As part of its commissioning, the Vera C. Rubin Observatory observed several fields repeatedly for a month with ComCam, an instrument that uses the same hardware as the LSST camera but covers a smaller field of view. We photometrically classify AT 2024ahzi, a transient discovered by ComCam, as a Type IIP supernova (SN IIP) using both ComCam and DECam photometry. We find that the duration, luminosity, and color of AT 2024ahzi's photometric plateau are all consistent with those from a large sample of SNe II. By comparing its multi-band light curves to SN II models and analytic relations, we place constraints on the SN progenitor, explosion dynamics, and circumstellar environment. We argue that the progenitor has an extended density profile indistinguishable from a slowly accelerating CSM. We discuss how a similar workflow can identify and characterize future Rubin SNe II.

There is a persistent tension of about 5 sigma-6 sigma between the value of the Hubble constant, as derived from the local distance ladder versus the cosmic microwave background, signaling either unaccounted for systematics in the measurements or "new physics," such as early dark energy. Determining the Hubble constant using Type Ia supernovae requires nontrivial and accurate corrections for dust extinction. To circumvent this obstacle, we here determine the Hubble constant from blue, and hence presumably unextinguished, supernovae. For two different sets of Type Ia supernova data and lightcurve fitting methods, we find that when using blue supernovae only, the derived Hubble constant is consistently lower by similar to 3 km s-1 Mpc-1 (70 +/- 2.1 and 70.3 +/- 3.0 km s-1 Mpc-1), and within 1 sigma of the cosmic microwave background measurement, compared to when using all supernovae. This is consistent with the hypothesis that systematic effects in dust corrections may affect standard supernova cosmology. However, the number of blue calibrating Type Ia supernovae is small (about six), and values of the Hubble constant for a range of different supernova colors are consistent at the 1.2 sigma level. Upcoming major transient surveys will discover numerous unextinguished SNe Ia, and thus be able to increase the precision of the Hubble constant measured from blue SNe Ia, heralding a promising path toward resolving the Hubble constant tension.

We present multi-wavelength observations and a comprehensive analysis of the nearby (D∼14 Mpc) Type IIb supernova (SN IIb) 2024iss. Observations of SN2024iss include an early ZTF detection at ∼40 minutes after first light and the earliest Hubble Space Telescope UV spectrum for a SN IIb to date at 7 days after first light. With the bolometric light curve and He-star models, we estimate an ejecta mass range of ∼ 1.1-3.3 M_⊙ and a ^56Ni mass of 0.11 ± 0.01 M_⊙. We fit shock-cooling emission models to the first peak in the light curve and estimate a progenitor radius of 100-320 R_⊙ and a H-rich envelope mass of 0.07-0.46 M_⊙. We also compared optical/UV spectra to binary progenitor model spectra, which indicate a stripped H-rich envelope mass of 0.19-0.28 M_⊙. We use early-time X-ray detections to calculate CSM densities that are consistent with a progenitor mass-loss rate of 5×10^-4 M_⊙ (v_w = 100km/s), corresponding to a period of significant mass ejection in the final 2-5 years before core collapse. In the UV spectrum, we observe strong Mg II emission extending to ∼15,000km/s as well as weak P-Cygni profiles of iron-group elements (e.g., Fe, Ti, Al, Ni) present in the outer SN ejecta during the end of shock cooling phase. We find that the overall spectroscopic evolution of SN2024iss is comparable to other SNe IIb, but that the increased brightness following the initial light curve peak is likely influenced by SN ejecta-CSM interaction. Finally, optical/NIR nebular spectroscopy of SN2024iss at ∼ 260-412 days reveals multi-peaked forbidden line profiles of O I and Mg I] indicative of inner ejecta asymmetry and/or clumping. We demonstrate the utility of a rich, multi-wavelength dataset for constraining the progenitor systems and explosion dynamics of SNe IIb.

In this paper, we first present observations of SN 2024acyl, a normal Type Ibn supernova (SN), with a large projected offset (∼35 kpc) from its host galaxy. The low star formation rate measured at the explosion site raises the possibility that the progenitor of SN 2024acyl may not have been a massive star. We then examine, more broadly, the spectral diversity of Type Ibn supernovae (SNe Ibn) around 20–35 days after peak brightness and identify two distinct groups: Group I, which shows bluer rest-frame optical color and narrower He I emission lines; and Group II, which shows redder rest-frame optical color and broader He I lines. Group I also tends to show higher peak luminosities. The diversity we identify appears to be closely connected to the diversity observed around the peak and to persist into late phases (> 80 days after peak). Given its redder color and broader He I lines, we classify SN 2024acyl as belonging to Group II. Based on the current dataset, we find no clear connection between this spectral diversity and either the host environments of SNe Ibn or their pre-explosion activity. The observed diversity in SNe Ibn likely reflects differences in circumstellar material properties and/or explosion energetics. These differences could result from a range of progenitor properties, such as different helium star mass, orbital period, and companion type if they are in binary systems, and may indicate fundamentally diverse progenitors. Whether a continuous distribution exists between the two groups remains to be determined and will require further data to explore.

The outer rings (ORs) of Supernova (SN) 1987A were ejected 20000 years before the explosion. Their characterisation is crucial for constraining the properties of the progenitor of this famous SN. While numerous studies investigated in detail the ejecta, equatorial ring (ER), and reverse shocks, few were dedicated to the ORs. We fill this gap and investigate the ORs physical properties. We analyse data obtained over a long temporal period, from multiple instruments, and over a wide wavelength range from optical to mid-infrared of the northern and southern ORs (NOR and SOR). We combine observations taken with HST between 1994 and 2022, VLT/MUSE in 2023, and JWST in 2022 and 2024. We measure emission flux in the ORs in HST and JWST/NIRCam images. We extract optical and mid-infrared spectra for the ORs in MUSE and JWST/MIRI/MRS data and measure line emission fluxes. We analyse the evolution of the ORs clumps' morphology over time with HST. The optical lightcurves of the ORs have shown a steady decline with time over the last 30 years. It is expected as the ORs were ionised by the initial SN UV-flash and are since then fading. The observations do not show any sign of interaction of the SN ejecta with the ORs. We estimated the decay times for [O III] to be 900 and 680 days for the NOR and SOR, and for Halpha+[N II] to be 15870 and 7160 days for the NOR and SOR. We constrained the temperature from the optical [N II] lines to 13400-16900K and 11800-14500K for the NOR and SOR. We constrained the electron density from the optical [S II] lines to 610-670cm-3 and 720-790cm-3 for the NOR and SOR. The spectra of the ORs differ significantly from the spectrum of the ER in lines detected and line ratios. The ORs will likely keep on fading for the next years, until the SN ejecta sweep them up. Continued monitoring of SN1987A and its ring system at all wavelengths is essential to capture this instant.

The multiply imaged SN 2022riv was discovered through a search of galaxy cluster fields as part of a Hubble Space Telescope (HST) SNAP program to find highly magnified stars. The supernova (SN) was detected in the last-to-arrive image of a galaxy at redshift z=1.522 strongly lensed by the foreground galaxy cluster RX J2129.7+0005. Follow up James Webb Space Telescope (JWST) NIRSpec G140M and PRISM spectroscopy yields a Type Ia SN classification. Using the SALT3-NIR light-curve fitter, we obtain a cosmology-independent measurement of the magnification of 5.35±1.01 for the last-to-arrive image of the SN, with multiple SALT SN spectral time-series models yielding consistent constraints. The last-to-arrive image of SN 2022riv we detect appeared adjacent to the brightest cluster galaxy (BCG) at a location with an exceptionally high stellar mass density (∼ 1-2 dex higher than that of SN Refsdal), where microlensing is expected to introduce a 20-50

The upcoming Vera C. Rubin Legacy Survey of Space and Time (LSST) will discover tens of thousands of astrophysical transients per night, far outpacing available spectroscopic follow-up capabilities. Carefully prioritising candidates for follow-up observations will maximise the scientific return from small telescopes with a single-object spectrograph. We introduce AAS2RTO, an astrophysical transient candidate prioritisation tool written in Python. AAS2RTO is flexible in that any number of criteria that consider observed properties of transients can be implemented. The visibility of candidates from a given observing site is also considered. The prioritised list of candidates provided by AAS2RTO is continually updated when new transient data are made available. Therefore, it can be applied to observing campaigns with a wide variety of scientific motivations. AAS2RTO uses a greedy algorithm to prioritise candidates. Candidates are represented by a single numerical value, or `score'. Scores are computed by constructing simple numerical factors which individually consider the competing facets of a candidate which make it suitable for follow-up observation. AAS2RTO is currently configured to work primarily with photometric data from the Zwicky Transient Facility (ZTF), distributed by certified LSST community brokers. We provide an example of how AAS2RTO can be used by defining a set of criteria to prioritise observations of type Ia supernovae (SNe Ia) close to peak brightness, in preparation for observations with the spectrograph at the Danish-1.54m telescope. Using a sample of archival alerts from ZTF, we evaluate the criteria we have designed to estimate the number of SNe Ia that we will be able to observe with a 1.5m telescope. Finally, we evaluate the performance of our criteria when applied to mock LSST observations of SNe Ia.

We present a detailed analysis of AT 2020nov, a tidal disruption event (TDE) in the center of its host galaxy, located at a redshift of z = 0.083. AT 2020nov exhibits unique features, including double-peaked Balmer emission lines, a broad UV/optical flare, and a peak log luminosity in the extreme-ultraviolet (EUV) estimated at ∼ 45.6 6 − 0.33 + 0.10 erg s − 1 . A late-time X-ray flare was also observed, reaching an absorbed luminosity of 1.67 × 1043 erg s−1 approximately 300 days after the UV/optical peak. Multiwavelength coverage, spanning optical, UV, X-ray, and mid-infrared (MIR) bands, reveals a complex spectral energy distribution (SED) that includes MIR flaring indicative of dust echoes, suggesting a dust covering fraction consistent with typical TDEs. Spectral modeling indicates the presence of an extended, quiescent disk around the central supermassive black hole with a radius of ∼ 5.0 6 − 0.77 + 0.59 × 1 0 4 R g . The multicomponent SED model, which includes a significant EUV component, suggests that the primary emission from the TDE is reprocessed by this extended disk, producing the observed optical and MIR features. The lack of strong active galactic nuclei signatures in the host galaxy, combined with the quiescent disk structure, highlights AT 2020nov as a rare example of a TDE occurring in a galaxy with a dormant but extended preexisting accretion structure.

We present a new photometric pipeline for the detection of pre-supernova (pre-SN) emission in the Young Supernova Experiment (YSE) sky survey. The method described is applied to SN 2020tlf, a type II SN (SN II) with precursor emission in the last 100 days before first light. We re-analyze the YSE griz-band light curves of SN 2020tlf and provide revised pre-explosion photometry that includes a robust list of confident detection and limiting magnitudes. Compared to the results of Jacobson-Galan et al. 2022a, this new analysis yields fewer total r/i/z-band pre-SN detections at phases > -100 days. Furthermore, we discourage the use of the blackbody modeling of the pre-explosion spectral energy distribution, the pre-SN bolometric light curve and the blackbody model parameters presented in Jacobson-Galan et al. 2022a. Nevertheless, binned photometry of SN 2020tlf confirms a consistent progenitor luminosity of 10^40 erg s^-1 before explosion.

Type Ia supernovae (SNe Ia) are fundamental to cosmology and galactic chemical evolution, yet the nature of their progenitor systems remains unresolved. Multiple evolutionary pathways, including single-degenerate, double-degenerate, and helium-donor systems, are thought to contribute to the SN Ia population, but direct observational constraints are limited. This uncertainty hampers our understanding of SN Ia diversity and introduces systematic uncertainties in their use as precision cosmological probes. By the 2040s, surveys such as Gaia, LSST, SDSS-V, 4MOST, and the gravitational-wave mission LISA will identify thousands of compact binaries in the Milky Way that are potential SN Ia progenitors. However, survey discoveries alone are insufficient. Robust identification and characterization require high-time-resolution, phase-resolved spectroscopy to determine fundamental parameters such as component masses, orbital inclinations, chemical compositions, and accretion states. Addressing these challenges demands new observational capabilities. The most compact binaries require continuous, dead-time-free spectroscopy with negligible readout noise, while the progenitor population spans a wide range of brightness and orbital periods. A modular, multi-aperture telescope array equipped with fast, low-noise spectrographs can flexibly combine collecting area for faint targets, observe bright systems efficiently, and deliver uninterrupted time series through staggered exposures. Such observations are difficult for single-aperture facilities.

In this paper, we first present observations of SN~2024acyl, a normal Type Ibn supernova with a large projected offset ($\sim$35~kpc) from its host galaxy. The low star-formation rate measured at the explosion site raises the possibility that the progenitor of SN~2024acyl may not have been a massive star. We then examine, more broadly, the spectral diversity of Type Ibn supernovae around 20--35 days after peak brightness and identify two distinct groups: Group I, which shows bluer rest-frame optical color and narrower He~I emission lines; and Group II, which shows redder rest-frame optical color and broader He~I lines. Group~I also tends to show higher peak luminosities. The diversity we identify appears to be closely connected to the diversity observed around peak and to persist into late phases ($>80$ days after peak). Given its redder color and broader He~I lines, we classify SN~2024acyl as belonging to Group II. Based on the current dataset, we find no clear connection between this spectral diversity and either the host environments of Type Ibn SNe or their pre-explosion activity. The observed diversity in Type Ibn SNe likely reflects differences in circumstellar material properties and/or explosion energetics. These differences could result from a range of progenitor properties, such as different helium star mass, orbital period and companion type if they are in binary systems, and may indicate fundamentally diverse progenitors. Whether a continuous distribution exists between the two groups remains to be determined and will require further data to explore.

Understanding the evolution of dust in galaxies is crucial because it affects the dynamics and cooling of gas, star formation, and chemical evolution. Recent work on dust removal in galaxies indicates timescales of gigayears, with old stellar populations and active galactic nuclei the primary drivers of this process. However, most statistically significant studies have focused on low redshifts, z < 0.4. Here we determine the dust removal timescale in galaxies over a wide range of redshifts, up to z similar to 5. We used publicly available catalogue data of infrared-selected galaxies observed by Herschel. Using the inferred dust masses, stellar masses, and stellar ages, we calculated the dust removal timescale in a sample of more than 120 000 galaxies. We find that, with increasing redshift, the dust removal timescale decreases from 1.8 Gyr at redshift z similar to 0.05 to less than 450 Myr at z > 3. Galaxies at higher redshifts undergo more efficient dust removal than galaxies at lower redshifts, likely driven by active galactic nucleus activity, supernova shocks, and astration. These findings indicate that dust removal evolves over cosmic time, reflecting the changing mechanisms regulating the dust content of galaxies as the Universe evolves.

Strongly lensed supernovae (SNe) are a rare class of transient that can offer tight cosmological constraints that are complementary to methods from other astronomical events. We present a follow-up study of one recently discovered strongly lensed SN, the quadruply imaged type Ia SN 2022qmx (aka “SN Zwicky”), at z = 0.3544. We measure updated, template-subtracted photometry for SN Zwicky and derive improved time delays and magnifications. This is possible because SNe are transient, fading away after reaching their peak brightness. Specifically, we measure point-spread-function photometry for all four images of SN Zwicky in three Hubble Space Telescope WFC3/UVIS passbands (F475W, F625W, and F814W) and one WFC3/IR passband (F160W), with template images taken ∼11 months after the epoch in which the SN images appear. We find consistency to within 2 σ between lens-model-predicted time delays (≲1 day) and measured time delays with HST colors (≲2 days), including the uncertainty from chromatic microlensing that may arise from stars in the lensing galaxy. The standardizable nature of SNe Ia allows us to estimate absolute magnifications for the four images, with images A and C being elevated in magnification compared to lens model predictions by about 6 σ and 3 σ , respectively, confirming previous work. We show that millilensing or differential dust extinction is unable to explain these discrepancies, and we find evidence for the existence of microlensing in images A, C, and potentially D that may contribute to the anomalous magnification.

Quenching of star formation plays a fundamental role in galaxy evolution. This process occurs due to the removal of the cold interstellar medium (ISM) or stabilization against collapse, so that gas cannot be used in the formation of new stars. In this paper, we study the effect of different mechanisms of ISM removal. In particular, we revised the well-known Baldwin–Philips–Terlevich (BPT) and EW H α versus [N ii ]/H α (WHAN) emission-line ratio diagnostics, so that we could classify all galaxies, even those not detected at some emission lines, introducing several new spectral classes. We use spectroscopic data and several physical parameters of 2409 dusty early-type galaxies in order to find out the dominant ionization source (active galactic nuclei, AGNs, young massive stars, hot low-mass evolved stars, HOLMES) and its effect on the ISM. We find that strong AGNs can play a significant role in the ISM removal process only for galaxies with ages lower than 10 9.4 yr, but we cannot rule out the influence of weak AGNs at any age. For older galaxies, HOLMES/planetary nebulae contribute significantly to the ISM removal process. Additionally, we provide the BPT and WHAN classifications not only for the selected sample but also for all 300,000 galaxies in the Galaxy And Mass Assembly survey fields.

Type Ia supernovae (SNe Ia) are standardizable candles: their peak magnitudes can be corrected for correlations between light-curve properties and their luminosities to precisely estimate distances. Understanding SN Ia standardization across wavelength improves methods for correcting SN Ia magnitudes. Using 150 SNe Ia from the Foundation Supernova Survey and Young Supernova Experiment, we present the first study focusing on SN Ia standardization properties in the z band. Straddling the optical and near-infrared, SN Ia light in the z band is less sensitive to dust extinction and can be collected alongside the optical on CCDs. Pre-standardization, SNe Ia exhibit less residual scatter in z-band peak magnitudes than in the g and r bands. SNe Ia peak z-band magnitudes still exhibit a significant dependence on light-curve shape. Post-standardization, the z-band Hubble diagram has a total scatter of root mean square=0.195 mag. We infer a z-band mass step of mag, gamma(z)=-0.105 +/- 0.031 which is consistent within 1 sigma of that estimated from gri data, assuming . R-V=2.61. When assuming different R(v )values for high and low mass host galaxies, the z band and optical mass steps remain consistent within 1 sigma . Based on current statistical precision, these results suggest dust reddening cannot fully explain the mass step. SNe Ia in the z band exhibit complementary standardizability properties to the optical that can improve distance estimates. Understanding these properties is important for the upcoming Vera Rubin Observatory and Nancy G. Roman Space Telescope, which will probe the rest-frame z band to redshifts 0.1 and 1.8.

We present the second and final release of optical spectroscopy of Type Ia Supernovae (SNe Ia) obtained during the first and second phases of the Carnegie Supernova Project (CSP-I and CSP-II). The newly released data consist of 148 spectra of 30 SNe Ia observed in the course of the CSP-I, and 234 spectra of 127 SNe Ia obtained during the CSP-II. We also present 216 optical spectra of 46 historical SNe Ia, including 53 spectra of 30 SNe Ia observed by the Calán/Tololo Supernova Survey. We combine these observations with previously published CSP data and publicly-available spectra to compile a large sample of measurements of spectroscopic parameters at maximum light, consisting of pseudo-equivalent widths and expansion velocities of selected features, for 232 CSP and historical SNe Ia (including more than 1000 spectra). Finally, we review some of the strongest correlations between spectroscopic and photometric properties of SNe Ia. Specifically, we define two samples: one consisting of SNe Ia discovered by targeted searches (most of them CSP-I objects) and the other composed of SNe Ia discovered by untargeted searches, which includes most of the CSP-II objects. The analysed correlations are similar for both samples. We find a larger incidence of SNe Ia belonging to the Cool (CL)and Broad Line (BL) Branch subtypes among the events discovered by targeted searches, Shallow Silicon (SS) SNe Ia are present with similar frequencies in both samples, while Core Normal (CN) SNe Ia are more frequent in untargeted searches.

We present a comprehensive analysis of the photometric and spectroscopic evolution of SN 2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow (approximate to 300-600 km s-1) absorption lines of hydrogen, helium, calcium, and iron together with broad helium emission lines with a full width at half-maximum (FWHM) of similar to 6000 km s-1. For a steady, wind mass-loss regime, light-curve modeling results in an ejecta mass of similar to 8 M circle dot and circumstellar material (CSM) mass below 1 M circle dot, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of approximate to 2 M circle dot yr-1. This mass-loss rate is 3 orders of magnitude larger than derived for normal Type II supernovae. We estimate that the bulk of the CSM of SN 2021foa must have been expelled within half a year, about 12 yr ago. Our analysis suggests that SN 2021foa had a helium-rich ejecta that swept up a dense shell of hydrogen-rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the redshifted emission of the hydrogen and helium lines, which results in an observed blueshift (similar to-3000 km s-1). Strong mass-loss activity prior to explosion, such as those seen in SN 2009ip-like objects and SN 2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star.

We present UV-optical-near-infrared observations and modeling of supernova (SN) 2024ggi, a type II supernova (SN II) located in NGC 3621 at 7.2 Mpc. Early-time ("flash") spectroscopy of SN 2024ggi within +0.8 days of discovery shows emission lines of H i, He i, C iii, and N iii with a narrow core and broad, symmetric wings (i.e., "IIn-like") arising from the photoionized, optically thick, unshocked circumstellar material (CSM) that surrounded the progenitor star at shock breakout (SBO). By the next spectral epoch at +1.5 days, SN 2024ggi showed a rise in ionization as emission lines of He ii, C iv, N iv/v, and O v became visible. This phenomenon is temporally consistent with a blueward shift in the UV-optical colors, both likely the result of SBO in an extended, dense CSM. The IIn-like features in SN 2024ggi persist on a timescale of t(IIn) = 3.8 +/- 1.6 days, at which time a reduction in CSM density allows the detection of Doppler-broadened features from the fastest SN material. SN 2024ggi has peak UV-optical absolute magnitudes of M-w2 = -18.7 mag and M-g = -18.1 mag, respectively, that are consistent with the known population of CSM-interacting SNe II. Comparison of SN 2024ggi with a grid of radiation hydrodynamics and non-local thermodynamic equilibrium radiative-transfer simulations suggests a progenitor mass-loss rate of M-center dot=10-2M(circle dot) yr(-1) (v(w) = 50 km s(-1)), confined to a distance of r < 5 x 10(14) cm. Assuming a wind velocity of v(w) = 50 km s(-1), the progenitor star underwent an enhanced mass-loss episode in the last similar to 3 yr before explosion.

Supernova (SN) H0pe is a gravitationally lensed, triply-imaged, Type Ia SN (SN Ia) discovered in James Webb Space Telescope imaging of the PLCK G165.7+67.0 cluster of galaxies. Well-observed multiply-imaged SNe provide a rare opportunity to constrain the Hubble constant ($H_0$), by measuring the relative time delay between the images and modeling the foreground mass distribution. SN H0pe is located at $z=1.783$, and is the first SN Ia with sufficient light curve sampling and long enough time delays for an $H_0$ inference. Here we present photometric time-delay measurements and SN properties of SN H0pe. Using JWST/NIRCam photometry we measure time delays of $\Delta t_{ab}=-116.6^{+10.8}_{-9.3}$ and $\Delta t_{cb}=-48.6^{+3.6}_{-4.0}$ observer-frame days relative to the last image to arrive (image 2b; all uncertainties are $1\sigma$), which corresponds to a $\sim5.6\%$ uncertainty contribution for $H_0$ assuming $70 \rm{km s^{-1} Mpc^{-1}}$. We also constrain the absolute magnification of each image to $\mu_{a}=4.3^{+1.6}_{-1.8}$, $\mu_{b}=7.6^{+3.6}_{-2.6}$, $\mu_{c}=6.4^{+1.6}_{-1.5}$ by comparing the observed peak near-IR magnitude of SN H0pe to the non-lensed population of SNe Ia.

A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class that can be characterised by both rapidly evolving light curves and persistent narrow He I lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multi-wavelength data of the Type Ibn SN 2020nxt, including $HST$/STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a $\lesssim4 {\rm M}_\odot$ helium star that lost its $\sim 1\,{\rm M}_\odot$ He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive ($>30 {\rm M}_\odot$) Wolf-Rayet-like stars.