  We present optical and near-IR spectroscopic observations of the luminous
blue variable SN 2009ip during its remarkable photometric evolution of 2012.
The spectra sample three key points in the SN 2009ip lightcurve, corresponding
to its initial brightening in August (2012-A) and its dramatic rebrightening in
early October (2012-B). Based on line fluxes and velocities measured in our
spectra, we find a surprisingly low I(H-alpha)/I(H-beta) ~ 1.3-1.4 in the
2012-B spectra. Such a ratio implies either a rare Case B recombination
scenario where H-alpha, but not H-beta, is optically thick, or an extremely
high density for the circumstellar material of n_e > 10^13 cm^(-3) The H-alpha
line intensity yields a minimum radiating surface area of >~20,000 AU^2 in
H-alpha at the peak of SN 2009ip's photometric evolution. Combined with the
nature of this object's spectral evolution in 2012, a high circumstellar
density and large radiating surface area imply the presence of a thin disk
geometry around the central star (and, consequently, a possible binary
companion), suggesting that the observed 2012-B rebrightening of SN 2009ip can
be attributed to the illumination of the disk's inner rim by fast-moving ejecta
produced by the underlying events of 2012-A.

Solar and Stellar Astrophysics