The diversity of massive star outbursts. I. Observations of SN2009ip, UGC2773 OT2009-1, and their progenitors

Despite both being outbursts of luminous blue variables (LBVs), SN2009ip and UGC2773 OT2009-1 have very different progenitors, spectra, circumstellar environments, and possibly physical mechanisms that generated the outbursts. From pre-eruption Hubble Space Telescope images, we determine that SN2009ip and UGC2773 OT2009-1 have initial masses of ≳ 60 and ≳ 25 M _⊙, respectively. Optical spectroscopy shows that at peak, SN2009ip had a 10,000K 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 UGC2773 OT2009-1, which also have narrow Hα 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 2000-7000kms^-1 are present in later spectra of SN2009ip - 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 UGC2773 OT2009-1 that is consistent with 2100K dust emission. We compare the properties of these two events and place them in the context of other known massive star outbursts such as η Car, NGC300 OT2008-1, and SN2008S. This qualitative analysis suggests that massive star outbursts have many physical differences that can manifest as the different observables seen in these two interesting objects.