We examine the spatial distribution and mass segregation of dense molecular cloud cores in a number of nearby star forming regions (the region L1495 in Taurus, Aquila, Corona Australis, and W43) that ...span about four orders of magnitude in star formation activity. We used an approach based on the calculation of the minimum spanning tree, and for each region, we calculated the structure parameter 𝒬 and the mass segregation ratio ΛMSR measured for various numbers of the most massive cores. Our results indicate that the distribution of dense cores in young star forming regions is very substructured and that it is very likely that this substructure will be imprinted onto the nascent clusters that will emerge out of these clouds. With the exception of Taurus in which there is nearly no mass segregation, we observe mild-to-significant levels of mass segregation for the ensemble of the 6, 10, and 14 most massive cores in Aquila, Corona Australis, and W43, respectively. Our results suggest that the clouds’ star formation activity are linked to their structure, as traced by their population of dense cores. We also find that the fraction of massive cores that are the most mass segregated in each region correlates with the surface density of star formation in the clouds. The Taurus region with low star forming activity is associated with a highly hierarchical spatial distribution of the cores (low 𝒬 value) and the cores show no sign of being mass segregated. On the other extreme, the mini-starburst region W43-MM1 has a higher 𝒬 that is suggestive of a more centrally condensed structure. Additionally, it possesses a higher fraction of massive cores that are segregated by mass. While some limited evolutionary effects might be present, we largely attribute the correlation between the star formation activity of the clouds and their structure to a dependence on the physical conditions that have been imprinted on them by the large scale environment at the time they started to assemble.
Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper, we introduce our recently developed chemical kinetic model (ChemKM). We show that the results of our ...HD 189733b model are in good agreement with previously published results, except at the bar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as Fe/H, log(g), and C/O. In addition, we find that the "methane valley," a region between 800 and 1500 K where above a certain C/O threshold value a greater chance of CH4 detection is expected, still exists after including the vertical mixing. The first robust CH4 detection on an irradiated planet (HD 102195b) places this object within this region, supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by the James Webb Space Telescope and suggest focusing on the targets with Teff between 1000 and 1800 K, orbiting around M dwarfs, and having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color maps suggests that the main two color populations are largely insensitive to the vertical mixing. Therefore, any deviation of observational points from these populations is likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (Teff < 900 K) with very low C/O ratios (<0.25) show significant deviations, making these planets interesting cases for further investigation.
We conduct a pebble-driven planet population synthesis study to investigate the formation of planets around very low-mass stars and brown dwarfs in the (sub)stellar mass range between 0.01
M
⊙
and ...0.1
M
⊙
. Based on the extrapolation of numerical simulations of planetesimal formation by the streaming instability, we obtain the characteristic mass of the planetesimals and the initial mass of the protoplanet (largest body from the planetesimal populations), in either the early self-gravitating phase or the later non-self-gravitating phase of the protoplanetary disk evolution. We find that the initial protoplanets form with masses that increase with host mass and orbital distance, and decrease with age. Around late M-dwarfs of 0.1
M
⊙
, these protoplanets can grow up to Earth-mass planets by pebble accretion. However, around brown dwarfs of 0.01
M
⊙
, planets do not grow to the masses that are greater than Mars when the initial protoplanets are born early in self-gravitating disks, and their growth stalls at around 0.01 Earth-mass when they are born late in non-self-gravitating disks. Around these low-mass stars and brown dwarfs we find no channel for gas giant planet formation because the solid cores remain too small. When the initial protoplanets form only at the water-ice line, the final planets typically have ≳15% water mass fraction. Alternatively, when the initial protoplanets form log-uniformly distributed over the entire protoplanetary disk, the final planets are either very water rich (water mass fraction ≳15%) or entirely rocky (water mass fraction ≲5%).
Observations suggest an abundance of water and a paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible ...causes. Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley. In this work, we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: (1) by heating up the photosphere of colder planets and (2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: (1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water-dominated spectra of colder carbon-poor exoplanets, and (2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes. We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color maps and find that clouds can explain some of the high-contrast observations by IRAC's channel 1 and 2.
A carbon-to-oxygen ratio (C/O) of around unity is believed to act as a natural separator of water- and methane-dominated spectra when characterizing exoplanet atmospheres. In this paper, we quantify ...the C/O ratios at which this separation occurs by calculating a large self-consistent grid of cloud-free atmospheric models in chemical equilibrium using the latest version of petitCODE. Our study covers a broad range of parameter space: 400 K < Teff < 2600 K, 2.0 < log(g) < 5.0, −1.0 < Fe/H < 2.0, 0.25 < C/O < 1.25, and stellar types from M to F. We make the synthetic transmission and emission spectra, as well as the temperature structures, publicly available. We find that the transition C/O ratio depends on many parameters, such as effective temperature, surface gravity, metallicity, and spectral type of the host star, and could have values less than, equal to, or higher than unity. By mapping all of the transition C/O ratios, we propose a "four-class" classification scheme for irradiated planets in this temperature range. We find a parameter space where methane always remains the cause of dominant spectral features. Detection of CH4 in this region, or the lack of it, provides a diagnostic tool to identify the prevalence of cloud formation and nonequilibrium chemistry. As another diagnostic tool, we construct synthetic Spitzer Infrared Array Camera color diagrams showing two distinguishable populations of planets. Since most of the exoplanet atmospheres appear cloudy when studied in transmission, we regard this study as a starting point of how such a C/O-sensitive observation-based classification scheme should be constructed. This preparatory work will have to be refined by future cloudy and nonequilibrium modeling to further investigate the existence and exact location of the classes, as well as the color-diagram analysis.
The formation of stars shapes the structure and evolution of entire galaxies. The rate and efficiency of this process are affected substantially by the density structure of the individual molecular ...clouds in which stars form. The most fundamental measure of this structure is the probability density function of volume densities (ρ-PDF), which determines the star formation rates predicted with analytical models. This function has remained unconstrained by observations. We have developed an approach to quantify ρ-PDFs and establish their relation to star formation. The ρ-PDFs instigate a density threshold of star formation and allow us to quantify the star formation efficiency above it. The ρ-PDFs provide new constraints for star formation theories and correctly predict several key properties of the star-forming interstellar medium.
We report the results of radiation-magnetohydrodynamics calculations in the context of high-mass star formation, using for the first time a self-consistent model for photon emission (i.e., via ...thermal emission and in radiative shocks) and with the high resolution necessary to properly resolve magnetic braking effects and radiative shocks on scales <100 AU. We investigate the combined effects of magnetic field, turbulence, and radiative transfer on the early phases of the collapse and the fragmentation of massive dense cores. We identify a new mechanism that inhibits initial fragmentation of massive dense cores where magnetic field and radiative transfer interplay. We show that this interplay becomes stronger as the magnetic field strength increases. Magnetic braking is transporting angular momentum outward and is lowering the rotational support and is thus increasing the infall velocity. This enhances the radiative feedback owing to the accretion shock on the first core. We speculate that highly magnetized massive dense cores are good candidates for isolated massive star formation while moderately magnetized massive dense cores are more appropriate forming OB associations or small star clusters.
Origin of the RNA world Pearce, Ben K. D.; Pudritz, Ralph E.; Semenov, Dmitry A. ...
Proceedings of the National Academy of Sciences - PNAS,
10/2017, Letnik:
114, Številka:
43
Journal Article
Recenzirano
Odprti dostop
Before the origin of simple cellular life, the building blocks of RNA (nucleotides) had to form and polymerize in favorable environments on early Earth. At this time, meteorites and interplanetary ...dust particles delivered organics such as nucleobases (the characteristic molecules of nucleotides) to warm little ponds whose wet–dry cycles promoted rapid polymerization. We build a comprehensive numerical model for the evolution of nucleobases in warm little ponds leading to the emergence of the first nucleotides and RNA. We couple Earth’s early evolution with complex prebiotic chemistry in these environments. We find that RNA polymers must have emerged very quickly after the deposition of meteorites (less than a few years). Their constituent nucleobases were primarily meteoritic in origin and not from interplanetary dust particles. Ponds appeared as continents rose out of the early global ocean, but this increasing availability of “targets” for meteorites was offset by declining meteorite bombardment rates. Moreover, the rapid losses of nucleobases to pond seepage during wet periods, and to UV photodissociation during dry periods, mean that the synthesis of nucleotides and their polymerization into RNA occurred in just one to a few wet–dry cycles. Under these conditions, RNA polymers likely appeared before 4.17 billion years ago.
Surface processes on cosmic solids in cold astrophysical environments lead to gas-phase depletion and molecular complexity. Most astrophysical models assume that the molecular ice forms a thick ...multilayer substrate, not interacting with the dust surface. In contrast, we present experimental results demonstrating the importance of the surface for porous grains. We show that cosmic dust grains may be covered by a few monolayers of ice only. This implies that the role of dust surface structure, composition, and reactivity in models describing surface processes in cold interstellar, protostellar, and protoplanetary environments has to be reevaluated.
The most massive stars can form via standard disk accretion--despite the radiation pressure generated--due to the fact that the massive accretion disk yields a strong anisotropy in the radiation ...field, releasing most of the radiation pressure perpendicular to the disk accretion flow. Here, we analyze the self-gravity of the forming circumstellar disk as the potential major driver of the angular momentum transport in the massive disks responsible for the high accretion rates needed for the formation of massive stars. For this purpose, we perform self-gravity radiation hydrodynamic simulations of the collapse of massive pre-stellar cores. The formation and evolution of the resulting circumstellar disk is investigated in (1) axially symmetric simulations using an Delta *a-shear-viscosity prescription and (2) a three-dimensional simulation in which the angular momentum transport is provided self-consistently by developing gravitational torques in the self-gravitating accretion disk. The simulation series of different strengths of the Delta *a viscosity shows that the accretion history of the forming star is mostly independent of the Delta *a-viscosity parameter. The accretion history of the three-dimensional run driven by self-gravity is more time dependent than the viscous disk evolution in axial symmetry. The mean accretion rate, i.e., the stellar mass growth rate, is nearly identical to the Delta *a-viscosity models. We conclude that the development of gravitational torques in self-gravitating disks around forming massive stars provides a self-consistent mechanism to efficiently transport angular momentum to outer disk radii. The formation of the most massive stars can therefore be understood in the standard accretion disk scenario.
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