The coldest brown dwarf, WISE 0855, is the closest known planetary-mass, free-floating object and has a
temperature nearly as cold as the solar system gas giants. Like Jupiter, it is predicted to have an atmosphere rich
in methane, water, and ammonia, with clouds of volatile ices. WISE 0855 is faint at near-infrared wavelengths
and emits almost all its energy in the mid-infrared. Skemer et al. (2016) presented a spectrum of WISE 0855
from 4.5–5.1 µm (M band), revealing water vapor features. Here, we present a spectrum of WISE 0855 in
L band, from 3.4–4.14 µm. We present a set of atmosphere models that include a range of compositions
(metallicities and C/O ratios) and water ice clouds. Methane absorption is clearly present in the spectrum.
The mid-infrared color can be better matched with a methane abundance that is depleted relative to solar
abundance. We find that there is evidence for water ice clouds in the M band spectrum, and we find a lack of
phosphine spectral features in both the L and M band spectra. We suggest that a deep continuum opacity source
may be obscuring the near-infrared flux, possibly a deep phosphorous-bearing cloud, ammonium dihyrogen
phosphate. Observations of WISE 0855 provide critical constraints for cold planetary atmospheres, bridging
the temperature range between the long-studied solar system planets and accessible exoplanets. JWST will soon
revolutionize our understanding of cold brown dwarfs with high-precision spectroscopy across the infrared,
allowing us to study their compositions and cloud properties, and to infer their atmospheric dynamics and
formation processes.
Keywords: planets and satellites: atmospheres, planets and satellites: gaseous planets, stars: brown dwarfs,
stars: atmospheres 

posted Apr 24, 2018, 6:14 PM by Shifu RC

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