*3.2. Giant Eruptions*

From a non-specialist's point of view, the observed giant eruptions have too much diversity. Some of them are supernova impostors (i.e., the star survives), while others may be genuine supernovae modified by surrounding material (Section 5.4 below). Both cases are sometimes classified as "Type IIn SNae," which implies narrower emission lines than normal SNae. Major radiation-driven eruptions have several traits:


An excellent example is SN2011ht [41,42], whose brightness and timescale resembled a supernova (Figure 2). It may have been either a supernova impostor or else a true SN within a dense envelope of prior ejecta (Section 5.4 below); but in either case the observed display was a radiation-driven outflow. Its spectrum (Figure 3) had characteristics explained in Section 4 below, with outward speeds of several hundred km s<sup>−</sup><sup>1</sup> and no hint of a blast wave before the brightness declined. Broad emission line wings were caused by Thomson scattering rather than bulk motion (Section 4.2), and the kinetic energy of visible ejecta was much smaller than in a normal SN [42].

About two months after maximum, the visual-wavelength brightness abruptly decreased by a factor of 60 (Figure 2). Since normal dust formation does not account for this change [42], the simplest interpretation is that most of the trapped radiation escaped through the photosphere just before that time. A normal core-collapse supernova would have remained substantially brighter due to radioactive decays in the ejecta. Some authors *assumed* that 2011ht was a supernova in a discussion of the light curve [43]; but the lack of a radioactive afterglow was decidedly peculiar in that case, and the light curve was reasonable for a non-SN instability (Section 5 below). Spectroscopy gives far more definite information than the shape of a light curve, and strongly implied an opaque continuum-driven outflow far above the Eddington Limit [42,44,45].

**Figure 2.** Luminosity record of SN2011ht based on visual-wavelength brightness [42]. The vertical scale, expressed in solar units, neglects variations in the bolometric correction but is adequate for conceptual purposes. The relative faintness after *t* ∼ 130 d is highly abnormal if this object was a true supernova.

**Figure 3.** Spectrum of SN2011ht at three different times [42], cf. Figure 2. Both scales are logarithmic, the three tracings have differing vertical offsets, and the marks near 6100 Å indicate *fλ* = 10−<sup>14</sup> erg cm −2 s<sup>−</sup><sup>1</sup> Å−1. Gaps at *λ* > 7000 Å are obscured by terrestrial atmospheric features. Concerning the line profiles, see Section 4.2 and Figure 6.

Historically, the first observed giant eruption was P Cygni about 400 years ago [14,35,46]. Its maximum luminosity was of the order of 106.5 *L*, quite small by giant eruption standards; but, unlike normal LBV events, that amount significantly exceeded the quiescent brightness. P Cyg's outburst (actually two or more episodes) persisted for years so the radiative energy output was probably more

than 10<sup>48</sup> ergs. Today this star is located in the LBV instability strip in the HR Diagram, but it has not exhibited an LBV event. Possibly this is a hint that the interval between episodes is correlated with the strength of the most recent instance, analogous to some forms of relaxation oscillators.

Among the eruptors discovered since 2000, some had luminosities comparable to *η* Car and/or P Cyg, but behaved very differently. SN2000ch, for instance, has exhibited multiple outbursts considerably brighter than the familiar type of LBV outburst, but not as bright as *η* Car's Great Eruption [47–49]. Those events were hotter than an LBV outburst, with higher outflow speeds, much shorter durations, and the luminosity may have increased appreciably on each occasion. Altogether its behavior differs from LBV's, *η* Car, and the brighter giant eruptions noted below. P Cyg might have appeared similar in the years 1600–1650, but this is merely a speculation.

A more extreme object with a different kind of multiplicity was described in [50]. PSN J09132750+7627410, a SN impostor in NGC 2748, attained a luminosity of the order of 107.3 *L*, comparable to *η* Car's maximum, for several months—even though its quiescent luminosity was probably less than 105.5 *L*. Near maximum its spectrum resembled SN2011ht described above. Its chief peculiarity was the existence of several distinct outflow velocities in each absorption feature: −400, −1100, and −1600 km s<sup>−</sup>1. These may signify either a series of mass-loss episodes, or structure in the observed episode, or separate ejecta from more than one star. The two larger speeds are much faster than an LBV outflow. Multiple velocities have been seen in a few other eruptive stars—e.g., see [11,51].

Two pre-2000 giant eruptions, SN1954J [52] and SN1961V [2,46], have had enough time to show whether their stars survived. The SN1954J event had a maximum luminosity of the order of 10<sup>7</sup> *L* with a duration less than a year [8,9,53]. The surviving star, a.k.a. V12 in NGC 2403, has a likely mass around 20 *M* and is seriously obscured by circumstellar dust. Its spectrum includes Thomson-scattered emission line profiles, indicating a present-day opaque outflow (Section 4.2 below). This fact is strong evidence that the observed object really is the survivor of a giant eruption. The star was probably in a post-RSG state when the event occurred [53]. SN1961V, on the other hand, remains doubtful. It achieved a peak luminosity well above 10<sup>8</sup> *L* with an overall event duration longer than a normal supernova, but no survivor has been identified with high confidence [3,4].

An unexpected development since 2000 has been the occurrence of precursor eruptions—i.e., giant eruptions that were followed several years later by real supernova events. At first sight this seems unlikely, because the final stages of core evolution have timescales of days, hours, and minutes rather than years. A few years is a likely timescale in the outer layers (Section 5 below), but in the standard view those regions "don't know" the precise state of the core. Hence the precursor events most likely arise in or near the core; but that assessment is too glib to be entirely satisfying, as noted in Section 5 below. The most notorious example of this phenomenon was SN 2009ip, whose blast wave explosion was not observed until 2012 [11]. That object exhibited other events between 2009 and 2012. Evidently some part of the star became unstable a few years before the SN event, but then the observed timescale didn't accelerate with the core evolution. Or perhaps the 2012 shock wave did not represent the real terminal event [10,54]! See comments in Section 5.3 below.

SN 1994W, SN 2009kn, and SN 2011ht probably ejected material months or years before their terminal explosions [42,44,55]. Since those objects became strangely faint at the stage when 56Ni decay normally produces luminosity after a core-collapse SN event, some authors suspect that core collapse did not occur—e.g., [44].

As outlined above, giant eruptions are usually easy to distinguish from LBV events. They have far greater mass loss rates, substantial increases in luminosity, and shorter durations in most cases. A few LBV's, however, have mistakenly been given SN designations. SN 2002kg, for example, is a luminous LBV also known as V37 in NGC 2403 [8,53,56].
