Estudio del acoplamiento entre dinámica y química en la formación de una estrella de masa solar

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra y Astrofísica, leída el 15-12-2021Astrochemistry is the study of the chemical composition of interstellar objects and the chemical reactions occurring in space. How species interact have a great impact in the process ofstar formation. In fact, determining aspects for the dynamical behavior of interstellar matterare the gas cooling rate and the ionization degree, highly influenced by the gas chemistryand molecular abundances. Indeed, efficient gas cooling diminishes thermal supportagainst gravity, leading to fragmentation and collapse of molecular filaments into pre-stellarcores. Here, elemental depletions of Carbon (C) and Oxygen (O) are key to determine thecooling rate of the gas, as CO and CII are the main coolants in molecular clouds. The ionizationfraction of the gas is involved in gas dynamics as well, since it controls the couplingof the gas with the magnetic fields. Magnetic fields thus drive the dissipation of turbulenceand angular momentum transfer in the cloud collapse and the dynamics of accretion disks.The ionization fraction of the gas is, in absence of other ionization agents like X-rays, UVphotons, and shocks, proportional to ³H2 , the cosmic-ray ionization rate for H2 molecules.Other factors that affect the ionization degree of the gas are the molecular abundances andelemental depletion factors. Towards the surface of a cloud (Av Ç 4 mag), Carbon is the maindonor of electrons. Deeper into the cloud however, in the range Av » 4¡7 mag, Sulphur (S)becomes the main donor of electrons. This range of extinctions encompasses a large fractionof the molecular cloud’s mass...La astroquímica es el área de estudio que investiga la composición química de los objetos interestelares y las reacciones químicas que tienen lugar el ellos. La interacción entre especies químicas tiene un gran impacto en el proceso de formación estelar. Dos aspectos fundamentales para el comportamiento dinámico de la materia interestelar son el ritmo de enfriamiento del gas interestelar y su grado de ionización, altamente influenciados por la química en fase gaseosa y las abundancias químicas presentes. En efecto, un enfriamiento eficiente del gas disminuye el soporte térmico de éste contra la fuerza de la gravedad, provocando la fragmentación y colapso de filamentos moleculares en núcleos pre-estelares. La depleción de los elementos carbono (C) y oxígeno (O) es crucial en este aspecto, ya que CO y CII son los principales refrigerantes del gas en nubes moleculares. Al igual que el ritmo de enfriamiento del gas, su fracción de ionización juega un papel importante en su dinámica, ya que controla su acoplamiento con los campos magnéticos. Estos campos dirigen la disipación de la turbulencia y la transferencia de momento angular en el colapso de una nube, y la dinámica de los discos de acreción. La fracción de ionización del gas es, en ausencia de otros agentes ionizantes como los rayos X, fotones ultravioleta o choques, proporciona la ³H2 , la tasa de ionización por rayos cósmicos para moléculas de hidrógeno. Otros factores que afectan al grado de ionización del gas son las abundancias moleculares y los factores de depleción elementales. Así, en la superficie de una nube molecular (Av Ç 4 mag), el carbono es el principal donador de electrones, mientras que el azufre (S) lo es en regiones internas en las que Av » 4¡7 mag. Este rango de extinciones comprende una fracción importante de la masa de la nube...Fac. de Ciencias FísicasTRUEunpu

AB Aur, a Rosetta stone for studies of planet formation (I): chemical study of a planet-forming disk

AB Aur is a Herbig Ae star that hosts a prototypical transition disk. The disk shows a plethora of features connected with planet formation mechanisms. Understanding the physical and chemical characteristics of these features is crucial to advancing our knowledge of planet formation. We aim to characterize the gaseous disk around the Herbig Ae star AB Aur. A complete spectroscopic study was performed using NOEMA to determine the physical and chemical conditions. We present new observations of the continuum and 12CO, 13CO, C18O, H2CO, and SO lines. We used the integrated intensity maps and stacked spectra to derive estimates of the disk temperature. By combining our 13CO and C18O observations, we computed the gas-to-dust ratio along the disk. We also derived column density maps for the different species and used them to compute abundance maps. The results of our observations were compared with Nautilus astrochemical models. We detected continuum emission in a ring that extends from 0.6 to 2.0 arcsec, peaking at 0.97 and with a strong azimuthal asymmetry. The molecules observed show different spatial distributions, and the peaks of the distributions are not correlated with the binding energy. Using H2CO and SO lines, we derived a mean disk temperature of 39 K. We derived a gas-to-dust ratio that ranges from 10 to 40. The comparison with Nautilus models favors a disk with a low gas-to-dust ratio (40) and prominent sulfur depletion. From a very complete spectroscopic study of the prototypical disk around AB Aur, we derived, for the first time, the gas temperature and the gas-to-dust ratio along the disk, providing information that is essential to constraining hydrodynamical simulations.Moreover, we explored the gas chemistry and, in particular, the sulfur depletion. The derived sulfur depletion is dependent on the assumed C/O ratio. Our data are better explained with C/O ~ 0.7 and S/H=8e-8.Comment: 13 figures, 6 table

Gas phase Elemental abundances in Molecular cloudS (GEMS). IX. Deuterated compounds of H2S in starless cores

H2S is thought to be the main sulphur reservoir in the ice, being therefore a key molecule to understand sulphur chemistry in the star formation process and to solve the missing sulphur problem. The H2S deuterium fraction can be used to constrain its formation pathways. We investigate for the first time the H2S deuteration in a large sample of starless cores (SC). We use observations of the GEMS IRAM 30m Large Program and complementary IRAM 30m observations. We consider a sample of 19 SC in Taurus, Perseus, and Orion, detecting HDS in 10 and D2S in five. The H2S single and double deuterium fractions are analysed with regard to their relation with the cloud physical parameters, their comparison with other interstellar sources, and their comparison with deuterium fractions in early stage star-forming sources of c-C3H2, H2CS, H2O, H2CO, and CH3OH. We obtain a range of X(HDS)/X(H2S)~0.025-0.2 and X(D2S)/X(HDS)~0.05-0.3. H2S single deuteration shows an inverse relation with the cloud kinetic temperature. H2S deuteration values in SC are similar to those observed in Class 0. Comparison with other molecules in other sources reveals a general trend of decreasing deuteration with increasing temperature. In SC and Class 0 objects H2CS and H2CO present higher deuteration fractions than c-C3H2, H2S, H2O, and CH3OH. H2O shows single and double deuteration values one order of magnitude lower than those of H2S and CH3OH. Differences between c-C3H2, H2CS and H2CO deuterium fractions and those of H2S, H2O, and CH3OH are related to deuteration processes produced in gas or solid phases, respectively. We interpret the differences between H2S and CH3OH deuterations and that of H2O as a consequence of differences on the formation routes in the solid phase, particularly in terms of the different occurrence of the D-H and H-D substitution reactions in the ice, together with the chemical desorption processes.Comment: Accepted for publication in A&

Gas phase Elemental abundances in Molecular cloudS (GEMS) VIII. Unlocking the CS chemistry: the CH + S→\rightarrow CS + H and C2_2 + S→\rightarrow CS + C reactions

We revise the rates of reactions CH + S -> CS + H and C_2 + S -> CS + C, important CS formation routes in dark and diffuse warm gas. We performed ab initio calculations to characterize the main features of all the electronic states correlating to the open shell reactants. For CH+S we have calculated the full potential energy surfaces for the lowest doublet states and the reaction rate constant with a quasi-classical method. For C_2+S, the reaction can only take place through the three lower triplet states, which all present deep insertion wells. A detailed study of the long-range interactions for these triplet states allowed to apply a statistic adiabatic method to determine the rate constants. This study of the CH + S reaction shows that its rate is nearly independent on the temperature in a range of 10-500 K with an almost constant value of 5.5 10^{-11} cm^3/s at temperatures above 100~K. This is a factor \sim 2-3 lower than the value obtained with the capture model. The rate of the reaction C_2 + S depends on the temperature taking values close to 2.0 10^{-10} cm^3/s at low temperatures and increasing to 5. 10^{-10} cm^3/s for temperatures higher than 200~K. Our modeling provides a rate higher than the one currently used by factor of \sim 2. These reactions were selected for involving open-shell species with many degenerate electronic states, and the results obtained in the present detailed calculations provide values which differ a factor of \sim 2-3 from the simpler classical capture method. We have updated the sulphur network with these new rates and compare our results in the prototypical case of TMC1 (CP). We find a reasonable agreement between model predictions and observations with a sulphur depletion factor of 20 relative to the sulphur cosmic abundance, but it is not possible to fit all sulphur-bearing molecules better than a factor of 10 at the same chemical time.Comment: 13 pages, 10 figure

The sulphur depletion problem in molecular clouds: The H

Sulphur is one of the most abundant elements in the Universe and plays a crucial role in biological systems. However, sulphuretted molecules in the ISM are not as abundant as expected and there is no clear answer of where the missing Sulphur is yet. To shed light onto this open question, we focus our attention on the chemistry of H2S, thought to be an important reservoir of Sulphur and formed mainly by grain-phase reactions. To understand the formation of H2S, the growth of ices, and the chemical desorption process, we study the CO, CH3OH, N2H+, and H2S abundances towards Barnard 1b, a Sulphur-rich cloud hosting a first Larson core. We look for correlations between gas-phase abundances of H2S and CH3OH that better constrain the location of the CO snowline in dark cores. Finally, this provides additional data to benchmark models for a deeper insight on the chemical desorption process and its efficiency