Gaia, White Dwarfs, and the Age of the Galaxy

The Milky Way is composed of four major stellar populations: the thin disk, thick disk, bulge, and halo. At present, we do not know the age of any of these populations to better than one or two billion years. This lack of knowledge keeps us from answering fundamental questions about the Galaxy: When did the thin disk, thick disk, and halo form? Did they form over an extended period, and if so, how long? Was star formation continuous across these populations or instead occur in distinct episodes? The Gaia satellite is providing precise trigonometric parallaxes for a plethora of white dwarfs in each of these populations. We combine these parallaxes (and hence, distances) with photometry and analyze them using a modeling technique that relies on Bayesian statistics. This allows us to derive precise ages for individual white dwarfs and determine the age distribution and star formation history for each of the constituents of our Galaxy. Here we will present current progress in this endeavor, with emphasis on the ages of individual white dwarfs in the Hyades. Measuring the ages of individual white dwarfs in well-studied clusters provides proof of concept for our technique, as well exploration of any systematic offsets caused from timescales from main sequence models, as well as the initial-final mass relation

A CHIME/FRB Study of Burst Rate and Morphological Evolution of the Periodically Repeating FRB 20180916B

FRB 20180916B is a repeating fast radio burst (FRB) with a 16.3 day periodicity in its activity. In this study, we present morphological properties of 60 FRB 20180916B bursts detected by CHIME/FRB between 2018 August and 2021 December. We recorded raw voltage data for 45 of these bursts, enabling microseconds time resolution in some cases. We studied variation of spectro-temporal properties with time and activity phase. We find that the variation in dispersion measure (DM) is ≲1 pc cm−3 and that there is burst-to-burst variation in scattering time estimates ranging from ∼0.16 to over 2 ms, with no discernible trend with activity phase for either property. Furthermore, we find no DM and scattering variability corresponding to the recent change in rotation measure from the source, which has implications for the immediate environment of the source. We find that FRB 20180916B has thus far shown no epochs of heightened activity as have been seen in other active repeaters by CHIME/FRB, with its burst count consistent with originating from a Poissonian process. We also observe no change in the value of the activity period over the duration of our observations and set a 1σ upper limit of 1.5 × 10−4 day day−1 on the absolute period derivative. Finally, we discuss constraints on progenitor models yielded by our results, noting that our upper limits on changes in scattering and DM as a function of phase do not support models invoking a massive binary companion star as the origin of the 16.3 day periodicity.</p

Gaia, White Dwarfs, and the Age of the Galaxy

The Milky Way is composed of four major stellar populations: the thin disk, thick disk, bulge, and halo. At present, we do not know the age of any of these populations to better than one or two billion years. This lack of knowledge keeps us from answering fundamental questions about the Galaxy: When did the thin disk, thick disk, and halo form? Did they form over an extended period, and if so, how long? Was star formation continuous across these populations or instead occur in distinct episodes? The Gaia satellite is providing precise trigonometric parallaxes for a plethora of white dwarfs in each of these populations. We combine these parallaxes (and hence, distances) with photometry and analyze them using a modeling technique that relies on Bayesian statistics. This allows us to derive precise ages for individual white dwarfs and determine the age distribution and star formation history for each of the constituents of our Galaxy. Here we will present current progress in this endeavor, with emphasis on the ages of individual white dwarfs in the Hyades. Measuring the ages of individual white dwarfs in well-studied clusters provides proof of concept for our technique, as well exploration of any systematic offsets caused from timescales from main sequence models, as well as the initial-final mass relation

A comprehensive variable temperature study of the layered oxide, Ca2Mn3O8

Ca2Mn3O8 forms a delafossite-related layered structure, which crystallises with monoclinic C2/m symmetry. Compared with the delafossite-structure, the MnO6 layers in Ca2Mn3O8 exhibit an ordered cation void which forms a magnetic ‘bow-tie’ like connectivity of Mn4+ ion layers separated by Ca2+ ions. In-situ variable temperature diffraction data demonstrates that the structure is robust up to a temperature of approximately 1173 K before the material decomposes into the perovskite, CaMnO3 and marokite, CaMn2O4 phases. Simultaneous thermal analysis suggests that a very small amount of water remains within the layers post synthesis. Impedance spectroscopy indicates that Ca2Mn3O8 is an electronic conductor in the range ∼400–700 K with an activation energy of 0.50 ± 0.01 eV

The power of principled bayesian methods in the study of stellar evolution

It takes years of effort employing the best telescopes and instruments to obtain high-quality stellar photometry, astrometry, and spectroscopy. Stellar evolution models contain the experience of lifetimes of theoretical calculations and testing. Yet most astronomers fit these valuable models to these precious datasets by eye. We show that a principled Bayesian approach to fitting models to stellar data yields substantially more information over a range of stellar astrophysics. We highlight advances in determining the ages of star clusters, mass ratios of binary stars, limitations in the accuracy of stellar models, post-main-sequence mass loss, and the ages of individual white dwarfs. We also outline a number of unsolved problems that would benefit from principled Bayesian analyses

Potassium carbonate-silica: a highly effective stationary phase for the chromatographic removal of organotin impurities

Organotin impurities in product mixtures can be reduced from stoichiometric levels to similar to 15 parts per million by column chromatography using 10% w/w anhydrous potassium carbonate-silica as a stationary phase

Spatial Principles in Control of Focus in Reasoning with Mental Representations, Images, and Diagrams

The effective control of attentional focus is an essential requirement in mental reasoning based on mental models and mental images, as well as in the interaction with external diagrams. In this paper, we argue for spatial organization principles common to various mental subsystems that entail a noncentralistic control of focus. We give a brief overview of mental spatial reasoning and present a review of psychological findings related to cognitive control. We review existing modeling approaches that realize control of focus in imagery, scene recognition, and mental animation. Based on these foundations, we identify basic spatial organizing principles that are shared by the diverse subsystems collaborating in mental spatial reasoning. We discuss the implications of these principles in the framework of a computational modeling approach and give an outline of the conception of control of focus in our computational architecture Casimir