We present a suite of seven 3D supernova simulations of non-rotating low-mass progenitors using multi-group neutrino transport. Our simulations cover single star progenitors with zeroage main sequence masses between 9.6M and 12.5M and (ultra)stripped-envelope progenitors with initial helium core masses between 2.8M and 3.5M . We find explosion energies between 0.1 Bethe and 0.4 Bethe, which are still rising by the end of the simulations. Although less energetic than typical events, our models are compatible with observations of less energetic explosions of low-mass progenitors. In six of our models, the mass outflow rate already exceeds the accretion rate onto the proto-neutron star, and the mass and angular momentum of the compact remnant have closely approached their final value, barring the possibility of later fallback. While the proto-neutron star is still accelerated by the gravitational tug of the asymmetric ejecta, the acceleration can be extrapolated to obtain estimates for the final kick velocity. We obtain gravitational neutron star masses between 1.22M and 1.44M , kick velocities between 11 km s −1 and 695 km s −1 , and spin periods from 20 ms to 2.7 s, which suggests that typical neutron star birth properties can be naturally obtained in the neutrino-driven paradigm. We find a loose correlation between the explosion energy and the kick velocity. There is no indication of spin-kick alignment, but a correlation between the kick velocity and the neutron star angular momentum, which needs to be investigated further as a potential point of tension between models and observations.

Fallback in core-collapse supernovae is considered a major ingredient for explaining abundance anomalies in metal-poor stars and the natal kicks and spins of black holes (BHs). We present a first 3D simulation of BH formation and fallback in an "aborted" neutrino-driven explosion of a 40 solar mass zero-metallicity progenitor from collapse to shock breakout. We follow the phase up to BH formation using the relativistic CoCoNuT-FMT code. For the subsequent evolution to shock breakout we apply the moving-mesh code Arepo to core-collapse supernovae for the first time. Our simulation shows that despite early BH formation, neutrino-heated bubbles can survive for tens of seconds before being accreted, leaving them sufficient time to transfer part of their energy to sustain the shock wave as is propagates through the envelope. Although the initial net energy (∼ 2 Bethe) of the neutrino-heated ejecta barely equals the binding energy of the envelope, 11 M of hydrogen are still expelled with an energy of 0.23 Bethe. We find no significant mixing and only a modest BH kick and spin, but speculate that stronger effects could occur for slightly more energetic explosions or progenitors with less tightly bound envelopes.

SMSS J031300.36−670839.3 (hereafter SM0313−6708) is a sub-giant halo star, with no detectable Fe lines and large overabundances of C and Mg relative to Ca. We obtained VLT-UVES spectra extending to 3060Å showing strong OH A-X band lines enabling an oxygen abundance to be derived. The OH A-X band lines in SM0313−6708 are much stronger than the CH C-X band lines. Spectrum synthesis fits indicate an [O/C] ratio of 0.02 ± 0.175. Our high S/N UVES data also enabled us to lower the Fe abundance limit to [Fe/H] 3D ,NLTE < −7.52 (3 σ). These data support our previous suggestion that the star formed from the iron-poor ejecta of a single massive star Population III supernova.

We report the discovery of one extremely metal-poor (EMP; [Fe/H]< −3) and one ultra metal-poor (UMP; [Fe/H]< −4) star selected from the SDSS/SEGUE survey. These stars were identified as EMP candidates based on their medium-resolution (R ∼ 2, 000) spectra, and were followed-up with high-resolution (R ∼ 35, 000) spectroscopy with the Magellan-Clay Telescope. Their derived chemical abundances exhibit good agreement with those of stars with similar metallicities. We also provide new insights on the formation of the UMP stars, based on comparison with a new set of theoretical models of supernovae nucleosynthesis. The models were matched with 20 UMP stars found in the literature, together with one of the program stars (SDSS J1204+1201), with [Fe/H] = −4.34. From fitting their abundances, we find that the supernovae progenitors, for stars where carbon and nitrogen are measured, had masses ranging from 20.5 M ⊙ to 28 M ⊙ , and explosion energies from 0.3 to 0.9×10 51 erg. These results are highly sensitive to the carbon and nitrogen abundance determinations, which is one of the main drivers for future high-resolution follow-up of UMP candidates. In addition, we are able to reproduce the different CNO abundance patterns found in UMP stars with a single progenitor type, by varying its mass and explosion energy.1 Based on observations gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile

Fallback in core-collapse supernovae plays a crucial role in determining the properties of the compact remnants and of the ejecta composition. We perform three-dimensional simulations of mixing and fallback for selected non-rotating supernova models to study how explosion energy and asymmetries correlate with the remnant mass, remnant kick, and remnant spin. We find that the strongest kick and spin are imparted by partial fallback in an asymmetric explosion. Black hole (BH) kicks of several hundred $\mathrm{km}\, \mathrm{s}^{-1}$ and spin parameters of $\mathord {\sim }0.25$ can be obtained in this scenario. If the initial explosion energy barely exceeds the envelope binding energy, stronger fallback results, and the remnant kick and spin remain small. If the explosion energy is high with respect to the envelope binding energy, there is little fallback with a small effect on the remnant kick, but the spin-up by fallback can be substantial. For a non-rotating $12\, \mathrm{M}_\odot$ progenitor, we find that the neutron star is spun up to millisecond periods. The high specific angular momentum of the fallback material can also lead to disc formation around BHs. Fallback may thus be a pathway towards millisecond-magnetar or collapsar-type engines for hypernovae and gamma-ray bursts that does not require rapid progenitor rotation. Within our small set of simulations, none reproduced the peculiar layered fallback necessary to explain the metal-rich iron-poor composition of many carbon-enhanced metal-poor (CEMP) stars. Models with different explosion energy and different realizations of asymmetries may, however, be compatible with CEMP abundance patterns.