We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 d < t1/2 < 12 d, of which 28 have blue (g-r<-0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had high-S/N predominantly featureless spectra and luminous radio emission: AT2018lug and AT2020xnd. Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad H$\alpha$ in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (six) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 22 of the transients. All X-ray and radio observations resulted in non-detections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs, and use ZTF's SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate. 

https://iopscience.iop.org/article/10.3847/1538-4357/acc533/pdf

A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients

A model for the formation of CO in the expanding ejecta of SN 1987A suggests that the 4.6-micron and 2.3 micron emissions noted in the SN spectrum may originate from the inner metal-rich regions of the ejecta. Also reported are the assignment of a 2.26-micron excess feature in the high-resolution IR spectra to the v = 2 to 0 overtone of CO(+) and the possible identification of a 2.95-micron feature with the v = 2 to 0 transition of electronically excited CO. The results suggest that the regions in which CO formation takes place must be ionized to a substantially lower degree than regions which dominate the line emission of heavier metals. 60 refs.

CO Formation in the Metal-Rich Ejecta of SN1987A

We present photometric and spectroscopic observations of Supernova 2020oi (SN 2020oi), a nearby ($\sim$17 Mpc) type-Ic supernova (SN Ic) within the grand-design spiral M100. We undertake a comprehensive analysis to characterize the evolution of SN 2020oi and constrain its progenitor system. We detect flux in excess of the fireball rise model $\delta t \approx 2.5$ days from the date of explosion in multi-band optical and UV photometry from the Las Cumbres Observatory and the Neil Gehrels Swift Observatory, respectively. The derived SN bolometric luminosity is consistent with an explosion with $M_{\rm ej} = 0.81 \pm 0.03 M_{\odot}$, $E_{k}= 0.79 \pm 0.09 \times 10^{51} \rm{erg} \rm{s}^{-1}$, and $M_{\rm Ni56} = 0.08 \pm 0.02 M_{\odot}$. Inspection of the event's decline reveals the highest $\Delta m_{15,\rm{bol}}$ reported for a stripped-envelope event to date. Modeling of optical spectra near event peak indicates a partially mixed ejecta comparable in composition to the ejecta observed in SN 1994I, while the earliest spectrum shows signatures of a possible interaction with material of a distinct composition surrounding the SN progenitor. Further, Hubble Space Telescope (HST) pre-explosion imaging reveals a stellar cluster coincident with the event. From the cluster photometry, we derive the mass and age of the SN progenitor using stellar evolution models implemented in the BPASS library. Our results indicate that SN 2020oi occurred in a binary system from a progenitor of mass $M_{\rm ZAMS} \approx 9.5 \pm 1.0 M_{\odot}$, corresponding to an age of $27 \pm 7$ Myr. SN 2020oi is the dimmest SN Ic event to date for which an early-time flux excess has been observed, and the first in which an early excess is unlikely to be associated with shock-cooling. 

An Early-time Optical and Ultraviolet Excess in the Type-Ic SN 2020oi

Supernovae (SNe) with photospheric spectra devoid of Hydrogen and Helium features are generally classified as Type Ic SNe (SNe Ic). However, there is ongoing debate as to whether Helium can be hidden in the ejecta of SNe Ic (that is, Helium is present in the ejecta, but produces no obvious features in the spectra). We present the first application of the fast, 1-D radiative transfer code TARDIS to a SN Ic, and we investigate the question of how much Helium can be hidden in the outer layers of the SN Ic ejecta. We generate TARDIS models for the nearby, well-observed, and extensively modeled SN Ic 1994I, and we perform a code comparison to a different, well-established Monte Carlo based radiation transfer code. The code comparison shows that TARDIS produces consistent synthetic spectra for identical ejecta models of SN1994I. In addition, we perform a systematic experiment of adding outer He shells of varying masses to our SN1994I models. We find that an outer He shell of only $0.05M_{\odot}$ produces strong optical and NIR He spectral features for SN1994I which are not present in observations, thus indicating that the SN1994I ejecta is almost fully He deficient compared to the He masses of typical He-rich SN progenitors. Finally we show that the He I $\lambda$20851 line pseudo equivalent width of our modeled spectra for SN1994I could be used to infer the outer He shell mass which suggests that NIR spectral follow-up of SNe Ic will be critical for addressing the hidden helium question for a statistical sample of SNe Ic.

Modelling Type Ic Supernovae with TARDIS: Hidden Helium in SN1994I?

We present 75 near-infrared (NIR; 0.8$-$2.5 $\mu$m) spectra of 34 stripped-envelope core-collapse supernovae (SESNe) obtained by the Carnegie Supernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb, Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days past maximum. This unique data set constitutes the largest NIR spectroscopic sample of SESNe to date. NIR spectroscopy provides observables with additional information that is not available in the optical. Specifically, the NIR contains the resonance lines of He I and allows a more detailed look at whether Type Ic supernovae are completely stripped of their outer He layer. The NIR spectra of SESNe have broad similarities, but closer examination through statistical means reveals a strong dichotomy between NIR "He-rich" and "He-poor" SNe. These NIR subgroups correspond almost perfectly to the optical IIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the two groups is observed in the 2 $\mu$m region, near the He I $\lambda$2.0581 $\mu$m line. The division between the two groups is not an arbitrary one along a continuous sequence. Early spectra of He-rich SESNe show much stronger He I $\lambda$2.0581 $\mu$m absorption compared to the He-poor group, but with a wide range of profile shapes. The same line also provides evidence for trace amounts of He in half of our SNe in the He-poor group. 

Carnegie Supernova Project-II: Near-infrared Spectroscopy of Stripped-envelope Core-collapse Supernovae*

We present near-infrared and optical observations of the Type Ic Supernova (SN) 2020oi in the galaxy M100 and the broad-lined Type Ic SN2020bvc in UGC 9379, using Gemini, LCO, SOAR, and other ground-based telescopes. The near-IR spectrum of SN2020oi at day 63 since the explosion shows strong CO emissions and a rising K-band continuum, which is the first unambiguous dust detection from a Type Ic SN. Non-LTE CO modeling shows that CO is still optically thick, and that the lower limit to the CO mass is 0.001 Msun. The dust temperature is 810 K, and the dust mass is ~10^(-5) Msun. We explore the possibilities that the dust is freshly formed in the ejecta, heated dust in the pre-existing circumstellar medium, and an infrared echo. The light curves of SN2020oi are consistent with a STELLA model with canonical explosion energy, 0.07 Msun Ni mass, and 0.7 Msun ejecta mass. A model of high explosion energy of ~10^(52) erg, 0.4 Msun Ni mass, 6.5 Msun ejecta mass with the circumstellar matter, reproduces the double-peaked light curves of SN2020bvc. We observe temporal changes of absorption features of the IR Ca~II triplet, S~I at 1.043 micron, and Fe~II at 5169 Angstrom. The blue-shifted lines indicate high velocities, up to 60,000 km/s for SN2020bvc and 20,000 km/s for SN2020oi, and the expansion velocity rapidly declines before the optical maximum. We present spectral signatures and diagnostics of CO and SiO molecular bands between 1.4 and 10 microns.

/pdf/near-infrared-and-optical-observations-of-type-ic-sn2020oi-4lzwq8mkof.pdf

Near-Infrared and Optical Observations of Type Ic SN2020oi and broad-lined Ic SN2020bvc: Carbon Monoxide, Dust and High-Velocity Supernova Ejecta

We present near-IR (NIR) and optical observations of the Type Ic supernova (SN Ic) SN 2020oi in the galaxy M100 and the broad-lined SN Ic SN 2020bvc in UGC 9379, using Gemini, Las Cumbres Observatory, Southern Astrophysical Telescope, and other ground-based telescopes. The NIR spectrum of SN 2020oi at day 63 since the explosion shows strong CO emissions and a rising K-band continuum, which is the first unambiguous dust detection from an SN Ic. Non-LTE CO modeling shows that CO is still optically thick and that the lower limit to the CO mass is 10?3 M⊙. The dust temperature is 810 K, and the dust mass is ～10?5 M⊙. We explore the possibilities that the dust is freshly formed in the ejecta, heated dust in the preexisting circumstellar medium, and an infrared echo. The light curves of SN 2020oi are consistent with a STELLA model with canonical explosion energy, 0.07 M⊙ Ni mass, and 0.7 M⊙ ejecta mass. A model of high explosion energy of 1052 erg, 0.4 M⊙ Ni mass, and 6.5 M⊙ ejecta mass with the circumstellar matter reproduces the double-peaked light curves of SN 2020bvc. We observe temporal changes of absorption features of the IR Ca ii triplet, S i at 1.043 μm, and Fe ii at 5169 ?. The blueshifted lines indicate high velocities, up to 60,000 km s?1 for SN 2020bvc and 20,000 km s?1 for SN 2020oi, and the expansion velocity rapidly declines before the optical maximum. We present modeled spectral signatures and diagnostics of CO and SiO molecular bands between 1.4 and 10 μm.

/pdf/near-infrared-and-optical-observations-of-type-ic-sn-2020oi-4nikp1t5jp.pdf

Near-infrared and Optical Observations of Type Ic SN 2020oi and Broad-lined Type Ic SN 2020bvc: Carbon Monoxide, Dust, and High-velocity Supernova Ejecta

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Near-Infrared and Optical Observations of Type Ic SN2020oi and broad-lined Ic SN2020bvc: Carbon Monoxide, Dust and High-Velocity Supernova Ejecta

Near-infrared and Optical Observations of Type Ic SN 2020oi and Broad-lined Type Ic SN 2020bvc: Carbon Monoxide, Dust, and High-velocity Supernova Ejecta