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Register a new userCloud Properties of Brown Dwarf Binaries Across the L/T Transition
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We present a new suite of atmosphere models with flexible cloud parameters to investigate the effects of clouds on brown dwarfs across the L/T transition. We fit these models to a sample of 13 objects with well-known masses, distances, and spectral types spanning L3-T5. Our modelling is guided by spatially-resolved photometry from the Hubble Space Telescope and the W. M. Keck Telescopes covering visible to near-infrared wavelengths. We find that, with appropriate cloud parameters, the data can be fit well by atmospheric models with temperature and surface gravity in agreement with the predictions of evolutionary models. We see a clear trend in the cloud parameters with spectral type, with earlier-type objects exhibiting higher-altitude clouds with smaller grains (0.25-0.50 micron) and later-type objects being better fit with deeper clouds and larger grains ($geq$1 micron). Our results confirm previous work that suggests L dwarfs are dominated by submicron particles, whereas T dwarfs have larger particle sizes.
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Current atmospheric models cannot reproduce some of the characteristics of the transition between the L dwarfs with cloudy atmospheres and the T dwarfs with dust-depleted photospheres. It has been proposed that a majority of the L/T transition brown dwarfs could actually be a combinaison of a cloudy L dwarf and a clear T dwarf. Indeed binarity seems to occur more frequently among L/T transition brown dwarfs. We aim to refine the statistical significance of the seemingly higher frequency of binaries. Co-eval binaries would also be interesting test-beds for evolutionary models. We obtained high-resolution imaging for six mid-L to late-T dwarfs, with photometric distances between 8 and 33pc, using the adaptive optics systems NACO at the VLT, and the Lick system, both with the laser guide star. We resolve none of our targets. Combining our data with published results, we obtain a frequency of resolved L/T transition brown dwarfs of (31+21-15)%, compared to (21+10-7)% and (14+14-7)% for mid-L and T dwarfs (90% of confidence level). These fractions do not significantly support, nor contradict, the hypothesis of a larger binary fraction in the L/T transition. None of our targets has companions with effective temperatures as low as 360-1000K at separations larger than 0.5.
Time-resolved observations of brown dwarfs rotational modulations provide powerful insights into the properties of condensate clouds in ultra-cool atmospheres. Multi-wavelength light curves reveal cloud vertical structures, condensate particle sizes, and cloud morphology, which directly constrain condensate cloud and atmospheric circulation models. We report results from Hubble Space Telescope/Wide Field Camera 3 near-infrared G141 taken in six consecutive orbits observations of HN Peg B, an L/T transition brown dwarf companion to a G0V type star. The best-fit sine wave to the $1.1-1.7mu$m broadband light curve has the amplitude of $1.206pm0.025%$ and period of $15.4pm0.5$ hr. The modulation amplitude has no detectable wavelength dependence except in the 1.4 $mu$m water absorption band, indicating that the characteristic condensate particle sizes are large ($>1mu$m). We detect significantly ($4.4sigma$) lower modulation amplitude in the 1.4$mu$m water absorption band, and find that HN Peg Bs spectral modulation resembles those of early T type brown dwarfs. We also describe a new empirical interpolation method to remove spectral contamination from the bright host star. This method may be applied in other high-contrast time-resolved observations with WFC3.
We report the identification of 17 candidate brown dwarf binaries whose components straddle the L dwarf/T dwarf transition. These sources were culled from a large near-infrared spectral sample of L and T dwarfs observed with the Infrared Telescope Facility SpeX spectrograph. Candidates were selected on the basis of spectral ratios which segregate known (resolved) L dwarf/T dwarf pairs from presumably single sources. Composite templates, constructed by combining 13581 pairs of absolute flux-calibrated spectra, are shown to provide statistically superior fits to the spectra of our seventeen candidates as compared to single templates. Ten of these candidates appear to have secondary components that are significantly brighter than their primaries over the 1.0-1.3 micron band, indicative of rapid condensate depletion at the L dwarf/T dwarf transition. Our results support prior indications of enhanced multiplicity amongst early-type T dwarfs; 53+/-7% of the T0-T4 dwarfs in our spectral sample are found to be either resolved or unresolved (candidate) pairs, although this is consistent with an intrinsic (volume complete) brown dwarf binary fraction of only 15%. If verified, this sample of spectral binaries more than doubles the number of known L dwarf/T dwarf transition pairs, enabling a broader exploration of this poorly-understood phase of brown dwarf atmospheric evolution.
Most directly imaged giant exoplanets are fainter than brown dwarfs with similar spectra. To explain their relative underluminosity unusually cloudy atmospheres have been proposed. However, with multiple parameters varying between any two objects, it remained difficult to observationally test this idea. We present a new method, sensitive time-resolved Hubble Space Telescope near-infrared spectroscopy, to study two rotating L/T transition brown dwarfs (2M2139 and SIMP0136). The observations provide spatially and spectrally resolved mapping of the cloud decks of the brown dwarfs. The data allow the study of cloud structure variations while other parameters are unchanged. We find that both brown dwarfs display variations of identical nature: J- and H-band brightness variations with minimal color and spectral changes. Our light curve models show that even the simplest surface brightness distributions require at least three elliptical spots. We show that for each source the spectral changes can be reproduced with a linear combination of only two different spectra, i.e. the entire surface is covered by two distinct types of regions. Modeling the color changes and spectral variations together reveal patchy cloud covers consisting of a spatially heterogenous mix of low-brightness, low-temperature thick clouds and brighter, thin and warm clouds. We show that the same thick cloud patches seen in our varying brown dwarf targets, if extended to the entire photosphere, predict near-infrared colors/magnitudes matching the range occupied by the directly imaged exoplanets that are cooler and less luminous than brown dwarfs with similar spectral types. This supports the models in which thick clouds are responsible for the near infrared properties of these underluminous exoplanets.
We present the first results from our high-precision infrared (IR) astrometry program at the Canada-France-Hawaii Telescope. We measure parallaxes for 83 ultracool dwarfs (spectral types M6--T9) in 49 systems, with a median uncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide the first parallaxes for 48 objects in 29 systems, and for another 27 objects in 17 systems, we significantly improve upon published results, with a median (best) improvement of 1.7x (5x). Three systems show astrometric perturbations indicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and 2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). In addition, we present here a large set of Keck adaptive optics imaging that more than triples the number of binaries with L6--T5 components that have both multi-band photometry and distances. Our data enable an unprecedented look at the photometric properties of brown dwarfs as they cool through the L/T transition. Going from approxL8 to approxT4.5, flux in the Y and J bands increases by approx0.7 mag and approx0.5 mag, respectively (the Y- and J-band bumps), while flux in the H, K, and L bands declines monotonically. This wavelength dependence is consistent with cloud clearing over a narrow range of temperature, since condensate opacity is expected to dominate at 1.0--1.3 micron. Interestingly, despite more than doubling the near-IR census of L/T transition objects, we find a conspicuous paucity of objects on the color--magnitude diagram just blueward of the late-L/early-T sequence. This L/T gap occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, and implies that the last phases of cloud evolution occur rapidly. Finally, we provide a comprehensive update to the absolute magnitudes of ultracool dwarfs as a function of spectral type using a combined sample of 314 objects.
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