On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

We substantially update the capabilities of the open-source software instrument Modules for Experiments in Stellar Astrophysics (MESA). MESA can now simultaneously evolve an interacting pair of differentially rotating stars undergoing transfer and loss of mass and angular momentum, greatly enhancing the prior ability to model binary evolution. New MESA capabilities in fully coupled calculation of nuclear networks with hundreds of isotopes now allow MESA to accurately simulate advanced burning stages needed to construct supernova progenitor models. Implicit hydrodynamics with shocks can now be treated with MESA, enabling modeling of the entire massive star lifecycle, from pre-main sequence evolution to the onset of core collapse and nucleosynthesis from the resulting explosion. Coupling of the GYRE non-adiabatic pulsation instrument with MESA allows for new explorations of the instability strips for massive stars while also accelerating the astrophysical use of asteroseismology data. We improve treatment of mass accretion, giving more accurate and robust near-surface profiles. A new MESA capability to calculate weak reaction rates "on-the-fly" from input nuclear data allows better simulation of accretion induced collapse of massive white dwarfs and the fate of some massive stars. We discuss the ongoing challenge of chemical diffusion in the strongly coupled plasma regime, and exhibit improvements in MESA that now allow for the simulation of radiative levitation of heavy elements in hot stars. We close by noting that the MESA software infrastructure provides bit-for-bit consistency for all results across all the supported platforms, a profound enabling capability for accelerating MESA's development.

Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

In this paper, we present a new mechanism for core-collapse supernova explosions that relies upon acoustic power generated in the inner core as the driver. In our simulation using an 11-M ⊙ progenitor, an advective-acoustic oscillationà la Foglizzo with a period of ∼25−30 milliseconds (ms) arises ∼200 ms after bounce. Its growth saturates due to the generation of secondary shocks, and kinks in the resulting shock structure funnel and regulate subsequent accretion onto the inner core. However, this instability is not the primary agent of explosion. Rather, it is the acoustic power generated early on in the inner turbulent region stirred by the accretion plumes, and most importantly, but later on, by the excitation and sonic damping of core g-mode oscillations. An ℓ = 1 mode with a period of ∼3 ms grows at late times to be prominent around ∼500 ms after bounce. The accreting protoneutron star is a self-excited oscillator, "tuned" to the most easily excited core g-mode. The associated acoustic power seen in our 11-M ⊙ simulation is sufficient to drive the explosion >550 milliseconds after bounce. The angular distribution of the emitted sound is fundamentally aspherical. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. The ultimate source of the acoustic power is the gravitational energy of infall and the core oscillation acts like a transducer to convert this accretion energy into sound. An advantage of the acoustic mechanism is that acoustic power does not abate until accretion subsides, so that it is available as long as it may be needed to explode the star. This suggests a natural means by which the supernova is self-regulating.

We present here the first 2D rotating, multi-group, radiation magnetohydrodynamics (RMHD) simulations of supernova core collapse, bounce, and explosion. In the context of rapid rotation, we focus on the dynamical effects of magnetic stresses and the creation and propagation of MHD jets. We find that a quasi-steady state can be quickly established after bounce, during which a well-collimated MHD jet is maintained by continuous pumping of power from the differentially rotating core. If the initial spin period of the progenitor core is < ∼ 2 seconds, the free energy reservoir in the secularly evolving protoneutron star is adequate to power a supernova explosion, and may be enough for a hypernova. The jets are well collimated by the infalling material and magnetic hoop stresses, and maintain a small opening angle. We see evidence of sausage instabilities in the emerging jet stream. Neutrino heating is sub-dominant in the rapidly rotating models we explore, but can contribute 10−25% to the final explosion energy. Our simulations suggest that even in the case of modest or slow rotation, a supernova explosion might be followed by a secondary, weak MHD jet explosion, which, because of its weakness may to date have gone unnoticed in supernova debris. Furthermore, we suggest that the generation of a non-relativistic MHD precursor jet during the early protoneutron star/supernova phase is implicit in both the collapsar and "millisecond magnetar" models of GRBs. The multi-D, multi-group, rapidly rotating RMHD simulations we describe here are a start along the path towards more realistic simulations of the possible role of magnetic fields in some of Nature's most dramatic events.

We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering 100 ms, and starting from azimuthal-averaged twodimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4 M (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polarenhanced neutrino luminosities, dominated byν e and "ν μ " neutrinos at the peak, although ν e emission may be stronger at late times. We obtain typical peak neutrino energies for ν e ,ν e , and "ν μ " of ∼ 12, ∼ 16, and ∼ 22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with Ṁ ∼ 10 −3 M s −1 and baryon-loading in the polar regions, preventing any production of a γ -ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that 10 −4 M of material with an electron fraction in the range 0.1-0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the ν iνi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as ∼t −1.8 , decrease over our 100 ms window from a few ×10 50 to ∼ 10 49 erg s −1 , equivalent to a few ×10 54 to ∼ 10 53 e − e + pairs per second.

Using non-LTE time-dependent radiative-transfer calculations, we study the impact of mixing and non-thermal processes associated with radioactive decay on SN IIb/Ib/Ic light curves (LCs) and spectra. Starting with short-period binary models of \leq5Msun He-rich stars (18-25Msun on the main-sequence), we produce 1.2B ejecta which we artificially mix to alter the chemical stratification. While the total 56Ni mass influences the LC peak, the spatial distribution of 56Ni, controlled by mixing processes, impacts both the multi-band LCs and spectra. With enhanced mixing, our synthetic LCs start their post-breakout re-brightening phase earlier, follow a more gradual rise to peak, appear redder, and fade faster after peak due to enhanced gamma-ray escape. Non-thermal electrons, crucial for the production of HeI lines, deposit a dominant fraction of their energy as heat. Because energy deposition is generally local well after the LC peak, the broad HeI lines characteristic of maximum-light SN IIb/Ib spectra require mixing that places 56Ni and helium nuclei to within a gamma-ray mean-free-path. This requirement indicates that SNe IIb and Ib most likely arise from the explosion of stripped-envelope massive stars (main-sequence masses \leq25Msun) that have evolved through mass-transfer in a binary system, rather than from more massive single WR stars. In contrast, the lack of HeI lines in SNe Ic may result from a variety of causes: A genuine helium deficiency; strongly-asymmetric mixing; weak mixing; or a more massive, perhaps single, progenitor characterized by a larger oxygen-rich core. Our models, subject to different mixing magnitudes, can produce a variety of SN types, including IIb, IIc, Ib, and Ic. As it is poorly constrained by explosion models, mixing challenges our ability to infer the progenitor and explosion properties of SNe IIb/Ib/Ic.Comment: 22 pages, 14 figures, accepted to MNRA

We explore the properties of Type II-Plateau (II-P) supernovae (SNe) together with their redsupergiant (RSG) star progenitors. Using MESA STAR, we modulate the parameters (e.g., mixing length, overshoot, rotation, metallicity) that control the evolution of a 15 M ⊙ mainsequence star to produce a variety of physical pre-SN models and SN II-P ejecta. We extend previous modeling of SN II-P radiation to include photospheric and nebular phases, as well as multi-band light curves and spectra. Our treatment does not assume local thermodynamic equilibrium, is time dependent, treats explicitly the effects of line blanketing, and incorporates non-thermal processes. We find that the color properties of SNe II-P require large model atoms for Fe I and Fe II, much larger than adopted in Dessart & Hillier (2011). The color properties also imply RSG progenitors of limited extent (∼ 500 R ⊙ ) -larger progenitor stars produce a SN II-P radiation that remains too blue for too long. This finding calls for a reduction of RSG radii, perhaps through a strengthening of convective energy transport in RSG envelopes. Increased overshoot and rotation reduce the ratio of ejecta to helium-core mass, similarly to an increase in main-sequence mass, and thus complicate the inference of progenitor masses. In contrast to the great sensitivity on progenitor radius, SN II-P color evolution appears insensitive to variations in explosion energy. Finally, we document the numerous SN II-P signatures that vary with progenitor metallicity, revealing their potential for metallicity determinations in the nearby and distant Universe.