Galactic Center Pulsars with the ngVLA

Pulsars in the Galactic Center (GC) are important probes of General Relativity, star formation, stellar dynamics, stellar evolution, and the interstellar medium. Despite years of searching, only a handful of pulsars in the central 0.5 deg are known. The high-frequency sensitivity of ngVLA will open a new window for discovery and characterization of pulsars in the GC. A pulsar in orbit around the GC black hole, Sgr A*, will provide an unprecedented probe of black hole physics and General Relativity.Comment: To be published in the ASP Monograph Series, "Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA

Diachronous end-Permian terrestrial ecosystem collapse with its origin in wildfires

The Permian-Triassic Mass Extinction (PTME) is the greatest biodiversity crisis in Earth history and while the marine crisis is increasingly well constrained, the timing and cause(s) of terrestrial losses remain poorly understood. There have been suggestions that the End-Permian Terrestrial Collapse (EPTC) pre-dated, was synchronous with or post-dated the marine crisis, or even occurred asynchronously in different regions. We address these conflicting interpretations through a detailed geochemical study of a terrestrial sequence in the Liujiang Coalfield on the North China Plate (NCP) in which we apply zircon U-Pb dating of tuffaceous claystone, kerogen identification, and analysis of organic carbon isotopic composition (δ13Corg), total organic carbon (TOC), continental weathering (via the chemical index of alteration; CIA) and Ni concentrations. Our study constrains the Permian-Triassic boundary (PTB) near the base of bed 20 in our sequence at approximately 251.9 ± 1.1 Ma, immediately above a Ni anomaly also known from other terrestrial sequences and the marine PTME. Organic carbon isotope chemostratigraphy together with evidence for algal blooms and the presence of mudstone clasts suggests that the onset of the EPTC in the NCP was synchronous with the crisis in low latitudes (e.g., South China), but was about 310 kyr later than the EPTC in higher southerly latitudes (e.g., Australia). The EPTC predates the marine PTME. Kerogen macerals suggest that a phase of increased wildfire was sustained from the onset of the EPTC in the NCP until the marine PTME interval, implicating wildfire as a major driver of the EPTC (at least in low latitudes) that, in turn, had devastating consequences for the marine realm

Water promoted photocatalytic Cβ-O bonds hydrogenolysis in lignin model compounds and lignin biomass conversion to aromatic monomers

Photocatalysis has proved its potential in cleaving the Cβ-O linkages between the natural aromatic units in lignin biomass and converting abundant lignin biomass to valuable aromatic monomer products. However, the slow reaction rate and low selectivity for aromatic monomers still hinder its future industrial implementation. To address these challenges in photocatalytic Cβ-O bond fragmentation, a Zn/S rich phase zinc indium sulfide photocatalyst was developed to promote hydrogenolysis of Cβ-O linkages in lignin. In this work, water is for the first time, used as the hydrogen donor and can significantly promote the photocatalytic process by eliminating the limitation of protons supply. The reaction selectivity for aromatic monomers increased by 170% and PP-ol conversion rate raised by 58% comparing to the reaction condition without water. Notably, complete conversion of lignin model compounds with an expectational improved reaction rate and over 90% selectivity for aromatic monomers have been achieved in this study. The isotopic labeling experiments and kinetic isotope effects (KIE) measurements also indicate that the dissociation of the O–H bond in water which provides protons to the Cβ-O bond hydrogenolysis process is a critical step to this reaction. Mechanistic studies reveal that the dehydrogenated radical intermediates are initially generated by the oxidation of photogenerated holes, and the protons generated from photocatalytic water splitting are superior in facilitating the subsequently hydrogenolysis process of Cβ-O bonds. This study provides a new and effective strategy to promote the cleavage of Cβ-O linkages and is helpful for the future development of photocatalytic lignin valorization

The impact of frequent wildfires during the Permian-Triassic transition: Floral change and terrestrial crisis in southwestern China

Wildfires are considered to have played an important role in the land plants crisis during the Permian–Triassic (P–T) transition. However, the nature and impact of wildfires in the P–T terrestrial crisis remains unclear. Organic petrology data from a terrestrial sequence from southwestern China show that the inertinite content ranges from 21.3% to 80.9% (mean 44.5%), suggesting that wildfires were a frequent phenomenon in low-latitude tropical rainforests during the P–T transition. Abundant inertinite and Hg/TOC peaks in earliest Triassic strata support the co-existence of wildfires and volcanism at that time. Volcanic emissions were potentially lethal for plants and adjacent arc volcanism represents a possible source of ignition. Inertinite reflectance values are used to estimate wildfire combustion temperatures, which themselves are a function of wildfire type. Inertinite with reflectances higher than 4.5% have concentrations between 47% and 65% in the P–T transitional strata. Crown fires with high combustion temperatures were prevalent in wetland settings in the latest Permian. However, surface fires with lower combustion temperatures became dominant during the major terrestrial extinction phase as a result of the sparse, scrubby vegetation that dominated at that time. The subsequent spread of gymnosperms in the earliest Triassic resulted in the re-establishment of high-temperature crown fires. Wildfires associated with the onset of volcanism in the late Permian likely contributed to ecological disturbance in terrestrial settings, which occurred notably earlier than that seen in marine environments. Thus, enhanced wildfire activity destabilised wetlands and increased ecological stress in the late Permian. Wildfire activity on land potentially had devastating consequences for late Permian marine environments via a complex cascade of terrestrial denudation, runoff, and nutrient flux

An Investigation of the Required MR Bone Attenuation Correction for Quantitative Whole-Body PET/MR Imaging Using Clinical NaF PET/CT Studies

Ai, H. , Mawlawi, O. , Stafford, R. , Bankson, J. , Shao, Y. , Guindani, M. and Wendt III, R. (2018) An Investigation of the Required MR Bone Attenuation Correction for Quantitative Whole-Body PET/MR Imaging Using Clinical NaF PET/CT Studies. International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 7, 273-295. doi: 10.4236/ijmpcero.2018.73023.https://openworks.mdanderson.org/mdacc_imgphys_pubs/1003/thumbnail.jp

Testing Gravity with Pulsars in the SKA Era

The Square Kilometre Array (SKA) will use pulsars to enable precise measurements of strong gravity effects in pulsar systems, which yield tests of gravitational theories that cannot be carried out anywhere else. The Galactic census of pulsars will discover dozens of relativistic pulsar systems, possibly including pulsar -- black hole binaries which can be used to test the "cosmic censorship conjecture" and the "no-hair theorem". Also, the SKA's remarkable sensitivity will vastly improve the timing precision of millisecond pulsars, allowing probes of potential deviations from general relativity (GR). Aspects of gravitation to be explored include tests of strong equivalence principles, gravitational dipole radiation, extra field components of gravitation, gravitomagnetism, and spacetime symmetries.Comment: 20 pages, 4 figures, to be published in: "Advancing Astrophysics with the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)04

An astronomical timescale for the Permian-Triassic mass extinction reveals a two-step, million-year-long terrestrial crisis in South China

The Permian-Triassic Mass Extinction (PTME) is the greatest biotic crisis of the Phanerozoic. In terrestrial settings, the PTME appears to have been diachronous and it has been suggested that losses initiated before the marine crisis. We examine organic carbon-isotope (δ13Corg) and geochemical proxies for environmental change in a palaeotropical wetland succession from southwest China. A newly constructed astronomical timescale provides a temporal framework for constraining the timing of the terrestrial PTME. Two major, negative carbon isotope excursions (CIEs) of 5.3‰ and 3.9‰ are observed between the top of the (Permian) Xuanwei Formation and the middle of the (Permian-Triassic) Kayitou Formation respectively. Our cyclostratigraphic model suggests that carbon cycle destabilization lasted ~0.6 ± 0.1 Myr. We calculate total erosion rates for basaltic landscapes as a proxy for volumes of bare soil resulting from deforestation. Two phases of accelerated erosion saw denudation rates rise over a ~1 Myr period from ~150 t/km2/yr in the upper Xuanwei Formation (Permian) to >2000 t/km2/yr at the base of the Dongchuan Formation (Triassic). Calibrating the collapse of terrestrial ecosystems indicates that although the equatorial terrestrial PTME initiated before the marine crisis, it was a protracted process with the final coup-de-grâce not until ~ 700ky later. This has a bearing on extinction scenarios in which the terrestrial PTME is a causal factor in marine losses via enhanced nutrient runoff, because the final devastation on land post-dates the much more abrupt marine PTME

Astrometry with the Wide-Field InfraRed Space Telescope

The Wide-Field InfraRed Space Telescope (WFIRST) will be capable of delivering precise astrometry for faint sources over the enormous field of view of its main camera, the Wide-Field Imager (WFI). This unprecedented combination will be transformative for the many scientific questions that require precise positions, distances, and velocities of stars. We describe the expectations for the astrometric precision of the WFIRST WFI in different scenarios, illustrate how a broad range of science cases will see significant advances with such data, and identify aspects of WFIRST's design where small adjustments could greatly improve its power as an astrometric instrument.Comment: version accepted to JATI