SN?2015bh: NGC?2770’s 4th supernova or a luminous blue variable on its way to a Wolf-Rayet star?

摘要

Very massive stars in the final phases of their lives often show unpredictable outbursts that can mimic supernovae, so-called, “SN impostors”, but the distinction is not always straightforward. Here we present observations of a luminous blue variable (LBV) in NGC?2770 in outburst over more than 20 yr that experienced a possible terminal explosion as type IIn SN in 2015, named SN?2015bh. This possible SN (or “main event”) had a precursor peaking ~40 days before maximum. The total energy release of the main event is ~1.8 × 1049 erg, consistent with a <0.5?M⊙ shell plunging into a dense CSM. The emission lines show a single narrow P?Cygni profile during the LBV phase and a double P?Cygni profile post maximum suggesting an association of the second component with the possible SN. Since 1994 the star has been redder than an LBV in an S-Dor-like outburst. SN?2015bh lies within a spiral arm of NGC?2770 next to several small star-forming regions with a metallicity of ~0.5 solar and a stellar population age of 7–10 Myr. SN?2015bh shares many similarities with SN?2009ip and may form a new class of objects that exhibit outbursts a few decades prior to a “hyper eruption” or final core-collapse. If the star survives this event it is undoubtedly altered, and we suggest that these “zombie stars” may evolve from an LBV to a Wolf-Rayet star over the timescale of only a few years. The final fate of these stars can only be determined with observations a decade or more after the SN-like event.