Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with ...respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reaction network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on previous literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be depleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chemical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are accurately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains.
The paper focuses on the issues of merit, explored in its complex polysemy, meritocracy, and skills, placing them in relation to those of--educational, social, economic, cultural--poverty and social ...disadvantage, which afflict, with considerable aggravation especially in the era of Sars-Cov-2 pandemic, even Italy. The essay consists of three paragraphs. In the first two, a reference framework is outlined, on the one hand relating to the polysemic concepts of merit, and meritocracy; on the other, to the situation of poverty and social inequalities in Italy. The third paragraph, which represents the heart of the paper, is linked to the abovementioned frameworks: it highlights the key role that a widespread, accessible, and high-quality Early Childhood Education and Care (ECEC) play, both in supporting children's development, both promoting a democratic and inclusive society; a society characterized by effective "merit dynamics", open to all, so as to promote skills development and, above all, social ascent and redemption. Keywords. Merit - Meritocracy - Skills - Social disadvantage - Social redemption - Early Childhood Education and Care (ECEC)
In previous studies, we identified two classes of starless cores, thermally subcritical and supercritical, distinguished by different dynamical behaviour and internal structure. Here, we study the ...evolution of the dynamically unstable, thermally supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry. From an initial state as an unstable Bonnor–Ebert (BE) sphere, a contracting core evolves towards the configuration of a singular isothermal sphere by inside–out collapse. We follow the gas temperature and abundance of CO during the contraction. The temperature is predominantly determined by radiative equilibrium, but in the rapidly contracting centre of the core compressive heating raises the gas temperature by a few degrees over its value in static equilibrium. The time-scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical time-scale. The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile, and CO cannot be used as a chemical clock in starless cores. We use our non-local thermodynamic equilibrium (non-LTE) radiative transfer code mollie to predict observable CO and N2H+ line spectra, including the non-LTE hyperfine ratios of N2H+, during the contraction. These are compared against observations of the starless core L1544. The comparison indicates that the dust in L1544 has an opacity consistent with ice-covered rather than bare grains, the cosmic ray ionization rate is about 1 × 10−17 s−1 and the density structure of L1544 is approximately that of a BE sphere with a maximum central density of 2 × 107 cm−3, equivalent to an average density of 3 × 106 cm−3 within a radius of 500 au. The observed CO linewidths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic ray strikes alone, indicating that other desorption processes are also active.
Low-mass stars are understood to form by the gravitational collapse of the dense molecular clouds known as starless cores. However, it has proven impossible to use continuum observations to ...distinguish among the different hypotheses describing the collapse because the predicted density distributions for all spherical self-gravitating clouds are quite similar. However, the predicted velocities are quite different. We use two different molecular line transitions, H2O (1...-1...) and C...O (1-0), that are excited at different densities, 10... and 10... cm..., to measure the velocities at large and small radii in the contracting core L1544. We compare the observed spectra against those predicted for several different models of gravitational collapse including the Larson-Penston flow, the inside-out collapse of the singular isothermal sphere, the quasi-equilibrium contraction of an unstable Bonnor- Ebert sphere, and the non-equilibrium collapse of an overdense Bonnor-Ebert sphere. Only the model of the unstable quasi-equilibrium Bonnor-Ebert sphere is able to produce the observed shapes of both spectral lines. With this model, we interpret other molecular line observations of L1544 in the literature to find that the extended inward velocities seen in lines of CS(2-1) and N2H+ are located within the starless core itself, in particular in the region where the density profile follows an inverse square law. If these conclusions were to hold in the analysis of other starless cores, this would imply that the formation of hydrostatic clouds within the turbulent interstellar medium is not only possible but also not exceptional and may be an evolutionary phase in low-mass star formation. (ProQuest: ... denotes formulae/symbols omitted.)
The Herschel Space Observatory's recent detections of water vapour in the cold, dense cloud L1544 allow a direct comparison between observations and chemical models for oxygen species in conditions ...just before star formation. We explain a chemical model for gas-phase water, simplified for the limited number of reactions or processes that are active in extreme cold (<15 K). In this model, water is removed from the gas phase by freezing on to grains and by photodissociation. Water is formed as ice on the surface of dust grains from O and OH and released into the gas phase by photodesorption. The reactions are fast enough with respect to the slow dynamical evolution of L1544 that the gas-phase water is in equilibrium for the local conditions throughout the cloud. We explain the paradoxical radiative transfer of the H2O (1...-1...) line. Despite discouragingly high optical depth caused by the large Einstein A coefficient, the subcritical excitation in the cold, rarefied H2 causes the line brightness to scale linearly with column density. Thus, the water line can provide information on the chemical and dynamical processes in the darkest region in the centre of a cold, dense cloud. The inverse P-Cygni profile of the observed water line generally indicates a contracting cloud. This profile is reproduced with a dynamical model of slow contraction from unstable quasi-static hydrodynamic equilibrium (an unstable Bonnor-Ebert sphere). (ProQuest: ... denotes formulae/symbols omitted.)
In this work, we reexamine sulfur chemistry occurring on and in the ice mantles of interstellar dust grains, and report the effects of two new modifications to standard astrochemical models: namely, ...(a) the incorporation of cosmic-ray-driven radiation chemistry and (b) the assumption of fast, nondiffusive reactions for key radicals in the bulk. Results from our models of dense molecular clouds show that these changes can have a profound influence on the abundances of sulfur-bearing species in ice mantles, including a reduction in the abundance of solid-phase H2S and HS, and a significant increase in the abundances of OCS, SO2, as well as pure allotropes of sulfur, especially S8. These pure-sulfur species-though nearly impossible to observe directly-have long been speculated to be potential sulfur reservoirs and our results represent possibly the most accurate estimates yet of their abundances in the dense interstellar medium. Moreover, the results of these updated models are found to be in good agreement with available observational data. Finally, we examine the implications of our findings with regard to the as-yet-unknown sulfur reservoir thought to exist in dense interstellar environments.
We develop a self-consistent model for the equilibrium gas temperature and size-dependent dust temperature in cold, dense, prestellar cores, assuming an arbitrary power-law size distribution of dust ...grains. Compact analytical expressions applicable to a broad range of physical parameters are derived and compared with predictions of the commonly used standard model. It is suggested that combining the theoretical results with observations should allow us to constrain the degree of dust evolution and the cosmic-ray ionization rate in dense cores, and to help with discriminating between different regimes of cosmic-ray transport in molecular clouds. In particular, assuming a canonical MRN distribution of grain sizes, our theory demonstrates that the gas-temperature measurements in the prestellar core L1544 are consistent with an ionization rate as high as ∼10−16 s−1, an order of magnitude higher than previously thought.
ABSTRACT The detection of complex organic molecules (COMs) toward cold sources such as pre-stellar cores (with T < 10 K) has challenged our understanding of the formation processes of COMs in the ...interstellar medium. Recent modeling on COM chemistry at low temperatures has provided new insight into these processes predicting that COM formation depends strongly on parameters such as visual extinction and the level of CO freeze out. We report deep observations of COMs toward two positions in the L1544 pre-stellar core: the dense, highly extinguished continuum peak with AV ≥ 30 mag within the inner 2700 au; and a low-density shell with average AV ∼ 7.5-8 mag located at 4000 au from the core's center and bright in CH3OH. Our observations show that CH3O, CH3OCH3, and CH3CHO are more abundant (by factors of ∼2-10) toward the low-density shell than toward the continuum peak. Other COMs such as CH3OCHO, c-C3H2O, HCCCHO, CH2CHCN, and HCCNC show slight enhancements (by factors ≤3), but the associated uncertainties are large. This suggests that COMs are actively formed and already present in the low-density shells of pre-stellar cores. The modeling of the chemistry of O-bearing COMs in L1544 indicates that these species are enhanced in this shell because (i) CO starts freezing out onto dust grains driving an active surface chemistry; (ii) the visual extinction is sufficiently high to prevent the UV photo-dissociation of COMs by the external interstellar radiation field; and (iii) the density is still moderate to prevent severe depletion of COMs onto grains.
This paper contains a review of theoretical and empirical Italian pedagogical studies conducted from the Seventies to today on the topic of caregiver-child conversation, with particular reference to ...the role of adults in promoting the development of linguistic and communication skills, especially within Early Childhood Education and Care (ECEC) services for children aged zero-six years. The studies analysed here – considered a highly representative selection – show that research aimed at exploring the conversations between caregivers and children, conducted in Italy and especially over the last twenty years, has inspired reflection on linguistic interaction, also within Early Childhood Education and Care services. All in the belief that it is imperative to promote educators’ and teachers’ awareness of the key role that words play in children’s linguistic-communication and affective-emotional development, with a view to increasing the quality of ECEC.
PDF
