to be able to co-add from night-to-night? In
the end, we saw a faint trace after our first
two-hour observation block, and two months
later, we had a reasonable spectrum.
This observation could not have been done
anywhere else. The combination of Gemini’s low-emissivity silver primary coating,
queue-mode scheduling (that provided two
hours per night over 14 nights), dry Maunakea weather, and a fantastic observing
staff were all necessary to obtain such a faint
spectrum. Before our Gemini observation,
there had never been an M-band spectrum
of a brown dwarf or extrasolar planet colder
than 700 K.
As theoretical work suggested, WISE 0855
should have a spectrum dominated by water vapor. When we fit the Wise 0855 data
to our initial cloud-free model, all of the wiggles in the spectrum were indeed the result
of water vapor but their signature appeared
more muted. Borrowing a well-established
October 2016
technique from our friends who study Jupiter, we inserted an optically thick water
cloud deep in the photosphere of our model
atmosphere, to see if it would produce the
muting seen in our spectrum. The cloudy
model fit significantly better than the cloudfree one. However, water clouds are notoriously difficult to model. WISE 0855 is just our
first chance to apply these models to an extrasolar object.
Measuring Up
By far the closest analog to WISE 0855 is Jupiter, which has a temperature of ~130 K. We
compared our WISE 0855 spectrum to one
of Jupiter’s and noticed striking similarities
from 4.8-5.15 microns, where water vapor
absorption features dominate both objects.
Shortward of 4.8 microns, the spectra diverge. Jupiter shows phosphine absorption,
while WISE 0855 does not.
GeminiFocus
Figure 3.
Upper Left: Water cloud
models fit better than
cloud-free models.
Upper Right: WISE 0855
looks strikingly similar
to Jupiter from 4.8-5.15
microns. Shortward of
4.8 microns, the spectra
diverge as Jupiter is
dominated by phosphine,
while WISE 0855 is
dominated by water
vapor.
Lower Left: Our WISE
0855 spectrum is
sensitive to a Jupiter
abundance of phosphine,
but none is seen.
Lower Right: Our
WISE 0855 spectrum is
marginally sensitive to
deuterated methane, but
the feature is blended
with water vapor
features that are not well
understood.
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