Context. The total gas mass is one of the most fundamental properties of disks around young stars, because it controls their evolution and their potential to form planets. To measure disk gas masses, ...CO has long been thought to be the best tracer as it is readily detected at (sub)mm wavelengths in many disks. However, inferred gas masses from CO in recent ALMA observations of large samples of disks in the 1–5 Myr age range seem inconsistent with their inferred dust masses. The derived gas-to-dust mass ratios from CO are between one and two orders of magnitude lower than the ISM value of ~100 even if photodissociation and freeze-out are included. In contrast, Herschel measurements of hydrogen deuteride line emission of a few disks imply gas masses in line with gas-to-dust mass ratios of 100. This suggests that at least one additional mechanism is removing CO from the gas phase. Aims. Here we test the suggestion that the bulk of the CO is chemically processed and that the carbon is sequestered into less volatile species such as CO2, CH3OH, and CH4 in the dense, shielded midplane regions of the disk. This study therefore also addresses the carbon reservoir of the material which ultimately becomes incorporated into planetesimals. Methods. Using our gas-grain chemical code, we performed a parameter exploration and follow the CO abundance evolution over a range of conditions representative of shielded disk midplanes. Results. Consistent with previous studies, we find that no chemical processing of CO takes place on 1–3 Myr timescales for low cosmic-ray ionisation rates, <5 × 10−18 s−1. Assuming an ionisation rate of 10−17 s−1, more than 90% of the CO is converted into other species, but only in the cold parts of the disk below 30 K. This order of magnitude destruction of CO is robust against the choice of grain-surface reaction rate parameters, such as the tunnelling efficiency and diffusion barrier height, for temperatures between 20 and 30 K. Below 20 K there is a strong dependence on the assumed efficiency of H tunnelling. Conclusions. The low temperatures needed for CO chemical processing indicate that the exact disk temperature structure is important, with warm disks around luminous Herbig stars expected to have little to no CO conversion. In contrast, for cold disks around sun-like T Tauri stars, a large fraction of the emitting CO layer is affected unless the disks are young (<1 Myr). This can lead to inferred gas masses that are up to two orders of magnitude lower. Moreover, unless CO is locked up early in large grains, the volatile carbon composition of the icy pebbles and planetesimals forming in the midplane and drifting to the inner disk will be dominated by CH3OH, CO2 and/or hydrocarbons.
Context. Exoplanet atmospheres are thought be built up from accretion of gas as well as pebbles and planetesimals in the midplanes of planet-forming disks. The chemical composition of this material ...is usually assumed to be unchanged during the disk lifetime. However, chemistry can alter the relative abundances of molecules in this planet-building material. Aims. We aim to assess the impact of disk chemistry during the era of planet formation. This is done by investigating the chemical changes to volatile gases and ices in a protoplanetary disk midplane out to 30 AU for up to 7 Myr, considering a variety of different conditions, including a physical midplane structure that is evolving in time, and also considering two disks with different masses. Methods. An extensive kinetic chemistry gas-grain reaction network was utilised to evolve the abundances of chemical species over time. Two disk midplane ionisation levels (low and high) were explored, as well as two different makeups of the initial abundances (“inheritance” or “reset”). Results. Given a high level of ionisation, chemical evolution in protoplanetary disk midplanes becomes significant after a few times 105 yr, and is still ongoing by 7 Myr between the H2O and the O2 icelines. Inside the H2O iceline, and in the outer, colder regions of the disk midplane outside the O2 iceline, the relative abundances of the species reach (close to) steady state by 7 Myr. Importantly, the changes in the abundances of the major elemental carbon and oxygen-bearing molecules imply that the traditional “stepfunction” for the C/O ratios in gas and ice in the disk midplane (as defined by sharp changes at icelines of H2O, CO2 and CO) evolves over time, and cannot be assumed fixed, with the C/O ratio in the gas even becoming smaller than the C/O ratio in the ice. In addition, at lower temperatures (<29 K), gaseous CO colliding with the grains gets converted into CO2 and other more complex ices, lowering the CO gas abundance between the O2 and CO thermal icelines. This effect can mimic a CO iceline at a higher temperature than suggested by its binding energy. Conclusions. Chemistry in the disk midplane is ionisation-driven, and evolves over time. This affects which molecules go into forming planets and their atmospheres. In order to reliably predict the atmospheric compositions of forming planets, as well as to relate observed atmospheric C/O ratios of exoplanets to where and how the atmospheres have formed in a disk midplane, chemical evolution needs to be considered and implemented into planet formation models.
While semantic and episodic memory have been shown to influence each other, uncertainty remains as to how this interplay occurs. We introduce a behavioral representational similarity analysis ...approach to assess whether semantic space can be subtly re-sculpted by episodic learning. Eighty participants learned word pairs that varied in semantic relatedness, and learning was bolstered via either testing or restudying. Next-day recall is superior for semantically related pairs, but there is a larger benefit of testing for unrelated pairs. Analyses of representational change reveal that successful recall is accompanied by a pulling together of paired associates, with cue words in semantically related (but not unrelated) pairs changing more across learning than target words. Our findings show that episodic learning is associated with systematic and asymmetrical distortions of semantic space which improve later recall by making cues more predictive of targets, reducing interference from potential lures, and establishing novel connections within pairs.
Context. Near- to mid-infrared observations of molecular emission from protoplanetary disks show that the inner regions are rich in small organic volatiles (e.g., C2H2 and HCN). Trends in the data ...suggest that disks around cooler stars (Teff ≈ 3000 K) are potentially (i) more carbon-rich; and (ii) more molecule-rich than their hotter counterparts (Teff ≳ 4000 K). Aims. We explore the chemical composition of the planet-forming region (<10 AU) of protoplanetary disks around stars over a range of spectral types (from M dwarf to Herbig Ae) and compare with the observed trends. Methods. Self-consistent models of the physical structure of a protoplanetary disk around stars of different spectral types are coupled with a comprehensive gas-grain chemical network to map the molecular abundances in the planet-forming zone. The effects of (i) N2 self shielding; (ii) X-ray-induced chemistry; and (iii) initial abundances, are investigated. The chemical composition in the “observable” atmosphere is compared with that in the disk midplane where the bulk of the planet-building reservoir resides. Results. M dwarf disk atmospheres are relatively more molecule rich than those for T Tauri or Herbig Ae disks. The weak far-UV flux helps retain this complexity which is enhanced by X-ray-induced ion-molecule chemistry. N2 self shielding has only a small effect in the disk molecular layer and does not explain the higher C2H2/HCN ratios observed towards cooler stars. The models underproduce the OH/H2O column density ratios constrained in Herbig Ae disks, despite reproducing (within an order of magnitude) the absolute value for OH: the inclusion of self shielding for H2O photodissociation only increases this discrepancy. One possible explanation is the adopted disk structure. Alternatively, the “hot” H2O (T ≳ 300 K) chemistry may be more complex than assumed. The results for the atmosphere are independent of the assumed initial abundances; however, the composition of the disk midplane is sensitive to the initial main elemental reservoirs. The models show that the gas in the inner disk is generally more carbon rich than the midplane ices. This effect is most significant for disks around cooler stars. Furthermore, the atmospheric C/O ratio appears larger than it actually is when calculated using observable tracers only. This is because gas-phase O2 is predicted to be a significant reservoir of atmospheric oxygen. Conclusions. The models suggest that the gas in the inner regions of disks around cooler stars is more carbon rich; however, calculations of the molecular emission are necessary to definitively confirm whether the chemical trends reproduce the observed trends.
Highlights • A meta-analysis of cytokine responses to acute psychological stress was conducted. • Results showed stress-related increases in circulating markers of inflammation. • Reliable increases ...in concentrations of IL6, IL1β, IL10 and TNFα were observed. • Stress effects on circulating cytokines peaked between 31 and 90 min post stress. • Stress also associated with increased stimulated production of IL1β, IL4 and IFNγ.
We present spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) images of 12CO J = 3−2 emission from the protoplanetary disk around the Herbig Ae star, HD 100546. We expand upon ...earlier analyses of this data and model the spatially-resolved kinematic structure of the CO emission. Assuming a velocity profile which prescribes a flat or flared emitting surface in Keplerian rotation, we uncover significant residuals with a peak of ≈7δv, where δv = 0.21 km s-1 is the width of a single spectral resolution element. The shape and extent of the residuals reveal the possible presence of a severely warped and twisted inner disk extending to at most 100 au. Adapting the model to include a misaligned inner gas disk with (i) an inclination almost edge-on to the line of sight, and (ii) a position angle almost orthogonal to that of the outer disk reduces the residuals to <3δv. However, these findings are contrasted by recent VLT/SPHERE, MagAO/GPI, and VLTI/PIONIER observations of HD 100546 that show no evidence of a severely misaligned inner dust disk down to spatial scales of ~ 1 au. An alternative explanation for the observed kinematics are fast radial flows mediated by (proto)planets. Inclusion of a radial velocity component at close to free-fall speeds and inwards of ≈50 au results in residuals of ≈4δv. Hence, the model including a radial velocity component only does not reproduce the data as well as that including a twisted and misaligned inner gas disk. Molecular emission data at a higher spatial resolution (of order 10 au) are required to further constrain the kinematics within ≲100 au. HD 100546 joins several other protoplanetary disks for which high spectral resolution molecular emission shows that the gas velocity structure cannot be described by a purely Keplerian velocity profile with a universal inclination and position angle. Regardless of the process, the most likely cause is the presence of an unseen planetary companion.
This paper goes beyond -- transcends -- "pedagogy as justice," recognizing that justice, particularly in these present times, may not be enough. Its wager is with "pedagogies of and for life"; ...pedagogies that plant and cultivate, that push and enable other modes of living, despite the capitalist-modern-colonial-racist system, beyond the system, and in the system's margins, borders, fissures, and cracks. These pedagogies, as Catherine Walsh argues here, are necessarily tied to and constitutive of decolonial(izing) praxis, a praxis that, while not negating justice, takes us beyond it. The argument here is for the pedagogical-praxistical work being done now, and yet to be done; the actional thought, thoughtful action, and ongoing creation and construction of an otherwise "for" existence, "for" dignity, and "for" life in these times of multiple entwined pandemic-viruses, including capitalist greed, systemic racism, heteropatriarchy, and existential-territorial dispossession in which COVID is/was one more thread of the entwine.
Context. Detection of abundant O2 at 1–10% relative to H2O ice in the comae of comets 1P/Halley and 67P/Churyumov-Gerasimenko has motivated attempts to explain the origin of the high O2 ice ...abundance. Recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of O2 ice at the same abundance as that measured in the comet. Aims. We aim to carry out a thorough investigation to constrain the conditions under which O2 ice could have been produced through kinetic chemistry in the pre-solar nebula midplane. Methods. We have utilised an updated chemical kinetics code to evolve chemistry under pre-solar nebula midplane conditions. Four different chemical starting conditions and the effects of various chemical parameters have been tested. Results. Using the fiducial network, and for either reset conditions (atomic initial abundances) or atomic oxygen only conditions, the abundance level of O2 ice measured in the comets can be reproduced at an intermediate time, after 0.1–2 Myr of evolution, depending on ionisation level. When including O3 chemistry, the abundance of O2 ice is much lower than the cometary abundance (by several orders of magnitude). We find that H2O2 and O3 ices are abundantly produced (at around the level of O2 ice) in disagreement with their respective abundances or upper limits from observations of comet 67P. Upon closer investigation of the parameter space, and varying parameters for grain–surface chemistry, it is found that for temperatures 15–25 K, densities of 109−1010 cm−3, and a barrier for quantum tunnelling set to 2 Å, the measured level of O2 ice can be reproduced with the new chemical network, including an updated binding energy for atomic oxygen (1660 K). However, the abundances of H2O2 and O3 ices still disagree with the observations. A larger activation energy for the O + O2 → O3 reaction (Eact > 1000 K) helps to reproduce the non-detection of O3 ice in the comet, as well as reproducing the observed abundances of H2O2 and O2 ices. The only other case in which the O2 ice matches the observed abundance, and O3 and H2O2 ice are lower, is the case when starting with an appreciable amount of oxygen locked in O2. Conclusions. The parameter space investigation revealed a sweet spot for production of O2 ice at an abundance matching those in 67P and 1P, and O3 and H2O2 ice abundances matching those in 67P. This means that there is a radial region in the pre-solar nebula from 120–150 AU, within which O2 could have been produced in situ via ice chemistry on grain surfaces. However, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). Hence, because the more likely scenario starting with a percentage of elemental oxygen locked in O2 also reproduces the O2 ice abundance in 67P at early stages, this supports previous suggestions that the cometary O2 ice could have a primordial origin.
How are we to think about pedagogy and education in these present times when existence itself is in tension and question? As the pandemics of COVID-19, systemic racism, capitalist greed, and ...land-based plundering, displacement, and dispossession work together to reconfigure power and, relatedly, formal education, most especially in the Global South, what might it mean to think from and shift our gaze toward the decolonial 'cracks'? How do these 'cracks' - understood as the extant and nascent fissures in the dominant order - take form? Who are the 'crack-makers' and in what ways, through their ground up theorizing, practice, and praxis, are they giving substance and form to the pedagogical imperatives of resistance, re-existence, hope, and life, imperatives conspicuously absent in the conceptualization and rhetoric of 'quality education'? And finally, what might a reading and rereading of Paulo Freire offer in this regard?
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