Mergers of binary neutron stars (NSs) usually result in the formation of a hypermassive neutron star (HMNS). Whether-and when this remnant collapses to a black hole (BH) depends primarily on the equation of state and on angular momentum transport processes, both of which are uncertain. Here we show that the lifetime of the merger remnant may be directly imprinted in the radioactively powered kilonova emission following the merger. We employ axisymmetric, time-dependent hydrodynamic simulations of remnant accretion disks orbiting a HMNS of variable lifetime, and characterize the effect of this delay to BH formation on the disk wind ejecta. When BH formation is relatively prompt ( 100 ms), outflows from the disk are sufficiently neutron rich to form heavy r-process elements, resulting in ∼week-long emission with a spectral peak in the near-infrared (NIR), similar to that produced by the dynamical ejecta. In contrast, delayed BH formation allows neutrinos from the HMNS to raise the electron fraction in the polar direction to values such that potentially Lanthanidefree outflows are generated. The lower opacity would produce a brighter, bluer, and shorter-lived ∼ day-long emission (a 'blue bump') prior to the late NIR peak from the dynamical ejecta and equatorial wind. This new diagnostic of BH formation should be useful for events with a signal to noise lower than that required for direct detection of gravitational waveform signatures.

Expulsion of neutron-rich matter following the merger of neutron star (NS) binaries is crucial to the radioactively-powered electromagnetic counterparts of these events and to their relevance as sources of r-process nucleosynthesis. Here we explore the long-term (viscous) evolution of remnant black hole accretion disks formed in such mergers by means of two-dimensional, time-dependent hydrodynamical simulations. The evolution of the electron fraction due to charged-current weak interactions is included, and neutrino self-irradiation is modeled as a lightbulb that accounts for the disk geometry and moderate optical depth effects. Over several viscous times (∼ 1 s), a fraction ∼ 10% of the initial disk mass is ejected as a moderately neutronrich wind (Y e ∼ 0.2) powered by viscous heating and nuclear recombination, with neutrino self-irradiation playing a sub-dominant role. Although the properties of the outflow vary in time and direction, their mean values in the heavy-element production region are relatively robust to variations in the initial conditions of the disk and the magnitude of its viscosity. The outflow is sufficiently neutron-rich that most of the ejecta forms heavy r-process elements with mass number A 130, thus representing a new astrophysical source of r-process nucleosynthesis, distinct from that produced in the dynamical ejecta. Due to its moderately high entropy, disk outflows contain a small residual fraction ∼ 1% of helium, which could produce a unique spectroscopic signature.

We study the radioactively-powered transients produced by accretion disk winds following a compact object merger. Starting with the outflows generated in twodimensional hydrodynamical disk models, we use wavelength-dependent radiative transfer calculations to generate synthetic light curves and spectra. We show that the brightness and color of the resulting kilonova transients carry information about the merger physics. In the regions of the wind where neutrino irradiation raises the electron fraction to Y e 0.25, r-process nucleosynthesis halts before producing highopacity, complex ions (the lanthanides). The kilonova light curves thus show two distinct components: a brief (∼ 2 day) blue optical transient produced in the outer lanthanide-free ejecta, and a longer (∼ 10 day) infrared transient produced in the inner, lanthanide line-blanketed region. Mergers producing a longer-lived neutron star, or a more rapidly spinning black hole, have stronger neutrino irradiation, generate more lanthanide-free ejecta, and are optically brighter and bluer. At least some optical emission is produced in all disk wind models, which should enhance the detectability of electromagnetic counterparts to gravitational wave sources. However, the presence of even a small amount (10 −4 M ) of overlying, neutron-rich dynamical ejecta will act as a "lanthanide-curtain", obscuring the optical wind emission from certain viewing angles. Because the disk outflows have moderate velocities (∼ 10, 000 km s −1 ), numerous resolved line features are discernible in the spectra, distinguishing disk winds from fast-moving dynamical ejecta, and offering a potential diagnostic of the detailed composition of freshly produced r-process material.

Type Ia supernovae (SNe Ia), thermonuclear explosions of carbon-oxygen white dwarfs (CO-WDs), are currently the best cosmological "standard candles", but the triggering mechanism of the explosion is unknown. It was recently shown that the rate of head-on collisions of typical field CO-WDs in triple systems may be comparable to the SNe Ia rate. Here we provide evidence supporting a scenario in which the majority of SNe Ia are the result of such head-on collisions of CO-WDs. In this case, the nuclear detonation is due to a well understood shock ignition, devoid of commonly introduced free parameters such as the deflagration velocity or transition to detonation criteria. By using two-dimensional hydrodynamical simulations with a fully resolved ignition process, we show that zero-impact-parameter collisions of typical CO-WDs with masses 0.5 − 1 M ⊙ result in explosions that synthesize 56 Ni masses in the range of ∼ 0.1 − 1 M ⊙ , spanning the wide distribution of yields observed for the majority of SNe Ia. All collision models yield the same late-time ( ∼ > 60 days since explosion) bolometric light curve when normalized by 56 Ni masses (to better than 30%), in agreement with observations. The calculated widths of the 56 Ni-mass-weighted-line-of-sight velocity distributions are correlated with the calculated 56 Ni yield, agreeing with the observed correlation. The strong correlation, shown here for the first time, between 56 Ni yield and total mass of the colliding CO-WDs (insensitive to their mass ratio), is suggestive as the source for the continuous distribution of observed SN Ia features, possibly including the Philips relation.

We investigate the long-term evolution of black hole accretion disks formed in neutron star mergers. These disks expel matter that contributes to an r-process kilonova, and can produce relativistic jets powering short gamma-ray bursts. Here we report the results of a threedimensional, general-relativistic magnetohydrodynamic (GRMHD) simulation of such a disk which is evolved for long enough (∼ 9 s, or ∼ 6 × 10 5 r g /c) to achieve completion of mass ejection far from the disk. Our model starts with a poloidal field, and fully resolves the most unstable mode of the magnetorotational instability. We parameterize the dominant microphysics and neutrino cooling effects, and compare with axisymmetric hydrodynamic models with shear viscosity. The GRMHD model ejects mass in two ways: a prompt MHD-mediated outflow and a late-time, thermally-driven wind once the disk becomes advective. The total amount of unbound mass ejected (0.013M , or 40% of the initial torus mass) is twice as much as in hydrodynamic models, with higher average velocity (0.1c) and a broad electron fraction distribution with a lower average value (0.16). Scaling the ejected fractions to a disk mass of ∼ 0.1M can account for the red kilonova from GW170817 but underpredicts the blue component. About ∼ 10 −3 M of material should undergo neutron freezout and could produce a bright kilonova precursor in the first few hours after the merger. With our idealized initial magnetic field configuration, we obtain a robust jet and sufficient ejecta with Lorentz factor ∼ 1 − 10 to (over)produce the non-thermal emission from GW1708107.