A solid introduction to stable isotopes that can also be used as an instructive review for more experienced researchers and professionals. The book approaches the subject from the perspective of ...ecological and biological research, but its concepts can be applied within other disciplines.
Supernova triggers for end-Devonian extinctions Fields, Brian D.; Melott, Adrian L.; Ellis, John ...
Proceedings of the National Academy of Sciences - PNAS,
09/2020, Letnik:
117, Številka:
35
Journal Article
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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 146Sm or 244Pu 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.
We have designed a flexible ecological momentary assessment/intervention smartphone (EMA/EMI) "app". We examine the utility of this app for collecting real-time data, and assessing intra-subject ...variability, by using it to assess how freshman undergraduates spend their time. We also explore whether its use can promote greater self-awareness. Participants were randomly divided into an experimental group, who used the app, and a control group, who did not. We used the app to collect both randomized in-the-moment data as well as end-of-day data to assess time use. Using a posttest survey we asked participants questions about how they spent time throughout the school semester. We also asked the experimental group about their experience with the app. Among other findings, 80.49% participants indicated that they became more aware of how they spent their time using the app. Corroborating this report, among the experimental group, end-of-semester self-assessment of time spent wasted, and time spent using electronics recreationally, predicted semester GPA at a strength comparable to high school GPA and ACT score (two of the best single predictors for first semester college GPA), but had no correlation among controls. We discuss the advantages and limitations of using apps, such as ours, for EMA and/or EMI.
Motivated by recent measurements of deposits of 60Fe on the ocean floor and the lunar surface, we model the transport of dust grains containing 60Fe from a near-Earth (i.e., within 100 pc) supernova ...(SN). We inject dust grains into the environment of an SN remnant (SNR) and trace their trajectories by applying a 1D hydrodynamic description assuming spherical symmetry to describe the plasma dynamics, and we include a rudimentary, 3D magnetic field description to examine its influence on charged dust grains. We assume the interstellar medium (ISM) magnetic fields are turbulent and are amplified by the SNR shock, while the SN wind and ejecta fields are negligible. We examine the various influences on the dust grains within the SNR to determine when/if the dust decouples from the plasma, how much it is sputtered, and where within the SNR the dust grains are located. We find that Rayleigh-Taylor instabilities are important for dust survival, as they influence the location of the SN's reverse shock. We find that the presence of a magnetic field within the shocked ISM material limits the passage of SN dust grains, with the field either reflecting or trapping the grains within the heart of the SNR. These results have important implications for in situ 60Fe measurements and for dust evolution in SNRs generally.
There is a wealth of data on live, undecayed 60Fe (t1/2 = 2.6 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 ∼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 ≲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.
Abstract
The astrophysical sites where
r
-process elements are synthesized remain mysterious: it is clear that neutron star mergers (kilonovae (KNe)) contribute, and some classes of core-collapse ...supernovae (SNe) are also possible sources of at least the lighter
r
-process species. The discovery of
60
Fe on the Earth and Moon implies that one or more astrophysical explosions have occurred near the Earth within the last few million years, probably SNe. Intriguingly,
244
Pu has now been detected, mostly overlapping with
60
Fe pulses. However, the
244
Pu flux may extend to before 12 Myr ago, pointing to a different origin. Motivated by these observations and difficulties for
r
-process nucleosynthesis in SN models, we propose that ejecta from a KN enriched the giant molecular cloud that gave rise to the Local Bubble, where the Sun resides. Accelerator mass spectrometry (AMS) measurements of
244
Pu and searches for other live isotopes could probe the origins of the
r
-process and the history of the solar neighborhood, including triggers for mass extinctions, e.g., that at the end of the Devonian epoch, motivating the calculations of the abundances of live
r
-process radioisotopes produced in SNe and KNe that we present here. Given the presence of
244
Pu, other
r
-process species such as
93
Zr,
107
Pd,
129
I,
135
Cs,
182
Hf,
236
U,
237
Np, and
247
Cm should be present. Their abundances and well-resolved time histories could distinguish between the SN and KN scenarios, and we discuss prospects for their detection in deep-ocean deposits and the lunar regolith. We show that AMS
129
I measurements in Fe–Mn crusts already constrain a possible nearby KN scenario.
We consider the production and deposition on Earth of isotopes with half-lives in the range 10 super(5)-10 super(8) yr that might provide signatures of nearby stellar explosions, extending previous ...analyses of Core-Collapse Supernovae (CCSNe) to include Electron-Capture Supernovae (ECSNe), Super-Asymptotic Giant Branch (SAGB) stars, Thermonuclear/Type Ia Supernovae (TNSNe), and Kilonovae/Neutron Star Mergers (KNe). We revisit previous estimates of the super(60)Fe and super(26)Al signatures, and extend these estimates to include super(244)Pu and super(53)Mn. We discuss interpretations of the super(60)Fe signals in terrestrial and lunar reservoirs in terms of a nearby stellar ejection ~2.2Myr ago, showing that (1) the super(60)Fe yield rules out the TNSN and KN interpretations, (2) the super(60)Fe signals highly constrain SAGB interpretations but do not completely them rule out, (3) are consistent with a CCSN origin, and (4) are highly compatible with an ECSN interpretation. Future measurements could resolve the radioisotope deposition over time, and we use the Sedov blast wave solution to illustrate possible time-resolved profiles. Measuring such profiles would independently probe the blast properties including distance, and would provide additional constraints for the nature of the explosion.
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 supernovae, which is disfavored by nucleosynthesis simulations, or in an earlier kilonova event that seeded the nearby interstellar medium with 244Pu 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.
Several searches have found evidence of {sup 60}Fe deposition, presumably from a near-Earth supernova (SN), with concentrations that vary in different locations on Earth. This paper examines various ...influences on the path of interstellar dust carrying {sup 60}Fe from an SN through the heliosphere, with the aim of estimating the final global distribution on the ocean floor. We study the influences of magnetic fields, angle of arrival, wind, and ocean cycling of SN material on the concentrations at different locations. We find that the passage of SN material through the mesosphere/lower thermosphere has the greatest influence on the final global distribution, with ocean cycling causing lesser alteration as the SN material sinks to the ocean floor. SN distance estimates in previous works that assumed a uniform distribution are a good approximation. Including the effects on surface distributions, we estimate a distance of 46{sub −6}{sup +10} pc for an 8--10 M{sub ⊙} SN progenitor. This is consistent with an SN occurring within the Tuc-Hor stellar group ∼2.8 Myr ago, with SN material arriving on Earth ∼2.2 Myr ago. We note that the SN dust retains directional information to within 1{sup ◦} through its arrival in the inner solar system, so that SN debris deposition on inert bodies such as the Moon will be anisotropic, and thus could in principle be used to infer directional information. In particular, we predict that existing lunar samples should show measurable {sup 60}Fe differences.
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