Showing papers by "Brian D. Fields published in 2020"

Big-Bang Nucleosynthesis after Planck

Abstract: We assess the status of big-bang nucleosynthesis (BBN) in light of the final Planck data release and other recent developments, and in anticipation of future measurements. Planck data fix the cosmic baryon density to 0.9% precision, and determine the helium abundance and effective number of neutrinos with precision approaching that of astronomical and BBN determinations respectively. In addition, new high-redshift measurements give D/H to better precision than theoretical predictions, and new Li/H data reconfirm the lithium problem. We present new ${}^{7}{\rm Be}(n,p){}^{7}{\rm Li}$ rates using new neutron capture measurements; we have also examined the effect of proposed changes in the $d(p,\gamma){}^{3}{\rm He}$ rates. Using these results we perform a series of likelihood analyses. We assess BBN/CMB consistency, with attention to how our results depend on the choice of Planck data, as well as how the results depend on the choice of non-BBN, non-Planck data sets. Most importantly the lithium problem remains, and indeed is more acute given the very tight D/H observational constraints; new neutron capture data reveals systematics that somewhat increases uncertainty and thus slightly reduces but does not essentially change the problem. We confirm that $d(p,\gamma){}^{3}{\rm He}$ theoretical rates brings D/H out of agreement and slightly increases 7Li; new experimental data are needed at BBN energies. Setting the lithium problem aside, we find the effective number of neutrino species at BBN is $N_ u = 2.86 \pm 0.15$. Future CMB Stage-4 measurements promise substantial improvements in BBN parameters: helium abundance determinations will be competitive with the best astronomical determinations, and $N_{\rm eff}$ will approach sensitivities capable of detecting the effects of Standard Model neutrino heating of the primordial plasma. (Abridged)

Supernova triggers for end-Devonian extinctions

Abstract: The Late Devonian was a protracted period of low speciation resulting in biodiversity decline, culminating in extinction events near the Devonian–Carboniferous boundary. Recent evidence indicates that the final extinction event may have coincided with a dramatic drop in stratospheric ozone, possibly due to a global temperature rise. Here we study an alternative possible cause for the postulated ozone drop: a nearby supernova explosion that could inflict damage by accelerating cosmic rays that can deliver ionizing radiation for up to ∼ 100 ky. We therefore propose that the end-Devonian extinctions were triggered by supernova explosions at ∼ 20 pc , somewhat beyond the “kill distance” that would have precipitated a full mass extinction. Such nearby supernovae are likely due to core collapses of massive stars; these are concentrated in the thin Galactic disk where the Sun resides. Detecting either of the long-lived radioisotopes Sm 146 or Pu 244 in one or more end-Devonian extinction strata would confirm a supernova origin, point to the core-collapse explosion of a massive star, and probe supernova nucleosynthesis. Other possible tests of the supernova hypothesis are discussed.

The Impact of New d(p,\gamma)He3 Rates on Big Bang Nucleosynthesis

Abstract: We consider the effect on Big Bang Nucleosynthesis (BBN) of new measurements of the $d(p,\gamma){}^3$He cross section by the LUNA Collaboration. These have an important effect on the primordial abundance of D/H which is also sensitive to the baryon density at the time of BBN. We have re-evaluated the thermal rate for this reaction, using a world average of cross section data, which we describe with model-independent polynomials; our results are in good agreement with a similar analysis by LUNA. We then perform a full likelihood analysis combining BBN and Planck cosmic microwave background (CMB) likelihood chains using the new rate combined with previous measurements and compare with the results using previous rates. Concordance between BBN and CMB measurements of the anisotropy spectrum using the old rates was excellent. The predicted deuterium abundance at the Planck value of the baryon density was $({\rm D/H})_{\rm BBN+CMB}^{\rm old} = (2.57 \pm 0.13) \times 10^{-5}$ which can be compared with the value determined from quasar absorption systems $({\rm D/H})_{\rm obs} = (2.55 \pm 0.03) \times 10^{-5} $. Using the new rates we find $({\rm D/H})_{\rm BBN+CMB} = (2.51 \pm 0.11) \times 10^{-5}$. We thus find consistency among BBN theory, deuterium and ${}^4$He observations, and the CMB, when using reaction rates fit in our data-driven approach. We also find that the new reaction data tightens the constraints on the number of relativistic degrees of freedom during BBN, giving the effective number of light neutrino species $N_ u = 2.880 \pm 0.144$ in good agreement with the Standard Model of particle physics. Finally, we note that the observed deuterium abundance continues to be more precise than the BBN+CMB prediction, whose error budget is now dominated by $d(d,n){}^3$He and $d(d,p){}^{3}{\rm H}$.

Sandblasting the r -process: Spallation of Ejecta from Neutron Star Mergers

Abstract: Neutron star mergers (NSMs) are rapid neutron capture ($\\textit{r}$-process) nucleosynthesis sites that expel matter at high velocities, from $0.1c$ to as high as $0.6c$. Nuclei ejected at these speeds are sufficiently energetic to initiate spallation nuclear reactions with interstellar medium particles. We adopt a thick-target model for the propagation of high-speed heavy nuclei in the interstellar medium, similar to the transport of cosmic rays. We find that spallation may create observable perturbations to NSM isotopic abundances, particularly around the low-mass edges of the $\\textit{r}$-process peaks where neighboring nuclei have very different abundances. The extent to which spallation modifies the final NSM isotopic yields depends on: (1) the ejected abundances, which are determined by the NSM astrophysical conditions and the properties of nuclei far from stability, (2) the ejecta velocity distribution and propagation in interstellar matter, and (3) the spallation cross-sections. Observed solar and stellar $\\textit{r}$-process yields could thus constrain the velocity distribution of ejected neutron star matter, assuming NSMs are the dominant $\\textit{r}$-process source. We suggest avenues for future work, including measurement of relevant cross sections.

The origin of the elements: a century of progress.

Abstract: This review assesses the current state of knowledge of how the elements were produced in the Big Bang, in stellar lives and deaths, and by interactions in interstellar gas. We begin with statements...

Climate change via CO2 drawdown from astrophysically initiated atmospheric ionization

Abstract: Motivated by the occurrence of a moderately nearby supernova near the beginning of the Pleistocene, possibly as part of a long-term series beginning in the Miocene, we investigated whether nitrate rainout resulting from the atmospheric ionization of enhanced cosmic ray flux could have, through its fertilizer effect, initiated carbon dioxide drawdown. Such a drawdown could possibly reduce the greenhouse effect and induce the climate change that led to the Pleistocene glaciations. We estimate that the nitrogen flux enhancement onto the surface from an event at 50 pc would be of order 10%, probably too small for dramatic changes. We estimate deposition of iron (another potential fertilizer) and find it is also too small to be significant. There are also competing effects of opposite sign, including muon irradiation and reduction in photosynthetic yield caused by UV increase from stratospheric ozone layer depletion, leading to an ambiguous result. However, if the atmospheric ionization induces a large increase in the frequency of lightning, as argued elsewhere, the amount of nitrate synthesis should be much larger, dominate over the other effects and induce the climate change. More work needs to be done to clarify the effects on lightning frequency.

Spallation of r-Process Nuclei Ejected from a Neutron Star Merger

Abstract: Neutron star mergers (NSMs) are rapid neutron capture (r-process) nucleosynthesis sites, which eject materials at high velocities, from 0.1c to as high as 0.6c. Thus the r-process nuclei ejected from a NSM event are sufficiently energetic to initiate spallation reactions with the interstellar medium (ISM) particles. With a thick-target model for the propagation of high-speed heavy nuclei in the ISM, we find that spallation reactions may shift the r-process abundance patterns towards solar data, particularly around the low-mass edges of the r-process peaks where neighboring nuclei have very different abundances. The spallation effects depend both on the astrophysical conditions of the r-process nuclei and nuclear physics inputs for the nucleosynthesis calculations and the propagation process. This work extends that of [Wang et al.(2019)] by focusing on the influence of nuclear physics variations on spallation effects.

Witnessing History: Rates and Detectability of Naked-Eye Milky-Way Supernovae

Abstract: The Milky Way hosts on average a few supernova explosions per century, yet in the past millennium only five supernovae have been identified confidently in the historical record. This deficit of naked-eye supernovae is at least partly due to dust extinction in the Galactic plane. We explore this effect quantitatively, developing a formalism for the supernova probability distribution, accounting for dust and for the observer's flux limit. We then construct a fiducial axisymmetric model for the supernova and dust densities, featuring an exponential dependence on galactocentric radius and height, with core-collapse events in a thin disk and Type Ia events including a thick disk component. When no flux limit is applied, our model predicts supernovae are intrinsically concentrated in the Galactic plane, with Type Ia events extending to higher latitudes reflecting their thick disk component. We then apply a flux limit and include dust effects, to predict the sky distribution of historical supernovae. We use well-observed supernovae as light-curve templates, and introduce naked-eye discovery criteria. The resulting sky distributions are strikingly inconsistent with the locations of confident historical supernovae, none of which lie near our model's central peaks. Indeed, SN 1054 lies off the plane almost exactly in the anticenter, and SN 1181 is in the 2nd Galactic quadrant. We discuss possible explanations for these discrepancies. We calculate the percentage of all supernovae bright enough for historical discovery: $\simeq 13\%$ of core-collapse and $\simeq 33\%$ of Type Ia events. Using these and the confident historical supernovae, we estimate the intrinsic Galactic supernova rates, finding general agreement with other methods. Finally, we urge searches for supernovae in historical records from civilizations in the southern hemisphere.

Primordial element production studied beneath a mountain.

Abstract: Experiments conducted deep beneath a mountain have provided the most precise measurements yet of a key nuclear reaction that occurred seconds after the Big Bang — refining our knowledge of the constituents of the Universe. Measurement of a key process in Big Bang nucleosynthesis.

Spallation of r-Process Nuclei Ejected from a Neutron Star Merger

Abstract: Neutron star mergers (NSMs) are rapid neutron capture (r-process) nucleosynthesis sites, which eject materials at high velocities, from 0.1c to as high as 0.6c. Thus the r-process nuclei ejected from a NSM event are sufficiently energetic to initiate spallation reactions with the interstellar medium (ISM) particles. With a thick-target model for the propagation of high-speed heavy nuclei in the ISM, we find that spallation reactions may shift the r-process abundance patterns towards solar data, particularly around the low-mass edges of the r-process peaks where neighboring nuclei have very different abundances. The spallation effects depend both on the astrophysical conditions of the r-process nuclei and nuclear physics inputs for the nucleosynthesis calculations and the propagation process. This work extends that of [Wang et al.(2019)] by focusing on the influence of nuclear physics variations on spallation effects.