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 present a simple analytic model, that captures the key features of the emission of radiation from material ejected by the merger of neutron stars (NS), and construct the multi-band and bolometric luminosity light curves of the transient associated with GW170817, AT 2017gfo, using all available data. The UV to IR emission is shown to be consistent with a single ≈ 0.05 M ⊙ component ejecta, with a power-law velocity distribution between ≈ 0.1 c and > 0.3 c, a low opacity, κ < 1 cm 2 g −1 , and a radioactive energy release rate consistent with an initial Y e < 0.4. The late time spectra require an opacity of κ ν ≈ 0.1 cm 2 g −1 at 1 to 2µm. If this opacity is provided entirely by Lanthanides, their implied mass fraction is X Ln ≈ 10 −3 , approximately 30 times below the value required to account for the solar abundance. The inferred value of X Ln is uncertain due to uncertainties in the estimates of IR opacities of heavy elements, which also do not allow the exclusion of a significant contribution to the opacity by other elements (the existence of a slower ejecta rich in Lanthanides, that does not contribute significantly to the luminosity, can also not be ruled out). The existence of a relatively massive, ≈ 0.05 M ⊙ , ejecta with high velocity and low opacity is in tension with the results of numerical simulations of NS mergers.

We explore the implications of the observed low spin of GW150914 within the context of stellar astrophysics and progenitor models. We conclude that many of the recently proposed scenarios are in marked tension with this observation. We derive a simple model for the observed spin in the case that the progenitor system was a field binary composed of a black hole (BH) and a Wolf-Rayet star and explore the implications of the observed spin for this model. The spin observation allows us to place a lower limit for the delay time between the formation of the BH+BH binary and the actual merger, t merge . We use typical values for these systems to derive t merge ∼ > 10 8 yr, which proves to be an important diagnostic for different progenitor models. We anticipate the next series of events, and the associated spin parameters, will ultimately yield critical constraints on formation scenarios and on stellar parameters describing the late-stage evolution of massive stars.

We explore the consequences of dynamical evolution of field binaries composed of a primary black hole (BH) and a Wolf-Rayet (WR) star in the context of gravitational wave (GW) source progenitors. We argue, from general considerations, that the spin of the WR-descendent BH will be maximal in a significant number of cases due to dynamical effects. In other cases, the spin should reflect the natal spin of the primary BH which is currently theoretically unconstrained. We argue that the three currently published LIGO systems (GW150914, GW151226, LVT151012) suggest that this spin is small. The resultant effective spin distribution of gravitational wave sources should thus be bi-model if this classic GW progenitor channel is indeed dominant. While this is consistent with the LIGO detections thus far, it is in contrast to the three best-measured high-mass x-ray binary (HMXB) systems. A comparison of the spin distribution of HMXBs and GW sources should ultimately reveal whether or not these systems arise from similar astrophysical channels.

We discover clear doubly-peaked line profiles in 3 out of ∼ 20 type Ia supernovae (SNe Ia) with high-quality nebular-phase spectra. The profiles are consistently present in three well-separated Co/Fe emission features. The two peaks are respectively blueshifted and red-shifted relative to the host galaxies and are separated by ∼ 5000km/s. The doubly-peaked profiles directly reflect a bi-modal velocity distribution of the radioactive 56 Ni in the ejecta that powers the emission of these SNe. Due to their random orientations, only a fraction of SNe with intrinsically bi-modal velocity distributions will appear as doubly-peaked spectra. Therefore SNe with intrinsic bi-modality are likely common, especially among the SNe in the low-luminosity part on the Philips relation (∆m 15 (B) 1.3; ∼ 40% of all SNe Ia). Such bi-modality is naturally expected from direct collisions of white dwarfs (WDs) due to the detonation of both WDs and is demonstrated in a 3D 0.64M ⊙ -0.64M ⊙ WD collision simulation. In the future, with a large sample of nebular spectra and a comprehensive set of numerical simulations, the collision model can be unambiguously tested as the primary channel for type Ia SNe, and the distribution of nebular line profiles will either be a smoking gun or rule it out.