Title: Hubble PanCET: An isothermal day-side atmosphere for the bloated gas-giant HAT-P-32Ab,
Abstract: We present a thermal emission spectrum of the bloated hot Jupiter HAT-P-32Ab
from a single eclipse observation made in spatial scan mode with the Wide Field
Camera 3 (WFC3) aboard the Hubble Space Telescope (HST). The spectrum covers
the wavelength regime from 1.123 to 1.644 microns which is binned into 14
eclipse depths measured to an averaged precision of 104 parts-per million. The
spectrum is unaffected by a dilution from the close M-dwarf companion
HAT-P-32B, which was fully resolved. We complemented our spectrum with
literature results and performed a comparative forward and retrieval analysis
with the 1D radiative-convective ATMO model. Assuming solar abundance of the
planet atmosphere, we find that the measured spectrum can best be explained by
the spectrum of a blackbody isothermal atmosphere with Tp = 1995 +/- 17K, but
can equally-well be described by a spectrum with modest thermal inversion. The
retrieved spectrum suggests emission from VO at the WFC3 wavelengths and no
evidence of the 1.4 micron water feature. The emission models with temperature
profiles decreasing with height are rejected at a high confidence. An
isothermal or inverted spectrum can imply a clear atmosphere with an absorber,
a dusty cloud deck or a combination of both. We find that the planet can have
continuum of values for the albedo and recirculation, ranging from high albedo
and poor recirculation to low albedo and efficient recirculation. Optical
spectroscopy of the planet's day-side or thermal emission phase curves can
potentially resolve the current albedo with recirculation degeneracy.