We present new optical coronagraphic data of the bright star Fomalhaut
obtained with the HST in 2010/2012 using STIS. Fomalhaut b is recovered at both
epochs to high significance. The observations include the discoveries of
tenuous nebulosity beyond the main dust belt detected to at least 209 AU
projected radius and a ~50 AU wide azimuthal gap in the belt northward of Fom
b. The morphology of Fomalhaut b appears elliptical in the STIS detections. We
show that residual noise in the processed data can plausibly result in point
sources appearing extended. A MCMC analysis demonstrates that the orbit of Fom
b is highly eccentric, with e=0.8+/-0.1, a=177+/-68 AU, and q = 32+/-24 AU. Fom
b is apsidally aligned with the belt and 90% of allowed orbits have mutual
inclination 36 deg or less. Fomalhaut b's orbit is belt-crossing in projection,
but only 12% of possible orbits have nodes within a 25 AU wide belt annulus
(133-158 AU). The high e invokes a dynamical history where Fom b may have
experienced a significant dynamical interaction with a hypothetical planet
Fomalhaut c, and the current orbital configuration may be relatively
short-lived. The new value for the periastron distance diminishes the Hill
radius of Fom b and any weakly bound satellite system surrounding a planet
would be sheared and dynamically heated at periapse. We argue that Fom b's
minimum mass is that of a dwarf planet in order for a circumplanetary satellite
system to remain bound to a sufficient radius from the planet to be consistent
with the dust scattered light hypothesis. Fom b may be optically bright because
the recent passage through periapse and/or the ascending node has increased the
erosion rates of planetary satellites. In the coplanar case, Fomalhaut b will
collide with the main belt around 2032, and the subsequent emergent phenomena
may help determine its physical nature.Comment: 49 Pages, 33 Figures, 5 Tables; Submitted to ApJ, Dec. 31, 201
