arXiv subjects

Brian D. Fields

Publications and source records attributed to Brian D. Fields.

At least 19 recordsLinked to original sources

arXiv2026

Gardening on the Moon: An Advection-Diffusion Model to Guide the Search for Supernova Debris in the Lunar Regolith

The vertical redistribution of materials in the lunar regolith - ranging from continuously produced space-weathering products to sporadic pulses of supernova- or kilonova-derived isotopes - remains a fundamental problem in planetary science. We present a unified stochastic model of regolith gardening induced by the impact flux. Treating gardening as a competition between impact-driven advection and diffusion predicts the maturity profiles of Apollo cores over more than two orders of magnitude in time ($1.4 \times 10^7$ to $4.5 \times 10^8$ years). This model describes well the depth profiles of live Fe60 in Apollo regolith samples, suggesting that supernova dust capture is independent of native iron abundance, and is consistent with a uniform influx at the latitudes of the Apollo landing sites. We extend our model to predict lunar signals for live r-process species that might originate from supernovae or kilonovae: Pu244 tied to terrestrial detections, and I129, Hf182, and Cm247 based on r-process calculations. The Pu244/Fe60 depth profile can probe the origin of Pu244, motivating searches in Artemis regolith samples down to depths O(100) cm.

astro-ph.EP↗

arXiv2026

Searching for Neutron Star Mergers in the Absence of Gravitational Waves with Optical Afterglow Emission

With the forth observing run of the LIGO-Virgo-KAGRA gravitational-wave network, which enabled the discovery of the kilonova (KN) counterpart to GW170817, ending with no new confirmed neutron star mergers, the intrinsic rate of these events must be even lower than previously estimated. As a result, building a sample of KNe will remain challenging even with continued GW observations, motivating complementary discovery strategies that do not rely on gravitational-wave triggers. In this work, we consider how leveraging bright short gamma-ray burst afterglows can aid in the discovery on KNe with the Rubin Observatory's upcoming Legacy Survey of Space and Time (LSST), whose unprecedented depth will make such detections feasible. We find that nearly on-axis ($θ_{\rm view} \leq 30°$) afterglows can enhance KN detection rates in the LSST $g$-band from $29^{+51}_{-21} \ \rm yr^{-1}$ to $91^{+160}_{-65} \ \rm yr^{-1}$. We further show how the colors of the observed events can be used to distinguish between neutron star merger counterparts with and without KN emission. This study demonstrates how critical multi-wavelength and multi-survey observations are for these rare events, especially without context from gravitational waves. Fortunately, detectable events will likely be discovered near peak with LSST, allowing for rapid follow-up and confirmation. We discuss key uncertainties in our study, particularly volume rate of merger events, and the degeneracy between the empirically determined explosion energy and ambient medium density.

astro-ph.HE↗

arXiv2026

The LBT $Y_{\rm p}$ Project I: An Improved Determination of the Primordial Helium Abundance -- Project Description, Sample Selection, Observations, and Methodology

Extremely low metallicity HII regions have been observed with the goal of determining the primordial helium abundance ($Y_{\rm p}$). $Y_{\rm p}$, combined with standard big bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but $Y_{\rm p}$ must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of $Y_{\rm p}$. The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual HII regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a four-year period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of $Y_{\rm p}$ with a precision of $\sim$ 0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of $\sim$ 3%.

astro-ph.CO↗

arXiv2026

The LBT $Y_{\rm p}$ Project V: Cosmological Implications of a New Determination of Primordial $^4$He

The primordial abundance of $^4$He plays a central role in big-bang nucleosynthesis (BBN) and in the cosmic microwave background (CMB). The LBT $Y_{\rm p}$ Project's new measurement of the primordial $^4$He mass fraction $Y_{\rm p} =0.2458 \pm 0.0013$ is the most precise determination to date. In this paper, we combine our new $Y_{\rm p}$ value with the latest primordial deuterium measurement, and assess the consequences for cosmology. For Standard BBN, where the number of light neutrino species is fixed at $N_ν=3$, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining $Y_{\rm p}$ , D/H, BBN, and the CMB, gives the cosmic baryon-to-photon ratio $η= (6.120 \pm 0.038) \times 10^{-10}$, corresponding to a baryon density parameter $Ω_{\rm B} h^2 = 0.02236 \pm 0.00014$. We then allow $N_ν$ to vary and thus measure relativistic species present during nucleosynthesis. We find $η= (6.101 \pm 0.044) \times 10^{-10}$ or $Ω_{\rm B} h^2= 0.02229 \pm 0. 00016$, and $N_ν= 2.925 \pm 0.082$, and for $N_ν\ge 3$, $ΔN_ν= N_ν-3 \le 0.125$ (95\% CL) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics, and with the standard cosmology that links BBN at $\sim 1 \ \rm sec$ and the CMB at $\sim 400,000$ yr.

astro-ph.CO↗

arXiv2024

Nearby Supernova and Cloud Crossing Effects on the Orbits of Small Bodies in the Solar System

Supernova blasts envelop many surrounding stellar systems, transferring kinetic energy to small bodies in the systems. Geologic evidence from $^{60}\rm Fe$ points to recent nearby supernova activity within the past several Myr. Here, we model the transfer of energy and resulting orbital changes from these supernova blasts to the Oort Cloud, the Kuiper belt, and Saturn's Phoebe ring. For the Oort Cloud, an impulse approximation shows that a 50 pc supernova can eject approximately half of all objects less than 1 cm while altering the trajectories of larger ones, depending on their orbital parameters. For stars closest to supernovae, objects up to $\sim$100 m can be ejected. Turning to the explored solar system, we find that supernovae closer than 50 pc may affect Saturn's Phoebe ring and can sweep away Kuiper belt dust. It is also possible that the passage of the solar system through a dense interstellar cloud could have a similar effect; a numerical trajectory simulation shows that the location of the dust grains and the direction of the wind (from a supernova or interstellar cloud) has a significant impact on whether or not the grains will become unbound from their orbit in the Kuiper belt. Overall, nearby supernovae sweep micron-sized dust from the solar system, though whether the grains are ultimately cast towards the Sun or altogether ejected depends on various factors. Evidence of supernova-modified dust grain trajectories may be observed by New Horizons, though further modeling efforts are required.

astro-ph.SR↗

arXiv2024

Do we Owe our Existence to Gravitational Waves?

Two heavy elements essential to human biology are thought to have been produced by the astrophysical $r$-process, which occurs in neutron-rich environments: iodine is a constituent of thyroid hormones that affect many physiological processes including growth and development, body temperature and heart rate, and bromine is essential for tissue development and architecture. Collisions of neutron stars (kilonovae) have been identified as sources of $r$-process elements including tellurium, which is adjacent to iodine in the periodic table, and lanthanides. Neutron-star collisions arise from energy loss due to gravitational-wave emission from binary systems, leading us to suggest that gravitational waves have played a key role in enabling human life by producing iodine and bromine. We propose probing this proposal by searching in lunar material for live $^{129}$I deposited by a recent nearby kilonova explosion.

astro-ph.HE↗

arXiv2024

Limits on Non-Relativistic Matter During Big-Bang Nucleosynthesis

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures $\sim 10$ MeV to $\sim 100$ keV. Here, we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio, $η$, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on $η$ and $N_ν$ to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species to $N_{\rm eff} < 3$ for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on {\em Planck} CMB data alone find $N_ν = 2.800 \pm 0.294$, and when combined with standard BBN and the observations of D and \he4 give $N_ν = 2.898 \pm 0.141$. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit results are for a matter species that decays entirely electromagnetically with a lifetime $τ_X = 0.89 \ \rm sec$ and pre-decay density that is a fraction $ξ= (ρ_X/ρ_{\rm rad})|_{10 \ \rm MeV} = 0.0026$ of the radiation energy density at 10 MeV; similarly good fits are found over a range where $ξτ_X^{1/2}$ is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the $(τ_X,ξ)$ plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. $ξ=0$) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.

astro-ph.CO↗

arXiv2023

Could a Kilonova Kill: a Threat Assessment

Binary neutron star mergers (BNS) produce high-energy emissions from several physically different sources, including a gamma-ray burst (GRB) and its afterglow, a kilonova, and, at late times, a remnant many parsecs in size. Ionizing radiation from these sources can be dangerous for life on Earth-like planets when located too close. Work to date has explored the substantial danger posed by the GRB to on-axis observers: here we focus instead on the potential threats posed to nearby off-axis observers. Our analysis is based largely on observations of the GW 170817/GRB 170817A multi-messenger event, as well as theoretical predictions. For baseline kilonova parameters, we find that the X-ray emission from the afterglow may be lethal out to $\sim 5$ pc and the off-axis gamma-ray emission may threaten a range out to $\sim 4$ pc, whereas the greatest threat comes years after the explosion, from the cosmic rays accelerated by the kilonova blast, which can be lethal out to distances up to $\sim 11$ pc. The distances quoted here are typical, but the values have significant uncertainties and depend on the viewing angle, ejected mass, and explosion energy in ways we quantify. Assessing the overall threat to Earth-like planets, have a similar kill distance to supernovae, but are far less common. However, our results rely on the scant available kilonova data, and multi-messenger observations will clarify the danger posed by such events.

astro-ph.HE↗

arXiv2023

Distances to Recent Near-Earth Supernovae From Geological and Lunar 60Fe

Near-Earth supernova blasts which engulf the solar system have left traces of their ejecta in the geological and lunar records. There is now a wealth of data on live radioactive ${}^{60}$Fe pointing to a supernova at 3 Myr ago, as well as the recent discovery of an event at 7 Myr ago. We use the available measurements to evaluate the distances to these events. For the better analyzed supernova at 3 Myr, samples include deep-sea sediments, ferromanganese crusts, and lunar regolith; we explore the consistency among and across these measurements, which depends sensitively on the uptake of iron in the samples as well as possible anisotropies in the ${}^{60}$Fe fallout. There is also significant uncertainty in the astronomical parameters needed for these calculations. We take the opportunity to perform a parameter study on the effects that the ejected ${}^{60}$Fe mass from a core-collapse supernova and the fraction of dust that survives the remnant have on the resulting distance. We find that with an ejected ${}^{60}$Fe mass of $3\times10^{-5} M_\odot$ and a dust fraction of 10%, the distance range for the supernova 3 Myr ago is $D \sim 20 - 140$ pc, with the most likely range between $50 - 65$ pc. Using the same astrophysical parameters, the distance for the supernova at 7 Myr ago is $D \sim 110$ pc. We close with a brief discussion of geological and astronomical measurements that can improve these results.

astro-ph.SR↗

arXiv2023

X-Ray Luminous Supernovae: Threats to Terrestrial Biospheres

The spectacular outbursts of energy associated with supernovae (SNe) have long motivated research into their potentially hazardous effects on Earth and analogous environments. Much of this research has focused primarily on the atmospheric damage associated with the prompt arrival of ionizing photons within days or months of the initial outburst, and the high-energy cosmic rays that arrive thousands of years after the explosion. In this study, we turn the focus to persistent X-ray emission, arising in certain SNe that have interactions with a dense circumstellar medium, and observed months and/or years after the initial outburst. The sustained high X-ray luminosity leads to large doses of ionizing radiation out to formidable distances. We provide an assessment of the threat posed by these X-ray luminous SNe by analyzing the collective X-ray observations from Chandra, Swift-XRT, XMM-Newton, NuSTAR, and others. We find that this threat is particularly acute for SNe showing evidence of strong circumstellar interaction, such as Type IIn explosions, which have significantly larger ranges of influence than previously expected, and lethal consequences up to $\sim$ 50 pc away. Furthermore, X-ray bright SNe could pose a substantial and distinct threat to terrestrial biospheres, and tighten the Galactic habitable zone. We urge follow-up X-ray observations of interacting SNe for months and years after the explosion to shed light on the physical nature of the emission and its full time evolution, and to clarify the danger that these events pose for life in our Galaxy and other star-forming regions.

astro-ph.HE↗

arXiv2023

Supernova Dust Evolution Probed by Deep-sea 60Fe Time History

There is a wealth of data on live, undecayed 60Fe ($t_{1/2} = 2.6 \ \rm Myr$) in deep-sea deposits, the lunar regolith, cosmic rays, and Antarctic snow, which is interpreted as originating from the recent explosions of at least two near-Earth supernovae. We use the 60Fe profiles in deep-sea sediments to estimate the timescale of supernova debris deposition beginning $\sim 3$ Myr ago. The available data admits a variety of different profile functions, but in all cases the best-fit 60Fe pulse durations are $>1.6$ Myr when all the data is combined. This timescale far exceeds the $\lesssim 0.1$ Myr pulse that would be expected if 60Fe was entrained in the supernova blast wave plasma. We interpret the long signal duration as evidence that 60Fe arrives in the form of supernova dust, whose dynamics are separate from but coupled to the evolution of the blast plasma. In this framework, the $>1.6$ Myr is that for dust stopping due to drag forces. This scenario is consistent with the simulations in Fry et. al (2020), where the dust is magnetically trapped in supernova remnants and thereby confined around regions of the remnant dominated by supernova ejects, where magnetic fields are low. This picture fits naturally with models of cosmic-ray injection of refractory elements as sputtered supernova dust grains and implies that the recent 60Fe detections in cosmic rays complement the fragments of grains that survived to arrive on the Earth and Moon. Finally, we present possible tests for this scenario.

astro-ph.HE↗

arXiv2023

Proposed Lunar Measurements of $r$-Process Radioisotopes to Distinguish Origin of Deep-sea 244Pu

244Pu has recently been discovered in deep-sea deposits spanning the past 10 Myr, a period that includes two 60Fe pulses from nearby supernovae. 244Pu is among the heaviest $r$-process products, and we consider whether it was created in the supernovae, which is disfavored by nucleosynthesis simulations, or in an earlier kilonova event that seeded 244Pu in the nearby interstellar medium that was subsequently swept up by the supernova debris. We discuss how these possibilities can be probed by measuring 244Pu and other $r$-process radioisotopes such as 129I and 182Hf, both in lunar regolith samples returned to Earth by missions such as Chang'e and Artemis, and in deep-sea deposits.

astro-ph.HE↗

arXiv2023

The Neutron Mean Life and Big Bang Nucleosynthesis

We explore the effect of neutron lifetime and its uncertainty on standard big-bang nucleosynthesis (BBN). BBN describes the cosmic production of the light nuclides $^1{\rm H}$, ${\rm D}$, $^3{\rm H}$+$^3{\rm He}$, $^4{\rm He}$, and $^7{\rm Li}$+$^7{\rm Be}$ in the first minutes of cosmic time. The neutron mean life $τ_n$ has two roles in modern BBN calculations: (1) it normalizes the matrix element for weak $n \leftrightarrow p$ interconversions, and (2) it sets the rate of free neutron decay after the weak interactions freeze out. We review the history of the interplay between $τ_n$ measurements and BBN, and present a study of the sensitivity of the light element abundances to the modern neutron lifetime measurements. We find that $τ_n$ uncertainties dominate the predicted $^4{\rm He}$ error budget, but these theory errors remain smaller than the uncertainties in $^4{\rm He}$ observations, even with the dispersion in recent neutron lifetime measurements. For the other light-element predictions, $τ_n$ contributes negligibly to their error budget. Turning the problem around, we combine present BBN and cosmic microwave background (CMB) determinations of the cosmic baryon density to $\textit{predict}$ a "cosmologically preferred" mean life of $τ_{n}({\rm BBN+CMB}) = 870 \pm 16 \ \rm sec$, which is consistent with experimental mean life determinations. We go on to show that if future astronomical and cosmological helium observations can reach an uncertainty of $σ_{\rm obs}(Y_p) = 0.001$ in the $^4{\rm He}$ mass fraction $Y_p$, this could begin to discriminate between the mean life determinations.

astro-ph.CO↗

arXiv2022

Probing Physics Beyond the Standard Model: Limits from BBN and the CMB Independently and Combined

We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via the number $N_ν$ of equivalent neutrino species. We use the latest light element observations, neutron mean lifetime, and update our evaluation for the nuclear rates $d+d \rightarrow He3 + n$ and $d+d \rightarrow H3 + p$. Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs $N_ν$ and $η$ prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives $N_ν= 2.898 \pm 0.141$, resulting in $N_ν< 3.180$ at $2σ$. We apply these limits to a wide variety of new physics scenarios including right-handed neutrinos, dark radiation, and a stochastic gravitational wave background. We also search for limits on potential {\em changes} in $N_ν$ and/or the baryon-to-photon ratio $η$ between the two epochs. The present data place strong constraints on the allowed changes in $N_ν$ between BBN and CMB decoupling; for example, we find $-0.708 < N_ν^{\rm CMB}-N_ν^{\rm BBN} < 0.328$ in the case where $η$ and the primordial helium mass fraction $Y_p$ are unchanged between the two epochs; we also give limits on the allowed variations in $η$ or in $(η,N_ν)$ jointly. Looking to the future, we forecast the tightened precision for $N_ν$ arising from both CMB Stage 4 measurements as well as improvements in astronomical \he4 measurements. We find that CMB-S4 combined with present BBN and light element observation precision can give $σ(N_ν) \simeq 0.03$. Such future precision would reveal the expected effect of neutrino heating ($N_{\rm eff}-3=0.044$) of the CMB during BBN, and would be near the level to reveal any particle species ever in thermal equilibrium with the standard model.

astro-ph.CO↗

arXiv2022

Near-Earth Supernovae in the Past 10 Myr: Implications for the Heliosphere

We summarize evidence that multiple supernovae exploded within 100 pc of Earth in the past few Myr. These events had dramatic effects on the heliosphere, compressing it to within ~20 au. We advocate for cross-disciplinary research of nearby supernovae, including on interstellar dust and cosmic rays. We urge for support of theory work, direct exploration, and study of extrasolar astrospheres.

astro-ph.IM↗

arXiv2022

Heliospheric Compression due to Recent Nearby Supernova Explosions

The widespread detection of 60Fe in geological and lunar archives provides compelling evidence for recent nearby supernova explosions within $\sim 100$ pc around 3 Myr and 7 Myr ago. The blasts from these explosions had a profound effect on the heliosphere. We perform new calculations to study the compression of the heliosphere due to a supernova blast. Assuming a steady but non-isotropic solar wind, we explore a range of properties appropriate for supernova distances inspired by recent 60Fe data, and for a 20 pc supernova proposed to account for mass extinctions at the end-Devonian period. We examine the locations of the termination shock decelerating the solar wind and the heliopause that marks the boundary between the solar wind and supernova material. Pressure balance scaling holds, consistent with studies of other astrospheres. Solar wind anisotropy does not have an appreciable effect on shock geometry. We find that supernova explosions at 50 pc (95 pc) lead to heliopause locations at 16 au (23 au) when the forward shock arrives. Thus, the outer solar system was directly exposed to the blast, but the inner planets -- including the Earth -- were not. This finding reaffirms that the delivery of supernova material to the Earth is not from the blast plasma itself, but likely is from supernova dust grains. After the arrival of the forward shock, the weakening supernova blast will lead to a gradual rebound of the heliosphere, taking $\sim100$s of kyr to expand beyond 100 au. Prospects for future work are discussed.

astro-ph.SR↗

arXiv2022

Implications of the Non-Observation of ${}^{6}{\rm Li}$ in Halo Stars for the Primordial ${}^{7}{\rm Li}$ Problem

The primordial Lithium Problem is intimately connected to the assumption that ${}^{7}{\rm Li}$ observed in metal-poor halo stars retains its primordial abundance, which lies significantly below the predictions of standard big-bang nucleosynthesis. Two key lines of evidence have argued that these stars have not significantly depleted their initial ${}^{7}{\rm Li}$: i) the lack of dispersion in Li abundances measured at low metallicity; and ii) the detection of the more fragile ${}^{6}{\rm Li}$ isotope in at least two halo stars. The purported ${}^{6}{\rm Li}$ detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of ${}^{6}{\rm Li}$. This concordance left little room for depletion of ${}^{6}{\rm Li}$ depletion, and implied that the more robust ${}^{7}{\rm Li}$ largely evaded destruction. Recent (re)-observations of halo stars challenge the evidence against ${}^{7}{\rm Li}$ depletion: i) lithium abundances now show significant dispersion, and ii) sensitive ${}^{6}{\rm Li}$ searches now reveal only firm upper limits to the ${}^{6}{\rm Li}/{}^{7}{\rm Li}$ ratio. The tight new ${}^{6}{\rm Li}$ upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial ${}^{6}{\rm Li}$ depletion has occurred--by factors up to 50. We show that in stars with ${}^{6}{\rm Li}$ limits and thus lower bounds on ${}^{6}{\rm Li}$ depletion, an equal amount of ${}^{7}{\rm Li}$ depletion is more than sufficient to resolve the primordial ${}^{7}{\rm Li}$ Problem. This picture is consistent with stellar models in which ${}^{7}{\rm Li}$ is less depleted than ${}^{6}{\rm Li}$, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas. (abridged)

astro-ph.GA↗

arXiv2022

Simulations of $^{60}$Fe entrained in ejecta from a near-Earth supernova: Effects of observer motion

Recent studies have shown that live (not decayed) radioactive $^{60}$Fe is present in deep-ocean samples, Antarctic snow, lunar regolith, and cosmic rays. $^{60}$Fe represents supernova (SN) ejecta deposited in the Solar System around $3 \, \rm Myr$ ago, and recently an earlier pulse $\approx 7 \ \rm Myr $ ago has been found. These data point to one or multiple near-Earth SN explosions that presumably participated in the formation of the Local Bubble. We explore this theory using 3D high-resolution smooth-particle hydrodynamical simulations of isolated supernovae with ejecta tracers in a uniform interstellar medium (ISM). The simulation allows us to trace the supernova ejecta in gas form and those eject in dust grains that are entrained with the gas. We consider two cases of diffused ejecta: when the ejecta are well-mixed in the shock and when they are not. In the latter case, we find that these ejecta remain far behind the forward shock, limiting the distance to which entrained ejecta can be delivered to $\approx 100$ pc in an ISM with $n_\mathrm{H}=0.1\; \rm cm^{-3}$ mean hydrogen density. We show that the intensity and the duration of $^{60}$Fe accretion depend on the ISM density and the trajectory of the Solar System. Furthermore, we show the possibility of reproducing the two observed peaks in $^{60}$Fe concentration with this model by assuming two linear trajectories for the Solar System with $30$-km s$^{-1}$ velocity. The fact that we can reproduce the two observed peaks further supports the theory that the $^{60}$Fe signal was originated from near-Earth SNe.

astro-ph.HE↗