SN 2022acko: The First Early Far-ultraviolet Spectra of a Type IIP Supernova

Type II (hydrogen-rich) core-collapse supernovae arise from massive stars (≳8 solar masses) but many details of their evolution and explosions remain uncertain. Multiwavelength, multi-epoch observations probe different aspects of the explosion, yet UV spectroscopy has been scarce. The early UV, especially far-UV (<1700 Å), is rich in metal features that can constrain outer envelope composition, circumstellar material (CSM), and the density/temperature structure, contrasting with nearly featureless early optical spectra. However, early UV observations are challenging: UV emission fades quickly as the shocked envelope cools and severe metal line blanketing suppresses flux within weeks; UV access requires space telescopes, and HST scheduling is not optimized for very rapid ToOs with additional bright-object safety constraints. Prior to this work, the only early FUV spectra of normal Type IIP/L SNe were limited (e.g., SN 1999em at ~11–12 d post-explosion) and often affected by interaction or atypical progenitors (e.g., SN 1980K, SN 1987A). The putative uniformity of NUV spectra suggested from small, >1 week post-explosion samples is untested. This study aims to obtain and analyze the earliest FUV/NUV spectra of a normal Type IIP/L SN to identify dominant ions, constrain ejecta properties, and place the UV evolution in context with optical behavior.

• Early UV studies of Type II SNe are limited. IUE spectra exist for unusual events (SN 1980K with interaction; SN 1987A from a blue supergiant), while the earliest FUV spectrum of a normal Type II was SN 1999em at ~11–12 days post-explosion, already showing strong iron line blanketing.
• NUV observations (e.g., SN 2005cs, SN 2013ej, SN 2020fqv, SN 2021yja, SN 2022wsp, SN 2005ay) were often obtained >1 week post-explosion, had low S/N, or were significantly reddened.
• Prior claims of uniform NUV spectra among Type II SNe (e.g., GALEX-based SN 2005ay study) were based on small samples and did not include early-time FUV; diversity is more evident when higher-quality, earlier data are available.
• Modeling and theory emphasize the importance of metal line blanketing in the UV and its sensitivity to metallicity and ejecta conditions (e.g., Dessart & Hillier and collaborators), as well as the role of CSM interaction in shaping UV features for some events.
• This work builds on these by providing the earliest FUV spectra for a Type IIP/L SN and a comprehensive UV+optical modeling framework to identify ions and quantify ejecta properties.

• Target selection and triggering: SN 2022acko was discovered by DLT40 on 2022-12-06 UT, with an ATLAS non-detection within the previous 24 hours used as the explosion epoch (JD 2459918.67). Swift UV/optical imaging, obtained ~14.5 h after discovery, verified UV brightness and vetted the field for bright sources. The SN was spectroscopically classified as Type II within ~24 hours. A disruptive HST ToO was submitted ~28 hours after discovery; bright-object protection review completed by ~44 hours; observations executed within ~2.5 days.
• HST UV spectroscopy: STIS FUV/NUV spectra obtained at 5.2, 6.0, 7.3, and 18.9 days (FUV+NUV where scheduled) and an additional NUV spectrum at 20.8 days. Gratings: G140L (FUV-MAMA, 1150–1730 Å, R1000) and G230L (NUV-MAMA, 1570–3180 Å, R500). One second-epoch visit suffered guide-star acquisition failure; a make-up NUV epoch was executed at 20.7 days. Reduced data obtained from MAST (doi:10.17909/gaze-k021).
• Ground/space photometry: High-cadence UBVgri and Open-filter photometry from Las Cumbres Observatory and SkyNet PROMPT (DLT40), ATLAS forced photometry in o-band, and Swift/UVOT in uvw2, uvm2, uvw1, u, b, v. Standard pipelines used (e.g., BANZAI for LCO; IRAF/photutils/UVOT routines for Swift). Aperture choices and background regions assessed for late-time UV limits due to host contamination.
• Ground-based spectroscopy: Multiple telescopes/instruments (e.g., LCO FLOYDS, MMT/Binospec, Bok/B&C, SALT/RSS, SOAR/Goodman, LBT/MODS, INT/IDS). Reductions used IRAF or instrument-specific pipelines; spectra scaled to Swift and gri photometry via constant or linear adjustments.
• Distance and extinction: Adopted distance 19.0 ± 2.9 Mpc from PHANGS (numerical action method). Milky Way and host extinction estimated from Na I D1/D2 equivalent widths in medium-resolution spectra (MMT/Binospec, SALT/RSS) using Poznanski et al. (2012), cross-checked with Schlafly & Finkbeiner (2011). Adopted E(B−V)_MW = 0.026 ± 0.001 mag and E(B−V)_host = 0.04 ± 0.02 mag.
• Spectral modeling: Time-dependent NLTE radiative transfer with CMFGEN. Progenitor evolved with MESA: 12 M⊙ ZAMS at solar metallicity (Z=0.014), final mass 9.7 M⊙, R ~500 R⊙, with extrapolated low-density outer layers (~0.02 M⊙) for shock breakout suitability. Explosion via V1D thermal bomb between 1.55–1.60 M⊙ yielding ejecta mass 8.16 M⊙ and kinetic energy 6×10^50 erg; mass cut at 1.55 M⊙ (neutron star gravitational mass). CMFGEN modeling assumes homologous expansion from ~5 days; model truncated at v_min=2000 km/s, excludes 56Ni and non-thermal effects. Model flux scaled to Swift UV photometry at 5.5 days and applied uniformly across epochs to preserve model flux evolution. Ion identification via omission of bound-bound transitions in formal solution to isolate contributions.
• UV flux fraction analysis: Constructed combined UV+optical spectra (interpolating across ~3000 Å gap), scaled to contemporaneous photometry, and integrated flux redward of specific filter blue edges (uvw2, U, B, V) at days 5, 7, and 19 to quantify observed flux fractions given typical datasets.

• Earliest FUV spectra of a normal Type IIP/L SN: HST/STIS FUV and NUV spectra obtained at 5.2, 6.0, and 7.3 days post-explosion reveal strong, Doppler-broadened metal features in the FUV/NUV not previously observed this early for Type IIP/L SNe. UV flux declines rapidly over the first week.
• Strong line blanketing: By ~19–21 days, UV flux is nearly completely suppressed due to severe metal line blanketing, with a conspicuous ~2970 Å feature identified as a low-absorption window (not emission) among Fe II, Cr II, Ti II complexes.
• Photometric properties: Peak absolute magnitude V = −15.4 mag; plateau length ~115 days; plateau slope s_50,V ≈ 0.35 mag per 50 days. Light-curve evolution resembles low-luminosity SNe (LLSNe), particularly SN 2012A in UV/blue and SN 2018lab in red optical bands.
• Extinction and distance: Adopted D = 19.0 ± 2.9 Mpc; E(B−V)_MW = 0.026 ± 0.001 mag; E(B−V)_host = 0.04 ± 0.02 mag.
• Spectral modeling and progenitor/explosion: CMFGEN models (time-dependent NLTE) provide a good qualitative match using a low-mass red supergiant progenitor (12 M⊙ ZAMS, solar metallicity) with ejecta kinetic energy E_kin = 6 × 10^50 erg and ejecta mass 8.16 M⊙. Without parameter optimization, a uniform flux scaling (factor ~2) was required; a lower energy model (~4×10^50 erg) matched flux levels better but not line details.
• Ion identification: Early UV spectra are dominated by Fe III and Fe II, with significant contributions from C III, C II, Ti III, Si IV, Si III, Si II, S III, S II, Ni III, Cr III, Al III, Al II, and Mg II. Several relatively isolated FUV lines allow clear identification of C and Si features.
• Narrow absorption features: Numerous narrow ISM/CSM absorption lines detected at Milky Way and host redshifts (e.g., Si III 1206, C II 1335–1336, Si IV 1394–1403, Fe II 1608, Mg II 2796–2802), confirming host redshift and indicating approximately solar (or slightly higher) metallicity along the line of sight.
• Photospheric evolution (from modeling and EPM): Temperature declines from ~12,700 K at day 5.5 to ~6,000 K by day 23.5 (onset of hydrogen recombination); photospheric velocity decreases from ~9,500 km/s (day 5.5) to ~6,000 km/s (day 23); photospheric radius grows from ~4.5×10^14 cm (~6500 R⊙) to ~1.24×10^15 cm (~18,000 R⊙) by day 23.5. These values place SN 2022acko within the range of normal Type II SNe (between SN 2006bp/1999em and SN 2005cs).
• UV flux fraction captured by typical filters (fraction of total 1150–10150 Å flux redward of filter blue edge): uvw2: 0.96 (day 5), 0.98 (day 7), 1.00 (day 19); U: 0.66, 0.76, 0.99; B: 0.53, 0.63, 0.95; V: 0.35, 0.42, 0.79. Thus, relying on optical-only data misses nearly half the flux at day 5 if B is the bluest filter.
• Comparison to other SNe: At similar phases, SN 2022acko shows stronger early FUV features than SN 1999em (already strongly blanketed by day 12). Diversity is evident among Type II UV spectra; some events (e.g., SN 2021yja, SN 2022wsp) retain NUV flux to ~20 days, possibly due to weak interaction. SN 2022acko shows no clear narrow/intermediate-width emission or P-Cygni profiles in the UV indicative of ongoing CSM interaction.
• Observational firsts/context: First FUV spectra of a Type IIP within a week of explosion; UV rise in Swift NUV filters captured; SN 2022acko also notable as the first SN with JWST used to identify its HST-detected progenitor and first core-collapse SN with JWST spectroscopy (in prep. works).

The earliest-ever FUV observations of a normal Type IIP/L SN reveal that the early UV regime is rich in metal features providing strong constraints on ejecta temperature, density, velocity, and composition—information largely absent from early optical spectra. CMFGEN modeling indicates SN 2022acko arose from a low-mass red supergiant at approximately solar metallicity, with modest kinetic energy (6×10^50 erg). The model reproduces the UV flux evolution and feature morphology well after a uniform flux scaling, supporting the inferred ejecta conditions and ion identifications, particularly the dominant role of Fe-group line blanketing in governing UV flux suppression within ~3 weeks. The photospheric evolution (T, R, v) falls squarely within the range of normal Type II SNe, indicating SN 2022acko is not atypical despite being UV-faint by ~20 days. The absence of narrow/intermediate-width UV emission features suggests no strong, persistent CSM interaction at early times; any CSM is either weak, at low density, or swept up very early. The quantified fraction of observed flux versus filter coverage underscores that optical-only datasets substantially underestimate early-time bolometric output unless UV is included. Comparisons with other Type II SNe show significant diversity in UV behavior, contradicting earlier suggestions of uniformity and emphasizing the need for early, high-S/N UV spectroscopy to map this diversity and link it to progenitor/explosion properties (e.g., metallicity, CSM, energy, radius).

This work presents the first FUV spectra of a Type IIP/L supernova obtained within a week of explosion, demonstrating that early UV spectra contain a wealth of diagnostic features dominated by Fe-group line blanketing. SN 2022acko is a low-luminosity Type II SN (V ≈ −15.4 mag) with a ~115-day plateau and photospheric properties typical of normal Type II SNe. Time-dependent NLTE CMFGEN modeling with a 12 M⊙ solar-metallicity RSG progenitor and E_kin = 6×10^50 erg reproduces the UV spectral evolution and identifies key contributing ions (Fe II/III, Mg, Si, C, Al, Cr, S, Ni, Ti). The study quantifies how much early flux is missed without UV coverage, highlighting the necessity of rapid UV observations to accurately estimate bolometric luminosities and probe ejecta/CSM conditions. Future work should: (1) expand the sample of early FUV/NUV spectra to characterize diversity; (2) optimize modeling (explosion energy, progenitor radius, CSM) to reduce flux-scaling mismatches; (3) use UV spectral diagnostics to constrain metallicity and CSM properties; and (4) leverage rapid-response UV-capable missions (e.g., UVEX) to probe even earlier phases and possible CSM interaction signatures.

• Observational constraints: HST scheduling limited cadence (e.g., final early sequence epoch delayed; one visit lost guide stars); bright-object protection requirements constrain rapid UV ToOs. Late-time UV photometry was limited by host galaxy background without template subtraction at the time of analysis.
• Sample size: The number of high-quality early UV spectra for Type II SNe remains very small, hindering robust population-level conclusions about diversity.
• Modeling simplifications: CMFGEN model parameters were not fully optimized; an overall flux scaling (~factor of two) was applied. The model excludes inner ejecta below 2000 km/s, omits 56Ni and associated non-thermal effects, and assumes homologous expansion starting at ~5 days. Degeneracies among explosion energy, progenitor radius, distance, extinction, and potential CSM contribution remain.
• Extinction uncertainties: Possible underestimation of host extinction; adding E(B−V) ~0.03 mag improves early optical fits but worsens the day 18.9 optical epoch. Precise extinction impacts flux calibration and derived parameters.
• UV flux fraction estimates: Computed over 1150–10150 Å; contributions at longer wavelengths were not included, potentially biasing late-time flux fractions slightly.