We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted on to planets forming by core ...accretion from evolving protoplanetary discs. The astrochemistry of the host disc is computed using an ionization-driven, non-equilibrium chemistry network within viscously evolving disc models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted on to the protoplanet. We choose a fiducial disc model and evolve planets in three traps – water ice line, dead zone and heat transition. For a disc with a lifetime of 4.1 Myr, we produce two hot Jupiters (M = 1.43, 2.67 M
Jupiter, r = 0.15, 0.11 au) in the heat transition and ice line trap and one failed core (M = 0.003 M
Jupiter, r = 3.7 au) in the dead zone. These planets are found with mixing ratios for CO and H2O of 1.99 × 10−4 and 5.0 × 10−4, respectively, for both hot Jupiters. Additionally, for these planets we find CO2 and CH4, with mixing ratios of 1.8 × 10−6 → 9.8 × 10−10 and 1.1 × 10−8 → 2.3 × 10−10, respectively. These ranges correspond well with the mixing ratio ranges that have been inferred through the detection of emission spectra from hot Jupiters by multiple authors. We compute a carbon-to-oxygen ratio of 0.227 for the ice line planet and 0.279 for the heat transition planet. These planets accreted their gas inside the ice line, hence the sub-solar C/O.
Here a physical model for terminating giant planet formation is outlined and compared to other methods of late-stage giant planet formation. As has been pointed out before, gas accreting into a gap ...and onto the planet will encounter the planetary dynamo-generated magnetic field. The planetary magnetic field produces an effective cross section through which gas is accreted. Gas outside this cross section is recycled into the protoplanetary disk, hence only a fraction of mass that is accreted into the gap remains bound to the planet. This cross section inversely scales with the planetary mass, which naturally leads to stalled planetary growth late in the formation process. We show that this method naturally leads to Jupiter-mass planets and does not invoke any artificial truncation of gas accretion, as has been done in some previous population synthesis models. The mass accretion rate depends on the radius of the growing planet after the gap has opened, and we show that so-called hot-start planets tend to become more massive than cold-start planets. When this result is combined with population synthesis models, it might show observable signatures of cold-start versus hot-start planets in the exoplanet population.
Abstract
The detection of Earth-size exoplanets around low-mass stars –in stars such as Proxima Centauri and TRAPPIST-1– provide an exceptional chance to improve our understanding of the formation of ...planets around M stars and brown dwarfs. We explore the formation of such planets with a population synthesis code based on a planetesimal-driven model previously used to study the formation of the Jovian satellites. Because the discs have low mass and the stars are cool, the formation is an inefficient process that happens at short periods, generating compact planetary systems. Planets can be trapped in resonances and we follow the evolution of the planets after the gas has dissipated and they undergo orbit crossings and possible mergers. We find that formation of planets above Mars mass and in the planetesimal accretion scenario, is only possible around stars with masses M⋆ ≥ 0.07Msun and discs of Mdisc ≥ 10−2 Msun. We find that planets above Earth-mass form around stars with masses larger than 0.15 Msun, while planets larger than 5 M⊕ do not form in our model, even not under the most optimal conditions (massive disc), showing that planets such as GJ 3512b form with another, more efficient mechanism. Our results show that the majority of planets form with a significant water fraction; that most of our synthetic planetary systems have 1, 2 or 3 planets, but planets with 4,5,6 and 7 planets are also common, confirming that compact planetary systems with many planets should be a relatively common outcome of planet formation around small stars.
We have developed a new model for the astrochemical structure of a viscously evolving protoplanetary disc that couples an analytic description of the disc's temperature and density profile, chemical ...evolution and an evolving dust distribution. We compute evolving radial distributions for a range of dust grain sizes, which depend on coagulation, fragmentation and radial drift processes. In particular, we find that the water ice line plays an important role in shaping the radial distribution of the maximum grain size because ice-coated grains are significantly less susceptible to fragmentation than their dry counterparts. This in turn has important effects on disc ionization and therefore on the location of dead zones. In comparison to a simple constant gas-to-dust ratio model for the dust as an example, we find that the new model predicts an outer dead zone edge that moves in by a factor of about 3 at 1 Myr (to 5 au) and by a factor of about 14 by 3 Myr (to 0.5 au). We show that the changing position of the dead zone and heat transition traps have important implications for the formation and trapping of planets in protoplanetary discs. Finally, we consider our results in light of recent Atacama Large Millimeter Array observations of HL Tau and TW Hya.
Inflammasomes are protein complexes assembled upon recognition of infection or cell damage signals, and serve as platforms for clustering and activation of procaspase-1. Oligomerisation of initiating ...proteins such as AIM2 (absent in melanoma-2) and NLRP3 (NOD-like receptor family, pyrin domain-containing-3) recruits procaspase-1 via the inflammasome adapter molecule ASC (apoptosis-associated speck-like protein containing a CARD). Active caspase-1 is responsible for rapid lytic cell death termed pyroptosis. Here we show that AIM2 and NLRP3 inflammasomes activate caspase-8 and -1, leading to both apoptotic and pyroptotic cell death. The AIM2 inflammasome is activated by cytosolic DNA. The balance between pyroptosis and apoptosis depended upon the amount of DNA, with apoptosis seen at lower transfected DNA concentrations. Pyroptosis had a higher threshold for activation, and dominated at high DNA concentrations because it happens more rapidly. Gene knockdown showed caspase-8 to be the apical caspase in the AIM2- and NLRP3-dependent apoptotic pathways, with little or no requirement for caspase-9. Procaspase-8 localised to ASC inflammasome 'specks' in cells, and bound directly to the pyrin domain of ASC. Thus caspase-8 is an integral part of the inflammasome, and this extends the relevance of the inflammasome to cell types that do not express caspase-1.
The photon-the quantum excitation of the electromagnetic field-is massless but carries momentum. A photon can therefore exert a force on an object upon collision
. Slowing the translational motion of ...atoms and ions by application of such a force
, known as laser cooling, was first demonstrated 40 years ago
. It revolutionized atomic physics over the following decades
, and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen
, the antimatter atom consisting of an antiproton and a positron. By exciting the 1S-2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation
, we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude-with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S-2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic
and gravitational
studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules.
Abstract
We present the next step in a series of papers devoted to connecting the composition of the atmospheres of forming planets with the chemistry of their natal evolving protoplanetary discs. ...The model presented here computes the coupled chemical and dust evolution of the disc and the formation of three planets per disc model. Our three canonical planet traps produce a Jupiter near 1 AU, a Hot Jupiter and a Super-Earth. We study the dependence of the final orbital radius, mass, and atmospheric chemistry of planets forming in disc models with initial disc masses that vary by 0.02 M⊙ above and below our fiducial model (
$M_{\rm {\rm disc},0} = 0.1\,\, \mathrm{M}_{\odot }$
). We compute C/O and C/N for the atmospheres formed in our three models and find that C/Oplanet ∼ C/O
$_{\rm {\rm disc}}$
, which does not vary strongly between different planets formed in our model. The nitrogen content of atmospheres can vary in planets that grow in different disc models. These differences are related to the formation history of the planet, the time and location that the planet accretes its atmosphere, and are encoded in the bulk abundance of NH3. These results suggest that future observations of atmospheric NH3 and an estimation of the planetary C/O and C/N can inform the formation history of particular planetary systems.
ABSTRACT
The final architecture of planetary systems depends on the extraction of angular momentum and mass-loss processes of the discs in which they form. Theoretical studies proposed that ...magnetohydrodynamic winds launched from the discs (MHD disc winds) could govern accretion and disc dispersal. In this work, we revisit the observed disc demographics in the framework of MHD disc winds, combining analytical solutions of disc evolution and a disc population synthesis approach. We show that MHD disc winds alone can account for both disc dispersal and accretion properties. The decline of disc fraction over time is reproduced by assuming that the initial accretion time-scale (a generalization of the viscous time-scale) varies from disc to disc and that the decline of the magnetic field strength is slower than that of the gas. The correlation between accretion rate and disc mass, and the dispersion of the data around the mean trend as observed in Lupus, is then naturally reproduced. The model also accounts for the rapidity of the disc dispersal. This paves the way for planet formation models in the paradigm of wind-driven accretion.
Proteins of the mammalian PYHIN (IFI200/HIN-200) family are involved in defence against infection through recognition of foreign DNA. The family member absent in melanoma 2 (AIM2) binds cytosolic DNA ...via its HIN domain and initiates inflammasome formation via its pyrin domain. AIM2 lies within a cluster of related genes, many of which are uncharacterised in mouse. To better understand the evolution, orthology and function of these genes, we have documented the range of PYHIN genes present in representative mammalian species, and undertaken phylogenetic and expression analyses.
No PYHIN genes are evident in non-mammals or monotremes, with a single member found in each of three marsupial genomes. Placental mammals show variable family expansions, from one gene in cow to four in human and 14 in mouse. A single HIN domain appears to have evolved in the common ancestor of marsupials and placental mammals, and duplicated to give rise to three distinct forms (HIN-A, -B and -C) in the placental mammal ancestor. Phylogenetic analyses showed that AIM2 HIN-C and pyrin domains clearly diverge from the rest of the family, and it is the only PYHIN protein with orthology across many species. Interestingly, although AIM2 is important in defence against some bacteria and viruses in mice, AIM2 is a pseudogene in cow, sheep, llama, dolphin, dog and elephant. The other 13 mouse genes have arisen by duplication and rearrangement within the lineage, which has allowed some diversification in expression patterns.
The role of AIM2 in forming the inflammasome is relatively well understood, but molecular interactions of other PYHIN proteins involved in defence against foreign DNA remain to be defined. The non-AIM2 PYHIN protein sequences are very distinct from AIM2, suggesting they vary in effector mechanism in response to foreign DNA, and may bind different DNA structures. The PYHIN family has highly varied gene composition between mammalian species due to lineage-specific duplication and loss, which probably indicates different adaptations for fighting infectious disease. Non-genomic DNA can indicate infection, or a mutagenic threat. We hypothesise that defence of the genome against endogenous retroelements has been an additional evolutionary driver for PYHIN proteins.
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