SN 2025aico: Early observations of a faint Type IIb supernova with a low-mass envelope

Abstract: We aimed to investigate the physical properties and the underlying explosion mechanisms of the Type IIb SN 2025aico. Through a comprehensive analysis of early-phase optical light curves and spectroscopic data, we aim to constrain the fundamental explosion parameters and evaluate the physical state of the event. We present early multi-band optical imaging and low-resolution optical spectroscopic follow-up observations of the Type IIb SN 2025aico, spanning approximately 70 days from the explosion. We constrain the properties of SN 2025aico by utilizing a hybrid model that combines shock-cooling emission and radioactively powered diffusion, as well as by analyzing the spectroscopic evolution. We use various approaches to constrain the 56Ni mixing from early data, and also compared our spectra with models to constrain the properties of the progenitor. The explosion epoch of SN 2025aico is estimated to be MJD 61032.69, while the rise time in the r_M-band is 22.30 +/- 0.70 days. The peak pseudo-bolometric luminosity in the optical bands is L_opt = (4.07 +/- 0.10) x 10^41 erg/s. The fitting yields a moderate to relatively low 56Ni mass of M_Ni = 0.033 +0.006/-0.004 M_sun and an ejecta mass of M_ej = 2.79 +0.21/-0.18 M_sun. The photospheric velocity near the bolometric peak, measured from the Fe II lambda 5169 line, is 6450 +180/-160 km/s. The derived envelope properties suggest a compact He-star progenitor possessing an H-rich envelope of M_env approx. 0.01 M_sun and a radius of R_env approx. 6-10 R_sun. The derived physical properties of SN 2025aico indicate an origin from a moderate-mass, stripped He-star in a compact binary system, characterized by a minimal residual hydrogen envelope. The explosion itself demonstrates weak to moderate 56Ni mixing throughout the ejecta.

• The paper constrains progenitor properties using early shock-cooling light curve modeling, determining an envelope mass of 0.017–0.018 M⊙ and a compact radius of 6–9 R⊙.
• The paper demonstrates weak 56Ni mixing through a distinctive U-shaped color evolution and a modest 56Ni yield of 0.033 ± 0.005 M⊙, which shapes the light curve.
• The paper links these observations to binary progenitor channels in a sub-solar metallicity environment, offering critical constraints for future binary evolution models.

Early Observations and Progenitor Constraints of the Faint Type IIb SN 2025aico

Introduction

SN 2025aico offers a rare opportunity to study the physical properties, explosion mechanism, and progenitor configuration of a relatively faint Type IIb supernova characterized by a low-mass envelope. Through extensive optical and UV photometry and time-resolved spectroscopy, the analysis delineates the early shock-cooling phase, the color evolution—a key diagnostic of $^{56}$Ni mixing—and the radiative transfer processes dictating the observed light curve and spectral sequence. The context and implications of these findings bear directly on the progenitor scenarios for stripped-envelope supernovae, and provide incisive constraints on binary evolutionary pathways in sub-solar metallicity environments.

Photometric Evolution and Light Curve Modelling

SN 2025aico was discovered at $z=0.00455$ in the outskirts of the metal-poor dwarf galaxy LEDA 35384, presenting prompt follow-up with both ground-based and space-based facilities. The multi-band apparent light curve exhibits a rapid rise to $r_\mathrm{M}$-band maximum at $22.30\pm0.70$ days post-explosion, and a pseudo-bolometric peak luminosity $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$ erg s$^{-1}$, firmly in the faint regime for SNe IIb.

Figure 1: Multi-band UV and optical apparent light curve of SN 2025aico. The dashed vertical line denotes $r_\mathrm{M}$-band maximum, with upper limits marked and filter offsets applied for clarity.

A salient feature is the brief, blue shock-cooling phase in the earliest days, yielding a decline of $2.3$ magnitudes in $U$-band within $3.7$ days post-explosion. Modelling the initial light curve segment with hybrid shock-cooling plus radioactively powered diffusion models recovers strict constraints on the H-rich envelope: the best-fit envelope mass is $z=0.00455$0--$z=0.00455$1, envelope radius $z=0.00455$2--$z=0.00455$3, and expansion velocities in the $z=0.00455$4--$z=0.00455$5 km s$z=0.00455$6 range, consistent across both Piro (2015) and Sapir & Waxman (2017) models.

Figure 2: Bolometric light curve model with contributions from radioactive (RD) and shock-cooling (SC) components, illustrating the fits and uncertainty bounds.

The global light curve fit yields ejecta mass $z=0.00455$7, $z=0.00455$8Ni mass $z=0.00455$9, and kinetic energy $r_\mathrm{M}$0 erg for moderate opacity and typical velocities ($r_\mathrm{M}$1 km s$r_\mathrm{M}$2). These values are on the low-mass, low-energy tail for SE-SNe.

Figure 3: Pseudo-bolometric light curve comparison with other SNe IIb, emphasizing the fainter and shorter shock-cooling phase for SN 2025aico.

Importantly, the rapid post-peak decline and the observed recombination plateau suggest that He-recombination is a non-negligible luminosity contribution, supported by model fits failing when recombination is omitted.

Early Color Evolution and $r_\mathrm{M}$3Ni Mixing

The color evolution is critical: SN 2025aico’s intrinsic $r_\mathrm{M}$4 and $r_\mathrm{M}$5 colors exhibit a pronounced “U-shaped” time evolution, with initial rapid reddening, followed by a transient blueward excursion, and then post-peak monotonic reddening. Comparison with state-of-the-art, parameterized $r_\mathrm{M}$6Ni mixing models demonstrates that such color behavior can only be reproduced by weak or, at most, half $r_\mathrm{M}$7Ni mixing. Stronger mixing would produce a monotonic color progression inconsistent with the data.

Figure 4: Intrinsic color evolution of SN 2025aico compared to models with varying $r_\mathrm{M}$8Ni mixing degrees; the “U-shaped” feature requires weak mixing.

Thus, in conjunction with the modest $r_\mathrm{M}$9Ni yield, the results robustly support a weakly mixed explosion, with nickel largely confined to the inner core, which impacts both light curve morphology and the timing of non-thermal excitation in the ejecta.

Spectral Sequence and Line Evolution

The time-series spectroscopy captures the emergence and transformation of Balmer and He~I lines, as well as the transition in line velocities and excitation mechanism. Early spectra (pre-maximum) are dominated by blue continua and broad, blueshifted P Cygni profiles of H$22.30\pm0.70$0 and He~I~$22.30\pm0.70$15876, tracing bulk velocities up to $22.30\pm0.70$2 km s$22.30\pm0.70$3. As the photosphere recedes, both the width and strength of hydrogen lines decrease rapidly.

Figure 5: Time-ordered spectral sequence, with Balmer and He~I transitions marked; continuum blue at early times, evolving towards line-rich as temperature decreases.

Ejecta velocity evolution inferred from Fe~II~$22.30\pm0.70$45169—regarded as a robust tracer for the receding photosphere—shows an initial increase post-shock-cooling then a gradual decline to $22.30\pm0.70$5 km s$22.30\pm0.70$6 at late times. Comparison with models again supports weak $22.30\pm0.70$7Ni mixing.

Figure 6: Evolution of line velocities (Fe II, H$22.30\pm0.70$8, He~I) and blackbody temperature compared to mixing models; only weak/half-mixed scenarios adequately match both the velocity and temperature trends.

A noteworthy line evolution is the appearance of a blueshifted emission peak in He~I post-shock-cooling, with an increasing photospheric/outflow velocity during the rise due to the progression of the non-thermal excitation front as $22.30\pm0.70$9-rays penetrate optically thinner helium-dominated layers. Late-time spectra converge to standard SE-SN morphology, with strong P Cygni He~I features and diminishing H content.

Figure 7: Line profile evolution of He~I~$L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$05876 and $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$17065, showing early absence of P Cygni absorption (indicative of weak mixing) and subsequent strengthening.

Progenitor Scenario and Host Environment

The inferred total ejected mass, minimal residual hydrogen, compact envelope, and faint shock-cooling peak all support a progenitor that is a compact He star in a close binary, subject to Case B mass transfer. The metallicity of the SN site, measured via nebular emission lines, is $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$2—significantly subsolar. Such metallicity disfavors strong winds as the main envelope-stripping mechanism, thereby reinforcing a binary interaction path.

Direct comparison of SN 2025aico spectra at maximum with radiative transfer models from synthetic binary progenitors (e.g., Dessart 2024), combined with the low envelope mass from light curve modeling, point toward an orbital period $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$3--$L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$4 days in the LMC-like metallicity regime as the likely progenitor configuration. Solar metallicity binary models only reproduce the observations for $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$5 days and predict more massive hydrogen envelopes than observed, which further constrains the evolutionary channel.

Figure 8: Observed peak spectrum compared to synthetic binary progenitor models with varying periods and hydrogen envelope mass; the best fit corresponds to a short-period, minimal-H envelope system.

Implications and Prospects

These results establish SN 2025aico as a benchmark for cIIb (compact Type IIb) events bridging the classic H-rich (extended IIb) and H-poor (Ib) regimes. The unambiguous evidence for a minimal H envelope, weak $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$6Ni mixing, and a compact progenitor supports an evolutionary continuum wherein residual hydrogen in stripped-envelope SNe is set primarily by binary interaction timescales and host galaxy metallicity.

In practice, this constrains population synthesis models for SE-SNe and binary star evolution, as well as providing empirical requirements for future hydrodynamical and radiative transfer calculations. The shock-cooling constraint also has direct impact on understanding the first-light properties, informing both SN detection strategies and early-phase explosion physics.

Looking ahead, systems like Mephisto, combined with real-time color data and X-ray monitoring, will be essential in systematically sampling the diversity of early-phase SE-SNe, mapping the $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$7Ni mixing parameter, and connecting observed transient classes to detailed binary stellar models.

Conclusion

SN 2025aico exemplifies a faint, compact Type IIb supernova from a low-mass, binary-stripped progenitor with a minimal H-rich envelope and weak $L_\mathrm{opt} = (4.07 \pm 0.10) \times 10^{41}$8Ni mixing. Its early light curve, color evolution, and spectral sequence critically constrain the progenitor mass, explosion geometry, and chemical mixing processes. The findings underscore the necessity of high-cadence, multi-wavelength follow-up of SE-SNe to robustly link population characteristics to late-stage binary stellar evolution in varying metallicity environments.