pupil coronagraph, feeding a imager and integral field spectrograph. This will allow imaging and photometry of mature nearby planets and zodiacal disks in reflected light, as well as spectroscopy of the brightest targets. I will discuss the scientific motivations of the mission and show simulated science capabilities, and discuss the process towards definition of a science mission.