An infrared echo from a circumstellar disk in the hydrogen- and helium-poor SN 2024aecx

TL;DR

This work tackles the origin of a remarkable NIR excess in SN 2024aecx, a hydrogen- and helium-poor Type Ic SN. By combining multi-instrument NIR spectroscopy (1–2.4 μm) and dust emission modeling, the authors show that the observed IR continuum emerges around 32 days post peak and is best explained as an infrared echo from pre-existing circumstellar dust heated by the SN peak light, rather than in-situ dust formation. Dust-fitting reveals a single-temperature component evolving in time, with optically thin models yielding a dust mass of ~$10^{-4}$–$10^{-3}\,M_\odot$ and an optically thick scenario giving a lower-limit mass >$10^{-3}\,M_\odot$, while the implied geometry is a thick face-on disk whose inner edge lies at $r_{ m inner} \approx (3.1\text{--}8.3)\times10^{16}\ \mathrm{cm}$; the SN shock would interact with this CSM at ~$(440\pm200)$ days. Overall, the study constrains the close-in, hydrogen-poor CSM and provides insight into late-stage mass-loss processes in stripped-envelope progenitors, with implications for the diversity of SNe Ic and their circumstellar environments. The inferred inner edge and disk-like CSM geometry, together with the modest mass-loss rate, suggest mass loss mechanisms beyond simple case B binary interaction and motivate further IR monitoring (including JWST) to fully map dust evolution and future CSM interaction in such events.

Abstract

We present near-infrared (NIR) spectroscopy of the hydrogen- and helium-poor (Type Ic) supernova (SN) 2024aecx that displays a strong NIR excess emerging 32 days post peak. SN 2024aecx is a peculiar SN Ic that exhibited luminous shock-cooling emission at early times, suggestive of close-in circumstellar medium (CSM), unexpected for this class of SNe. Its early NIR spectra are typical for a SN Ic but with strong CI absorption features. By ~32 days post peak, the spectra show a strong NIR excess, while maintaining normal optical colors, unprecedented for SNe Ic. We find that the NIR excess is well fit with a single-temperature, optically thin dust model with declining temperature, increasing mass, and roughly constant luminosity over time. The NIR excess appears too promptly for dust to have formed in the SN ejecta, indicating an IR echo from pre-existing dust in the CSM. The IR echo is likely powered by the relatively slowly evolving SN peak light, and not the brief shock cooling emission, as the latter requires unrealistically high CSM densities to explain the observed dust mass. We consider different potential CSM geometries and find that a thick face-on disk with an inner edge around $5\times 10^{16}$ cm can best explain the dust mass and temperature evolution. In this scenario, the SN shock should start interacting with this CSM $440\pm200$ days post explosion. CSM around SN Ic is rare, and follow-up observations of SN 2024aecx will probe the mass-loss process responsible for removing hydrogen and helium from their progenitor star.

Figures (12)

• Figure 1: NIR spectra of SN 2024aecx from $-18$ to 60 days from peak. Prominent spectral features are marked. Early spectra are dominated by a hot continuum with prominent C1 absorptions, marked by the shaded regions. Between 12 and 32 days, a strong NIR continuum emerges and strengthens towards the end of spectral sequence shown here. Spectra in this phase show boxy Mg1 1.5033 $\mu$m. Comparatively weak CO first overtone is detected starting potentially from 37 days. The Ca2 triplet remains strong at all phases.
• Figure 2: Optical light curves of SN 2024aecx in the $uBVgri$ bands. The magnitudes are in the AB system. The photometry has not been corrected for extinction. Red ticks on top label epochs for our NIR spectra.
• Figure 3: Top: NIR spectrum of SN 2024aecx at 12 days post peak compared with those of a typical SN Ic 2013ge at 7.8 days Shahbandeh2022, C-rich SN Ic 2016adj at 6.3 days Stritzinger2024, and a helium-rich SN IIb 2022ngb at 18 days Tinyanont2024. C1 lines are plotted in dotted red lines. Other lines present are Mg1 1.5003 $\mu$m and Mg2 1.0938 $\mu$m, plotted in dashed magenta lines. The conspicuous helium absorption associated with the 2.059 $\mu$m line seen in SN 2022ngb is absent in SN 2024aecx. Bottom: NIR spectra of SN 2024aecx representative of different phases, compared with the spectra of a typical SN Ic 2013ge Shahbandeh2022; a strongly reddened SN Ic 2016adj with early CO formation Stritzinger2024; the CO- and dust-forming SNe 2020oi Rho2021 and 2021krf Ravi2023. This comparison highlights the extreme nature of the IR excess in SN 2024aecx.
• Figure 4: Top: Color evolution of SN 2024aecx $B-V$ (left), $V-r$ (middle), and $r-i$ (right). Unfilled red circles mark the observed color. Filled cyan squares mark the colors corrected for MW extinction with $A_{V, \rm MW} = 0.16$ mag and $R_{V, \rm MW} = 3.1$. Filled blue squares mark the colors corrected for both MW and host extinction with $A_{V, \rm host} = 0.39 \pm 0.09$ mag and $R_{V, \rm host} = 0.89 \pm 0.16$. Shaded bands mark the range of color templates for SNe IIb, Ib, and Ic from CSP-I Stritzinger2018. The Ic template is used to determine the host extinction parameters. Bottom: Synthetic NIR colors evolution of SN 2024aecx: $Y-J$ (left), $J-H$ (middle), and $H-K$ (right), compared with those of comparison events SNe 2013ge, 2016adj, 2020oi, and 2021krf. The synthetic colors are computed from spectra presented in Figure \ref{['fig:spec']}. Transparent circles and triangles show photometric colors of SNe Ib and Ic, respectively, from CSP I Stritzinger2018phot. Note that CSP I does not have $K$ band observations.
• Figure 5: NIR Spectra of SN 2024aecx at regions around Pa$\beta$ (left) and the He1 2.059 $\mu$m and Br$\gamma$ (right). We do not detect hydrogen or uncontaminated helium lines in the NIR at any phase.