Asymmetries in core-collapse supernovae from maps of radioactive 44Ti in Cassiopeia A

Presolar Grains as Probes of Supernova Nucleosynthesis

We provide an overview of the isotopic signatures of presolar supernova grains, specifically focusing on 44Ti-containing grains with robustly inferred supernova origins and their implications for nucleosynthesis and mixing mechanisms in supernovae. Recent technique advancements have enabled the differentiation between radiogenic (from 44Ti decay) and nonradiogenic 44Ca excesses in presolar grains, made possible by enhanced spatial resolution of Ca-Ti isotope analyses with the Cameca NanoSIMS (Nano-scale Secondary Ion Mass Spectrometer) instrument. Within the context of presolar supernova grain data, we discuss (i) the production of 44Ti in supernovae and the impact of interstellar medium heterogeneities on the galactic chemical evolution of 44Ca/40Ca, (ii) the nucleosynthesis processes of neutron bursts and explosive H-burning in Type II supernovae, and (iii) challenges in identifying the progenitor supernovae for 54Cr-rich presolar nanospinel grains. Drawing on constraints and insights derived from presolar supernova grain data, we also provide an overview of our current understanding of the roles played by various supernova types - including Type II, Type Ia, and electron capture supernovae - in accounting for the diverse array of nucleosynthetic isotopic variations identified in bulk meteorites and meteoritic components. We briefly overview the potential mechanisms that have been proposed to explain these nucleosynthetic variations by describing the transport and distribution of presolar dust carriers in the protoplanetary disk. We highlight existing controversies in the interpretation of presolar grain data and meteoritic nucleosynthetic isotopic variations, while also outlining potential directions for future research.

Gamma-ray observations of cosmic nuclei

Gamma rays from nuclear processes such as radioactive decay and de-excitations are among the most-direct tools to witness the production and existence of specific nuclei and isotopes in and near cosmic nucleosynthesis sites. With space-borne instrumentation such as NuSTAR and SPI/INTEGRAL, and experimental techniques to handle a substantial instrumental background from cosmic-ray activations of the spacecraft and instrument, unique results have been obtained, from diffuse emissions of nuclei and positrons in interstellar surroundings of sources, as well as from observations of cosmic explosions and their radioactive afterglows. These witness non-sphericity in supernova explosions and a flow of nucleosynthesis ejecta through superbubbles as common source environments. Next-generation experiments that are awaiting space missions promise a next level of observational nuclear astrophysics.

Sub-nanograin metal based high efficiency multilayer reflective optics for high energies

The present finding illuminates the physics of the formation of interfaces of metal based hetero-structures near layer continuous limit as an approach to develop high-efficiency W/B4C multilayer (ML) optics with ML periodicity varying d = 1.86-1.23 nm at a fixed number of layer pairs N = 400. The microstructure of metal layers is tailored near the onset of grain growth to control the surface density of grains resulting in small average sizes of grains to sub-nanometers. This generates concurrently desirable atomically sharp interfaces, high optical contrast, and desirable stress properties over a large number of periods, which have evidence through the developed ML optics. We demonstrate significantly high reflectivities of ML optics measured in the energy range 10-20 keV, except for d = 1.23 nm due to quasi-continuous layers. The reflectivities at soft gamma-rays are predicted.

High-entropy ejecta plumes in Cassiopeia A from neutrino-driven convection

Recent multi-dimensional simulations suggest that high-entropy buoyant plumes help massive stars to explode^1,2. Outwardly protruding iron (Fe)-rich fingers of gas in the galactic supernova remnant^3,4 Cassiopeia A seem to match this picture. Detecting the signatures of specific elements synthesized in the high-entropy nuclear burning regime (that is, α-rich freeze out) would constitute strong substantiating evidence. Here we report observations of such elements-stable titanium (Ti) and chromium (Cr)-at a confidence level greater than 5 standard deviations in the shocked high-velocity Fe-rich ejecta of Cassiopeia A. We found that the observed Ti/Fe and Cr/Fe mass ratios require α-rich freeze out, providing evidence of the existence of the high-entropy ejecta plumes that boosted the shock wave at explosion. The metal composition of the plumes agrees well with predictions for strongly neutrino-processed proton-rich ejecta^2,5,6. These results support the operation of the convective supernova engine via neutrino heating in the supernova that produced Cassiopeia A.

The exceptional X-ray evolution of SN 1996cr in high resolution

We present X-ray spectra spanning 18 yr of evolution for SN 1996cr, one of the five nearest SNe detected in the modern era. Chandra HETG exposures in 2000, 2004, and 2009 allow us to resolve spectrally the velocity profiles of Ne, Mg, Si, S, and Fe emission lines and monitor their evolution as tracers of the ejecta-circumstellar medium interaction. To explain the diversity of X-ray line profiles, we explore several possible geometrical models. Based on the highest signal-to-noise 2009 epoch, we find that a polar geometry with two distinct opening angle configurations and internal obscuration can successfully reproduce all of the observed line profiles. The best-fitting model consists of two plasma components: (1) a mildly absorbed (2 × 1021 cm-2), cooler (≈2 keV) with high Ne, Mg, Si, and S abundances associated with a wide polar interaction region (half-opening angle ≈58°); (2) a moderately absorbed (2 × 1022 cm-2), hotter (≳20 keV) plasma with high Fe abundances and strong internal obscuration associated with a narrow polar interaction region (half-opening angle ≈20°). We extend this model to seven further epochs with lower signal-to-noise ratio and/or lower spectral-resolution between 2000 and 2018, yielding several interesting trends in absorption, flux, geometry, and expansion velocity. We argue that the hotter and colder components are associated with reflected and forward shocks, respectively, at least at later epochs. We discuss the physical implications of our results and plausible explosion scenarios to understand the X-ray data of SN 1996cr.

Heavy Element Nucleosynthesis

This contribution deals with the important subject of the nucleosynthesis of heavy elements in the Galaxy. After an overview of several observational features, the physical processes responsible mainly for the formation of heavy elements will be described and linked to possible stellar sites and to galactic chemical evolution. In particular, we focus on the neutron-capture processes, namely the s-process (slow neutron capture) and the r-process (rapid neutron capture) and discuss some problems in connection with their sites and their outcome. The aim is to give a brief overview on the exciting subject of the heavy element nucleosynthesis in the Galaxy, emphasizing its importance to trace the galactic chemical evolution and illustrating the challenge of this subject.

COORDINATED ANALYSIS OF TWO GRAPHITE GRAINS FROM THE CO3.0 LAP 031117 METEORITE: FIRST IDENTIFICATION OF A CO NOVA GRAPHITE AND A PRESOLAR IRON SULFIDE SUBGRAIN

Presolar grains constitute the remnants of stars that existed before the formation of the solar system. In addition to providing direct information on the materials from which the solar system formed, these grains provide ground-truth information for models of stellar evolution and nucleosynthesis. Here we report the in situ identification of two unique presolar graphite grains from the primitive meteorite LaPaz Icefield 031117. Based on these two graphite grains, we estimate a bulk presolar graphite abundance of 5 - 3 + 7 ppm in this meteorite. One of the grains (LAP-141) is characterized by an enrichment in ¹²C and depletions in ^33,34S, and contains a small iron sulfide subgrain, representing the first unambiguous identification of presolar iron sulfide. The other grain (LAP-149) is extremely ¹³C-rich and ¹⁵N-poor, with one of the lowest ¹²C/¹³C ratios observed among presolar grains. Comparison of its isotopic compositions with new stellar nucleosynthesis and dust condensation models indicates an origin in the ejecta of a low-mass CO nova. Grain LAP-149 is the first putative nova grain that quantitatively best matches nova model predictions, providing the first strong evidence for graphite condensation in nova ejecta. Our discovery confirms that CO nova graphite and presolar iron sulfide contributed to the original building blocks of the solar system.

Accurate lineshape spectroscopy and the Boltzmann constant

Spectroscopy has an illustrious history delivering serendipitous discoveries and providing a stringent testbed for new physical predictions, including applications from trace materials detection, to understanding the atmospheres of stars and planets, and even constraining cosmological models. Reaching fundamental-noise limits permits optimal extraction of spectroscopic information from an absorption measurement. Here, we demonstrate a quantum-limited spectrometer that delivers high-precision measurements of the absorption lineshape. These measurements yield a very accurate measurement of the excited-state (6P1/2) hyperfine splitting in Cs, and reveals a breakdown in the well-known Voigt spectral profile. We develop a theoretical model that accounts for this breakdown, explaining the observations to within the shot-noise limit. Our model enables us to infer the thermal velocity dispersion of the Cs vapour with an uncertainty of 35 p.p.m. within an hour. This allows us to determine a value for Boltzmann's constant with a precision of 6 p.p.m., and an uncertainty of 71 p.p.m.

Supernovae. The bubble-like interior of the core-collapse supernova remnant Cassiopeia A

The death of massive stars is believed to involve aspheric explosions initiated by the collapse of an iron core. The specifics of these catastrophic explosions remain uncertain, due partly to limited observational constraints on asymmetries deep inside the star. Here we present near-infrared observations of the young supernova remnant Cassiopeia A, descendant of a type IIb core-collapse explosion, and a three-dimensional map of its interior unshocked ejecta. The remnant's interior has a bubble-like morphology that smoothly connects to and helps explain the multiringed structures seen in the remnant's bright reverse-shocked main shell of expanding debris. This internal structure may originate from turbulent mixing processes that encouraged outwardly expanding plumes of radioactive (56)Ni-rich ejecta. If this is true, substantial amounts of its decay product, (56)Fe, may still reside in these interior cavities.

Reflective multilayer optic as hard X-ray diagnostic on laser-plasma experiment

A multilayer-based optic was tested for use as an X-ray diagnostic on a laser-plasma experiment. The multilayer optic was employed to selectively pass X-rays between 55 and 100 keV. An order of magnitude improvement in signal-to-noise ratio is achieved compared to a transmission crystal spectrometer. A multilayer response model, taking into account the source size and spectral content, is constructed and the outlook for application above 500 keV is briefly discussed. LLNL-JRNL-664311.

Demonstration of multilayer reflective optics at photon energies above 0.6 MeV

Focusing optics operating in the soft gamma-ray photon energy range can advance a range of scientific and technological applications that benefit from the large improvements in sensitivity and resolution that true imaging provides. An enabling technology to this end is multilayer coatings. We show that very short period multilayer coatings deposited on super-polished substrates operate efficiently above 0.6 MeV. These experiments demonstrate that Bragg scattering theory established for multilayer applications as low as 1 eV continues to work well into the gamma-ray band.