Padova-Asiago Supernova Group
Giant outburst and eruptions of very massive stars
Luminous blue variables (LBVs) are very massive star that occasionally experience luminous eruptions, becoming the most luminous objects in the galaxies. The most famous example in the Milky Way is Eta Car, which experienced a Giant Eruption in the mid-19th Century. That eruption produced the complex circumstellar environment knows as the Homunculus (Fig. 1).

Fig. 1 HST image showing the Homunculus Nebula surrounding the Eta Carinae. The str ucture was produced in the 19th Century Giant Eruption of this Galactic LBV. Credits: N. Smith, and NASA.

Occasionally, LBVs and massive stars may experience single-episode outbursts, which reach luminosity comparable to that of giant eruption (e.g. Tartaglia et al. 2016, ApJ, 823, L23), but with a rapid evolutionary time scale (weeks to a few months). In other cases, LBVs may experience multiple outbursts during an eruptive phase which lasts years to decades, and whose individual light curve peaks reach absolute magnitude -14 (Pastorello et al. 2010, MNRAS, 408, 181; Pastorello et al. 2013, ApJ, 767,1). Although the massive progenitor survives these eruptive events, these ILOTs mimic, in terms of energetics and spectral appearance, true SN explosions. In particular, their spectra resemble those of Type IIn SNe (Fig. 2). For this reason, extragalactic LBV-like outbursts are frequently dubbed SN impostors (Van Dyk et al. 2000, PASP, 112, 1532).

Fig. 2 Spectral evolution of the single-outburst ILOT PSN J09132750+7627410; and comparisons with similar transients. Figure from Tartaglia et al. 2016, ApJ, 823, L2.

One of the most exciting astrophysical discoveries in recent years is that a growing number of SN impostors are likely followed by genuine SN explosions a few months or years later, usually producing Type IIn SNe (Tartaglia et al. 2016, MNRAS, 459, 1039; Elias-Rosa et al. 2016, MNRAS, 463, 3894; Pastorello et al. 2017, MNRAS, submitted (arXiv:1707.00611); see Fig. 3).

Fig. 3 Pre- and post-explosion light curves of a sample of Type IIn SNe. Figure from Pastorello et al. 2017.

Although SN impostors seem to preferentially herald Type IIn SNe, in one case a stellar outburst was observed before the explosion of a SN Ibn. This single outburst occurred in October 2004 in the galaxy UGC 4904, and was followed after two years by the explosion of the Type Ibn SN 2006jc (Pastorello et al. 2007, Nature, 447, 829; Fig. 4). The 2004 event was interpreted as an outburst of a Wolf-Rayet star, which exploded as a luminous core-collapse SN.

Fig. 4 UGC 4904 in 2001 (a), 2004 (b), 2005 (c) and 2006 (d). A stellar outburst of a Wolf-Rayet star is detected in 2004 (panel b), two years before the explosion of SN 2006jc (panel d). Image from Pastorello et al. 2007.

Leading researchers: N. Elias-Rosa, P. Ochner, A. Pastorello, L. Tartaglia

Collaborators: M. Fraser, R. Kotak, E. Kankare, J. Maund, S. Mattila, F. Bufano, V. Goranskij, E. Barsukova, G. Pignata, and the Padova SN group.