SN 2009ip-like Events: Phenomenology and Models

• SN 2009ip-like events are interacting transients defined by a two-stage (Event A and B) light curve, erratic pre-outburst variability, and dense circumstellar material.
• Observational studies reveal rapid spectral evolution and brightness changes that support both core-collapse and mergerburst scenarios with distinct energy and mass-loss profiles.
• High CSM density diagnostics and diverse progenitor environments imply overlapping progenitor demographics and complexities in the explosion mechanisms.

SN 2009ip-like events are a small, empirically defined class of hydrogen-rich interacting transients centered on the prototype SN 2009ip. They are characterized by erratic pre-outburst variability, a fainter first luminous episode conventionally termed “Event A,” a brighter second episode termed “Event B,” Balmer-dominated spectra shaped by dense circumstellar material (CSM), and a persistent dispute over whether the luminous phase marks terminal core collapse or an extreme non-terminal eruption. As the sample has expanded to include objects such as SN 2015bh, SN 2016bdu, SN 2016jbu, SN 2021foa, SN 2023ldh, SN 2024hpj, and SN 2016cvk, the subclass has become observationally more coherent even while its physical interpretation has remained contested (Pastorello et al., 2017, Pastorello et al., 29 Mar 2025, Salmaso et al., 20 Aug 2025).

1. Historical emergence of the class

The prototype SN 2009ip was already exceptional before 2012 because archival HST imaging linked the transient to a very luminous progenitor candidate, with $m_{\mathrm{F606W}}=21.84\pm0.25$ mag and $M_V \approx -10.3$ mag, and because the source had shown years of strong variability before its major eruptive episodes. Optical spectroscopy during the 2009 event showed a $\sim 10{,}000$ K continuum and narrow Balmer emission, while later spectra developed blueshifted absorption at $2000$–$7000\ {\rm km\ s^{-1}}$, which was argued to indicate “a supersonic explosion, rather than a subsonic outburst” (Foley et al., 2010).

The 2012 activity of SN 2009ip established the now-canonical Event A/Event B morphology. During the initial 2012 phase the source was near $M_V \approx -14.5$ mag; by September 2012 it showed Balmer emission with broad P-Cygni profiles, broad emission FWHM $\sim 8000\ {\rm km\ s^{-1}}$, and absorption extending to $-13{,}000\ {\rm km\ s^{-1}}$. After a brief fading, it rapidly rebrightened to $M_R=-17.5$ mag in about two days and then leveled near $-18$ mag, while the spectrum transformed from one briefly exposing fast ejecta to one resembling an optically thick Type IIn interaction phase (Mauerhan et al., 2012).

A major step toward class formation came when SN 2009ip was compared with SN 2010mc. Their light curves were described as nearly identical, including a precursor-like event roughly $M_V \approx -10.3$0 days before the rapid rise to the main maximum, and their spectra were argued to match as well. In that framework, SN 2009ip-like transients were interpreted as core-collapse Type IIn supernovae from compact blue supergiants, with the fainter first peak corresponding to the actual explosion and the brighter second peak to delayed ejecta-CSM interaction (Smith et al., 2013).

2. Defining observational phenomenology

The basic observational template is a two-stage light curve. In SN 2016bdu, Event A reached $M_V \approx -10.3$1 mag and Event B, more than one month later, reached $M_V \approx -10.3$2 mag; the paper treated this $M_V \approx -10.3$3 mag contrast as characteristic of the family. More recent additions preserve the same architecture with some diversity in duration and luminosity. SN 2023ldh showed a slow-rising Event A peaking at $M_V \approx -10.3$4 mag, a dip of about 1 mag lasting about two weeks, and Event B peaking at $M_V \approx -10.3$5 mag, while SN 2024hpj showed Event A at $M_V \approx -10.3$6 mag and Event B at $M_V \approx -10.3$7 mag after a $M_V \approx -10.3$8 day rise (Pastorello et al., 2017, Pastorello et al., 29 Mar 2025, Salmaso et al., 20 Aug 2025).

The spectroscopic sequence is similarly recurrent. Event A spectra often show multicomponent P-Cygni profiles of H I and Fe II, sometimes already containing material at several $M_V \approx -10.3$9. During the rise to and around Event B maximum, the continuum becomes blue and hot, and the spectrum is dominated by narrow or relatively narrow Balmer emission with Lorentzian wings, indicating dense interaction. Later phases develop broader or intermediate-width H$\sim 10{,}000$0, metal P-Cygni features, and increasingly prominent Ca II. In SN 2023ldh, for example, Event A spectra showed multicomponent P-Cygni profiles of H I and Fe II, the Event B rise was dominated by Balmer Lorentzians with FWHM $\sim 10{,}000$1, and late phases displayed strong Ca II and a boxy intermediate-width H$\sim 10{,}000$2 profile (Pastorello et al., 29 Mar 2025).

A transitional edge of the class is illustrated by SN 2021foa. Its $\sim 10{,}000$3-band light curve was described as very similar to those of SN 2009ip-like events, with a faint Event A followed by a much brighter Event B, but its spectra showed unusually strong He I, with He I $\sim 10{,}000$4 reaching roughly half of the H$\sim 10{,}000$5 luminosity at $\sim 10{,}000$6 d. This led to the proposal that SN 2021foa is transitional between H-rich SN 2009ip-like events and He-rich Type Ibn supernovae, indicating that the subclass may overlap with a broader continuum of interacting transients (Reguitti et al., 2022).

3. Circumstellar interaction, density diagnostics, and geometry

CSM interaction is central to virtually every interpretation of the class, and several of the most distinctive diagnostics constrain the CSM rather than the inner engine directly. In SN 2009ip, the timing of photometric bumps during 2012-B was interpreted as interaction between fast ejecta and an inhomogeneous CSM produced by earlier eruptions. Graham et al. expressed the expected interaction time as

$\sim 10{,}000$7

and used $\sim 10{,}000$8 and $\sim 10{,}000$9 to relate 2012 light-curve structure to prior mass-loss episodes (Graham et al., 2014).

Line-ratio diagnostics imply extreme densities in at least some cases. During the 2012-B rebrightening of SN 2009ip, the Balmer decrement

$2000$0

fell to $2000$1–1.4. That ratio was argued to imply either a rare case in which H$2000$2 is optically thick while H$2000$3 remains optically thin, or an electron density $2000$4. The same study estimated a minimum H$2000$5-radiating area of about $2000$6 and favored a thin disk or ring, rather than a spherical shell, because the Balmer emission remained centered near systemic velocity and remarkably symmetric (Levesque et al., 2012).

Other analyses likewise point to strong asymmetry. Smith et al. argued that the persistence of broad H$2000$7 at $2000$8 after the brightest interaction phase is difficult to reconcile with spherical symmetry, because most fast ejecta would have been decelerated if the CSM fully covered them. They used a disk with opening angle $2000$9 as an illustrative geometry, in which less than $7000\ {\rm km\ s^{-1}}$0 of the solid angle is intercepted, allowing a large fraction of ejecta to expand relatively freely. In the same core-collapse framework, the changing H$7000\ {\rm km\ s^{-1}}$1 equivalent width—eventually exceeding $7000\ {\rm km\ s^{-1}}$2 Å in SN 2009ip and $7000\ {\rm km\ s^{-1}}$3 Å in SN 2010mc—was treated as typical of SNe IIn rather than LBV winds (Smith et al., 2013).

Simple interaction scalings were also used to estimate the required CSM density. For SN 2009ip, Mauerhan et al. employed

$7000\ {\rm km\ s^{-1}}$4

and, with $7000\ {\rm km\ s^{-1}}$5 and $7000\ {\rm km\ s^{-1}}$6, obtained a wind-density parameter of $7000\ {\rm km\ s^{-1}}$7. Adopting $7000\ {\rm km\ s^{-1}}$8, they inferred a pre-SN outburst mass-loss “rate” of order $7000\ {\rm km\ s^{-1}}$9, while also stressing that the CSM was probably shell-like and asymmetric rather than a stationary wind (Mauerhan et al., 2012).

4. Competing physical interpretations

One major interpretation treats SN 2009ip-like events as genuine terminal explosions whose observable peculiarities are imposed by compact progenitors and dense recent mass loss. In that view, the faint first peak is compatible with explosion from a blue supergiant of radius $M_V \approx -14.5$0, while the second brighter peak is powered by interaction when SN ejecta overtake CSM expelled in precursor episodes. Smith et al. argued that the persistence of broad lines for months requires several solar masses of opaque, high-velocity ejecta, with $M_V \approx -14.5$1–$M_V \approx -14.5$2 as a lower bound and kinetic energy at least $M_V \approx -14.5$3 erg, plausibly $M_V \approx -14.5$4 erg, even if the radiated energy is only $M_V \approx -14.5$5 erg (Smith et al., 2013).

A competing interpretation explains SN 2009ip as a mergerburst. In that model, the 2012b outburst was powered by the merger of two massive stars, with earlier outbursts associated with periastron passages of an eccentric binary. The paper estimated a total radiated energy $M_V \approx -14.5$6 erg and total event energy $M_V \approx -14.5$7 erg, predicted $M_V \approx -14.5$8–$M_V \approx -14.5$9, and required $\sim 8000\ {\rm km\ s^{-1}}$0 erg, with a surviving hot giant-like remnant expected once the ejecta became optically thin (Soker et al., 2012).

More radical proposals invoke jets as the dominant power source. In SN 2019zrk, Soker argued that jets plausibly powered the precursor, the very energetic main event, and the late bump, and estimated that the main explosion required $\sim 8000\ {\rm km\ s^{-1}}$1 erg, too large for ordinary neutrino-driven CCSN models. That paper left open whether the central engine was a CCSN or a CEJSN-like system, but used SN 2019zrk to argue that at least some SN 2009ip-like events belong to a broader jet-powered continuum of stellar transients (Soker, 2022).

Other studies preserve the ambiguity rather than resolve it. The analysis of SN 2015bh treated it as almost a “carbon copy” of SN 2009ip and argued that the family may define a distinct phenomenological class of “zombie stars,” either terminal core-collapse explosions or non-terminal hyper-eruptions. Its main-event radiated energy, $\sim 8000\ {\rm km\ s^{-1}}$2 erg, was modeled with a $\sim 8000\ {\rm km\ s^{-1}}$3 shell plunging into dense CSM, and the authors explicitly stated that only observations years later could determine whether the star had survived (Thöne et al., 2016).

5. Progenitors, environments, and demographic inferences

Direct and indirect progenitor constraints span a wide range. For SN 2009ip, pre-eruption HST imaging implied an initial mass of at least $\sim 8000\ {\rm km\ s^{-1}}$4, and other summaries quote a progenitor luminosity $\sim 8000\ {\rm km\ s^{-1}}$5 and initial mass $\sim 8000\ {\rm km\ s^{-1}}$6–$\sim 8000\ {\rm km\ s^{-1}}$7. SN 2011fh was placed in a similarly massive regime: HST photometry of the surrounding cluster implied an age of $\sim 8000\ {\rm km\ s^{-1}}$8 Myr and a progenitor mass of $\sim 8000\ {\rm km\ s^{-1}}$9 in single-star models, or $-13{,}000\ {\rm km\ s^{-1}}$0–$-13{,}000\ {\rm km\ s^{-1}}$1 for a binary; the paper also argued that the progenitor exceeded the classical Eddington limit by a large factor before the 2011 luminous outburst, favoring strong super-Eddington winds (Foley et al., 2010, Pessi et al., 2021).

At the same time, environments often complicate a simple single-star LBV endpoint picture. SN 2015bh lay in a spiral arm of NGC 2770 near small star-forming regions with metallicity $-13{,}000\ {\rm km\ s^{-1}}$2 solar and a stellar population age of $-13{,}000\ {\rm km\ s^{-1}}$3–$-13{,}000\ {\rm km\ s^{-1}}$4 Myr, while SN 2024hpj was presented as residing in a star-forming dwarf-galaxy environment and used, together with rate statistics, to argue for progenitor masses around $-13{,}000\ {\rm km\ s^{-1}}$5–$-13{,}000\ {\rm km\ s^{-1}}$6 and lower. That rate analysis estimated SN 2009ip-like events to be roughly $-13{,}000\ {\rm km\ s^{-1}}$7 of CCSNe within 50 Mpc, implying a much broader and potentially lower-mass progenitor demographic than the most luminous direct progenitor detections alone would suggest (Thöne et al., 2016, Salmaso et al., 20 Aug 2025).

SN 2009ip itself remains the most striking environmental outlier. Late-time HST imaging showed that the transient occurred in the remote outskirts of NGC 7259, with no nearby H II region and no evidence for anything like 30 Doradus, the Carina Nebula, or even Orion within $-13{,}000\ {\rm km\ s^{-1}}$8–$-13{,}000\ {\rm km\ s^{-1}}$9 kpc, except possibly exactly under the unresolved SN position. Smith, Andrews, and Mauerhan therefore argued that if the progenitor truly looked like a $M_R=-17.5$0–$M_R=-17.5$1 star, then binary rejuvenation through merger or mass transfer offered a natural explanation for its presence in an older-looking environment (Smith et al., 2016).

A decade-late HST test of the prototype shifted the balance of the terminality debate. By 2021, the source at the SN 2009ip position had faded steadily to $M_R=-17.5$2 mag below the pre-explosion progenitor in the same F606W filter and $M_R=-17.5$3–$M_R=-17.5$4 mag below the pre-SN outbursts. The optical continuum faded at nearly the same rate in all visual filters, with no significant color change, which was argued to rule out both a dust-obscured survivor and a hotter post-LBV survivor; the nearly constant UV flux since 2015 was instead interpreted as unresolved OB-star light from an underlying young cluster (Smith et al., 2022).

6. Expansion of the sample and the question of class coherence

Subsequent analogues have reinforced both the uniformity and the diversity of the class. SN 2016bdu showed the canonical $M_R=-17.5$5 mag Event A, $M_R=-17.5$6 mag Event B sequence and late-time spectra that developed broad Balmer and metal P-Cygni features resembling a normal Type II SN; because all members of the SN 2009ip-like group were found to be remarkably similar to SN 2005gl, whose progenitor disappeared, the authors suggested that the family may consist of genuine SNe even if unequivocal nebular nucleosynthetic signatures were still missing (Pastorello et al., 2017).

SN 2023ldh was described as a “clone of SN 2009ip,” with a long-lasting Event A, a dip, and Event B peaking at $M_R=-17.5$7 mag. Its spectra followed the familiar sequence from Event A P-Cygni H I and Fe II lines to a blue interaction-dominated peak and late Ca II-rich phases. The authors concluded that the “surprisingly homogeneous observational properties of SN 2009ip-like events may indicate similar explosion scenarios and similar progenitor parameters,” though they also noted that SN 2023ldh is slightly more luminous and slower than some earlier members (Pastorello et al., 29 Mar 2025).

SN 2016cvk added two especially important elements. First, it showed early flash-ionization features of C III, N III, and He II lasting $M_R=-17.5$8 d, a rare diagnostic within the subclass that points to dense nearby CSM and, in comparison with model spectra, abundances more consistent with a red, yellow, or blue supergiant than with a classical LBV. Second, its $M_R=-17.5$9 d spectrum showed [Ca II], [Fe II], and particularly clear [O I], which the authors regarded as possible evidence of nucleosynthesized material from a terminal SN explosion. At the same time, its lack of a strong double-peaked Balmer profile was attributed to differences in CSM structure or viewing angle, showing that geometry can generate substantial diversity within a still recognizable class (Matilainen et al., 2 Sep 2025).

Taken together, the literature supports an observationally coherent but physically unresolved category. SN 2009ip-like events repeatedly show dense recent mass loss, a two-stage Event A/Event B light curve, Balmer-dominated interaction spectra, and long-lived late-time CSM interaction. What remains unsettled is whether all such events are terminal, whether some are merger- or jet-powered nonterminal explosions, or whether the label groups multiple channels with comparable CSM-regulated observables. The most stable conclusion is therefore phenomenological: SN 2009ip-like events are defined less by a single proven engine than by the recurrent coupling of eruptive prehistory, structured nearby CSM, and a luminous interaction-dominated outburst sequence (Salmaso et al., 20 Aug 2025, Reguitti et al., 2022).