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We present new M and L-dwarfs confirmed through follow-up of 2MASS color-selected objects with the CorMASS near-infrared spectrograph (R ˜ 300) on the Palomar 60-inch telescope as part of a continuing follow-up survey. Most of the objects are bright (Ks < 13).
We construct Ks-band light curves for nine field L and T brown dwarfs using the Palomar 60 inch Telescope. Results of a robust statistical analysis indicate that about half the targets show significant evidence for variability. Two of these variable targets have marginally significant peaks in the Lomb-Scargle periodogram. The phased light curves show evidence for periodic behavior on timescales of about 1.5 and 3.0 hours.
A sophisticated approach to condensate opacity is required to properly model the atmospheres of L and T dwarfs. Here we review different models for the treatment of condensates in brown dwarf atmospheres. We conclude that models which include both particle sedimentation and upwards transport of condensate (both gas and particles) provide the best fit for the L dwarf colors. While a globally uniform cloud model fits the L dwarf data, it turns to the blue in J - K too slowly to fit the T dwarfs. Models which include local clearings in the global cloud deck, similar to Jupiter's prominent five-micron hot spots, better reproduce the available photometric data and also account for the observed resurgence of FeH absorption in early type T dwarfs.
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 describe in this work the BASS survey for brown dwarfs in young moving groups of the solar neighborhood, and summarize the results that it generated. These include the discovery of the 2MASS J01033563–5515561 (AB)b and 2MASS J02192210–3925225 B young companions near the deuterium-burning limit as well as 44 new low-mass stars and 69 new brown dwarfs with a spectroscopically confirmed low gravity. Among those, ~20 have estimated masses within the planetary regime, one is a new L4 γ bona fide member of AB Doradus, three are TW Hydrae candidates with later spectral types (L1–L4) than all of its previously known members and six are among the first contenders for low-gravity ≥ L5 β/γ brown dwarfs, reminiscent of WISEP J004701.06+680352.1, PSO J318.5338–22.8603 and VHS J125601.92–125723.9 b. Finally, we describe a future version of this survey, BASS-Ultracool, that will specifically target ≥ L5 candidate members of young moving groups. First experimentations in designing the survey have already led to the discovery of a new T dwarf bona fide member of AB Doradus, as well as the serendipitous discoveries of an L9 subdwarf and an L5 + T5 brown dwarf binary.
In the “Weather on Other Worlds” Spitzer Exploration Science program, we surveyed 44 nearby L3–T8 dwarfs for spot-induced rotational variability. Among single L3–L9.5 dwarfs, we found that 80% are variable at >0.2% in the 3–5 μm wavelength range, while 36% of T0–T8 were variable at >0.4%. Taking into account viewing angle and sensitivity considerations, both of these findings are consistent with spots being present on ~100% of L3–T8 dwarfs. Intriguingly, we find a tentative association (92% confidence) between low surface gravity and high-amplitude variability among L3–L5.5 dwarfs. Although we can not confirm whether lower gravity is also correlated with a higher incidence of variables, the result is promising for the characterization of directly imaged young extrasolar planets through variability.
We report on our search for L dwarf flares using NASA's Kepler mission. Spectroscopically confirmedflares were detected with the original Kepler mission from an L1 dwarf stars. We discuss the physicalcharacteristics of these white light flares and compare them to M dwarf flares. For “habitable zone” planets, the apparent flare brightnesses would be comparable to the most powerful M dwarf flares. Weare monitoring more L dwarfs with the Kepler K2 mission. We discussthe prospect for more detections during the remainder of the K2 mission.
Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.
This report gives an update on developments since the last General Assembly in Rio de Janeiro. Classification – both photometric and spectral – continues to play a vital role in stellar astrophysics and stellar surveys. During the past three years, rapid progress has been made in the classification of brown dwarfs, with the discovery of the first Y dwarfs and the introduction of a near-IR classification system for M- and L-dwarfs. The number of known L-dwarfs now exceeds 1000, and so peculiar types are beginning to show up. For instance, there is now enough material to define a low-gravity spectral sequence for the L0 – L5 dwarfs. In addition, a number of unusally blue L-dwarfs are now known. Large-area surveys, always of interest to Commission 45, have proliferated during this period, including RAVE, SEGUE, and WISE with many more in the planning stages.
Indirect and direct spectroscopic studies of exoplanets are beginning to probe the most prominent chemical constituents and processes in their atmospheres. However, studies of equivalently low-temperature brown dwarfs have been taking place for over a decade. In this review, I summarize some of the results of detailed spectroscopic, photometric and polarimetric studies of brown dwarfs of various effective temperatures, surface gravities and metallicities, highlighting the insight gained into the chemistry and cloud formation of planetary-like atmospheres. Nonequilibrium chemistry and variations in cloud properties are singled out as critical ingredients for interpreting exoplanet spectra. I also discuss recent direct spectroscopic studies of exoplanet atmospheres, both close to and widely-separated from their host star, and propose that the latter are better analogs to isolated brown dwarfs.
Brown dwarfs are natural clocks, cooling and dimming over time due to insufficient core fusion. They are also numerous and present in nearly all Galactic environments, making them potentially useful chronometers for a variety of Galactic studies. For this potential to be realized, however, precise and accurate ages for individual sources are required, a prospect made difficult by the complex atmospheres and spectra of low-temperature brown dwarfs; degeneracy between mass, age and luminosity; and the lack of useful age trends in magnetic activity and rotation. In this contribution, I review five ways in which ages for brown dwarfs are uniquely determined, discuss their applicability and limitations, and give current empirical precisions.
We present a study of the dynamics and magnetic activity of M dwarfs using the largest spectroscopic sample of low-mass stars ever assembled. The age at which strong surface magnetic activity (as traced by Hα) ceases in M dwarfs has been inferred to have a strong dependence on mass (spectral type, surface temperature) and explains previous results showing a large increase in the fraction of active stars at later spectral types. Using spectral observations of more than 40000 M dwarfs from the Sloan Digital Sky Survey, we show that the fraction of active stars decreases as a function of vertical distance from the Galactic plane (a statistical proxy for age), and that the magnitude of this decrease changes significantly for different M spectral types. Adopting a simple dynamical model for thin disk vertical heating, we assign an age for the activity decline at each spectral type, and thus determine the activity lifetimes for M dwarfs. In addition, we derive a statistical age-activity relation for each spectral type using the dynamical model, the vertical distance from the Plane and the Hα emission line luminosity of each star (the latter of which also decreases with vertical height above the Galactic plane).
We report on the progress of the Brown Dwarf Kinematics Project (BDKP), which aims to measure the 6D positions and velocities of all known brown dwarfs within 20 pc of the Sun and select sources of scientific interest. In this paper we report on the status of the 33 targets on our parallax list as well as the results of our proper motion survey where we have measured over 400 new proper motions for known late M, L and T dwarfs.
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