Aimee L. Hungerford

Constraints on the progenitor of cassiopeia A

We compare a suite of three-dimensional explosion calculations and stellar models incorporating advanced physics with observational constraints on the progenitor of Cassiopeia A. We consider binary and single stars from 16 to 40 M☉ with a range of explosion energies and geometries. The parameter space allowed by observations of nitrogen-rich high-velocity ejecta, ejecta mass, compact remnant mass, and 44Ti and 56Ni abundances individually and as an ensemble is considered. A progenitor of 15-25 M☉ that loses its hydrogen envelope to a binary interaction and undergoes an energetic explosion can match all the observational constraints.

Constraints on Type Ib/c Supernovae and Gamma‐Ray Burst Progenitors

Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these models, we find that single stars cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends that we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.Although there is strong support for the collapsar engine as the power source of long-duration gamma-ray bursts (GRBs), we still do not definitively know the progenitor of these explosions. Here we review the current set of progenitor scenarios for long-duration GRBs and the observational constraints on these scenarios. Examining these, we find that single-star models cannot be the only progenitor for long-duration GRBs. Several binary progenitors can match the solid observational constraints and also have the potential to match the trends we are currently seeing in the observations. Type Ib/c supernovae are also likely to be produced primarily in binaries; we discuss the relationship between the progenitors of these explosions and those of the long-duration GRBs.

Swift and NuSTAR observations of GW170817: Detection of a blue kilonova

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Ultraviolet and x-ray observations of a binary neutron star merger show a hot kilonova but no jet along the line of sight. With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and x-ray observations by Swift and the Nuclear Spectroscopic Telescope Array of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading UV emission indicates a high mass (≈0.03 solar masses) wind-driven outflow with moderate electron fraction (Ye ≈ 0.27). Combined with the x-ray limits, we favor an observer viewing angle of ≈30° away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultrarelativistic, highly collimated ejecta (a γ-ray burst afterglow).

Spectra of Type Ia Supernovae from Double Degenerate Mergers

The merger of two white dwarfs (aka double-degenerate merger) has often been cited as a potential progenitor of Type Ia supernovae. Here we combine population synthesis, merger, and explosion models with radiation-hydrodynamics light-curve models to study the implications of such a progenitor scenario on the observed Type Ia supernova population. Our standard model, assuming double-degenerate mergers do produce thermonuclear explosions, produces supernova light curves that are broader than the observed type Ia sample. In addition, we discuss how the shock breakout and spectral features of these double-degenerate progenitors will differ from the canonical bare Chandrasekhar-massed explosion models. We conclude with a discussion of how one might reconcile these differences with current observations.

Gamma-ray lines from asymmetric supernovae

We present three-dimensional smoothed particle hydrodynamics simulations of supernova explosions from 100 s to 1 yr after core bounce. By extending our modeling efforts to a three-dimensional hydrodynamics treatment, we are able to investigate the effects of explosion asymmetries on mixing and γ-ray line emergence in supernovae. A series of initial explosion conditions are implemented, including jetlike and equatorial asymmetries of varying degree. For comparison, symmetric explosion models are also calculated. A series of time slices from the explosion evolution are further analyzed using a three-dimensional Monte Carlo γ-ray transport code. The emergent hard X- and γ-ray spectra are calculated as a function of both viewing angle and time, including trends in the γ-ray line profiles. We find significant differences in the velocity distribution of radioactive nickel between the symmetric and asymmetric explosion models. The effects of this spatial distribution change are reflected in the overall high-energy spectrum, as well as in the individual γ-ray line profiles.

NuGrid stellar data set. I. Stellar yields from H to Bi for stars with metallicities Z = 0.02 and Z = 0.01

We provide a set of stellar evolution and nucleosynthesis calculations that applies established physics assumptions simultaneously to low- and intermediate-mass and massive star models. Our goal is to provide an internally consistent and comprehensive nuclear production and yield database for applications in areas such as presolar grain studies. Our non-rotating models assume convective boundary mixing (CBM) where it has been adopted before. We include 8 (12) initial masses for Z = 0.01 (0.02). Models are followed either until the end of the asymptotic giant branch phase or the end of Si burning, complemented by simple analytic core-collapse supernova (SN) models with two options for fallback and shock velocities. The explosions show which pre-SN yields will most strongly be effected by the explosive nucleosynthesis. We discuss how these two explosion parameters impact the light elements and the s and p process. For low- and intermediate-mass models, our stellar yields from H to Bi include the effect of CBM at the He-intershell boundaries and the stellar evolution feedback of the mixing process that produces the ¹³C pocket. All post-processing nucleosynthesis calculations use the same nuclear reaction rate network and nuclear physics input. We provide a discussion of the nuclear production across the entire mass range organized by element group. The entirety of our stellar nucleosynthesis profile and time evolution output are available electronically, and tools to explore the data on the NuGrid VOspace hosted by the Canadian Astronomical Data Centre are introduced.

EXPLOSIVE NUCLEOSYNTHESIS FROM GAMMA-RAY BURST AND HYPERNOVA PROGENITORS: DIRECT COLLAPSE VERSUS FALLBACK

The collapsar engine behind long-duration gamma-ray bursts extracts the energy released from the rapid accretion of a collapsing star onto a stellar mass black hole. In a collapsing star, this black hole can form in two ways: the direct collapse of the stellar core into a black hole and the delayed collapse of a black hole caused by fallback in a weak supernova explosion. In the case of a delayed-collapse black hole, the strong collapsar-driven explosion overtakes the weak supernova explosion before shock breakout, and it is very difficult to distinguish this black hole formation scenario from the direct-collapse scenario. However, the delayed-collapse mechanism, with its double explosion, produces explosive nucleosynthetic yields that are very different from those in the direct-collapse scenario. We present one-dimensional studies of the nucleosynthetic yields from both black hole formation scenarios, deriving differences and trends in their nucleosynthetic yields.

Trends in 44Ti and 56Ni from Core-collapse Supernovae

We compare the yields of 44Ti and 56Ni produced from post-processing the thermodynamic trajectories from three different core-collapse models—a Cassiopeia A progenitor, a double shock hypernova progenitor, and a rotating two-dimensional explosion—with the yields from exponential and power-law trajectories. The peak temperatures and densities achieved in these core-collapse models span several of the distinct nucleosynthesis regions we identify, resulting in different trends in the 44Ti and 56Ni yields for different mass elements. The 44Ti and 56Ni mass fraction profiles from the exponential and power-law profiles generally explain the tendencies of the post-processed yields, depending on which regions are traversed by the model. We find that integrated yields of 44Ti and 56Ni from the exponential and power-law trajectories are generally within a factor two or less of the post-process yields. We also analyze the influence of specific nuclear reactions on the 44Ti and 56Ni abundance evolution. Reactions that affect all yields globally are the 3α, p(e–, νe)n and . The rest of the reactions are ranked according to their degree of impact on the synthesis of 44Ti. The primary ones include 44Ti(α, p)47V, 40Ca(α, γ)44Ti, 45V(p, γ)46Cr, 40Ca(α, p)43Sc, 17F(α, p)20Ne, 21Na(α, p)24Mg, 41Sc(p, γ)42Ti, 43Sc(p, γ)44Ti, 44Ti(p, γ)45V, and 57Ni(p, γ)58Cu, along with numerous weak reactions. Our analysis suggests that not all 44Ti need to be produced in an α-rich freeze-out in core-collapse events, and that reaction rate equilibria in combination with timescale effects for the expansion profile may account for the paucity of 44Ti observed in supernova remnants.

Spectra and light curves of failed supernovae

Astronomers have proposed a number of mechanisms to produce supernova explosions. Although many of these mechanisms are now not considered primary engines behind supernovae (SNe), they do produce transients that will be observed by upcoming ground-based surveys and NASA satellites. Here, we present the first radiation-hydrodynamics calculations of the spectra and light curves from three of these failed SNe: SNe with considerable fallback, accretion-induced collapse of white dwarfs, and energetic helium flashes (also known as type Ia SNe).

Trends in Ti44 and Ni56 from Core-Collapse Supernovae

We compare the yields of 44Ti and 56Ni produced from post-processing the thermodynamic trajectories from three different core-collapse models—a Cassiopeia A progenitor, a double shock hypernova progenitor, and a rotating two-dimensional explosion—with the yields from exponential and power-law trajectories. The peak temperatures and densities achieved in these core-collapse models span several of the distinct nucleosynthesis regions we identify, resulting in different trends in the 44Ti and 56Ni yields for different mass elements. The 44Ti and 56Ni mass fraction profiles from the exponential and power-law profiles generally explain the tendencies of the post-processed yields, depending on which regions are traversed by the model. We find that integrated yields of 44Ti and 56Ni from the exponential and power-law trajectories are generally within a factor two or less of the post-process yields. We also analyze the influence of specific nuclear reactions on the 44Ti and 56Ni abundance evolution. Reactions that affect all yields globally are the 3α, p(e–, νe)n and . The rest of the reactions are ranked according to their degree of impact on the synthesis of 44Ti. The primary ones include 44Ti(α, p)47V, 40Ca(α, γ)44Ti, 45V(p, γ)46Cr, 40Ca(α, p)43Sc, 17F(α, p)20Ne, 21Na(α, p)24Mg, 41Sc(p, γ)42Ti, 43Sc(p, γ)44Ti, 44Ti(p, γ)45V, and 57Ni(p, γ)58Cu, along with numerous weak reactions. Our analysis suggests that not all 44Ti need to be produced in an α-rich freeze-out in core-collapse events, and that reaction rate equilibria in combination with timescale effects for the expansion profile may account for the paucity of 44Ti observed in supernova remnants.