XUV-driven mass loss from extrasolar giant planets orbiting active stars

Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmospheres of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. In this study, we focus on Extrasolar Giant Planets (EPGs) orbiting K and M dwarf stars. XUV spectra for three different stars – ∊ Eridani, AD Leonis and AU Microscopii – are constructed using a coronal model. Neutral density and temperature profiles in the upper atmosphere of hypothetical EGPs orbiting these stars are then obtained from a fluid model, incorporating atmospheric chemistry and taking atmospheric escape into account. We find that a simple scaling based solely on the host star’s X-ray emission gives large errors in mass loss rates from planetary atmospheres and so we have derived a new method to scale the EUV regions of the solar spectrum based upon stellar X-ray emission. This new method produces an outcome in terms of the planet’s neutral upper atmosphere very similar to that obtained using a detailed coronal model of the host star. Our results indicate that in planets subjected to radiation from active stars, the transition from Jeans escape to a regime of hydrodynamic escape at the top of the atmosphere occurs at larger orbital distances than for planets around low activity stars (such as the Sun)

Effect of stellar flares on the upper atmospheres of HD 189733b and HD 209458b

Stellar flares are a frequent occurrence on young low-mass stars around which many detected exoplanets orbit. Flares are energetic, impulsive events, and their impact on exoplanetary atmospheres needs to be taken into account when interpreting transit observations. We have developed a model to describe the upper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. The model simulates thermal escape from the upper atmospheres of close-in EGPs. Ionisation by solar radiation and electron impact is included and photochemical and diffusive transport processes are simulated. This model is used to study the effect of stellar flares from the solar-like G star HD209458 and the young K star HD189733 on their respective planets. A hypothetical HD209458b-like planet orbiting the active M star AU Mic is also simulated. We find that the neutral upper atmosphere of EGPs is not significantly affected by typical flares. Therefore, stellar flares alone would not cause large enough changes in planetary mass loss to explain the variations in HD189733b transit depth seen in previous studies, although we show that it may be possible that an extreme stellar proton event could result in the required mass loss. Our simulations do however reveal an enhancement in electron number density in the ionosphere of these planets, the peak of which is located in the layer where stellar X-rays are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare levels and enhanced electron densities last from about 3 to 10 hours after the onset of the flare. The strength of the flare and the width of its spectral energy distribution affect the range of altitudes that see enhancements in ionisation. A large broadband continuum component in the XUV portion of the flaring spectrum in very young flare stars, such as AU Mic, results in a broad range of altitudes affected in planets orbiting this star.Comment: accepted for publication in A&

Modelling of the upper atmosphere of gas-giant exoplanets irradiated by low-mass stars

Upper atmospheres of Hot Jupiters are subject to extreme radiation conditions that can result in rapid atmospheric escape. The composition and structure of the upper atmosphere of these planets are affected by the high-energy spectrum of the host star. This emission depends on stellar type and age, which are thus important factors in understanding the behaviour of exoplanetary atmospheres. The work descried in this thesis details the development of a new 1D ionospheric model to describe the upper atmospheres of Extrasolar Giant Plants (EGPs). The model is time-dependent and includes photo-chemistry and diffusive transport. Electron-impact ionisation processes are taken into account through coupling with a suprathermal electron transport code. Neutral composition and temperature profiles are obtained by using a thermospheric model that incorporates atmospheric escape. Atmospheres composed of H, H2, He, and their associated ions are considered. Efforts have been made to obtain accurate X-ray and Extreme Ultraviolet (EUV) spectral irradiance of the stars studied. To this effect, synthetic spectra are used originating from a detailed coronal model for three different low-mass stars of different activity levels: epsilon Eridani, AD Leonis and AU Microscopii. This work is the first study of the ionosphere of EGPs that takes into account the different spectral energy distribution of low-mass stars. In planets subjected to radiation from active stars, the transition from slow, Jeans escape to a regime of rapid hydrodynamic escape at the top of the atmosphere is found to occur at larger orbital distances than for planets around low activity stars (such as the Sun). To correctly estimate the critical orbital distance of this transition, the spectral shape of stellar XUV radiation is important. A novel method to scale the EUV region of the solar spectrum based upon stellar X-ray emission is developed in this work. This new method produces an outcome in terms of the planet's upper atmosphere and escape regime that is very similar to that obtained using a detailed coronal model of the host star. EGP ionospheres at all orbital distances and around all stars studied are dominated by the long-lived H+ ion. In addition, planets in the Jeans escape regime also have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, H3+ densities undergo significant diurnal variations, their peak value being determined by the stellar X-ray flux. In contrast, H+ densities show very little day/night variability and their value is determined by the level of stellar EUV flux. The H3+ peak in EGPs in the rapid hydrodynamic escape regime under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (C, O, etc.).Open Acces

EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars

The composition and structure of the upper atmospheres of extrasolar giant planets (EGPs) are affected by the high-energy spectrum of their host stars from soft X-rays to the extreme ultraviolet (EUV). This emission depends on the activity level of the star, which is primarily determined by its age. In this study, we focus upon EGPs orbiting K- and M-dwarf stars of different ages – ϵ Eridani, AD Leonis, AU Microscopii – and the Sun. X-ray and EUV (XUV) spectra for these stars are constructed using a coronal model. These spectra are used to drive both a thermospheric model and an ionospheric model, providing densities of neutral and ion species. Ionisation – as a result of stellar radiation deposition – is included through photo-ionisation and electron-impact processes. The former is calculated by solving the Lambert-Beer law, while the latter is calculated from a supra-thermal electron transport model. We find that EGP ionospheres at all orbital distances considered (0.1−1 AU) and around all stars selected are dominated by the long-lived H+ ion. In addition, planets with upper atmospheres where H2 is not substantially dissociated (at large orbital distances) have a layer in which H3+ is the major ion at the base of the ionosphere. For fast-rotating planets, densities of short-lived H3+ undergo significant diurnal variations, with the maximum value being driven by the stellar X-ray flux. In contrast, densities of longer-lived H+ show very little day/night variability and the magnitude is driven by the level of stellar EUV flux. The H3+ peak in EGPs with upper atmospheres where H2 is dissociated (orbiting close to their star) under strong stellar illumination is pushed to altitudes below the homopause, where this ion is likely to be destroyed through reactions with heavy species (e.g. hydrocarbons, water). The inclusion of secondary ionisation processes produces significantly enhanced ion and electron densities at altitudes below the main EUV ionisation peak, as compared to models that do not include electron-impact ionisation. We estimate infrared emissions from H3+, and while, in an H/H2/He atmosphere, these are larger from planets orbiting close to more active stars, they still appear too low to be detected with current observatories

Faraday: Synthetic Smart Meter Generator for the smart grid

Access to smart meter data is essential to rapid and successful transitions to electrified grids, underpinned by flexibility delivered by low carbon technologies, such as electric vehicles (EV) and heat pumps, and powered by renewable energy. Yet little of this data is available for research and modelling purposes due consumer privacy protections. Whilst many are calling for raw datasets to be unlocked through regulatory changes, we believe this approach will take too long. Synthetic data addresses these challenges directly by overcoming privacy issues. In this paper, we present Faraday, a Variational Auto-encoder (VAE)-based model trained over 300 million smart meter data readings from an energy supplier in the UK, with information such as property type and low carbon technologies (LCTs) ownership. The model produces household-level synthetic load profiles conditioned on these labels, and we compare its outputs against actual substation readings to show how the model can be used for real-world applications by grid modellers interested in modelling energy grids of the future.Comment: Published as a workshop paper at Tackling Climate Change with Machine Learning, ICLR 202

A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b

Exoplanets orbiting close to their parent stars could lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to suggest that hot rocky planets might have begun as Neptune-like, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start ~2 h before, and end >3 h after the ~1 h optical transit, which is substantially different from one previous claim (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of ~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.Comment: Published in Nature on 25 June 2015. Preprint is 28 pages, 12 figures, 2 table

Effects of Blood Flow Restriction Resistance Training on Muscle Thickness and Quality

By decreasing venous drainage, blood flow restriction in combination with resistance exercise (BFR), stimulates similar signals of fatigue and adaptation at much lower intensity than standard training. PURPOSE: The purpose was to study the effects of BFR on muscle thickness and muscle quality (MQ: strength/unit muscle) following an 8-week intervention. METHODS: 13 male and 17 female college students were recruited and placed in control (CON; n=13) or BFR (n=17) groups who completed training 3x/week, at 50% of occlusion pressure, using 20% of one-repetition maximum, for four compound exercises at 30, 15, 15, and 15 reps. Images captured with Terason ultrasound for biceps brachii (BB), rectus femoris (RF), and biceps femoris (BF) were analyzed with ImageJ to obtain muscle thickness. Maximal isotonic strength for elbow flexion, knee extension, and knee flexion via a HUMAC Norm was used to calculate MQ. RESULTS: No differences were seen in muscle thicknesses of BB (CON: male -10.983±11.078, BFR: -1.062±12.910; CON: female 11.155±10.415; B2RT: female 10.145±9.796), RF (CON: male -13.407±11.334, BFR: -2.98±13.209; CON: female -9.038 ± 10.657; B2RT: female -9.842 ± 10.022), and BF (CON: male -2.356±13.509, BFR: –6.136±15.744 CON: female 10.276±12.701; B2RT: 18.067±11.946) and no differences were observed in MQ percent change over time at any site. CONCLUSIONS: The BFR training protocol in this population was ineffective at promoting discernible changes in muscular thickness or MQ. Further efforts to examine higher intensities or higher occlusion pressures in a more homogenous population are warranted to fully elucidate BFR effects on muscle morphology and performance

Blood Flow Restriction Training Effects on Pennation Angles

Pennation angle (PA) represents the angle between muscle fibers and the force-generating axis of the muscle. Blood flow resistance (BFR) training involves the occlusion of venous blood flow in working muscles to promote muscular growth signaling with reduced training intensity. PURPOSE: The purpose is to examine if hypertrophy through BFR may result in PA alterations to accommodate muscle fiber volume alterations. METHODS: Males (N=11, 20.35±1.59 yrs, 177.97±9.22 cm, 83.66±20.69 kg), and females (N=12, 23.20±9.53 yrs, 164.60±6.42 cm, 63.2±10.65 kg) participated in resistance training with BFR for 3 times/week, for 8 weeks at 60% occlusion pressure, at 30% of one-repetition maximum in four compound exercises performed for 4 sets of 30, 15, 15, and 15 reps. A Tearson ultrasound was used to quantify PA for Biceps Brachii (BB), Rectus Femoris (RF), and Biceps Femoris (BF). RESULTS: Comparing percent change (PC) in PA, for males, there was similar PC in B2RT and control for BB (8.56±12.27 vs. 9.03±14.19%, p\u3e0.05), RF (8.64±16.53 vs. -0.310±19.11%, p\u3e0.05), but significantly higher PC for B2RT then control for BF (55.85±15.43 vs. 4.10±17.82%, p=0.029). In females, there was similar PC in B2RT and control for BB (11.61±13.98 vs. 16.49±11.77%, p\u3e0.05), BF (55.24±17.59 vs. 46.74±14.80%, p\u3e0.05), but PC trended higher in PA in B2RT than the control for RF (37.86±18.84 vs. 11.50±15.85%, p=0.272). CONCLUSION: This short-term protocol demonstrated altered muscle architecture in select lower-extremity muscles for males and potential trends in females which justifies additional studies to discern muscle specific adaptations

Effects of Blood Flow Restriction Training on Muscle Quality

Blood flow restricted resistance training (B2RT) has been shown to enhance effectiveness of low-intensity resistance training. While B2RT is increasingly popular, its specific effects on muscle quality have yet to be specified. PURPOSE: The purpose of this intervention was to investigate the effects of B2RT on muscle quality (MQ) of biceps brachii, triceps brachii, biceps femoris, and rectus femoris. METHODS: 30 undergraduates (11M, 19F) were broken into control (CON, n=15, 21.1±2.1 years, 23.9±3.5 kg/m2) and B2RT (n=15, 19.9±1.4 years, 26.3±5.1 kg/m2) groups. Control maintained normal activity while B2RT participated in an 8-week B2RT training regimen consisting of 3 sessions/week, with 4 sets of 30, 15, 15, and 15 reps at 60% occlusion pressure and 30% of one-repetition maximum for squats, Romanian deadlifts, bench rows, and bench presses. Maximal isometric strength was assessed for each muscle group with a HUMAC Norm and cross-sectional thickness was captured on a Terason ultrasound. Image J software was utilized to measure size (cm) changes within the muscles. For each image, a pixel to cm ratio was calculated before the image was measured. Muscle quality (strength/thickness) for each muscle was calculated and analyzed with SPSS. RESULTS: No significant findings were observed for most variables with the exception of significant percent changes in triceps brachii thickness in male B2RT vs. CON (pCONCLUSION: This short short-term B2RT protocol does not seem to systematically alter MQ although further research could focus on altered training variables to yield more conclusive evidence