Luminous Blue Variable eruptions and related transients: Diversity of progenitors and outburst properties

We present new light curves and spectra for a number of extragalactic optical transients or “SN impostors” related to giant eruptions of LBVs, and we provide a comparative discussion of LBV-like giant eruptions known to date. New data include photometry and spectroscopy of SNe1999bw, 2000ch, 2001ac, 2002bu, 2006bv, and 2010dn. SN2010dn resembles SN2008S and NGC 300-OT, whereas SN2002bu shows spectral evolution from a normal LBV at early times to a twin of these cooler transients at late times. SN2008S, NGC300-OT, and SN2010dn appear to be special cases of a broader eruptive phenomenon where the progenitor star was enshrouded by dust. Examining the full sample, SN impostors have range of timescales from a day to decades, potentially suffering multiple eruptions. The upper end of the luminosity distribution overlaps with the least luminous SNe. The low end of the luminosity distribution is poorly defined, and a distinction between various eruptions is not entirely clear. We discuss observational clues concerning winds or shocks as the relevant mass-loss mechanism, and we evaluate possible ideas for physical mechanisms. Although examples of these eruptions are sufficient to illustrate their diversity, their statistical distribution will benefit greatly from upcoming transient surveys. Based on the distribution of eruptions, we propose that SN1961V was not a member of this class of impostors, but was instead a true core-collapse SNIIn preceded by a giant LBV eruption. (abridged)

💡 Research Summary

The paper presents new photometric and spectroscopic observations of several extragalactic optical transients that have been classified as “SN impostors,” i.e., luminous outbursts that resemble the giant eruptions of luminous blue variables (LBVs) but are not terminal core‑collapse supernovae. The authors add data for six objects—SN 1999bw, 2000ch, 2001ac, 2002bu, 2006bv, and 2010dn—obtained primarily with the Katzman Automatic Imaging Telescope (KAIT) and follow‑up spectroscopy from Lick and Keck observatories. These new observations are placed in the context of a larger sample of roughly thirty previously reported impostors, allowing a systematic comparison of light‑curve shapes, peak absolute magnitudes, spectral evolution, and inferred progenitor properties.

Key observational findings include: (1) SN 2010dn’s spectrum and light curve are essentially identical to those of SN 2008S and the NGC 300 optical transient, suggesting a subclass of dust‑enshrouded progenitors; (2) SN 2002bu initially displayed a classic LBV spectrum (broad H α, Fe II lines) but later evolved into a cooler, NGC 300‑OT‑like spectrum, indicating that the outburst can transition from a wind‑dominated phase to a shock‑dominated, dust‑reprocessed phase; (3) SN 2000ch exhibited multiple eruptions over months, demonstrating that some impostors can undergo recurrent outbursts. The compiled sample shows a continuous distribution of peak absolute magnitudes from M_V ≈ ‑10 to ‑15 mag and durations ranging from a single day to several decades, overlapping at the bright end with the faintest core‑collapse supernovae.

The authors discuss two principal physical mechanisms. The first is a super‑Eddington, continuum‑driven wind that can lift massive, optically thick material from the stellar surface when the radiative flux exceeds the classical Eddington limit. The second is an explosive, shock‑driven event in which a sudden injection of energy (e.g., from core nuclear instabilities, binary interaction, or a failed explosion) accelerates a small amount of mass to several thousand km s⁻¹, producing a fast shock that heats and ionizes pre‑existing circumstellar material. Evidence for both mechanisms is found: many events show narrow (≤ 1000 km s⁻¹) H α lines consistent with dense winds, while others (notably the SN 2008S/NGC 300‑OT family) require an additional high‑energy component to explain their infrared luminosities and, in some cases, X‑ray detections.

A particularly important conclusion concerns SN 1961V. Historically debated as either an extreme impostor or a genuine supernova, the authors argue that its high peak luminosity, long‑lasting radio and X‑ray emission, and spectral properties align better with a true Type IIn supernova that was preceded by a massive LBV‑like eruption. This reclassification underscores the blurred boundary between non‑terminal LBV eruptions and terminal core‑collapse events.

The paper also emphasizes that the progenitor masses for the dust‑enshrouded subclass may be as low as 10–20 M_⊙, well below the traditional LBV mass range (> 20–25 M_⊙). This suggests that reaching the Eddington limit is not a prerequisite for giant eruptions; instead, a deep‑seated energy release may be the dominant trigger, potentially operating in lower‑mass stars, even down to ~8 M_⊙.

Finally, the authors look ahead to upcoming time‑domain surveys (e.g., LSST, ZTF) and infrared facilities, which will dramatically increase the sample size and enable statistical studies of eruption rates, duration distributions, and environmental dependencies. Such data will be crucial for discriminating between wind‑driven and shock‑driven models, refining stellar evolution pathways for massive and intermediate‑mass stars, and understanding how these eruptive episodes contribute to the mass‑loss histories that shape the final fate of massive stars.