We present 56Ni mass estimates for 110 normal Type II supernovae (SNe II), computed here from their luminosity in the radioactive tail. This sample consists of SNe from the literature, with at least three photometric measurements in a single optical band within 95–320 d since explosion. To convert apparent magnitudes to bolometric ones, we compute bolometric corrections (BCs) using 15 SNe in our sample having optical and near-IR photometry, along with three sets of SN II atmosphere models to account for the unobserved flux. We find that the I- and i-band are best suited to estimate luminosities through the BC technique. The 56Ni mass distribution of our SN sample has a minimum and maximum of 0.005 and 0.177 M⊙, respectively, and a selection-bias-corrected average of 0.037 ± 0.005 M⊙. Using the latter value together with iron isotope ratios of two sets of core-collapse (CC) nucleosynthesis models, we calculate a mean iron yield of 0.040 ± 0.005 M⊙ for normal SNe II. Combining this result with recent mean 56Ni mass measurements for other CC SN subtypes, we estimate a mean iron yield <0.068 M⊙ for CC SNe, where the contribution of normal SNe II is >36 per cent. We also find that the empirical relation between 56Ni mass and steepness parameter (S) is poorly suited to measure the 56Ni mass of normal SNe II. Instead, we present a correlation between 56Ni mass, S, and absolute magnitude at 50 d since explosion. The latter allows to measure 56Ni masses of normal SNe II with a precision around 30 per cent.

We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star–black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg2 for the 90th percentile best localization), covering a total of 51 deg2 and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an r-band decline rate of 0.68 mag day−1, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most −17.8 mag (50% confidence). Our data are not constraining for “red” kilonovae and rule out “blue” kilonovae with M > 0.5 M ⊙ (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles <17° assuming an initial jet opening angle of ∼5.°2 and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.

We present DELIGHT, or Deep Learning Identification of Galaxy Hosts of Transients, a new algorithm designed to automatically and in real time identify the host galaxies of extragalactic transients. The proposed algorithm receives as input compact, multiresolution images centered at the position of a transient candidate and outputs two-dimensional offset vectors that connect the transient with the center of its predicted host. The multiresolution input consists of a set of images with the same number of pixels, but with progressively larger pixel sizes and fields of view. A sample of 16,791 galaxies visually identified by the Automatic Learning for the Rapid Classification of Events broker team was used to train a convolutional neural network regression model. We show that this method is able to correctly identify both relatively large (10″ < r < 60″) and small (r ≤ 10″) apparent size host galaxies using much less information (32 kB) than with a large, single-resolution image (920 kB). The proposed method has fewer catastrophic errors in recovering the position and is more complete and has less contamination (<0.86%) recovering the crossmatched redshift than other state-of-the-art methods. The more efficient representation provided by multiresolution input images could allow for the identification of transient host galaxies in real time, if adopted in alert streams from new generation of large -etendue telescopes such as the Vera C. Rubin Observatory.

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The amount of cosmic dust contributed by stellar sources in galaxies at all cosmic epochs remains a controversial topic, particularly whether or not supernovae (SNe) have an important role to play given the dust-hostile environments provided by SNe. To date, freshly-formed dust has been observed in a handful of core collapse (CC) SNe, both in the ejecta in-situ and in the interactions between the ejecta and circumstellar medium (CSM). As yet, there exists no clear observational evidence for dust formation in Type Ia SNe despite predictions of $3×10^{-4}$- $0.2$ Msun of dust forming per Ia. Here we report evidence of dust formation in the ejecta-CSM interaction in the Type Ia (SNIa) SN2018evt just three years after the explosion, characterized by a staggering rise in the mid-infrared (MIR) flux accompanied by an accelerated decline in the optical. This hypothesis is strengthened by the concurrent evolution of the profiles of the Hα emission lines. A preexisting hydrogen-rich torus which itself may be dusty before the SN explosion, provides a natural explanation of the observed data. SN 2018evt is the first SNIa with clear evidence of both dust destruction and formation in its ejecta and surroundings. The amount of the newly formed dust follows a steep power law rise of index 4 with time after the explosion. This steep rise indicates that the newly formed dust is formed in the SN ejecta, which is compressed by the shock interaction with the CSM.