ALBERT

Description: By means of radiative transfer simulation, we study the evolution of the far-infrared colours of protoplanetary discs undergoing inside-out dispersal, often referred to as transition discs. We show that a brightening of the mid- and far-infrared emission from these objects is a natural consequence of the removal of the inner disc. Our results can fully explain recent observations of transition discs in the Chamaleon and Lupus star-forming regions from the Herschel Gould Belt Survey, which shows a higher median for the 70 μm ( Herschel PACS 1) band of known transition objects compared with primordial discs. Our theoretical results hence support the suggestion that the 70 μm band may be a powerful diagnostic for the identification of transition discs from photometry data, provided that the inner hole is larger than tens of au, depending on spectral type. Furthermore, we show that a comparison of photometry in the K , 12 μm and 70 μm bands to model tracks can provide a rough, but quick estimate of the inner hole size of these objects, provided their inclination is below ~85° and the inner hole size is again larger than tens of au.

Description: Chondrule formation remains one of the most elusive early Solar system events. Here, we take the novel approach of employing numerical simulations to investigate chondrule origin beyond purely cosmochemical methods. We model the transport of generically produced chondrules and dust in a 1D viscous protoplanetary disc model in order to constrain the chondrule formation events. For a single formation event we are able to match analytical predictions of the memory they retain of each other (complementarity), finding that a large mass accretion rate (10 –7 M  yr –1 ) allows for delays on the order of the disc's viscous time-scale between chondrule formation and chondrite accretion. Further, we find older discs to be severely diminished of chondrules, with accretion rates 10 –9 M  yr –1 for nominal parameters. We then characterize the distribution of chondrule origins in both space and time, as functions of disc parameters and chondrule formation rates, in runs with continuous chondrule formation and both static and evolving discs. Our data suggest that these can account for the observed diversity between distinct chondrite classes, if some diversity in accretion time is allowed for.

Description: Photoevaporation of protoplanetary discs by high-energy radiation from the central young stellar object is currently the favourite model to explain the sudden dispersal of discs from the inside out. While several theoretical works have provided a detailed pictured of this process, the direct observational validation is still lacking. Emission lines produced in these slow-moving protoplanetary disc winds may bear the imprint of the wind structure and thus provide a potential diagnostic of the underlying dispersal process. In this paper, we primarily focus on the collisionally excited neutral oxygen line at 6300 Å. We compare our models predictions to observational data and demonstrate a thermal origin for the observed blueshifted low-velocity component of this line from protoplanetary discs. Furthermore, our models show that while this line is a clear tell-tale sign of a warm, quasi-neutral disc wind, typical of X-ray photoevaporation, its strong temperature dependence makes it unsuitable to measure detailed wind quantities like mass-loss rate.

Description: The ubiquity of M dwarfs, combined with the relative ease of detecting terrestrial-mass planets around them, has made them prime targets for finding and characterizing planets in the ‘habitable zone’ (HZ). However, Kepler finds that terrestrial-mass exoplanets are often born with voluminous H/He envelopes, comprising mass-fractions ( M env / M core ) 1 per cent. If these planets retain such envelopes over Gyr time-scales, they will not be ‘habitable’ even within the HZ. Given the strong X-ray/UV fluxes of M dwarfs, we study whether sufficient envelope mass can be photoevaporated away for these planets to become habitable. We improve upon previous work by using hydrodynamic models that account for radiative cooling as well as the transition from hydrodynamic to ballistic escape. Adopting a template active M dwarf XUV spectrum, including stellar evolution, and considering both evaporation and thermal evolution, we show that: (1) the mass-loss is (considerably) lower than previous estimates that use an ‘energy-limited’ formalism and ignore the transition to Jeans escape; (2) at the inner edge of the HZ, planets with core mass 0.9 M  can lose enough H/He to become habitable if their initial envelope mass-fraction is ~1 per cent; (3) at the outer edge of the HZ, evaporation cannot remove a ~1 per cent H/He envelope even from cores down to 0.8 M . Thus, if planets form with bulky H/He envelopes, only those with low-mass cores may eventually be habitable. Cores 1 M , with 1 per cent natal H/He envelopes, will not be habitable in the HZ of M dwarfs.

Description: Late-type main-sequence stars exhibit an X-ray to bolometric flux ratio that depends on ${\tilde{R}o}$ , the ratio of rotation period to convective turnover time, as ${\tilde{R}o}^{-\zeta }$ with 2 ≤ ≤ 3 for ${\tilde{R}o} \gt 0.13$ , but saturates with || 〈 0.2 for ${\tilde{R}o} \lt 0.13$ . Saturated stars are younger than unsaturated stars and show a broader spread of rotation rates and X-ray activity. The unsaturated stars have magnetic fields and rotation speeds that scale roughly with the square root of their age, though possibly flattening for stars older than the Sun. The connection between faster rotators, stronger fields, and higher activity has been established observationally, but a theory for the unified time-evolution of X-ray luminosity, rotation, magnetic field and mass loss that captures the above trends has been lacking. Here we derive a minimalist holistic framework for the time evolution of these quantities built from combining a Parker wind with new ingredients: (1) explicit sourcing of both the thermal energy launching the wind and the X-ray luminosity via dynamo produced magnetic fields; (2) explicit coupling of X-ray activity and mass-loss saturation to dynamo saturation (via magnetic helicity build-up and convection eddy shredding); (3) use of coronal equilibrium to determine how magnetic energy is divided into wind and X-ray contributions. For solar-type stars younger than the Sun, we infer conduction to be a subdominant power loss compared to X-rays and wind. For older stars, conduction is more important, possibly quenching the wind and reducing angular momentum loss. We focus on the time evolution for stars younger than the Sun, highlighting what is possible for further generalizations. Overall, the approach shows promise towards a unified explanation of all of the aforementioned observational trends.

Description: Winds from short-period Earth and Neptune mass exoplanets, driven by high-energy radiation from a young star, may evaporate a significant fraction of a planet's mass. If the momentum flux from the evaporative wind is not aligned with the planet/star axis, then it can exert a torque on the planet's orbit. Using steady-state one-dimensional evaporative wind models, we estimate this torque using a lag angle that depends on the product of the speed of the planet's upper atmosphere and a flow time-scale for the wind to reach its sonic radius. We estimate the regime of planet radius, mass and stellar radiation flux in which a wind is capable of exerting a significant torque on the planet's orbit, and we find that it could be important for some of the observed planets. We also estimate the momentum flux from time-dependent one-dimensional hydrodynamical simulations. Similar to the Yarkovsky effect, the wind causes the planet to drift outwards if atmospheric circulation is prograde (super-rotating) and in the opposite direction if the circulation is retrograde. A close-in super-Earth mass planet that loses a large fraction of its mass in a wind could drift a few per cent of its semimajor axis. While this change is small, it places constraints on the evolution of resonant pairs such as Kepler 36b and c.

Description: Photoevaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2D simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work, we explored how the formation of a giant planet triggers the dispersal of the inner disc by photoevaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photoevaporation alone. In this work, we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc, the disc is a non-accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to understand if it is long lived enough to be found observationally. Depending on the parameters, especially on the X-ray luminosity of the star, we find that the fraction of time spent as a non-accretor greatly varies. We build a population synthesis model to compare with observations and find that in general thermal sweeping is not effective enough to destroy the outer disc, leaving many transition discs in a relatively long lived phase with a gas-free hole, at odds with observations. We discuss the implications for transition disc evolution. In particular, we highlight the current lack of explanation for the missing non-accreting transition discs with large holes, which is a serious issue in the planet hypothesis.

Description: Cosmological models predict the oldest stars in the Galaxy should be found closest to the centre of the potential well, in the bulge. The Extremely Metal-poor BuLge stars with AAOmega survey (EMBLA) successfully searched for these old, metal-poor stars by making use of the distinctive SkyMapper photometric filters to discover candidate metal-poor stars in the bulge. Their metal-poor nature was then confirmed using the AAOmega spectrograph on the Anglo-Australian Telescope. Here we present an abundance analysis of 10 bulge stars with –2.8 〈 [Fe/H] 〈 –1.7 from MIKE/Magellan observations, in total determining the abundances of 22 elements. Combining these results with our previous high-resolution data taken as part of the Gaia -ESO Survey, we have started to put together a picture of the chemical and kinematic nature of the most metal-poor stars in the bulge. The currently available kinematic data are consistent with the stars belonging to the bulge, although more accurate measurements are needed to constrain the stars’ orbits. The chemistry of these bulge stars deviates from that found in halo stars of the same metallicity. Two notable differences are the absence of carbon-enhanced metal-poor bulge stars, and the α element abundances exhibit a large intrinsic scatter and include stars which are underabundant in these typically enhanced elements.

Description: We show that if young low-mass stars are subject to vigorous X-ray driven disc winds, then such winds may be rendered detectable in cluster environments through their interaction with ionizing radiation from massive stars. In particular we argue that in the Orion nebula cluster (ONC) one expects to see of order tens of ‘X-ray proplyds’ (i.e. objects with offset ionization fronts detectable through optical imaging) in the range 0.3–0.6 pc from 1 C Ori (the dominant O star in the ONC). Objects at this distance lie outside the central ‘FUV zone’ in the ONC where proplyd structures are instead well explained by neutral winds driven by external far-ultraviolet (FUV) emission from 1 C Ori. We show that the predicted numbers and sizes of X-ray proplyds in this region are compatible with the numbers of proplyds observed and that this may also provide an explanation for at least some of the far flung proplyds observed in the Carina nebula. We compare the sizes of observed proplyds outside the FUV region of the ONC with model predictions based on the current observed X-ray luminosities of these sources (bearing in mind that the current size is actually set by the X-ray luminosity a few hundred years previously, corresponding to the flow time to the ionization front). We discuss whether variability on this time-scale can plausibly explain the proplyd size data on a case-by-case basis. We also calculate the predicted radio free–free emission signature of X-ray proplyds and show that this is readily detectable. Monitoring is, however, required in order to distinguish such emission from non-thermal radio emission from active coronae. We also predict that it is only at distances more than a parsec from 1 C Ori that the free–free emission signature of such offset ionized structures would be clearly distinguishable from an externally driven ionized disc wind. We argue that the fortuitous proximity of massive stars in the ONC can be used as a beacon to light up internally driven X-ray winds and that this represents a promising avenue for observational tests of the X-ray photoevaporation scenario.

Description: The lack of observed transition discs with inner gas holes of radii greater than ~50 au implies that protoplanetary discs dispersed from the inside out must remove gas from the outer regions rapidly. We investigate the role of photoevaporation in the final clearing of gas from low mass discs with inner holes. In particular, we study the so-called ‘thermal sweeping’ mechanism which results in rapid clearing of the disc. Thermal sweeping was originally thought to arise when the radial and vertical pressure scalelengths at the X-ray heated inner edge of the disc match. We demonstrate that this criterion is not fundamental. Rather, thermal sweeping occurs when the pressure maximum at the inner edge of the dust heated disc falls below the maximum possible pressure of X-ray heated gas (which depends on the local X-ray flux). We derive new critical peak volume and surface density estimates for rapid radiative clearing which, in general, result in rapid dispersal happening less readily than in previous estimates. This less efficient clearing of discs by X-ray driven thermal sweeping leaves open the issue of what mechanism (e.g. far-ultraviolet heating) can clear gas from the outer disc sufficiently quickly to explain the non-detection of cold gas around weak line T Tauri stars.