1904.03353
Revisiting the biological ramifications of variations in Earth's magnetic field
Lingam
An Earth-like planetary magnetic field has been widely invoked as a requirement for habitability as it purportedly mitigates the fluxes of ionizing radiation reaching the surface and the escape of neutrals and ions from the atmosphere. Recent paleomagnetic evidence indicates that the nucleation of Earth's inner core, followed perhaps by an increase in geomagnetic field strength, might have occurred close to the Edicarian period. Motivated by this putative discovery, we explore the ensuing ramifications from the growth or reversals of Earth's dynamo. By reviewing and synthesizing emerging quantitative models, it is proposed that neither the biological radiation dose rates nor the atmospheric escape rates would vary by more than a factor of $\sim 2$ under these circumstances. Hence, we suggest that hypotheses seeking to explain the Cambrian radiation or mass extinctions via changes in Earth's magnetic field intensity are potentially unlikely. We also briefly discuss how variations in the planetary magnetic field may have impacted early Mars and could influence exoplanets orbiting M-dwarfs.
Revisiting the biological ramifications of variations in Earth's magnetic field
Lingam
An Earth-like planetary magnetic field has been widely invoked as a requirement for habitability as it purportedly mitigates the fluxes of ionizing radiation reaching the surface and the escape of neutrals and ions from the atmosphere. Recent paleomagnetic evidence indicates that the nucleation of Earth's inner core, followed perhaps by an increase in geomagnetic field strength, might have occurred close to the Edicarian period. Motivated by this putative discovery, we explore the ensuing ramifications from the growth or reversals of Earth's dynamo. By reviewing and synthesizing emerging quantitative models, it is proposed that neither the biological radiation dose rates nor the atmospheric escape rates would vary by more than a factor of $\sim 2$ under these circumstances. Hence, we suggest that hypotheses seeking to explain the Cambrian radiation or mass extinctions via changes in Earth's magnetic field intensity are potentially unlikely. We also briefly discuss how variations in the planetary magnetic field may have impacted early Mars and could influence exoplanets orbiting M-dwarfs.
1904.03364
Significant improvement in the accuracy of simulated chaotic $N$-body orbits by using smoothness
Hernandez
Symplectic integrators are a foundation to the study of dynamical $N$-body phenomena, at scales ranging from from planetary to cosmological. These integrators preserve the Poincar\'{e} invariants of Hamiltonian dynamics. The $N$-body Hamiltonian has another, perhaps overlooked, symmetry: it is perfectly smooth for particle separations greater than $0$. Many popular symplectic integrators, such as hybrid methods or block adaptive stepping methods, do not respect this symmetry and it is perhaps unclear whether they should. We investigate the importance of this symmetry by considering hybrid integrators, whose smoothness can be tuned easily. Hybrid methods are perfectly smooth, except at a finite number of phase space points. We study chaotic planetary orbits in a test considered by Wisdom. We find that increasing smoothness, at negligible extra computational cost in particular tests, improves the Jacobi constant error of the orbits by about $5$ orders of magnitude in long-term simulations. The results from this work suggest that smoothness of the $N$-body problem is a symmetry worth preserving in simulations.
1904.03547
Stellar rotation and the extended main sequence turnoff in the open cluster NGC 5822
Sun, et al
The origin of extended main sequence turnoffs (eMSTOs) in intermediate-age (1-3 Gyr) clusters is one of the most intriguing questions in current star cluster research. Unlike the split main sequences found in some globular clusters. which are caused by bimodal populations in age and/or chemical abundances, eMSTOs are believed to be owing to stellar rotation. We present a spectroscopic survey of MSTO stars in a nearby, intermediate-age (\unit[0.9]{Gyr}), low mass ($\sim 1.7\times10^3 M_\odot$) Galactic open cluster, NGC 5822. We derive a clean sample of member stars based on \textit{Gaia} proper motions and parallaxes and confirm the existence of an eMSTO. Using medium-resolution ($R\sim 4000$) Southern African Large Telescope (SALT) spectra, we derive the rotational velocities of 24 member stars (representing 20\% completeness around the eMSTO region) and find that the loci of the main sequence stars in the eMSTO region show a clear correlation with the projected rotational velocities in the sense that fast rotators are located on the red side of the eMSTO and slow rotators are found on the blue side. By comparison with a synthetic cluster model, we show that the stellar rotational velocities and the eMSTO of NGC 5822 can be well reproduced and we conclude that stellar rotation is the main cause of the eMSTO in NGC 5822.
1904.03747
Time dependent solar modulation of cosmic rays from solar minimum to solar maximum
Wang, et al
We study the time-dependent modulation effect and derive the local interstellar spectra (LIS) for the cosmic ray (CR) proton, helium, boron and carbon. A two-dimensional modulation model including the variation of the interplanetary environment with time is adopted to describe modulation process. The propagation equation of CRs in the heliosphere is numerically solved by the package Solarprop. We derive the LIS by fitting the latest results of several experiments, including Voyager 1, PAMELA, BESS-POLARII and ACE, during low solar activity periods. We further study the modulation in the polarity reversal periods with the PAMELA proton data. We find that the rigidity dependence of the diffusion coefficient is critical to explain the modulation effect during reversal periods. Our results also indicate a power law relation between the diffusion coefficient and the magnitude of the heliospheric magnetic field (HMF) at the Earth.
1904.03894
Probing galactic cosmic ray distribution with TeV gamma-ray sky
Cataldo, et al
The distribution of cosmic rays in the Galaxy at energies above few TeVs is still uncertain and this affects the expectations for the diffuse gamma flux produced by hadronic interactions of cosmic rays with the interstellar gas. We show that the TeV gamma-ray sky can provide interesting constraints. Namely, we compare the flux from the galactic plane measured by Argo-YBJ, HESS, HAWC and Milagro with the expected flux due to diffuse emission and point-like and extended sources observed by HESS showing that experimental data can already discriminate among different hyphoteses for cosmic ray distribution. The constraints can be strengthened if the contribution of sources not resolved by HESS is taken into account.
1904.03919
The paradoxes of the Late Hesperian Mars ocean
Turbet, Forget
The long-standing debate on the existence of ancient oceans on Mars has been recently revived by evidence for tsunami resurfacing events that date from the Late Hesperian geological era. It has been argued that these tsunami events originated from the impact of large meteorites on a deglaciated or nearly deglaciated ocean present in the northern hemisphere of Mars. Here we show that the presence of such a late ocean faces a paradox. If cold, the ocean should have been entirely frozen shortly after its formation, thus preventing the formation of tsunami events. If warm, the ice-free ocean should have produced fluvial erosion of Hesperian Mars terrains much more extensively than previously reported. To solve this apparent paradox, we suggest a list of possible tests and scenarios that could help to reconcile constraints from climate models with tsunami hypothesis. These scenarios could be tested in future dedicated studies.
1904.03944
Ensuring uninterrupted power supply to lunar installations through an organic Rankine Cycle
Fransisco, Bertolami
We propose using the temperature gradients between the Moon's surface and the soil at a certain depth to power an Organic Rankine Cycle that could supply a permanent installation, particularly at night, when solar power is not available. Our theoretical and engineering considerations show that, with existing working fluids and quite feasible technical requirements, it is possible to continuously yield $25\,{\rm kW}$ to sustain a 3 member crew.
1904.04310
Uinsg gamma ray monitoring to avoid missing the next Milky Way Type Ia supernova
Wang, et al
A Milky-Way Type Ia Supernova (SNIa) could be unidentified or even initially unnoticed, being dim in radio, X-rays, and neutrinos, and suffering large optical/IR extinction in the Galactic plane. But SNIa emit nuclear gamma-ray lines from $^{56}{\rm Ni}\to ^{56}{\rm Co}\to ^{56}{\rm Fe}$ radioactive decays. These lines fall within the Fermi/GBM energy range, and the $^{56}{\rm Ni}$ 158 keV line is detectable by Swift/BAT. Both instruments frequently monitor the Galactic plane, which is transparent to gamma rays. Thus GBM and BAT are ideal Galactic SNIa early warning systems. We simulate SNIa MeV light curves and spectra to show that GBM and BAT could confirm a Galactic SNIa explosion, followed by Swift localization and observation in X-rays and UVOIR band. The time of detection depends sensitively on the $^{56}{\rm Ni}$ distribution, and can be as early as a few days if $\gtrsim$ 10% of the $^{56}{\rm Ni}$ is present in the surface as suggested by SN2014J gamma data.
1904.04311
Astro2020 APT White Paper: "Mind the gap": a call to redesign astronomy graduate education
Moro-Martin
About one fifth of Ph.D's across all STEM disciplines secure a tenure track position in academia. It is also the case that science and engineering have evolved significantly and so has the nature of the labor market and the increasingly multidisciplinary nature of the greatest scientific challenges. These realities, however, have not altered the main objective of graduate STEM education: the training of unidisciplinary academic researchers. There is therefore a gap between what the students and society need and what graduate STEM education offers. At root of the problem is not only the lack of information regarding actual career opportunities but the lack of formation because, as the National Academies of Sciences, Engineering and Medicine recognized in its recent report entitled "Graduate STEM Education for the 21st Century", "many graduate programs do not adequately prepare students to translate their knowledge into impact in multiple careers". In that report, the National Academies of Sciences sets new standards for graduate STEM education, describes the actions required by each stakeholder, and urges all to strongly commit to this paradigm change. Astro2020 represents an opportunity for the astronomy and astrophysics community to show this commitment by issuing recommendations on the redesign of astronomy graduate education following the new guidelines set by the National Academies of Sciences. By "minding the gap" between expectations and opportunities, keeping in mind the evolving needs of the STEM workforce, we can better justify the use of tax-payers money with an investment that allows to do transformative science while conscientiously training highly qualified STEM professionals able to apply the power of science to problems and opportunities of generations to come, as envisioned by Vannevar Bush's science as the endless frontier.
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