An Improved Red Spectrum of the Methane or T-dwarf SDSS 1624+0029: Role of the Alkali Metals

A Keck~II low resolution spectrum shortward of ome-micron is presented for SDSS 1624+0029, the first field methane or T dwarf discovered in the Sloan Digital Sky Survey. Significant flux is detected down to the spectrum's short wavelength limit of 6200\AA. The spectrum exhibits a broad absorption feature centered at 7700\AA, which we interpret as the K~I 7665/7699 resonance doublet. The observed flux declines shortward of 7000\AA, due most likely to the red wing of the Na~I doublet. Both Cs~I doublet lines are detected more strongly than in an earlier red spectrum. Neither Li~I absorption nor H$\alpha$ emission are detected. An exploratory model fit to the spectrum suggests that the shape of the red spectrum can be primarily accounted for by the broad wings of the K~I and Na~I doublets. This behavior is consistent with the argument proffered by Burrows, Marley and Sharp that strong alkali absorption is principally responsible for depressing T dwarf spectra shortward of 1$\mu$m. In particular, there seems no compelling reason at this time to introduce dust or an additional opacity source in the atmosphere of the SDSS object. The width of the K~I and strengths of the Cs~I lines also indicate that the Sloan object is warmer than Gl~229B.Comment: accepted March 3, 2000 for Ap.J. Letters, LaTeX, 2 figure

Refining and relating fundamentals of functional theory

To advance the foundation of one-particle reduced density matrix functional theory (1RDMFT), we refine and relate some of its fundamental features and underlying concepts. We define by concise means the scope of a 1RDMFT, identify its possible natural variables, and explain how symmetries could be exploited. In particular, for systems with time-reversal symmetry, we explain why there exist six equivalent universal functionals, prove concise relations among them, and conclude that the important notion of v-representability is relative to the scope and choice of variable. All these fundamental concepts are then comprehensively discussed and illustrated for the Hubbard dimer and its generalization to arbitrary pair interactions W. For this, we derive by analytical means the pure and ensemble functionals with respect to both the real- and complex-valued Hilbert space. The comparison of various functionals allows us to solve the underlying v-representability problems analytically, and the dependence of its solution on the pair interaction is demonstrated. Intriguingly, the gradient of each universal functional is found to always diverge repulsively on the boundary of the domain. In that sense, this key finding emphasizes the universal character of the fermionic exchange force, recently discovered and proven in the context of translationally invariant one-band lattice models

On the Nature of the Unique Hα\alpha-Emitting T Dwarf 2MASS J12373919+6526148

We explore and discount the hypothesis that the strong, continual H$\alpha$-emitting T dwarf 2MASS J12373919+6526148 can be explained as a young, low gravity, very low mass brown dwarf. The source is already known to have a marginally-fainter absolute magnitude than similar T dwarfs with trigonometric parallax measurements, and has a tangential velocity consistent with old disk kinematics. Applying the technique of Burgasser, Burrows & Kirkpatrick on new near infrared spectroscopy for this source, estimates of its {\teff}, $\log{g}$ and metallicity ([M/H]) are obtained. 2M 1237+6526 has a {\teff} $\approx$ 800-850 K. If [M/H] is solar, $\log{g}$ is as high as $\sim$5.5 (cgs) and this source is older than 10 Gyr. We find a more plausible scenario to be a modestly subsolar metallicity ([M/H] = -0.2) and moderate $\log{g}$ $\sim$ 5.0, implying an age older than 2 Gyr and a mass greater than 0.035 M_{\sun}. The alternative explanation of the unique emission of this source, involving an interacting, close, double degenerate system, should be investigated further. Indeed, there is some evidence of a {\teff} $<$ 500 K companion to 2M 1237+6526 on the basis of a possible $Spitzer IRAC$ [3.6]--[4.5] color excess. This excess may, however, be caused by a subsolar metallicity.Comment: Astrophysical Journal, in press 15 pages, 5 figure

Refining and relating fundamentals of functional theory

To advance the foundation of one-particle reduced density matrix functional theory (1RDMFT) we refine and relate some of its fundamental features and underlying concepts. We define by concise means the scope of a 1RDMFT, identify its possible natural variables and explain how symmetries could be exploited. In particular, for systems with time-reversal symmetry, we explain why there exist six equivalent universal functionals, prove concise relations among them and conclude that the important notion of $v$-representability is relative to the scope and choice of variable. All these fundamental concepts are then comprehensively discussed and illustrated for the Hubbard dimer and its generalization to arbitrary pair interactions $W$. For this, we derive by analytical means the pure and ensemble functionals with respect to both the real- and complex-valued Hilbert space. The comparison of various functionals allows us to solve the underlying $v$-representability problems analytically and the dependence of its solution on the pair interaction is demonstrated. Intriguingly, the gradient of each universal functional is found to always diverge repulsively on the boundary of the domain. In that sense, this key finding emphasizes the universal character of the fermionic exchange force, recently discovered and proven in the context of translationally-invariant one-band lattice models

A Flaring L5 Dwarf: The Nature of H\alpha Emission in Very Low Mass (Sub)Stellar Objects

Time series spectrophotometry of the L5 dwarf 2MASS 01443536-0716142 showed strong H\alpha emission which declined by nearly 75% in four consecutive exposures. The line was not detected in emission on a spectrum obtained eleven months later. This behavior constrasts with that of 2MASSI J1315309-264951, an L5 dwarf which has shown even stronger H\alpha emission on four separate occasions. The observational database suggests that L dwarfs can be found in such strong flares only occasionally, with a duty cycle of order 1%. In contrast, the few, continuously-strong H\alpha emitters, including PC 0025+0447 and 2MASSI J1237392+652615, must either be (1) objects no older than 10-100 Myrs with continuously-active accretion and/or chromospheres, but which apparently formed in isolation from known young stellar clusters and associations, or (2) objects empowered by a different and unknown mechanism for the H\alpha energy.Comment: 11 pages, 2 figures, Astronomical Journal in press -- Jan 2003 issu

The Spectra of T Dwarfs. II. Red Optical Data

We present 6300--10100 {\AA} spectra for a sample of 13 T dwarfs observed using LRIS mounted on the Keck I 10m Telescope. A variety of features are identified and analyzed, including pressure-broadened K I and Na I doublets; narrow Cs I and Rb I lines; weak CaH, CrH, and FeH bands; strong H$_2$O absorption; and a possible weak CH$_4$ band. H$\alpha$ emission is detected in three of the T dwarfs, strong in the previously reported active T dwarf 2MASS 1237+6526 and weak in SDSS 1254$-$0122 and 2MASS 1047+2124. None of the T dwarfs exhibit Li I absorption. Guided by the evolution of optical spectral features with near-infrared spectral type, we derive a parallel optical classification scheme, focusing on spectral types T5 to T8, anchored to select spectral standards. We find general agreement between optical and near-infrared types for nearly all of the T dwarfs so far observed, including two earlier-type T dwarfs, within our classification uncertainties ($\sim$1 subtype). These results suggest that competing gravity and temperature effects compensate for each other over the 0.6--2.5 \micron spectral region. We identify one possible means of disentangling these effects by comparing the strength of the K I red wing to the 9250 {\AA} H$_2$O band. One of our objects, 2MASS 0937+2931, exhibits a peculiar spectrum, with a substantial red slope and relatively strong FeH absorption, both consequences of a metal-deficient atmosphere. Based on its near-infrared properties and substantial space motion, this object may be a thick disk or halo brown dwarf.Comment: 22 pages including 9 figures, accepted to ApJ v594 Sept. 200

Probing Dark Matter

Recent novel observations have probed the baryonic fraction of the galactic dark matter that has eluded astronomers for decades. Late in 1993, the MACHO and EROS collaborations announced in this journal the detection of transient and achromatic brightenings of a handful of stars in the Large Magellanic Cloud that are best interpreted as gravitational microlensing by low-mass foreground objects (MACHOS). This tantalized astronomers, for it implied that the population of cool, compact objects these lenses represent could be the elusive dark matter of our galactic halo. A year later in 1994, Sackett et al. reported the discovery of a red halo in the galaxy NGC 5907 that seems to follow the inferred radial distribution of its dark matter. This suggested that dwarf stars could constitute its missing component. Since NGC 5907 is similar to the Milky Way in type and radius, some surmised that the solution of the galactic dark matter problem was an abundance of ordinary low-mass stars. Now Bahcall et al., using the Wide-Field Camera of the recently repaired Hubble Space Telescope, have dashed this hope.Comment: 3 pages, Plain TeX, no figures, published as a News and Views in Nature 373, 191 (1995

SDSS J042348.57-041403.5AB: A Brown Dwarf Binary Straddling the L/T Transition

We present the discovery of SDSS J042348.57-041403.5 as a closely-separated (0"16) brown dwarf binary, resolved by the Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer. Physical association is deduced from the angular proximity of the components and constraints on their common proper motion. SDSS0423-0414AB appears to be composed of two brown dwarfs with spectral types L6+/-1 and T2+/-1. Hence, this system straddles the transition between L dwarfs and T dwarfs, a unique evolutionary phase of brown dwarfs characterized by substantial shifts in spectral morphology over an apparently narrow effective temperature range. Binarity explains a number of unusual properties of SDSS 0423-0414, including its overluminosity and high effective temperature compared to other early-type T dwarfs, and possibly its conflicting spectral classifications (L7.5 in the optical, T0 in the near infrared). The relatively short estimated orbital period of this system (~15-20 yr) and the presence of Li I absorption in its combined light spectrum make it an ideal target for both resolved spectroscopy and dynamical mass measurements. SDSS 0423-0414AB joins a growing list of late-L/early-T dwarf binaries, the high percentage of which (~50%) may provide a natural explanation for observed peculiarities across the L/T transition.Comment: 6 pages, 2 figures, accepted for publication in Astrophysical Journal Letter

Substellar Companions to Main Sequence Stars: No Brown Dwarf Desert at Wide Separations

We use three field L and T dwarfs which were discovered to be wide companions to known stars by the Two Micron All-Sky Survey (2MASS) to derive a preliminary brown dwarf companion frequency. Observed L and T dwarfs indicate that brown dwarfs are not unusually rare as wide (Delta >1000 A.U.) systems to F-M0 main-sequence stars (M>0.5M_sun, M_V<9.5), even though they are rare at close separation (Delta <3 A.U.), the ``brown dwarf desert.'' Stellar companions in these separation ranges are equally frequent, but brown dwarfs are >~ 10 times as frequent for wide than close separations. A brown dwarf wide-companion frequency as low as the 0.5% seen in the brown dwarf desert is ruled out by currently-available observations.Comment: ApJL, in pres