Dust temperature and time-dependent effects in the chemistry of photodissociation regions

When studying the chemistry of PDRs, time dependence becomes important as visual extinction increases, since certain chemical timescales are comparable to the cloud lifetime. Dust temperature is also a key factor, since it significantly influences gas temperature and mobility on dust grains, determining the chemistry occurring on grain surfaces. We present a study of the dust temperature impact and time effects on the chemistry of different PDRs, using an updated version of the Meijerink PDR code and combining it with the time-dependent code Nahoon. We find the largest temperature effects in the inner regions of high $G$$_{\mathrm{0}}$ PDRs, where high dust temperatures favour the formation of simple oxygen-bearing molecules (especially that of O$_2$), while the formation of complex organic molecules is much more efficient at low dust temperatures. We also find that time-dependent effects strongly depend on the PDR type, since long timescales promote the destruction of oxygen-bearing molecules in the inner parts of low $G$$_{\mathrm{0}}$ PDRs, while favouring their formation and that of carbon-bearing molecules in high $G$$_{\mathrm{0}}$ PDRs. From the chemical evolution, we also conclude that, in dense PDRs, CO$_2$ is a late-forming ice compared to water ice, and confirm a layered ice structure on dust grains, with H$_2$O in lower layers than CO$_2$. Regarding steady state, the PDR edge reaches chemical equilibrium at early times ($\lesssim$10$^5$ yr). This time is even shorter ($<$10$^4$ yr) for high $G$$_{\mathrm{0}}$ PDRs. By contrast, inner regions reach equilibrium much later, especially low $G$$_{\mathrm{0}}$ PDRs, where steady state is reached at $\sim$10$^6$-10$^7$ yr.Comment: 24 pages, 15 figures, 9 table

The first frost in the Pipe Nebula

Spectroscopic studies of ices in nearby star-forming regions indicate that ice mantles form on dust grains in two distinct steps, starting with polar ice formation (H2O rich) and switching to apolar ice (CO rich). We test how well the picture applies to more diffuse and quiescent clouds where the formation of the first layers of ice mantles can be witnessed. Medium-resolution near-infrared spectra are obtained toward background field stars behind the Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in seven lines of sight out of 21 sources for which observed spectra are successfully reduced. The peak optical depth of the water ice is significantly lower than those in Taurus with the same visual extinction. The source with the highest water-ice optical depth shows CO ice absorption at 4.7 micron as well. The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and about half as much as the values typically seen in low-mass star-forming regions. A small fractional abundance of CO ice is consistent with some of the existing simulations. Observations of CO2 ice in the early diffuse phase of a cloud play a decisive role in understanding the switching mechanism between polar and apolar ice formation.Comment: 17 pages, 8 figures, accepted by A&

Laboratory experiments on the sublimation of methane through ice dust layers and applications to cometary activity

Context. Comets are small celestial bodies made of ice, dust, and rock that orbit the Sun. Understanding their behavior as they warm up at perihelion unveils many pieces of information about the interior and general morphology of the ices hidden under the dust. Aims. The goal of this research is to study the sublimation of CH4 through amorphous solid water (ASW), with a focus on the structural changes in water and the influence of a layer of indene (as a proxy of the crust) during a period of thermal processing, which we use in a controlled laboratory setting to simulate cometary environments. Methods. Ices at a CH4 to H2O abundance ratio of about 0.01 are deposited and layered, or co-deposited, at 30 K and are heated until 200 K (or 140 K) with a ramp of either 1 or 5 K per min. We use mass spectrometry and infrared spectroscopy to analyze the results. Results. Depending on the heating ramp and type of deposition, the sublimation of methane through ASW varies, being lower in intensity and higher in temperature when the co-deposited structure is considered. When two temperature cycles are applied, the second one sees less intense CH4 desorptions. When indene is placed above the ice mixtures, we find that the thicker its layer, the later the methane desorption. Conclusions. The structural changes of water ice drive volatile and hyper-volatile desorption because of the transition from high to low intrinsic density and transformation from amorphous to crystalline. This desorption indicates that such material has been deposited at low temperatures in agreement with previous theories on cometary ices formed in the pre-stellar cloud.Comment: Accepted A&

Water delivery in the inner solar nebula : Monte Carlo simulations of forsterite hydration

This work is part of the Dutch Astrochemistry program financed by the Netherlands Organisation for Scientific Research, NWO.Context:  Endogenous or exogenous, dry or wet, various scenarios have been so far depicted for the origin of water on our Solar Systems rocky bodies. Hydrated silicates found in meteorites and in interplanetary dust particles together with observations of abundant water reservoirs in the habitable zone of protoplanetary disks are evidences that support aqueous alteration of silicate dust grains by water vapor condensation in a nebular setting. Aims:  We investigate the thermodynamics (temperature and pressure dependencies) and kinetics (adsorption rates and energies, surface diffusion and cluster formation) of water adsorption on surfaces of forsterite grains, constraining the location in the solar nebula where aqueous alteration of silicates by water vapor adsorption could occur efficiently and lead to the formation of phyllosilicates. We analyze the astrophysical conditions favorable for such hydration mechanism and the implications for water on solid bodies. Methods:  The protoplanetary disk model (ProDiMo) code is tuned to simulate the thermochemical disk structure of the early solar nebula at three evolutionary stages. Pressure, temperature and water vapor abundance within 1 au from the protosun were extracted and used as input for a Monte Carlo code to model water associative adsorption using adsorption energies that resemble the forsterite [100] crystal lattice. Results:  Hydration of forsterite surfaces by water vapor adsorption could have occurred within the nebula lifetime already at a density of 108 cm3, with increasing surface coverage for higher water vapor densities. Full surface coverage is attained for temperatures lower than 500 K, while for hotter grain surfaces water cluster formation plays a crucial role. Between 0.5 and 10 number of Earth’s oceans can arise from the agglomeration of hydrated 0.1 μm grains into an Earth-sized planet. However, if grain growth occurs dry and water vapor processes the grains afterwards, this value can decrease by two orders of magnitude. Conclusions: This work shows that water cluster formation enhances the water surface coverage and enables a stable water layer to form at high temperature and low water vapor density conditions. Finally, surface diffusion of physisorbed water molecules shortens the timescale for reaching steady state, enabling phyllosilicate formation within the solar nebula timescale.PostprintPeer reviewe

H2 formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations

Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H formation in a variety of interstellar environments.VW’s research is funded by an ERC Starting Grant (3DICE, grant agreement 336474). GV acknowledges financial support from the National Science Foundation’s Astronomy & Astrophysics Division (Grants No. 1311958 and 1615897). LH acknowledges support from ERC Consolidator Grant GRANN (grant agreement no. 648551). GN acknowledges support from the Swedish Research Council. VW, FD and SM acknowledge the CNRS program ”Physique et Chimie du Milieu Interstellaire” (PCMI) co-funded bythe Centre National d’Etudes Spatiales (CNES). SDP acknowledges funding from STFC, UK. V.V acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (MagneticYSOS project, grant agreement No 679937)

PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar

(Abridged) We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the HII region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. The NIRSpec data reveal a forest of lines including, but not limited to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. We observe numerous smaller scale structures whose typical size decreases with distance from Ori C and IR lines from CI, if solely arising from radiative recombination and cascade, reveal very high gas temperatures consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The H2 lines reveal multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star- and planet formation as well as galaxy evolution.Comment: 52 pages, 30 figures, submitted to A&

A targeted next-generation sequencing assay for the molecular diagnosis of genetic disorders with orodental involvement.

BACKGROUND: Orodental diseases include several clinically and genetically heterogeneous disorders that can present in isolation or as part of a genetic syndrome. Due to the vast number of genes implicated in these disorders, establishing a molecular diagnosis can be challenging. We aimed to develop a targeted next-generation sequencing (NGS) assay to diagnose mutations and potentially identify novel genes mutated in this group of disorders. METHODS: We designed an NGS gene panel that targets 585 known and candidate genes in orodental disease. We screened a cohort of 101 unrelated patients without a molecular diagnosis referred to the Reference Centre for Oro-Dental Manifestations of Rare Diseases, Strasbourg, France, for a variety of orodental disorders including isolated and syndromic amelogenesis imperfecta (AI), isolated and syndromic selective tooth agenesis (STHAG), isolated and syndromic dentinogenesis imperfecta, isolated dentin dysplasia, otodental dysplasia and primary failure of tooth eruption. RESULTS: We discovered 21 novel pathogenic variants and identified the causative mutation in 39 unrelated patients in known genes (overall diagnostic rate: 39%). Among the largest subcohorts of patients with isolated AI (50 unrelated patients) and isolated STHAG (21 unrelated patients), we had a definitive diagnosis in 14 (27%) and 15 cases (71%), respectively. Surprisingly, COL17A1 mutations accounted for the majority of autosomal-dominant AI cases. CONCLUSIONS: We have developed a novel targeted NGS assay for the efficient molecular diagnosis of a wide variety of orodental diseases. Furthermore, our panel will contribute to better understanding the contribution of these genes to orodental disease. TRIAL REGISTRATION NUMBERS: NCT01746121 and NCT02397824.journal articleresearch support, non-u.s. gov't2016 Feb2015 10 26importe