D. Saumon

Clouds and Chemistry: Ultracool Dwarf Atmospheric Properties from Optical and Infrared Colors

The optical and infrared colors of L and T dwarfs are sensitive to cloud sedimentation and chemical equilibrium processes in their atmospheres. The i-z versus J-K color-color diagram provides a window into diverse atmospheric processes mainly because different chemical processes govern each color, and cloud opacity largely affects J-K but not i-z. Using theoretical atmosphere models that include for the first time a self-consistent treatment of cloud formation, we present an interpretation of the i-z versus J-K color trends of known L and T dwarfs. We find that the i-z color is extremely sensitive to chemical equilibrium assumptions; chemical equilibrium models accounting for cloud sedimentation predict redder i-z colors—by up to 2 mag—than models that neglect sedimentation. We explore the previously known J-K color trends in which objects first become redder, then bluer with decreasing effective temperature. Only models that include sedimentation of condensates are able to reproduce these trends. We find that the exact track of a cooling brown dwarf in J-K (and i-z) is very sensitive to the details of clouds, in particular to the efficiency of sedimentation of condensates in its atmosphere. We also find that clouds still affect the strength of the J-, H-, and K-band fluxes of even the coolest T dwarfs. In addition, we predict the locus in the i-z versus J-K color-color diagram of brown dwarfs cooler than yet discovered.

Direct Detection of Galactic Halo Dark Matter

The Milky Way galaxy contains a large, spherical component which is believed to harbor a substantial amount of unseen matter.u2002Recent observations indirectly suggest that as much as half of this “dark matter” may be in the form of old, very cool white dwarfs, the remnants of an ancient population of stars as old as the galaxy itself. We conducted a survey to find faint, cool white dwarfs with large space velocities, indicative of their membership in the galaxys spherical halo component. The survey reveals a substantial, directly observed population of old white dwarfs, too faint to be seen in previous surveys. This newly discovered population accounts for at least 2 percent of the halo dark matter. It provides a natural explanation for the indirect observations, and represents a direct detection of galactic halo dark matter.

Pure Hydrogen Model Atmospheres for Very Cool White Dwarfs

Microlensing events observed in the line of sight toward the LMC indicate that a significant fraction of the mass of the dark halo of the Galaxy is probably composed of white dwarfs. In addition, white dwarf sequences have now be observed in the H-R diagrams of several globular clusters. Because of the unavailability of white dwarf atmospheres for Teff < 4000 K, the cooling timescales for white dwarfs older than ≈10 Gyr are very uncertain. Moreover, the identification of a MACHO white dwarf population by direct observation depends on a knowledge of the colors and bolometric corrections of very cool white dwarfs. In this Letter, we present the first detailed model atmospheres and spectra of very cool hydrogen white dwarfs for Teff < 4000 K. We include the latest description of the opacities of hydrogen, and, significantly, we introduce a nonideal equation of state in the atmosphere calculation. We find that due to strong absorption from H2 in the infrared, very old white dwarfs are brightest in the V, R, and I bands, and we confirm that they become bluer in most color indices as they cool below Teff ≈ 3500 K.

Infrared Observations and Modeling of One of the Coolest T Dwarfs: Gliese 570D

We have obtained a good-quality R ~ 400, 0.8-2.5 ?m spectrum as well as accurate photometry of Gliese 570D, one of the coolest and least-luminous brown dwarfs currently known. The spectrum shows that Gl 570D has deeper absorptions in the strong water and methane bands at 1.12-1.17, 1.33-1.45, 1.62-1.88, and 2.20-2.45 ?m and is both bluer at J - K and redder at K - L than previously observed T dwarfs. Data analysis using model spectra coupled with knowledge of the well-understood primary implies that for the same surface gravity, Gl 570D is about 160 K cooler than Gl 229B. For an age range of 2-5 Gyr, Gl 570D has an effective temperature in the range 784-824 K, a surface gravity log g in the range 5.00-5.27 (cm s-2), and a luminosity in the range ? 10-6 L?.

Observations of Ultracool White Dwarfs

We present new spectroscopic and photometric measurements of the white dwarfs LHS 3250 and WD 0346+246. Along with F351-50, these white dwarfs are the coolest ones known, all with effective temperatures below 4000 K. Their membership in the Galactic halo population is discussed, and detailed comparisons of all three objects with new atmosphere models are presented. The new models consider the effects of mixed H/He atmospheres and indicate that WD 0346+246 and F351-50 have predominantly helium atmospheres with only traces of hydrogen. LHS 3250 may be a double degenerate whose average radiative temperature is between 2000 and 4000 K, but the new models fail to explain this object.

Radiative Transfer in the Refractive Atmospheres of Very Cool White Dwarfs

We consider the problem of radiative transfer in stellar atmospheres where the index of refraction departs from unity and is a function of density and temperature. We present modified Feautrier and Λ-iteration methods to solve the equation of radiative transfer with refraction in a plane-parallel atmosphere. These methods are general and can be used in any problem with one-dimensional geometry where the index of refraction is a monotonically varying function of vertical optical depth. We present an application to very cool white dwarf atmospheres in which the index of refraction departs significantly from unity. We investigate how ray curvature and total internal reflection affect the limb darkening and the pressure-temperature structure of the atmosphere. Refraction results in a much weakened limb-darkening effect. We find that through the constraint of radiative equilibrium, total internal reflection warms the white dwarf atmosphere near the surface (τ 1). This effect may have a significant impact on studies of very cool white dwarf stars.