Beyond the T Dwarfs: Theoretical Spectra, Colors, and Detectability of the Coolest Brown Dwarfs

By Adam Burrows, D. Sudarsky, Jonathan I. Lunine. 2003

265 citations recorded by the source corpus.

We explore the spectral and atmospheric properties of brown dwarfs cooler than the latest known T dwarfs. Our focus is on the yet-to-be-discovered free-floating brown dwarfs in the Teff range from ~800 to ~130 K and with masses from 25 to 1 MJ. This study is in anticipation of the new characterization capabilities enabled by the launch of the Space Infrared Telescope Facility (SIRTF) and the eventual launch of the James Webb Space Telescope (JWST). In addition, it is in support of the continuing ground-based searches for the coolest substellar objects. We provide spectra from ~0.4 to 30 μm, highlight the evolution and mass dependence of the dominant H2O, CH4, and NH3 molecular bands, consider the formation and effects of water ice clouds, and compare our theoretical flux densities with the putative sensitivities of the instruments on board SIRTF and JWST. The latter can be used to determine the detection ranges from space of cool brown dwarfs. In the process, we determine the reversal point of the blueward trend in the near-infrared colors with decreasing Teff (a prominent feature of the hotter T dwarf family), the Teff's at which water and ammonia clouds appear, the strengths of gas-phase ammonia and methane bands, the masses and ages of the objects for which the neutral alkali metal lines (signatures of L and T dwarfs) are muted, and the increasing role as Teff decreases of the mid-infrared fluxes longward of 4 μm. These changes suggest physical reasons to expect the emergence of at least one new stellar class beyond the T dwarfs. Furthermore, studies in the mid-infrared could assume a new, perhaps transformational, importance in the understanding of the coolest brown dwarfs. Our spectral models populate, with cooler brown dwarfs having progressively more planet-like features, the theoretical gap between the known T dwarfs and the known giant planets. Such objects likely inhabit the Galaxy, but their numbers are as yet unknown.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/2240aa18054fe364941f7df2a93b136742b02a25

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Citations in the corpus

Ammonia as a Tracer of Chemical Equilibrium in the T7.5 Dwarf Gliese 570D (2006, background): “Its presence in very cool brown dwarf atmospheres has been expected for some time (Tsuji 1964;Marley et al. 1996; Fegley& Lodders 1996; Saumon et al. 2000, 2003; Allard et al. 2001; Lodders & Fegley 2002; Burrows et al. 2003).”
A STUDY OF THE DIVERSE T DWARF POPULATION REVEALED BY WISE (2013, background): “In W3 and W4, past the blackbody peak for the coolest T dwarf, the flux decreases and is further reduced by NH3 absorption (Saumon et al. 2003; Burrows et al. 2003).”
L and T Dwarf Models and the L to T Transition (2005, methodology): “Our always-evolving opacity/spectroscopy database would require a paper in itself to explain, but is partially described (with references) in Burrows et al. (1997,2001,2002a,2003).”
FURTHER DEFINING SPECTRAL TYPE “Y” AND EXPLORING THE LOW-MASS END OF THE FIELD BROWN DWARF MASS FUNCTION (2012, methodology): “…found that the assumed space density and measured color distri- bution of the 14 brown dwarf candidates most closely matched a Chabrier (2003) log-normal mass function, or a power-law mass function with α = 1.3, as long as the 4.5-µm flux of the Burrows et al. (2003) models was adjusted downward15.”
MID-INFRARED PHOTOMETRY OF COLD BROWN DWARFS: DIVERSITY IN AGE, MASS, AND METALLICITY (2010, background): “Also, as Teff decreases brown dwarfs emit significant flux at mid-infrared wavelengths (e.g. Burrows et al. 2003, Leggett et al. 2009) and it is expected that the recently launched Wide-Field Infrared Survey Explorer (WISE, Liu et al. 2008) will detect a significant number of cold brown dwarfs in…”
THE SPITZER DEEP, WIDE-FIELD SURVEY (2009, background): “This is primarily because their SEDs peak at ≈ 4.5 μm (e.g., Burrows et al. 2003), making them very faint at ground-based optical through near-infrared wavelengths.”
The UKIRT Infrared Deep Sky Survey ZY JHK photometric system: passbands and synthetic colours (2006, background): “The models however all suggest that the coolest brown dwarfs continue getting bluer in the nearinfrared with decreasing Teff for Teff > 400 K. Burrows et al. (2003) describe the reddening at cooler temperatures to be due to the appearance of water clouds and, more importantly, the collapse of flux…”
On the Luminosity of Young Jupiters (2006, unknown): “The assumption of cloud-free atmospheres is valid here, because, as we show below, effective temperatures for these planets cluster around 500–800 K at young ages, while water cloud condensation should not begin until Teff < 500 K (Burrows et al. 2003).”
A T8.5 BROWN DWARF MEMBER OF THE ξ URSAE MAJORIS SYSTEM (2012, background): “For an isolated brown dwarf, determination of mass and age are intertwined, such that a broad locus of mass and age will be consistent with a single measured effective temperature (Burrows et al. 2003).”
A variationally computed line list for hot NH3 (2010, background): “Modelling suggests that absorption by ammonia will be even more important in the yet-to-be discovered Y-dwarfs (Burrows et al. 2003) and it is possible that NH3 absorption bands may be the principal distinguishing feature of this new class of ultra-cool dwarf.”
FOMALHAUT'S DEBRIS DISK AND PLANET: CONSTRAINING THE MASS OF FOMALHAUT B FROM DISK MORPHOLOGY (2008, background): “As for uncertainties in model exoplanet atmospheres (Fortney et al. 2008; Burrows et al. 2003), these appear greater in H band than in F814W; the various models disagree with each other at near-infrared wavelengths by factors of a few.”
Synthetic Spectra and Colors of Young Giant Planet Atmospheres: Effects of Initial Conditions and Atmospheric Metallicity (2008, background): “Here we also limit our calculations toTeff > 400 K, since we also ignore opacity due to water clouds, which condense aroundTeff=400-500 K for these objects (Marley et al. 2002; Burrows et al. 2003).”
THE GEMINI/NICI PLANET-FINDING CAMPAIGN: THE FREQUENCY OF PLANETS AROUND YOUNG MOVING GROUP STARS (2013, background): “…to these detections, models predicted that young gas giant planets at moving group ages (10–300 Myr) would likely have cool photospheres with prominent methane absorption features (Baraffe et al. 2003; Burrows et al. 2003), i.e., that these objects would be spectral analogs to T-type brown dwarfs.”
L′ and M′ Photometry of Ultracool Dwarfs (2004, methodology): “Burrows et al. (1997, 2001, 2003) created 1–30 µm spectra of brown dwarfs and ex- trasolar giant planets (EGPs) of various masses and ages using model atmospheres that assume settled condensate clouds and thermochemical equilibrium.”
The UKIRT Infrared Deep Sky Survey (UKIDSS) (2006, background): “(Calculations based on Burrows et al. (2003) indicate that halo brown dwarfs will have an absolute Y magnitude >16 and so could be found to distances of 60pc.) (iii) We hope to find quasars at z = 7 and to detect the epoch of re-ionisation.”
THE DEUTERIUM-BURNING MASS LIMIT FOR BROWN DWARFS AND GIANT PLANETS (2010, methodology): “…brown dwarfs were improved by the application of nongray radiative transfer theory (Burrows et al. 1997, 1998; Burrows & Sharp 1999; Burrows 1999; Burrows et al. 2001, 2003; Baraffe et al. 2002, 2003; Allard et al. 2003; Chabrier et al. 2000b,a; Chabrier & Baraffe 2000; Sharp & Burrows 2007;…”