The Diversity of Massive Star Outbursts I: Observations of SN 2009ip,
  UGC 2773 OT2009-1, and Their Progenitors
  Despite both being outbursts of luminous blue variables (LBVs), SN 2009ip and
UGC 2773 OT2009-1 have very different progenitors, spectra, circumstellar
environments, and possibly physical mechanisms that generated the outbursts.
  From pre-eruption HST images, we determine that SN 2009ip and UGC 2773
OT2009-1 have initial masses of >60 and >25 M_sun, respectively. Optical
spectroscopy shows that at peak SN 2009ip had a 10,000 K photosphere and its
spectrum was dominated by narrow H Balmer emission, similar to classical LBV
giant outbursts, also known as "supernova impostors." The spectra of UGC 2773
OT2009-1, which also have narrow H alpha emission, are dominated by a forest of
absorption lines, similar to an F-type supergiant. Blueshifted absorption lines
corresponding to ejecta at a velocity of 2000 - 7000 km/s are present in later
spectra of SN 2009ip -- an unprecedented observation for LBV outbursts,
indicating that the event was the result of a supersonic explosion, rather than
a subsonic outburst. The velocity of the absorption lines increases between two
epochs, suggesting that there were two explosions in rapid succession. A rapid
fading and rebrightening event concurrent with the onset of the high-velocity
absorption lines is consistent with the double-explosion model. A near-infrared
excess is present in the spectra and photometry of UGC 2773 OT2009-1 that is
consistent with ~2100 K dust emission. We compare the properties of these two
events and place them in the context of other known massive star outbursts such
as eta Car, NGC 300 OT2008-1, and SN 2008S. This qualitative analysis suggests
that massive star outbursts have many physical differences which can manifest
as the different observables seen in these two interesting objects.
