How dust shapes protoplanetary discs and the implications to planet formation

Protoplanetary discs are the natal environments of planets and contain the building blocks from which planets form. It is therefore of crucial importance to understand how the dust growth and evolution shapes discs and what the implications are for planet formation. At the same time, our observational capabilities have improved in the recent years, providing us with more constraints that need to be considered in our theoretical studies. The goal of this thesis is to determine through numerical simulations how the dust shapes the (thermal) structure of the protoplanetary disc, how the conditions within the disc affect the growth of planets and how the forming planets affect the dust mass itself. We find that opacity models based only on micrometer-sized dust grains might not be a good approximation to simulate the disc's structure, especially for discs with significant viscous heating. There is a trade-off between the pebble isolation mass and the planetary growth timescale, which is important for the modeling of the growth of super-Earths via pebble accretion. We also find that the most favorable conditions for giant planet formation are high disc mass, early formation, and a large enough disc, however we conclude that their formation is mainly the outcome of a combination of beneficial factors or lack of adverse ones. Our findings strengthen the hypothesis that planet formation has already happened or is ongoing in Class II discs and we show that the assumption of an optically thin emission significantly underestimates the total dust mass in discs, if a giant planet is present that traps dust exterior to its orbit. We conclude that we should use the ever-increasing and improved observational data to better constrain the protoplanetary disc properties and connect the dots better to the observed exoplanets, based on our more sophisticated theoretical models

Influence of grain growth on the thermal structure of protoplanetary discs

The thermal structure of a protoplanetary disc is regulated by the opacity that dust grains provide. However, previous works have often considered simplified prescriptions for the dust opacity in hydrodynamical disc simulations, e.g. by considering only a single particle size. In the present work we perform 2D hydrodynamical simulations of protoplanetary discs where the opacity is self-consistently calculated for the dust population, taking into account the particle size, composition and abundance. We first compare simulations using single grain sizes to two different multi-grain size distributions at different levels of turbulence strengths, parameterized through the $\alpha$-viscosity, and different gas surface densities. Assuming a single dust size leads to inaccurate calculations of the thermal structure of discs, because the grain size dominating the opacity increases with orbital radius. Overall the two grain size distributions, one limited by fragmentation only and the other determined from a more complete fragmentation-coagulation equilibrium, give similar results for the thermal structure. We find that both grain size distributions give less steep opacity gradients that result in less steep aspect ratio gradients, in comparison to discs with only micrometer sized dust. Moreover, in the discs with a grain size distribution, the innermost outward migration region is removed and planets embedded is such discs experience lower migration rates. We also investigate the dependency of the water iceline position on the alpha-viscosity, the initial gas surface density at 1 AU and the dust-to-gas ratio and find $r_{ice} \propto \alpha^{0.61} \Sigma_{g,0}^{0.8} f_{DG}^{0.37}$ independently of the distribution used. The inclusion of the feedback loop between grain growth, opacities and disc thermodynamics allows for more self-consistent simulations of accretion discs and planet formation.Comment: Accepted by A&A, 27 pages, 19 figure

EDEN Survey: Small Transiting Planet Detection Limits and Constraints on the Occurrence Rates for Late M Dwarfs within 15 pc

Earth-sized exoplanets that transit nearby, late spectral type red dwarfs will be prime targets for atmospheric characterization in the coming decade. Such systems, however, are difficult to find via wide-field transit surveys like Kepler or TESS. Consequently, the presence of such transiting planets is unexplored and the occurrence rates of short-period Earth-sized planets around late M dwarfs remain poorly constrained. Here, we present the deepest photometric monitoring campaign of 22 nearby late M dwarf stars, using data from over 500 nights on seven 1-2 meter class telescopes. Our survey includes all known single quiescent northern late M dwarfs within 15 pc. We use transit-injection-and-recovery tests to quantify the completeness of our survey, successfully identify most ($>80\%$) transiting short-period (0.5-1 d) super-Earths ($R > 1.9 R_\oplus$), and are sensitive ($\sim50\%$) to transiting Earth-sized planets ($1.0-1.2 R_\oplus$). Our high sensitivity to transits with a near-zero false positive rate demonstrates an efficient survey strategy. Our survey does not yield a transiting planet detection, yet it provides the most sensitive upper limits on transiting planets orbiting our target stars. Finally, we explore multiple hypotheses about the occurrence rates of short-period planets (from Earth-sized planets to giant planets) around late M dwarfs. We show, for example, that giant planets at short periods ($<1$ day) are uncommon around our target stars. Our dataset provides some insight into occurrence rates of short-period planets around TRAPPIST-1-like stars, and our results can help test planetary formation and system evolution models, as well as guide future observations of nearby late M dwarfs.Comment: 27 pages, 11 figure

Perinatal and 2-year neurodevelopmental outcome in late preterm fetal compromise: the TRUFFLE 2 randomised trial protocol

Introduction: Following the detection of fetal growth restriction, there is no consensus about the criteria that should trigger delivery in the late preterm period. The consequences of inappropriate early or late delivery are potentially important yet practice varies widely around the world, with abnormal findings from fetal heart rate monitoring invariably leading to delivery. Indices derived from fetal cerebral Doppler examination may guide such decisions although there are few studies in this area. We propose a randomised, controlled trial to establish the optimum method of timing delivery between 32 weeks and 36 weeks 6 days of gestation. We hypothesise that delivery on evidence of cerebral blood flow redistribution reduces a composite of perinatal poor outcome, death and short-term hypoxia-related morbidity, with no worsening of neurodevelopmental outcome at 2 years. Methods and analysis: Women with non-anomalous singleton pregnancies 32+0 to 36+6 weeks of gestation in whom the estimated fetal weight or abdominal circumference is &lt;10th percentile or has decreased by 50 percentiles since 18-32 weeks will be included for observational data collection. Participants will be randomised if cerebral blood flow redistribution is identified, based on umbilical to middle cerebral artery pulsatility index ratio values. Computerised cardiotocography (cCTG) must show normal fetal heart rate short term variation (≥4.5 msec) and absence of decelerations at randomisation. Randomisation will be 1:1 to immediate delivery or delayed delivery (based on cCTG abnormalities or other worsening fetal condition). The primary outcome is poor condition at birth and/or fetal or neonatal death and/or major neonatal morbidity, the secondary non-inferiority outcome is 2-year infant general health and neurodevelopmental outcome based on the Parent Report of Children's Abilities-Revised questionnaire. Ethics and dissemination: The Study Coordination Centre has obtained approval from London-Riverside Research Ethics Committee (REC) and Health Regulatory Authority (HRA). Publication will be in line with NIHR Open Access policy. Trial registration number: Main sponsor: Imperial College London, Reference: 19QC5491. Funders: NIHR HTA, Reference: 127 976. Study coordination centre: Imperial College Healthcare NHS Trust, Du Cane Road, London, W12 0HS with Centre for Trials Research, College of Biomedical &amp; Life Sciences, Cardiff University. IRAS Project ID: 266 400. REC reference: 20/LO/0031. ISRCTN registry: 76 016 200

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P &lt; 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

Influence of grain growth on the thermal structure of inner protoplanetary discs

Protoplanetary discs surround young stars for the first few million years after their formation and they are the birthplaces of planetary systems. The thermal structure of the discs is regulated by their dust content and the opacity that it provides. The aim of this project is to investigate the effect grain growth has on the structure of the protoplanetary discs. Hydrodynamical simulations have been coupled to a new opacity model, which can calculate the opacity as a function of temperature for a dust population taking into account the particle size, composition and abundance. Single-size simulations are investigated for different turbulent strengths. Full size distributions are also explored that take into account coagulation and fragmentation of dust particles. For the single size simulations it was found that discs with small grains, from 0.1 to 10 μm have similar thermal structures at high turbulence. There exists a slight progressive drop with increasing orbital distance in the temperature or aspect ratio of these discs for the specified grain sizes. On the other hand, discs with grains of 1 mm are around 50% colder at midplane within the first few AU, compared to discs with small grains, but this difference is diminished after approximately 10 AU. The 0.1 mm grains lead to discs that remain 60% hotter even at the outer boundaries of the discs. The location of the iceline depends on the particle size, as it moves inwards as the particles size increases and it is inside 1 AU for the larger particles. In general, decreasing turbulence leads to colder discs and shrinks even further the differences between various grain sizes. The iceline in this case typically moves inside 1 AU even for smaller grain sizes. Two different full grain size distributions were modelled. In the first, the number density follows a power-law inspired by a coagulation/fragmentation equilibrium (Dohnanyi, 1969; Tanaka et al, 1996). The second begins with the same mass distribution, but takes into consideration the relative velocities for particles of different sizes and divides particles into regimes with different slopes for the mass distribution depending on their sizes and therefore aerodynamical properties (Birnstiel et al, 2011). Both models converge near the outer boundaries of the discs simulated here and they show a strong influence from the particles of around 0.1 mm in size. The inner parts of the disc simulated here, show a difference because of the upper boundary of each size distribution. In the more complex model the upper boundary is determined by the fragmentation barrier which leaves only smaller particles in the inner disc. On the contrary, the simple model following the Dohnanyi (1969) distribution has an arbitrarily fixed upper boundary which means that larger particles are present in this case. These have lower opacities and therefore enhance the cooling rate and decrease the disc’s temperature and aspect ratio. The iceline in both of the grain size distribution models is around 3 AU. The grain sizes distribution simulations show that in discs with significant viscous heating, often-used opacity models based on μm-sized dust grains only are not a good approximation in order to create more realistic theoretical models.Intresset för att studera protoplanetära diskar har ökat inom forskarvärlden i och med de observationer av dem som har gjorts de senaste åren. Observatoriet Atacama Large Millimeter Array (ALMA) i Chile har observerat flertalet protoplanetära diskar i enastående detalj. Dessa observationer har motiverat forskare till ytterligare teoretiska studier av protoplanetära diskar och deras dynamiska utveckling. Syftet bakom detta projekt är att skapa en mer realistisk modell för strukturen av protoplanetär diskar. Typiskt för äldre teoretiska modeller är att enbart en stoftstorlek och dess motsvarande opacitet inkluderas, men, observationer indikerar att stoftkornen växer och driver i den protoplanetära disken. Med det i åtanke, har en mer realistisk modell för distributionen av stoftstorlekar använts i detta projekt. I detta projektet utför jag hydrodynamiska simuleringar som initialt använder enbart en stoftstorlek och motsvarande medelvärden för opaciteten, som i sin tur är beroende på temperaturen. Detta tillåter oss att jämföra de strukturer som bildas, som en funktion av stoftstorleken. Dessa resultat ger då en första indikation påhur de olika stoftstorlekarna påverkar disk strukturen. Nästa steg är att skapa en distribution av stoftstorlekar och undersöka dess effekter påvärmestrukturen i disken. Vi finner att denna diskstruktur påverkas av att stoftet växer, och modeller som enbart inkluderar en stoftstorlek inte kan beskriva strukturerna väl. Dessa fynd är viktiga för att många av processerna bakom planetbildning (småstenar som driver i disken, planetesimalbildning, anhopningen av småstenar, samt planet migration) påverkas direkt av diskens temperatur

The growth of super-Earths

The conditions in the protoplanetary disk are determinant for the various planet formation mechanisms. We present a framework that combines self-consistent disk structures with the calculations of the growth rates of planetary embryos via pebble accretion, in order to study the formation of super-Earths. We first perform 2D hydrodynamical simulations of the inner disks, considering a grain size distribution with multiple chemical species and their corresponding size and composition dependent opacities. The resulting aspect ratios are almost constant with orbital distance, resulting in radially constant pebble isolation masses, the mass where pebble accretion stops. This supports the “peas-in-a-pod” constraint from the Kepler observations. The derived pebble sizes are used to calculate the growth rates of planetary embryos via pebble accretion. Disks with low levels of turbulence (expressed through the α-viscosity) and/or high dust fragmentation velocities allow larger particles, hence lead to lower pebble isolation masses, and the contrary. At the same time, small pebble sizes lead to low accretion rates. We find that there is a trade-off between the pebble isolation mass and the growth timescale; the best set of parameters is an α-viscosity of 10−3 and a dust fragmentation velocity of 10 m s−1, mainly for an initial gas surface density (at 1 AU) greater than 1000 g cm−2. A self-consistent treatment between the disk structures and the pebble sizes is thus of crucial importance for planet formation simulations

Influence of grain size and composition on the contraction rates of planetary envelopes and on planetary migration

A crucial phase during planetary growth is the migration, when the planetary core has been assembled but has not yet opened a deep gap. During this phase, the planet is subject to fast type-I migration, which is mostly directed inwards, and the planet can lose a significant fraction of its semi-major axis. The duration of this phase is set by the time required for the planetary envelope to contract before it reaches a mass similar to that of the planetary core, which is when runaway gas accretion can set in and the planet can open a deeper gap in the disc, transitioning into the slower type-II migration. This envelope contraction phase depends crucially on the planetary mass and on the opacity inside the planetary envelope. Here we study how different opacity prescriptions influence the envelope contraction time and how this in turn influences how far the planet migrates through the disc. We find within our simulations that the size distribution of the grains as well as the chemical composition of the grains crucially influences how far the planet migrates before reaches the runaway gas accretion phase. Grain size distributions with larger grain sizes result in less inward migration of the growing planet because of faster gas accretion enabled by more efficient cooling. In addition, we find that planets forming in water-poor environments can contract their envelope faster and therefore migrate less, implying that gas giants forming in water-poor environments might be located further away from their central star compared to gas giants forming in water-rich environments. Future studies of planet formation that aim to investigate the chemical composition of formed gas giants need to take these effects into account self-consistently

The water-ice line as a birthplace of planets: implications of a species-dependent dust fragmentation threshold

The thermodynamic structure of protoplanetary discs is determined by dust opacities, which depend on the size of the dust grains and their chemical composition. In the inner regions, the grain sizes are regulated by the level of turbulence (e.g. α viscosity) and by the dust fragmentation velocity that represents the maximal velocity that grains can have at a collision before they fragment. Here, we perform self-consistently calculated 2D hydrodynamical simulations that consider a full grain size distribution of dust grains with a transition in the dust fragmentation velocity at the water-ice line. This approach accounts for the results of previous particle collision laboratory experiments, in which silicate particles typically have a lower dust fragmentation velocity than water-ice particles. Furthermore, we probe the effects of variations in the water abundance, the dust-to-gas ratio, and the turbulence parameter on the disc structure. For the discs with a transition in the dust fragmentation velocity at the water-ice line, we find a narrow but striking zone of planetary outward migration, including for low viscosities. In addition, we find a bump in the radial pressure gradient profile that tends to be located slightly inside the ice line. Both of these features are present for all tested disc parameters. Thus, we conclude that the ice line can function both as a migration trap, which can extend the growth times of planets before they migrate to the inner edge of the protoplanetary disc, and as a pressure trap, where planetesimal formation can be initiated or enhanced