Modeling SiO Maser Emission from Late-Type Stars

We have performed a thorough study of both radiative and collisional pumping of the SiO masers around late-type stars, carefully considering the combined and separate actions of each type of pump in order to gauge its effectiveness. We find that collisional pumping is severely underestimated when the model calculations use a small number (less than about 18) of rotational levels in each vibrational state. We have developed a procedure that corrects this problem and gives results that are nearly independent of the number of levels utilized in the calculations. We recognize, but do not solve, an important problem that afflicts all escape probability treatments which include maser saturation effects on the level populations. Maser radiation is strongly beamed and the functional form of the beaming angle must be known to properly calculate the maser escape probability. However, the beam pattern for saturated masers in the presence of large velocity gradients has yet to be studied in the literature. Our model is based on observations and theoretical arguments that place the SiO masers in high-density clumps rather than in the smooth stellar wind. Significantly, general conclusions can be reached which are independent of the pumping mechanism. Most importantly, the overall molecular density is restricted to lie between ~109-1010 cm-3, regardless of the type of pumping. In addition, both collisional and radiative pumps result in the production of a maser chain within each vibrational state, as observed. There are some important differences, however, between the pumping mechanisms. All pumps based on stellar radiation become less efficient with distance from the star because of the rapid decline in pump rate. This prevents any radiative pump from being able to produce the observed maser emission over most of the observed maser region. The only feasible radiative pumps require fine tuning of physical conditions and produce inversion only over a narrow range of optical depths that depends sensitively on the size of the velocity gradient and the form of the escape probability expression. In addition, these radiative pumps have difficulty in explaining the simultaneous production of masers in the same rotational transitions of adjacent vibrational states as is observed. We find that collisional pumping produces the strongest maser emission and, in contrast to radiative pumping, generates maser radiation over the entire observed region and does not require fine tuning of the physical parameters for its operation. Furthermore, there is a significant range of overlapping column densities where collisional pumping produces maser emission in the same rotational transitions of adjacent vibrational states, as observed. Collisional pumping thus appears to be the primary pumping mechanism responsible for the SiO maser phenomenon

Excited-State OH Masers and Supernova Remnants

The collisionally pumped, ground-state 1720 MHz maser line of OH is widely recognized as a tracer for shocked regions and observed in star-forming regions and supernova remnants. Whereas some lines of excited states of OH have been detected and studied in star-forming regions, the subject of excited-state OH in supernova remnants-where high collision rates are to be expected-is only recently being addressed. Modeling of collisional excitation of OH demonstrates that 1720, 4765, and 6049 MHz masers can occur under similar conditions in regions of shocked gas. In particular, the 6049 and 4765 MHz masers become more significant at increased OH column densities where the 1720 MHz masers begin to be quenched. In supernova remnants, the detection of excited-state OH line maser emission could therefore serve as a probe of regions of higher column densities. Using the Very Large Array, we searched for excited-state OH in the 4.7, 7.8, 8.2, and 23.8 GHz lines in four well-studied supernova remnants with strong 1720 MHz maser emission (Sgr A East, W28, W44 and IC 443). No detections were made, at typical detection limits of around 10 mJy beam-1. The search for the 6 GHz lines were done using Effelsberg since the VLA receivers did not cover those frequencies, and are reported on in an accompanying letter (Fish and coworkers). We also cross-correlated the positions of known supernova remnants with the positions of 1612 MHz maser emission obtained from blind surveys. No probable associations were found, perhaps except in the Sgr A East region. The lack of detections of excited-state OH indicates that the OH column densities suffice for 1720 MHz inversion but not for inversion of excited-state transitions, consistent with the expected results for C-type shocks

Multi-ancestry genome-wide association study of 21,000 cases and 95,000 controls identifies new risk loci for atopic dermatitis

Genetic association studies have identified 21 loci associated with atopic dermatitis risk predominantly in populations of European ancestry. To identify further susceptibility loci for this common, complex skin disease, we performed a meta-analysis of >15 million genetic variants in 21,399 cases and 95,464 controls from populations of European, African, Japanese and Latino ancestry, followed by replication in 32,059 cases and 228,628 controls from 18 studies. We identified ten new risk loci, bringing the total number of known atopic dermatitis risk loci to 31 (with new secondary signals at four of these loci). Notably, the new loci include candidate genes with roles in the regulation of innate host defenses and T cell function, underscoring the important contribution of (auto)immune mechanisms to atopic dermatitis pathogenesis

Global urban environmental change drives adaptation in white clover

Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale