Ultraviolet Mg II emission from fast neutral ejecta around Eta Carinae
  We present the first images of the nebula around eta Carinae obtained with
HST/WFC3, including a UV image in the F280N filter that traces MgII emission,
plus contemporaneous imaging in the F336W, F658N, and F126N filters that trace
near-UV continuum, [NII], and [FeII], respectively. The F336W and F658N images
are consistent with previous images in these filters, and F126N shows that for
the most part, [FeII] 12567 traces clumpy shocked gas seen in [NII]. The F280N
image, however, reveals MgII emission from structures that have not been seen
in any previous line or continuum images of eta Carinae. This image shows
diffuse MgII emission immediately outside the bipolar Homunculus nebula in all
directions, but with the strongest emission concentrated over the poles. The
diffuse structure with prominent radial streaks, plus an anticorrelation with
ionized tracers of clumpy shocked gas, leads us to suggest that this is
primarily MgII resonant scattering from unshocked, neutral atomic gas. We
discuss the implied structure and geometry of the MgII emission, and its
relation to the Homunculus lobes and various other complex nebular structures.
An order of magnitude estimate of the neutral gas mass traced by MgII is
0.02Msun, with a corresponding kinetic energy around 1e47erg. This may provide
important constraints on polar mass loss in the early phases of the Great
Eruption. We argue that the MgII line may be an excellent tracer of significant
reservoirs of freely expanding, unshocked, and otherwise invisible neutral
atomic gas in a variety of stellar outflows.
