Based on a simple thermodynamical argument, we proposed a cloudy model with a warm dust cloud deep in the photosphere. We showed, for the first time, that a single grid of model photospheres (800 ≲ Teff ≳ 2600 K) offers a natural explanation not only for the division of dwarfs cooler than M into the two distinct types L and T but also for the changes of the spectra and colors along the L – T spectral sequence.

Procedure and results of computation of optical and infrared spectra of ultracool (1000 < Teff < 3000 K) objects are discussed. LTE synthetic spectra and spectral energy distributions in wide spectral region λλ 0.6–10 microns) are computed for model atmospheres from grids of different authors taking into account opacities provided by water and other molecules.

Essential interest will be paid for problems of realization of lithium and deuterium tests which can be used for assessment of objects of sub-stellar masses and big planets and refining of scenarios of evolution of young stars and substellar objects.

The growing numbers of brown dwarfs have largely been divided into two new spectral classes, L and T. Subclassification systems for L, based on optical (0.6–1.0 μm) spectra, have existed since 1999, but there is a need for infrared systems for both spectral classes. The differences in the infrared (1.0–2.5 μm) characteristics of L spectra and T spectra observed prior to 2000 are huge, all then-known T dwarfs showing strong methane absorption bands, and all L dwarfs devoid of these bands. However, in the last 2–3 years a significant number of brown dwarfs with infrared spectra in transition between the L and first observed T types have been discovered. The spectra of late L dwarfs and these “transition objects” can be put into a well ordered sequence of increasing H2O and CH4 band strengths and decreasing CO band strengths. Specific infrared spectral indicators have been found that allow the L and T sequences to be linked and the boundary between L and T to be precisely defined. In current T classification schemes the transition objects are defined as early T dwarfs. We recommend that the boundary between the L and T classes be defined as the first appearance of methane absorption just longward of 1.60 μm, in the H band.

The discovery of many cool brown dwarfs similar to Gliese 229B has led to the definition of a new spectral class, the T dwarfs, whose 1–2.5 μm spectra exhibit signatures of CH4 absorption. Two classification schemes have been proposed for these objects by Burgasser et al. and Geballe et al. We discuss and compare these schemes, and describe a joint classification scheme currently in development that closely follows the guidelines of the MK process. We also address future work toward establishing gravity classes, classifying at other wavelengths, and identifying those features that may signify the end of the T spectral class.

This paper describes the status of a new 0.9 – 2.4 μm spectroscopic survey of low-mass stars and brown dwarfs. Flux-calibrated spectra of 30 objects from M6 to T8 have been obtained with a uniform spectral resolution of R=2,000. For the J-band alone, 60 objects have been observed. In addition, we have obtained the first sequence of spectra at R=20,000 (15 km/s) showing the detailed behavior of the J-band potassium doublets over the same range in spectral type. A few sources have been monitored for radial velocity variations, but none have been detected at the ±1–2 km/s level. Preliminary infrared spectral classification indices have been derived and several young brown dwarfs show clear spectral signatures of lower gravity. Our survey will be available on-line in the future.

We present the initial results of a medium resolution, 0.8 to 4.2 μm spectroscopic survey of M, L, and T dwarfs. We have identified the most prominent molecular and atomic absorption features found in the spectra of these late-type dwarfs. We have also compared the spectra to a laboratory FeH emission spectrum and identified nearly 100 features common to the FeH spectrum and the dwarf spectra from 0.99 to 1.8 μm.

In this contribution, I review a subset of our recent efforts in spectral and opacity modeling aimed at guiding the study of brown dwarfs, L dwarfs, and T dwarfs. I discuss theoretical calculations of the alkali line profiles, newly generated CrH opacities, new evidence for refractory rainout in T dwarfs from optical spectral measurements, and the controversy concerning the distinction between brown dwarfs and planets.

In this work we report recent spectral analyses of L dwarfs and our success in measuring Teff and log(g). Using dust filled atmospheres for early L dwarfs and rained out atmospheres for late L dwarfs we could derive Teff of 1400 to 2000 K for L8 to M9.5 dwarfs respectively. We also give an outlook what we can achieve with future models that are improving the fits to intermediate L dwarfs and IR spectra.

We present first results from the first parallax program to use an infrared camera to target the observation of a sample of methane T-dwarfs, using the ESO 3.5m NTT.

Trigonometric parallaxes have been measured by Dahn et al. (2002) for 28 cool dwarfs and brown dwarfs, including 17 L dwarfs and three T dwarfs. Broadband CCD and near-IR photometry (VRIz*JHK) have been obtained for these objects and for 24 additional late-type dwarfs. These data have been supplemented with astrometry and photometry from the literature, including parallaxes for the brighter companions of ten L and two T dwarfs. The absolute magnitudes and colors are reviewed here. The I - J color and the spectral type are both good predictors of absolute magnitude for late-M and L dwarfs. MJ becomes monotonically fainter with I - J color and with spectral type through late-L dwarfs, then brightens for early-T dwarfs. In contrast, the J - K color correlates poorly with absolute magnitude for L dwarfs. Using several other parameters from the literature (Li detection, Hα emission strength, projected rotation velocity, and tangential velocity), we fail to uncover any measurable parameter that correlates with the anomalous J - K color.

We present new accurate masses at the bottom of the main sequence as well as an improved empirical mass-luminosity relation for very low mass stars in the visible and near infrared. Masses were obtained by combining very accurate radial velocities and adaptive optics images of multiple stars obtained at different orbital phases.

We obtained near–infrared spectra of a sample of very low mass objects as a function of age in order to investigate the temperature and surface gravity sensitivity of several features in the J– and K–bands.

We have updated our PHOENIX model atmospheres and theoretical spectra for ultracool dwarfs with new opacity data for methane based on line strength predictions with the STDS software. By extending the line list to rotational levels of J = 40 we can significantly improve the shape of the near-IR absorption features of CH4, and in addition find an enhanced blanketing effect, resulting in up to 50% more flux emerging in the J band than seen in previous models, which may thus contribute to the brightening in J and blue IR colors observed in T dwarfs.

The first infrared photometry of an L-type brown dwarf, DENIS J0255-4700, includes N-band and narrow-band 8.8-μm detections, with upper limits in narrow-band 10.3 and 11.7 μm. Model-independent blackbody fits of existing data yield Teff = 1250 – 1750 K, with models favoring the lower end of that range. Dusty atmospheric models by Allard, Burrows and Marley which match the near-infrared photometry are not completely consistent with our mid-infrared photometry.

We have obtained near-infrared spectra of three L dwarfs discovered by 2MASS, 1146+22 (L3), 1507-16 (L5), and 0920+35 (L6.5). From the comparison of the H and K band spectra of these L dwarfs, we have found the presence of methane absorption in 0920+35. This implies that detectable methane absorption in the H and K bands, usually considered the signature of a T dwarf, can be present in objects classified optically as late L. Methane detection in L dwarfs is consistent with the presence of a dust layer deep in the atmosphere as the unified model of Tsuji suggests. (This work has been published in Nakajima, Tsuji & Yanagisawa (2001).)

One of the triumphs of the last 2 decades has been the establishment of the relation between stellar rotation and magnetic activity in solar-type stars. Rapid rotation produces strong activity, which in turn provides magnetic braking to reduce rotation. A solar-type dynamo cannot operate in fully convective stars, so it is of interest to study mid and late M stars. Hints that a dramatic change occurs in very low-mass stars and substellar objects appeared in 1995. The past 7 years have seen substantial progress on this question, with the conclusion that the rotation-activity connection indeed breaks down. As one goes to the bottom of the main sequence and below, the amount of magnetic activity takes a sudden fall, with a concomitant increase in the spindown times of the objects. We summarize these results, and some theoretical work which helps explain them. We also present some remaining mysteries, such as why very young objects seem excessively active, and flaring in objects with no other signs of magnetic activity.

The kinematics of late M and certainly L dwarfs indicate that they have a mean age of no more than a few Gyr. This result does not rule out a mix of young brown dwarfs and ZAMS stars. Relative Hα emission strengths begin to decline in the late M dwarfs, even though about 100% of the M7-8 dwarfs show the weaker emission. Both the frequency and likely strength of this emission declines with spectral type among the L's and T's. A larger sample of late M and L dwarfs, which may provide more definitive tests of the kinematic and activity properties, is being assembled from our NStars project.

Two unusual objects, one an M9.5e and the other a T6e, show persistent Hα emission at least an order of magnitude stronger than their counterparts of similar spectral type. If these are hypothesized to be very young stellar objects (≲10 Myr) like some observed in Rho Oph, Sigma Ori and Taurus – evolutionary models would require them to have very low masses: In particular, for an entity to fade to a late T dwarf temperature and luminosity within 10 Myr, its mass should be no more than several Jupiters. This seems implausible for an isolated object not near any obvious site of recent star formation. However, our attempt to show that the emission might be due to accretion in a binary system was unsuccessful.

We report on two recent XMM-Newton observations of Brown Dwarfs in the Pleiades cluster and in the field aiming to constrain the age dependence of X-ray emission from substellar objects.

I review recent observations of brown dwarfs by the Chandra X-ray Observatory. These observations fall in 2 categories, young stellar clusters which contain brown dwarfs and brown dwarf candidates and directed pointings at brown dwarfs and very low mass stars. Surprisingly, there are already over 60 published detections of brown dwarfs by Chandra. A review of the X–ray characteristics shows these objects are subject to flaring and their temperatures and luminosities have a vast range which is related to age.

We report on a J and H photometric variability survey of 10 T dwarfs. Four brown dwarfs in the sample show significant J band variability at the 99% confidence level or higher. The amplitude of variation ranges from 2 to 17%. Our data show evidence of variability for late as well as early T dwarfs.