Microlensing Events from Measurements of the Deflection Angle

Microlensing events are now regularly being detected by monitoring the flux of a large number of potential sources and measuring the combined magnification of the images. This phenomenon could also be detected directly from the gravitational deflection, by means of high precision astrometry using interferometry. Relative astrometry at the level of 10\muas may become possible in the near future. The gravitational deflection can be measured by astrometric monitoring of a bright star having a background star within a small angular separation. This type of monitoring program will be carried out for the independent reasons of discovering planets from the angular motion they induce on the nearby star around which they are orbiting, and for measuring parallaxes, proper motions and orbits of binary stars. We discuss three applications of the measurement of gravitational deflections by astrometric monitoring: measuring the mass of the bright stars that are monitored, measuring the mass of brown dwarfs or giant planets around the bright stars, and detecting microlensing events by unrelated objects near the line of sight to the two stars. We discuss the number of stars whose mass could be measured by this procedure. We also give expressions for the number of expected microlensing events by unrelated objects, which could be stars, brown dwarfs, or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter

An assessment of the tea bag index method as a proxy for organic matter decomposition in intertidal environments

This work was supported by the Natural Environment Research Council (grant NE/R010846/1) Carbon in Storage in Intertidal Environments (C-SIDE) project.Intertidal wetlands capture and store carbon (C) for long periods of time, helping to reduce the concentration of CO2 in the atmosphere. Yet the processes which govern the decomposition and subsequent long‐term storage of organic matter (OM) and C in these habitats remains poorly understood. The Tea Bag Index (TBI) uses a standardized OM (green and Rooibos tea) and has the potential to shed light on OM decomposition across habitats, including saltmarshes. Here, we apply the TBI method at two saltmarshes within the same estuary with the aim of (i) reducing the influence of climatic variables and (ii) determining the role of the environment, including the soil characteristics, in the decomposition of OM. We extended the standard (3 months) incubation period over a full year in order to investigate the longer‐term decomposition processes at each site. The initial results partially support previous studies that the early stages of decomposition (leaching of the water‐soluble fraction) is governed by climatic conditions, but may be further enhanced by tidal flushing in saltmarshes. By extending the incubation period, we observed the initiation of mid‐stage OM decomposition (Cellulose degradation) upon which the soil characteristics appear to be the dominant control. These results highlight the importance of long‐term TBI incubations to understand early‐stage OM decomposition. The relationship between tea mass (OM) loss and C loss in these intertidal environments is not straightforward and we would caution the use of the TBI as a direct universal proxy for soil C degradation in such intertidal wetlands.Publisher PDFPeer reviewe

Spherical Harmonics for the 1D Radiative Transfer Equation II: Thermal Emission

Approximate methods to estimate solutions to the radiative transfer equation are essential for the understanding of atmospheres of exoplanets and brown dwarfs. The simplest and most popular choice is the "two-stream method" which is often used to produce simple yet effective models for radiative transfer in scattering and absorbing media. Toon et al. (1989) (Toon89) outlined a two-stream method for computing reflected light and thermal spectra and was later implemented in the open-source radiative transfer model PICASO. In Part~I of this series, we developed an analytical spherical harmonics method for solving the radiative transfer equation for reflected solar radiation (Rooney et al. 2023), which was implemented in PICASO to increase the accuracy of the code by offering a higher-order approximation. This work is an extension of this spherical harmonics derivation to study thermal emission spectroscopy. We highlight the model differences in the approach for thermal emission and benchmark the 4-term method (SH4) against Toon89 and a high-stream discrete-ordinates method, CDISORT. By comparing the spectra produced by each model we demonstrate that the SH4 method provides a significant increase in accuracy, compared to Toon89, which can be attributed to the increased order of approximation and to the choice of phase function. We also explore the trade-off between computational time and model accuracy. We find that our 4-term method is twice as slow as our 2-term method, but is up to five times more accurate, when compared with CDISORT. Therefore, SH4 provides excellent improvement in model accuracy with minimal sacrifice in numerical expense.Comment: Submitted ApJ; 17 pages; 7 figures; Code available at https://github.com/natashabatalha/picaso; Zenodo release at https://zenodo.org/record/7765171#.ZC3G7uzMI8Y; Tutorials/figure reproducibility at https://natashabatalha.github.io/picaso/notebooks/10c_AnalyzingApproximationsThermal.html

Energy Efficiency Ratios of Surface Mining Systems for a Small Iowa Coal Mine

The normal criterion used to determine if a mining venture should be started or continued is its financial profitability. Energy efficiency is another criterion for dealing with public energy decisions. The energy efficiency ratios (output/input) of a large bulldozer, a scraper ripper, a small dragline, and a large dragline system were calculated for the Iowa Coal Project Demonstration Mine #1 (ICPDM #1) located between Oskaloosa and Bussey, Iowa. Even though the large dragline had the lowest cost and highest energy efficiency, it would not be used on this site without having other nearby sites available to allow long-term use