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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)

We present the first simultaneous, multi-wavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-ray Observatory, and the Kitt Peak 4-m telescope. We detect strongly variable and periodic radio emission (P=3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of L_X/L_{bol}, Submitted to ApJ; 26 pages, 15 figures

We present analysis of Hubble Space Telescope images of 82 nearby field late-M and L dwarfs. We resolve 13 of these systems into double M/L dwarf systems and identify an additional possible binary. Combined with previous observations of 20 L dwarfs, we derive an observed binary fraction for ultracool dwarfs of 17+4-3%, where the statistics included systems with separations in the range 1.6-16 A.U. We argue that accounting for biases and incompleteness leads to an estimated binary fraction 15+-5% in the range 1.6-16 A.U. No systems wider than 16 A.U. are seen, implying that the wide companion frequency is less than 1.7%; the distribution of orbital separation is peaked at ~2-4 A.U. and differs greatly from the G dwarf binary distribution. Indirect evidence suggests that the binary fraction is ~5+-3% for separations less than 1.6 A.U. We find no evidence for differences in the binary fraction between stellar late-M and L dwarfs and substellar L dwarfs. We note, however, that the widest (>10 A.U.) systems in our sample are all of earlier (M8-L0) spectral type; a larger sample is needed determine if this is a real effect. One system with a spectral type of L7 has a secondary that is fainter in the HST F814W filter but brighter in F1042M; we argue that this secondary is an early-T dwarf., To appear in June 2003 Astronomical Journal

An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12000 K, and the density of these stars for 15, Comment: Accepted for AJ; 43 pages, including 12 figures and 5 tables

Trigonometric parallax determinations are presented for 28 late type dwarfs and brown dwarfs, including eight M dwarfs with spectral types between M7 and M9.5, 17 L dwarfs with spectral types between L0 and L8, and three T dwarfs. Broadband photometry at CCD wavelengths (VRIz) and/or near-IR wavelengths (JHK) are presented for these objects and for 24 additional late-type dwarfs. Supplemented with astrometry and photometry from the literature, including ten L and two T dwarfs with parallaxes established by association with bright, usually HIPPARCOS primaries, this material forms the basis for studying various color-color and color-absolute magnitude relations. The I-J color is a good predictor of absolute magnitude for late-M and L dwarfs. M_J becomes monotonically fainter with I-J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. The combination of zJK colors alone can be used to classify late-M, early-L, and T dwarfs accurately, and to predict their absolute magnitudes, but is less effective at untangling the scatter among mid- and late-L dwarfs. The mean tangential velocity of these objects is found to be slightly less than that for dM stars in the solar neighborhood, consistent with a sample with a mean age of several Gyr. Using colors to estimate bolometric corrections, and models to estimate stellar radii, effective temperatures are derived. The latest L dwarfs are found to have T_eff ~ 1360 K., Comment: 48 pages, including 7 figures and 6 tables. Accepted for AJ

Precise trigonometric parallax measurements orders of magnitude more accurate than a milliarcsecond will contribute greatly to our understanding of peculiar, low luminosity stellar objects of several types. First, the volume of space out to which luminosities may be determined to the accuracy of the best, very-nearby stars will be increased greatly. For the relatively rare field Population II stars, this will lead to the first accurate empirical calibrations of the main sequence at the low mass end, for comparison with globular clusters of various metallicities. Parallaxes at 1 kpc or farther will be adequate to help in the discovery or confirmation of the rare carbon dwarfs — main sequence stars with carbon-rich atmospheres. For cool white dwarfs, luminosities accurate to a few per cent or better will identify unresolved binaries, and objects of unusually high and low mass. For our most numerous solar neighbors, the M dwarfs and especially those near the stellar mass limit, accurate luminosities can help in the determinations of the chemical composition and age distributions.

Observations a haute dispersions de la region spectrale Hα pour un echantillon d'etoiles naines M tres avancees. On en derive les vitesses radiales et les mouvements spatiaux. On examine les resultats par rapport a l'appartenance a une population cinematique

The temperature, radius, and other atmospheric parameters are derived for the cool DC white dwarf Stein 2051B (=G175-34B=EG 180), whose mass was recently determined by Strand. New spectrophotometric scans of this star and its dwarf M companion are discussed; these and existing Stroemgren photometry are fitted to model atmospheres with hydrogen/metal deficient compositions, and a temperature of 7050 +- 400 K is determined. The resulting radius of 0.0111 +- 0.0015 R/sub sun/ is marginally smaller than that of 40 Eri B. (AIP)

The three X-ray selected mid-K stars with the highest values of optical flux ratios in the Einstein Medium Sensitivity Survey have been studied spectroscopically at moderate and high spectral resolution. One of these objects (1E1548.7 + 1125) is presented as a binary-star system with characteristics similar to the RS CVn class of binaries. A second of these stars (1E1751.0 + 7046) is a rapid rotator, while the third star (1E1339.9 + 6030) has not been shown to be either a binary system or a rapid rotator. The presence of both Ca II H and K emission core and high X-ray flux is attributed to chromospheric and coronal activity, but no single underlying cause is indicated.

Broad-band circular polarization measurements for 32 white dwarfs are presented, part of a continuing program to assess the magnetic field distribution and stellar parameters of magnetic white dwarfs. None of the stars discussed here represent statistically significant detections, though a few are marginal, and one in particular (LP777-1) deserves closer study. The null results for cool degenerates include the very low luminosity stars LP701-29, LP131-66, and Ross 193B (VB11), and support the hypothesis that the temperature distribution of magnetic stars is not signficantly different from the distribution of all white dwarfs. G35-26, a likely low-field case with Zeeman-split lines, does not show significant continuum circular polarization.

Image-tube scans are presented for three white dwarfs known to be magnetic from polarization measurements and for the polarized recurrent nova DQ Her. Five separate scans of short duration were obtained of GD229, G240-72, and G195-19. No convincing evidence for spectral variability was found for these stars, even for the detailed band structure of GD229. The very broad (A > 1000 A) depression found by Creenstein in G240-72 is confirmed. However, for A> 4400 A no other features previously reported for G195-19 and G240-72 are confirmed by these data. The two scans of DQ Her revealed emission features reported many years ago; the underlying absorption spectrum at maximum light was featureless to A7200. Key words: white-dwarf stars - magnetic stars - novae - spectrophotometry

The Einstein Observatory's imaging proportional counter has detected 150-eV soft X-ray radiation from the four hot DA white dwarfs EG 187, Gr 288 and 289, and LB 1663. The observed pulse height spectra suggest that the emission is generated by hot photospheres whose T(eff) lie in the 30,000-60,000 K range. The IUE spacecraft UV spectra and H-beta line profiles for the four stars have been fitted, along with the X-ray fluxes, with a grid of hot, high gravity, homogeneous model atmospheres of mixed H-He composition. In all cases, the data require the presence of some X-ray opacity in the photosphere. Attention is given to the implications of this result in the context of white dwarf surface layer diffusion theories. Also noted are the limits imposed on the hot white dwarf population by the Einstein Medium Sensitivity Survey.

Photometry and spectrophotometry of VB3, the distant companion to HR 3018, show it to be a white dwarf of low luminosity (Mv +15.6) and not an M dwarf. The pair provides the best available example of a very old white dwarf with a main-sequence star of sufficiently early type that a useful empirical limit may eventually be derivable for the ages of the oldest degenerate stars.

Lick image tube spectrophotometry is presented of suspected degenerate M (DM) stars and of the faintest of the suspected Hyades red subluminous (RSL) star suspects. All appear to be dwarf M's or subdwarfs. It is believed that these results, taken together with recent work by Greenstein and with Naval Observatory parallaxes, leave no remaining degenerate M or strong Hyades RSL candidates.

G200-39 is an unusual degenerate star showing relatively narrow lines of both HeI and H of comparable strengths. Six separate spectroscopic observations yield no evidence for spectrum variability. Line profiles are compared with published DB model profiles to yield estimates of T/sub eff/approx. =15000 K, log g similar to other DB stars, and n/sub He//n/sub H/approx.4 x 10/sup 3/. We argue that G200-39 is not composite, that it lacks a magnetic field stronger than 5 x 10/sup 5/ Gauss, and that the atmosphere is probably not stratified with a pure hydrogen upper layer. However, the spectroscopic data do not offer sufficient resolution to rule out a rotation rate (approx.500 k/s) that might have permitted mixing due to meridional circulation. We discuss the available data on other degenerate stars showing evidence for both atmospheric helium and hydrogen; these have been found over a wide range of temperatures (6000--50000 K), though the list may be greatly influenced by observational selection.

Image-tube scans show that the low-luminosity, proper-motion star G51-15 has an M-dwarf absorption spectrum later than Wolf 359 (dM6) and spectroscopic "flare" -star characteristics. The Balmer lines and Ca ii H and K were seen in emission, and an ultraviolet continuum excess was evident. An absorption feature at 550O-556(), recently identified with CaOH, appears very strongly in this star. Key words: spectrophotometry - late-type stars - emission-line stars - flare stars

Narrow-band photometry is presented for five stars selected from a survey of faint red stars near the north galactic pole. Four of the stars are dwarfs; the fifth is a halo giant. Spectrophotometry of the giant star supports the luminosity classification and indicates there are no strong molecular band peculiarities in the spectrum; we conclude that it lies at a distance of 10 kpc, yet has only a moderate metal deficiency, [M/Hj = -0.6 I 0.2. Key words: halo stars-photometry-spectrophotometry

Lick Observatory spectrophotometry is presented for three recognized 4670-A stars (G47-18, G99-37, and G257-38) and the DC star L1363-3. New C I features are reported for G47-18; G257-38 has no C I but definite C2; weak bands are also noted in L1363-3, which is a Wolf 219 group member. The magnetic CH star G99-37 is examined in greater detail. Model-atmosphere calculations show that it must have a helium-dominated atmosphere, since stringent upper limits on H-alpha and H-beta central Zeeman components are found. The CH 'G' band profile is compared with the theoretical calculations of Angel and Landstreet (1974). Other upper limits for atomic lines are discussed.