UV driven evaporation of close-in planets: energy-limited; recombination-limited and photon-limited flows

We have investigated the evaporation of close-in exoplanets irradiated by ionizing photons. We find that the properties of the flow are controlled by the ratio of the recombination time to the flow time-scale. When the recombination time-scale is short compared to the flow time-scale the the flow is in approximate local ionization equilibrium with a thin ionization front, where the photon mean free path is short compared to flow scale. In this "recombination limited" flow the mass-loss scales roughly with the square root of the incident flux. When the recombination time is long compared to the flow time-scale the ionization front becomes thick and encompasses the entire flow, with the mass-loss rate scaling linearly with flux. If the planet's potential is deep the flow is approximately "energy-limited"; however, if the planet's potential is shallow we identify a new limiting mass-loss regime, which we term "photon-limited". In this scenario the mass-loss rate is purely limited by the incoming flux of ionizing photons. We have developed a new numerical approach that takes into account the frequency dependence of the incoming ionizing spectrum and performed a large suite of 1D simulations to characterise UV driven mass-loss around low mass planets. We find the flow is "recombination-limited" at high fluxes but becomes "energy-limited" at low fluxes; however, the transition is broad occurring over several order of magnitude in flux. Finally, we point out the transitions between the different flow types does not occur at a single flux value, but depends on the planet's properties, with higher mass planets becoming "energy-limited" at lower fluxes.Comment: Published in Ap

Effects of Herbicide-Resistant Technology Fees on the Plant Population Decision for Cotton Production

This study evaluated effects on cotton net revenues of four herbicide-resistant technology policies used since 1996 by Monsanto. Results indicate that farmers may have an incentive to switch from narrow-row to wide-row cotton and to use a lower plant density when the technology fee is tied to the seeding rate.Farm Management,

OPTIMAL PLANT POPULATION FOR ULTRA-NARROW-ROW COTTON PRODUCTION AS INFLUENCED BY LINT AND TRANSGENIC SEED PRICES

Farmers are concerned about the high costs of transgenic seed and technology fees associated with the large plant population densities recommended for ultra-narrow row cotton. This study evaluated the effects of alternative plant population density decision criteria on net revenues under different lint price and transgenic seed cost scenarios. Results indicate that farmers may be able to maximize profits by seeding for a target plant population density of approximately 15.5 plants m-2.Crop Production/Industries,

Cognitive performance in multiple system atrophy

The cognitive performance of a group of patients with multiple system atrophy (MSA) of striato-nigral predominance was compared with that of age and IQ matched control subjects, using three tests sensitive to frontal lobe dysfunction and a battery sensitive to memory and learning deficits in Parkinson's disease and dementia of the Alzheimer type. The MSA group showed significant deficits in all three of the tests previously shown to be sensitive to frontal lobe dysfunction. Thus, a significant proportion of patients from the MSA group failed an attentional set-shifting test, specifically at the stage when an extra-dimensional shift was required. They were also impaired in a subject-ordered test of spatial working memory. The MSA group showed deficits mostly confined to measures of speed of thinking, rather than accuracy, on the Tower of London task. These deficits were seen in the absence of consistent impairments in language or visual perception. Moreover, the MSA group showed no significant deficits in tests of spatial and pattern recognition previously shown to be sensitive to patients early in the course of probable Alzheimer's disease and only a few patients exhibited impairment on the Warrington Recognition Memory Test. There were impairments on other tests of visual memory and learning relative to matched controls, but these could not easily be related to fundamental deficits of memory or learning. Thus, on a matching-to-sample task the patients were impaired at simultaneous but not delayed matching to sample, whereas difficulties in a pattern-location learning task were more evident at its initial, easier stages. The MSA group showed no consistent evidence of intellectual deterioration as assessed from their performance on subtests of the Wechsler Adult Intelligence Scale (WAIS) and the National Adult Reading Test (NART). Consideration of individual cases showed that there was some heterogeneity in the pattern of deficits in the MSA group, with one patient showing no impairment, even in the face of considerable physical disability. The results show a distinctive pattern of cognitive deficits, unlike those previously seen using the same tests in patients with Parkinson's and Alzheimer's diseases, and suggesting a prominent frontal-lobe-like component. The implications for concepts of 'subcortical' dementia and 'fronto-striatal' cognitive dysfunction are considered

The Motion of a Body in Newtonian Theories

A theorem due to Bob Geroch and Pong Soo Jang ["Motion of a Body in General Relativity." Journal of Mathematical Physics 16(1), (1975)] provides the sense in which the geodesic principle has the status of a theorem in General Relativity (GR). Here we show that a similar theorem holds in the context of geometrized Newtonian gravitation (often called Newton-Cartan theory). It follows that in Newtonian gravitation, as in GR, inertial motion can be derived from other central principles of the theory.Comment: 12 pages, 1 figure. This is the version that appeared in JMP; it is only slightly changed from the previous version, to reflect small issue caught in proo

Exoplanet atmosphere evolution: emulation with random forests

Atmospheric mass-loss is known to play a leading role in sculpting the demographics of small, close-in exoplanets. Understanding the impact of such mass-loss driven evolution requires modelling large populations of planets to compare with the observed exoplanet distributions. As the quality of planet observations increases, so should the accuracy of the models used to understand them. However, to date, only simple semi-analytic models have been used in such comparisons since modelling populations of planets with high accuracy demands a high computational cost. To address this, we turn to machine learning. We implement random forests trained on atmospheric evolution models, including XUV photoevaporation, to predict a given planet's final radius and atmospheric mass. This evolution emulator is found to have an RMS fractional radius error of 1$\%$ from the original models and is $\sim 400$ times faster to evaluate. As a test case, we use the emulator to infer the initial properties of Kepler-36b and c, confirming that their architecture is consistent with atmospheric mass loss. Our new approach opens the door to highly sophisticated models of atmospheric evolution being used in demographic analysis, which will yield further insight into planet formation and evolution.Comment: 5 pages, 3 figures. Submitted to MNRAS letter