Sharpening the Cutting Edge: Corporate Action for a Strong, Low-Carbon Economy

Outlines lessons learned from early efforts to create a low-carbon economy, current and emerging best practices, and next steps, including climate change metrics, greenhouse gas reporting, effective climate policy, and long-term investment choices

Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event

The cosmic origin of the elements heavier than iron has long been uncertain. Theoretical modelling shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted to power a distinctive thermal glow (a 'kilonova'). The discovery of an electromagnetic counterpart to the gravitational-wave source GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements. Here we report models that predict the detailed electromagnetic emission of kilonovae and enable the mass, velocity and composition of ejecta to be derived from the observations. We compare the models to the optical and infrared radiation associated with GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. Inferring the ejected mass and a merger rate from GW170817 implies that such mergers are a dominant mode of r-process production in the Universe.Comment: Natur

A Unified Picture of Short and Long Gamma-ray Bursts from Compact Binary Mergers

The recent detections of the $\sim10$-s long $\gamma$-ray bursts (GRBs) 211211A and 230307A followed by softer temporally extended emission (EE) and kilonovae, point to a new GRB class. Using state-of-the-art first-principles simulations, we introduce a unifying theoretical framework that connects binary neutron star (BNS) and black hole-NS (BH-NS) merger populations with the fundamental physics governing compact-binary GRBs (cbGRBs). For binaries with large total masses $M_{\rm tot}\gtrsim2.8\,M_\odot$, the compact remnant created by the merger promptly collapses into a BH, surrounded by an accretion disk. The duration of the magnetically arrested disk (MAD) phase sets the duration of the roughly constant power cbGRB and could be influenced by the disk mass, $M_d$: long cbGRBs such as 211211A are produced by massive disks ($M_d\gtrsim0.1\,M_\odot$), which form for large binary mass ratio $q\gtrsim1.2$ in BNS or $q\lesssim3$ in BH-NS mergers. Once the disk becomes MAD, the jet power drops with the mass accretion rate as $\dot{M}\sim t^{-2}$, establishing the EE decay. Two scenarios are plausible for short cbGRBs. They can be powered by BHs with less massive disks, which form for other $q$ values. Alternatively, for binaries with $M_{\rm tot}\lesssim2.8\,M_\odot$, mergers should go through a hypermassive NS (HMNS) phase, as inferred for GW170817. Magnetized outflows from such HMNSs, which typically live for $\lesssim1\,{\rm s}$, offer an alternative progenitor for short cbGRBs. The first scenario is challenged by the bimodal distribution of cbGRB durations and the fact that the Galactic BNS population peaks at sufficiently low masses that most mergers should go through a HMNS phase. HMNS-powered jets also more readily account for other light curve features, from precursor flares to EE characteristics

On the Conditions for Neutron-Rich Gamma-Ray Burst Outflows

We calculate the structure and neutron content of neutrino-heated MHD winds driven from the surface of newly-formed magnetars (``proto-magnetars'') and from the midplane of hyper-accreting disks, two of the possible central engines for gamma-ray bursts (GRBs) and hyper-energetic supernovae (SNe). Both the surface of proto-magnetars and the midplane of neutrino-cooled accretion flows (NDAFs) are electron degenerate and neutron-rich (neutron-to-proton ratio n/p >> 1). If this substantial free neutron excess is preserved to large radii in ultra-relativistic outflows, several important observational consequences may result. Weak interaction processes, however, can drive n/p to ~1 in the nondegenerate regions that obtain just above the surfaces of NDAFs and proto-magnetars. Our calculations show that mildly relativistic neutron-rich outflows from NDAFs are possible in the presence of a strong poloidal magnetic field. However, we find that neutron-rich winds possess a minimum mass-loss rate that likely precludes simultaneously neutron-rich and ultra-relativistic (Lorentz factor > 100) NDAF winds accompanying a substantial accretion power. In contrast, proto-magnetars are capable of producing neutron-rich long-duration GRB outflows ~10-30 seconds following core bounce for sub-millisecond rotation periods; such outflows would, however, accompany only extremely energetic events, in which the GRB + SN energy budget exceeds ~ 4e52 ergs. Neutron-rich highly relativistic outflows may also be produced during some short-duration GRBs by geometrically thick accretion disks formed from compact object mergers. The implications for r-process nucleosynthesis, optical transients due to non-relativistic neutron-rich winds, and Nickel production in proto-magnetar and NDAF winds are also briefly discussed.Comment: 24 pages, 7 figures, submitted to Ap

Proto-Neutron Star Winds with Magnetic Fields and Rotation

We solve the 1D neutrino-heated non-relativistic MHD wind problem for conditions that range from slowly rotating (spin period P > 10 ms) protoneutron stars (PNSs) with surface field strengths typical of radio pulsars (B < 10^13 G), to "proto-magnetars" with B ~ 10^14-10^15 G in their hypothesized rapidly rotating initial states (P ~ 1 ms). We use the simulations of Bucciantini et al. (2006) to map our monopole results onto a more physical dipole geometry and to estimate the spindown of PNSs when their winds are relativistic. We then quantify the effects of rotation and magnetic fields on the mass loss, energy loss, and r-process nucleosynthesis in PNS winds. We describe the evolution of PNS winds through the Kelvin-Helmholtz cooling epoch, emphasizing the transition between (1) thermal neutrino-driven, (2) non-relativistic magnetically-dominated, and (3) relativistic magnetically-dominated outflows. We find that proto-magnetars with P ~ 1 ms and B > 10^15 G drive relativistic winds with luminosities, energies, and Lorentz factors (magnetization sigma ~ 0.1-1000) consistent with those required to produce long duration gamma-ray bursts and hyper-energetic supernovae (SNe). A significant fraction of the rotational energy may be extracted in only a few seconds, sufficiently rapidly to alter the asymptotic energy of the SN remnant, its morphology, and, potentially, its nucleosynthetic yield. Winds from PNSs with more modest rotation periods (2 - 10 ms) and with magnetar-strength fields produce conditions significantly more favorable for the r-process than winds from slowly rotating PNSs. Lastly, we show that energy and momentum deposition by convectively-excited waves further increase the likelihood of successful r-process in PNS winds.Comment: 21 pages, 12 figures, final version accepted to Ap

Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole-Neutron Star Mergers

We present the first numerical simulations that track the evolution of a black hole-neutron star (BH-NS) merger from pre-merger to $r\gtrsim10^{11}\,{\rm cm}$. The disk that forms after a merger of mass ratio $q=2$ ejects massive disk winds ($3-5\times10^{-2}\,M_\odot$). We introduce various post-merger magnetic configurations, and find that initial poloidal fields lead to jet launching shortly after the merger. The jet maintains a constant power due to the constancy of the large-scale BH magnetic flux, until the disk becomes magnetically arrested (MAD), where the jet power falls off as $L_j\sim t^{-2}$. All jets inevitably exhibit either excessive luminosity due to rapid MAD activation when accretion rate is high, or excessive duration due to delayed MAD activation, compared to typical short gamma-ray burst (sGRBs). This provides a natural explanation to long sGRBs such as GRB 211211A, but also raises a fundamental challenge to our understanding of jet formation in binary mergers. One possible implication being the necessity of higher binary mass ratios or moderate BH spins to launch typical sGRB jets. For post-merger disks with a toroidal magnetic field, dynamo processes delay jet launching such that the jets break out of the disk winds after several seconds. We show for the first time that sGRB jets with initial magnetization $\sigma_0>100$ retain significant magnetization ($\sigma\gg1$) at $r>10^{10}\,{\rm cm}$, emphasizing the importance of magnetic processes in the prompt emission. The jet-wind interaction leads to a power-law angular energy distribution by inflating an energetic cocoon, whose emission is studied in a companion paper.Comment: For movies of the simulations, see https://oregottlieb.com/bhns.htm