Ejecta-CSM Interaction in Explosive Transients

• Ejecta-CSM interaction is the process where supernova ejecta meet pre-existing circumstellar material, initiating hydrodynamical, radiative, and chemical processes that reveal progenitor mass-loss history and explosion geometry.
• The interaction involves a forward shock in the circumstellar medium and a reverse shock in the ejecta, forming a cold, dense shell influenced by instabilities like Rayleigh–Taylor and Kelvin–Helmholtz.
• Radiative outputs, including spectral signatures, light curves, and polarization signals, provide diagnostic tools to probe progenitor properties and complex shock dynamics in explosive transients.

Ejecta-circumstellar material (CSM) interaction refers to the hydrodynamical, radiative, and chemical processes initiated when supernova or transient ejecta encounter pre-existing circumstellar material. This phenomenon is central to the emission properties of a variety of explosive and eruptive transients—including core-collapse and thermonuclear supernovae, fast blue optical transients, and even certain gamma-ray bursts—spanning a broad range of timescales, luminosities, and environments. The detailed nature of the ejecta–CSM interaction encodes information about the progenitor's mass-loss history, explosion geometry, and the structure of the near-stellar environment. Rigorous modeling and observation of ejecta–CSM interaction have provided both diagnostic tools for stellar evolution and access to extreme physical regimes of shock physics, nucleosynthesis, and radiation hydrodynamics.

1. Fundamental Hydrodynamics and Density Structures

The interaction is generally described by the propagation of fast, freely expanding ejecta into a CSM whose density profile derives from a range of pre-supernova mass-loss mechanisms. The ejecta density typically follows a broken power-law in velocity space:

• For velocities $v > v_t$, the outer ejecta density is $\rho_{\rm ej}(v,t) \propto t^{-3}v^{-n}$.
• The inner core often assumes a flatter profile, $\rho_{\rm ej}(v,t) \propto t^{-3}v^{-\delta}$, with $n \in [7,12]$ and $\delta \sim 0-1$ (Nagao et al., 2023, Liu et al., 2020, Wang et al., 23 Jan 2024).

The CSM density is parameterized as: $\rho_{\rm CSM}(r) = D\, r^{-s}$ where $s$ varies with the nature of mass loss: $s=2$ for a steady wind, $s\simeq0$ for a constant-density shell, and intermediate/steeper values for particular scenarios (e.g., $s \sim 2.9$ for Type Icn SNe) (Nagao et al., 2023, Kurfürst et al., 2019, Gagliardini et al., 13 Oct 2025). Realistic CSM can display pronounced asphericity (e.g., disks, torii, bipolar lobes) and strong temporal inhomogeneities due to eruptive or binary-driven mass loss (Wen et al., 30 Jul 2024, Suzuki et al., 2019, Reynolds et al., 23 Jan 2025).

2. Shock Structure and Instabilities

Upon collision, a forward shock (FS) is driven into the CSM, while a reverse shock (RS) propagates back into the ejecta. Energy and momentum transfer across these shocks generates a cold, dense shell (CDS) at the interface, whose subsequent evolution and radiative output depend on the CSM properties, ejecta kinetic energy, and geometry (Gagliardini et al., 13 Oct 2025, Nagao et al., 2023, Dewey et al., 2011). The dynamics are governed by the Euler equations, with boundary conditions and initial profiles encoded by the progenitor system (Dewey et al., 2011, Kurfürst et al., 2019, Suzuki et al., 2019, Orlando et al., 28 Apr 2025).

Key equations for the thin-shell approximation (Chevalier self-similar solution) capture the shock dynamics: $R_{\rm FS}(t) \propto t^{(n-3)/(n-s)}$

$$v_{\rm FS}(t) = \frac{dR_{\rm FS}}{dt} \propto t^{-(s-3)/(n-s)}$$

Post-shock temperatures can reach $T_{\rm shock} = \frac{3\mu m_p}{16 k_B} v_s^2$, with $v_s \sim 10^3$–$10^4$ km s$^{-1}$, implying temperatures from $10^7$ to $10^9$ K (Dewey et al., 2011, Gagliardini et al., 13 Oct 2025).

Hydrodynamic instabilities—Rayleigh–Taylor, Kelvin–Helmholtz, and thin-shell (Vishniac)—arise at the contact discontinuity, especially where steep gradients or shear flows exist (e.g., in aspherical disks or clumpy shells). These instabilities drive mixing, turbulence, and the fragmentation of the CDS, imprinting themselves on spectral line profiles and dust formation (Suzuki et al., 2019, Wen et al., 30 Jul 2024, Pan et al., 2013, Smith, 2012).

3. Radiation, Emission Mechanisms, and Light Curve Properties

The radiative signature of ejecta–CSM interaction is controlled by prompt conversion of kinetic energy at the shocks into photons, with the following scaling for instantaneous luminosity: $$L_{\rm int}(t) \approx \epsilon \, 2\pi R_{\rm sh}^2\, \rho_{\rm CSM}(R_{\rm sh})\, v_{\rm sh}^3$$ where $\epsilon$ is the efficiency of kinetic-to-radiative energy conversion (Liu et al., 2020, Nagao et al., 2023). For optically thick shells, radiative diffusion governs the observed light curve, with the emergent $L(t)$ given by Arnett-like integrals over the shock power and diffusion timescale $t_{\rm diff}$, typically: $$L(t) = \frac{1}{t_{\rm diff}} \exp\left(-\frac{t}{t_{\rm diff}}\right) \int_0^t \exp\left(\frac{t'}{t_{\rm diff}}\right) L_{\rm inp}(t') dt'$$ (Liu et al., 2017, Wang et al., 23 Jan 2024, Liu et al., 2020).

Radiative regimes split into:

• Radiative shocks: $t_{\rm cool} < t_{\rm flow}$; energy is rapidly radiated, producing hard X-rays, UV, and optical emission, often with plateau-like or double-peaked light curves (Pan et al., 2013, Smith, 2012).
• Adiabatic shocks: $t_{\rm cool} > t_{\rm flow}$; less efficient photon production, emission peaks when the shock crosses the shell and then declines (Pan et al., 2013).

Spectral signatures depend on shock regime and CSM geometry:

• Thermal bremsstrahlung and line-cooling dominate X-ray and UV bands.
• Emission line formation: Multi-component broad/intermediate/narrow line profiles trace different velocity and spatial zones (e.g., the quartet of H$\alpha$ components in KISS15s).
• Polarimetry: High continuum polarization indicates strong asphericity; disk/torus-type CSM can yield $p \gtrsim10$% at early times, with double-peaked polarization evolution as geometry and dominance of disk/pole fluxes change (Wen et al., 30 Jul 2024, Reynolds et al., 23 Jan 2025).
• View angle effects: Light curve rise times, peak luminosities, and line profiles depend sensitively on whether the disk/CSM is viewed face-on (short, bright, broad) or edge-on (longer rise, lower peak, narrow) (Suzuki et al., 2019).

4. CSM Mass, Geometry, and Progenitor Mass-loss Diagnosis

Accurate inference of mass-loss rates, geometry, and episodic structure follows from matching light curve shape, peak, and emission line diagnostics to analytic/semi-analytic models and radiation-hydrodynamic simulations:

• Disk or torus-shaped CSMs, as inferred for SN 2021irp, require $$\dot M_{\rm disk} \sim (0.035\text{–}0.1) M_\odot\,{\rm yr}^{-1}$$, half-opening angle $\theta_0\sim30$–$50^\circ$, and total $M_{\rm CSM}\gtrsim2\,M_\odot$ (Reynolds et al., 23 Jan 2025).
• Spherical dense shells may be formed in eruptions or binary-driven mass transfer, with masses up to $M_{\rm shell}\sim10\,M_\odot$ and inner radii $R\sim10^{14}$–$10^{16}$ cm (e.g., Eta Car, HSC16aayt) (Smith, 2012, Moriya, 2023).
• Flat ($s\lesssim1.5$) or shell-like CSM distributions extend the light curve rise to hundreds of days, requiring unsteady, episodic, or eruptive pre-SN mass loss with $\dot M$ substantially exceeding steady wind values (Moriya, 2023).
• In SN Ia and related thermonuclear explosions, late-time CSM interaction at radii $R\gtrsim10^{16}$ cm reveals shells laid down by nova eruptions or symbiotic winds (PTF11kx, SN 2015cp, SN 2020qxz) (Graham et al., 2018, Terwel et al., 6 Aug 2025).

Astrophysical implications extend to distinguishing single- from double-degenerate channels in Type Ia SNe, reconstructing binary/rotation-driven mass ejection in massive stars, and probing the final pre-explosion decades of stellar evolution using both early flash spectroscopy and late-time radio/UV follow-up (Graham et al., 2018, Terwel et al., 6 Aug 2025, Gagliardini et al., 13 Oct 2025).

5. Multi-messenger and Multi-dimensional Diagnostics

Observational methods for diagnosing ejecta–CSM interaction are fundamentally multi-modal:

• X-ray observations: High-energy shock signatures, emission line profiles, and time-variable absorption provide constraints on CSM density, geometry, and composition (Dewey et al., 2011, Pan et al., 2013, Orlando et al., 28 Apr 2025).
• Polarization mapping: Time-resolved continuum polarimetry and line polarization, including position angle and wavelength dependence, offer geometric diagnostics inaccessible to spectroscopy alone (Wen et al., 30 Jul 2024, Reynolds et al., 23 Jan 2025).
• Ultraviolet and optical photometry: Early-time space-based UV flash surveys (e.g., ULTRASAT) and ground-based wide-field monitoring enable detection of both breakout and CSM-powered emission; rise time, peak, and decay encode shell properties (Gagliardini et al., 13 Oct 2025).
• Radio and neutrino emission: For sufficiently dense and fast shocks, radio synchrotron (from accelerated electrons) and neutrino bursts (from hadronic $pp$ interactions) provide cross-checks on CSM interaction strength, geometry, and high-energy processes (Gagliardini et al., 13 Oct 2025, Hu et al., 2023).
• Radiative transfer and hydrodynamic modeling: Axisymmetric and full 3D simulations incorporating radiation–matter coupling, energy dissipation, and shock instabilities are essential for relating viewing angle and physical geometry to light curves and polarization (Suzuki et al., 2019, Kurfürst et al., 2019).

The growing synergy between large, time-domain surveys (e.g., ZTF, LSST) and dedicated rapid-response spectropolarimetry is reshaping the landscape of CSM interaction studies by capturing both rare and systematic events across the transient zoo (Terwel et al., 6 Aug 2025).

6. Diversity, Asphericity, and Viewing-Angle Effects

Ejecta–CSM interactions display enormous diversity reflecting differences in progenitor evolution, binary interactions, and the geometry of the pre-explosion mass-loss environment:

• Aspherical interaction geometries (disks, jets, bipolar CSM) can produce double-peaked polarization, angle-dependent light curves, and multi-component emission lines, dependent on the observer’s line-of-sight (Wen et al., 30 Jul 2024, Suzuki et al., 2019, Reynolds et al., 23 Jan 2025).
• In Type Icn SNe, diversity in spectroscopic features, early light curve evolution, and required high-energy outflows traces not only physical parameters but also orientation: e.g., SNe 2021ckj and 2021csp are "twins" with different viewing angles—one polar, one off-axis—resulting in differences in emission components of C II/C III lines and the strength of absorption features (Nagao et al., 2023).
• Ejecta–CSM interaction in relativistic and fast transients (e.g., GRBs, FBOTs) is similarly sensitive: moderate mass-loading of jets by dense CSM generates mildly relativistic, nearly spherical outflows (universal $\rho \propto v^{-5}$), and the observed UV/optical flash timescale and color evolution link directly to the CSM radius and density (Suzuki et al., 11 Jun 2024).
• Multi-shell models, invoking sequential shell and wind mass loss, explain multi-peaked light curves and can reconstruct decades of progenitor activity prior to explosion (Liu et al., 2017).

The interplay between dynamic, radiative, and geometric effects makes the ejecta–CSM interaction a central, diagnostic engine in time-domain astrophysics, with applications from core-collapse energetics to progenitor system classification.

7. Broader Astrophysical Implications

The paper of ejecta–CSM interaction informs:

• The structure, asphericity, and evolutionary pathways of massive stars near end-of-life, including episodic binary-driven or wave-driven mass loss, LBV eruptions, and the role of common-envelope events (Smith, 2012, Reynolds et al., 23 Jan 2025, Moriya, 2023).
• Stellar feedback in the interstellar medium and the chemical/dust enrichment from freshly-formed dust in post-shock shells (Kokubo et al., 2019).
• The calibration and diversity of explosion models for SNe IIn, SNe Ia, and FBOTs, including the implications for standardization in cosmological samples (Graham et al., 2018, Terwel et al., 6 Aug 2025).
• Multi-messenger astronomy: strong CSM interaction can produce detectable neutrino signals, constrain hadronic acceleration efficiency, and provide unique targets for future UV and high-energy missions (Gagliardini et al., 13 Oct 2025).

Ongoing developments in high-cadence, multi-wavelength, and polarimetric monitoring, combined with advances in simulation and modeling, continue to refine the connection between observed ejecta–CSM interaction signatures and the complex interplay of explosion physics and pre-supernova stellar evolution.