  We present one of the most precise emission spectra of an exoplanet observed
so far. We combine five secondary eclipses of the hot Jupiter WASP-18 b
(Tday=2900K) that we secured between 1.1 and 1.7 micron with the WFC3
instrument aboard the Hubble Space Telescope. Our extracted spectrum (S/N=50,
R=40) does not exhibit clearly identifiable molecular features but is poorly
matched by a blackbody spectrum. We complement this data with previously
published Spitzer/IRAC observations of this target and interpret the combined
spectrum by computing a grid of self-consistent, 1D forward models, varying the
composition and energy budget. At these high temperatures, we find there are
important contributions to the overall opacity from H- ions, as well as the
removal of major molecules by thermal dissociation (including water), and
thermal ionization of metals. These effects were omitted in previous spectral
retrievals for very hot gas giants, and we argue that they must be included to
properly interpret the spectra of these objects. We infer a new metallicity and
C/O ratio for WASP-18 b, and find them well constrained to be solar
([M/H]=-0.01 (0.35), C/O<0.85 at 3 sigma confidence level), unlike previous
work but in line with expectations for giant planets. The best fitting
self-consistent temperature-pressure profiles are inverted, resulting in an
emission feature at 4.5 micron seen in the Spitzer photometry. These results
further strengthen the evidence that the family of very hot gas giant
exoplanets commonly exhibit thermal inversions.
