The Persistent Eruption of UGC 2773-OT: finally, a decade-long extragalactic Eta Carinae analogue

Abstract

While supernova (SN) impostors resemble the Great Eruption of η Carinae in the sense that their spectra show narrow H lines and they have typical peak absolute magnitudes of −13 to −14 mag, most extragalactic events observed so far are quite different from η Car in duration. Their bright phases typically last for ∼100 d or less, rather than persisting for several years. The transient object UGC 2773-OT (discovered in 2009) had a similar peak absolute magnitude to other SN impostors, but with a gradual 5-yr prediscovery rise. In the ∼6 yr since discovery, it has faded very slowly (0.26 mag yr −1 ). Overall, we suggest that its decade-long eruption is so far the best known analog of η Car's 19th century eruption. We discuss extensive spectroscopy of the ongoing eruption. The spectra show interesting changes in velocity and line shape that we discuss in detail, including an asymmetric Hα emission line that we show is consistent with the ejection of a bipolar nebula that could be very much like the Homunculus of η Car. Moreover, changes in the line width, line profile, blue excess emission resembling that of Type IIn supernovae, and the intensity of Hα suggest the presence of strong circumstellar interaction in the eruption at late times. This supports the hypothesis that the extended plateau of η Car's eruption may have been powered by shock interaction as well. One interesting difference compared to η Car, however, is that UGC 2773-OT so far does not exhibit the repeated brief spikes in luminosity that have been associated with binary periastron events.

Light echoes from the plateau in Eta Carinae’s Great Eruption reveal a two-stage shock-powered event

“…Third, similar narrow He i emission was seen in spectra of UGC 2773-OT in its later phases dominated by CSM interaction (Smith et al 2016a). The He i emission was much narrower (about 100 km s −1 ) than Hα and other lines (600-1000 km s −1 ), very much like the case here.…”

“…Except for the Gemini spectrum (red), all the echo spectra of η Car (black) were obtained with IMACS. Also for comparison, Figure 8 shows the early and late phase spectra of the SN impostor UGC 2773-OT (blue) from Smith et al (2016a), which is thought to be a close analog of η Car.…”

“…Evolution of the low-res spectra of EC2. Included for comparison are the first epoch spectra of the EC1 echo discussed by Rest et al (2012), which samples an early peak in 1843 or 1838, and both early and late-time spectra of UGC 2773-OT (in blue), from Smith et al (2016a). All spectra of η Car's echoes have been dereddened by E(B − V )=1.0 mag; this amount of reddening is probably a minimum, but the true line-of-sight reddening is complicated by the fact that we are also observing reflected light.…”

Common Envelope Ejection for a Luminous Red Nova in M101

“…Members of the LBVs and ILOT classes also show outflow velocities well below 1000 kms −1 . The double-peaked Hα emission profile, tracing the bipolar structure of the ejecta, has also been observed in the asymmetric outflows of LBVs (Smith et al 2016a) and nebular phases of SNe IIn with bipolar circumstellar medium (CSM) (Smith et al 2015;Andrews et al 2016) or CCSNe, such as SN 1987A (Gröningsson et al 2008). Newly formed dust within the ejecta is responsible for the extinction of optical and NIR light.…”

“…As a consequence, the progenitor stars are generally living in a dusty environment, caused by previous episodes of mass ejections. The nonterminal eruptions of SN 2009ip (Fraser et al 2013;Mauerhan et al 2013;Pastorello et al 2013) and UGC 2773 OT2009-1 (Foley et al 2011;Smith et al 2016a) are examples of LBVs in their cool eruptive phase.…”