Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.Comment: Accepted for Astrophysical Journal. 16 pages (includes 3 figures

A framework for experimental-data-driven assessment of Magnetized Liner Inertial Fusion stagnation image metrics

A variety of spherical crystal x-ray imager (SCXI) diagnostics have been developed and fielded on Magnetized Liner Inertial Fusion (MagLIF) experiments at the Sandia National Laboratories Z-facility. These different imaging modalities provide detailed insight into different physical phenomena such as mix of liner material into the hot fuel, cold liner emission, or reduce impact of liner opacity. However, several practical considerations ranging from the lack of a consistent spatial fiducial for registration to different point-spread-functions and tuning crystals or using filters to highlight specific spectral regions make it difficult to develop broadly applicable metrics to compare experiments across our stagnation image database without making significant unverified assumptions. We leverage experimental data for a model-free assessment of sensitivities to instrumentation-based features for any specified image metric. In particular, we utilize a database of historical and recent MagLIF data including $N_{\text{scans}} = 139$ image plate scans gathered across $N_{\text{exp}} = 67$ different experiments to assess the impact of a variety of features in the experimental observations arising from uncertainties in registration as well as discrepancies in signal-to-noise ratio and instrument resolution. We choose a wavelet-based image metric known as the Mallat Scattering Transform for the study and highlight how alternate metric choices could also be studied. In particular, we demonstrate a capability to understand and mitigate the impact of signal-to-noise, image registration, and resolution difference between images. This is achieved by utilizing multiple scans of the same image plate, sampling random translations and rotations, and applying instrument specific point-spread-functions found by ray tracing to high-resolution datasets, augmenting our data in an effectively model-free fashion.Comment: 17 pages, 14 figure

On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars

We present a detailed analysis of a large spectroscopic and photometric sample of DZ white dwarfs based on our latest model atmosphere calculations. We revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz (12 stars) and analyze 147 new DZ white dwarfs discovered in the Sloan Digital Sky Survey. The inclusion of metals and hydrogen in our model atmosphere calculations leads to different atmospheric parameters than those derived from pure helium models. Calcium abundances are found in the range from log (Ca/He) = -12 to -8. We also find that fits of the coolest objects show peculiarities, suggesting that our physical models may not correctly describe the conditions of high atmospheric pressure encountered in the coolest DZ stars. We find that the mean mass of the 11 DZ stars with trigonometric parallaxes, = 0.63 Mo, is significantly lower than that obtained from pure helium models, = 0.78 Mo, and in much better agreement with the mean mass of other types of white dwarfs. We determine hydrogen abundances for 27% of the DZ stars in our sample, while only upper limits are obtained for objects with low signal-to-noise ratio spectroscopic data. We confirm with a high level of confidence that the accretion rate of hydrogen is at least two orders of magnitude smaller than that of metals (and up to five in some cases) to be compatible with the observations. We find a correlation between the hydrogen abundance and the effective temperature, suggesting for the first time empirical evidence of a lower temperature boundary for the hydrogen screening mechanism. Finally, we speculate on the possibility that the DZA white dwarfs could be the result of the convective mixing of thin hydrogen-rich atmospheres with the underlying helium convection zone.Comment: 67 pages, 32 figures, accepted for publication in Ap

Faint High Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results

We report the discovery of 39 Faint High Latitude Carbon Stars (FHLCs) from Sloan Digital Sky Survey commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0, and show a diversity of temperatures as judged by both colors and NaD line strengths. At the completion of the Sloan Survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper motion measures for each object, indicating that the sample is a mixture of extremely distant (>100 kpc) halo giant stars, useful for constraining halo dynamics, plus members of the recently-recognized exotic class of very nearby dwarf carbon (dC) stars. Motions, and thus dC classification, are inferred for 40-50 percent of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. (Abstract abridged).Comment: Accepted for publication in The Astronomical Journal, Vol. 124, Sep. 2002, 40 pages, 7 figures, AASTeX v5.

The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data

We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27 mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 sq. deg. and a detection limit of z* = 19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution.Comment: 9 pages, 3 figures, AASTeX, to appear in ApJ Letters, authors list update

Unconventional AGN from the SDSS

We discuss some of the most unusual active galactic nuclei (AGN) discovered to date by the Sloan Digital Sky Survey (SDSS): the first broad absorption line quasar seen to exhibit He II absorption, several quasars with extremely strong, narrow UV Fe II emission, and an AGN with an unexplained and very strange continuum shape.Comment: 6 pages, 4 figures, to appear in "Multiwavelength AGN Surveys," ed. R. Mujica & R. Maiolin