El ciclo de la materia en el medio interestellar : procesamiento energético del polvo y el hielo

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 05-05-2017El objetivo de esta tesis doctoral es el de profundizar en el conocimiento de los procesos que tienen lugar en el medio interestelar (MI) relacionados con el procesamiento energético de granos de polvo interestelares y los mantos de hielo que los recubren, y en particular, de los procesos de desorción inducida por fotones de las moléculas del hielo en las regiones frías del MI que son necesarios para explicar las abundancias de ciertas especies observadas en la fase gaseosa. En astrofísica no sólo se utilizan las observaciones astronómicas, sino que también se usan simulaciones experimentales en condiciones relevantes para la astrofísica y simulaciones teóricas para complementar la información obtenida a través de las observaciones y conseguir así un mejor entendimiento de los procesos que ocurren en el espacio en general y el MI en particular. Los resultados presentados en esta tesis han sido obtenidos combinando estas tres metodologías, aunque la astrofísica de laboratorio ha tenido un peso mayor. Tras los capítulos de introducción, en el capítulo 5 se presenta el estudio experimental del procesamiento por fotones UV de partículas análogas a los granos de polvo carbonáceos que se encuentran en el MI, a distintas temperaturas. La irradiación produce la formación de moléculas de H2 que luego difunden a través de las partículas y pasan a la fase gaseosa. Este proceso está controlado por el coeficiente de difusión, cuya dependencia con la temperatura fue el objeto de estudio, obteniéndose una energía de activación de 1660 K. Es en la superficie de estos granos de polvo donde se forman los mantos de hielo en las regiones más frías y más densas del MI. En el capítulo 6 se demuestra que las condiciones en las que se forman los hielos de CO afectan a las energías de enlace entre las moléculas, y por tanto a la morfología de los mismos. Sin embargo, las diferencias se desvanecen cuando los hielos son calentados. Los capítulos 7 -10 están dedicados al procesamiento con fotones UV de los mantos de hielo, prestando especial atención a los procesos de desorción inducidos por los fotones, que constituyen el núcleo de la tesis. En el capítulo 8 se estudia la irradiación de un hielo puro de CO2. Gracias a la combinación del espectrómetro IR (que observa la composición del hielo) y el cuadrupolo de masas (que monitoriza el gas) se pudo obtener una cuantificación completa de los procesos fotoquímicos y de fotodesorción. En los capítulos 9 y 10 se usaron un hielo puro de etanol, y una mezcla binaria rica en agua con moléculas de metano, más realista para explorar la posible formación (a temperaturas muy bajas en torno a 8 K) y la consiguiente desorción de moléculas de metanol y otras especies relacionadas. La formación de metanol sólo se produjo durante la irradiación de la mezcla binaria, pero no se detectó su fotodesorción. A pesar de todo se observó que la fotodesorción de otros fotoproductos puede seguir dos patrones distintos a medida que aumenta el tiempo de irradiación, según el mecanismo a través del es inducida la fotodesorción...The aim of this PhD thesis is to expand our knowledge of particular processes taking place in the interstellar medium (ISM). These processes are related in one way or another to the energetic processing of the interstellar dust grains and the ice mantles on top of them, and to the interplay between the solid and the gaseous phases of the ISM. In particular, photon-induced desorption processes of ice molecules in cold regions of the ISM are needed to explain the observed gas-phase abundances of several species. It is therefore a paradigmatic case of the interaction between the two phases of the ISM, and its importance is nowadays beyond any doubt. The core of this thesis (Part IV) is devoted to the study of such processes. Astrophysics is no longer about observations only. Both experimental simulations under astrophysically relevant conditions, and theoretical models are used to complement the observations and get a better understanding of the processes that are taking place in space and the ISM in particular. The results presented in this thesis have been obtained using a combination of these three methodologies, although more attention is paid to laboratory astrophysics. Parts III (Chapter 5 and IV (Chapters 6 - 11) deal with experimental simulations carried out in a high-vacuum (HV) and an ultra-high-vacuum (UHV) chambers. These chambers have base pressures slightly over (in the case of the HV) or similar (UHV) to those found in the densest regions of the ISM. A closed-cycle helium cryostat is used in both cases to reach the temperatures typically found in the cold and warm ISM (below 100 K, and as low as 10 K in the densest and coldest regions). Under these conditions, dust or ice analogs can be grown onto a substrate in order to simulate the energetic processing (photoprocessing in most chapters, but also thermal processing) of the solid component of the ISM...Fac. de Ciencias FísicasTRUEunpu

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons. III. Diffusion of photo-produced H2 as a function of temperature

Hydrogenated amorphous carbon (a-C:H) has been proposed as one of the carbonaceous solids detected in the interstellar medium. Energetic processing of the a-C:H particles leads to the dissociation of the C-H bonds and the formation of hydrogen molecules and small hydrocarbons. Photo-produced H2 molecules in the bulk of the dust particles can diffuse out to the gas phase and contribute to the total H2 abundance. We have simulated this process in the laboratory with plasma-produced a-C:H and a-C:D analogs under astrophysically relevant conditions to investigate the dependence of the diffusion as a function of temperature. Plasma-produced a-C:H analogs were UV-irradiated using a microwave-discharged hydrogen flow lamp. Molecules diffusing to the gas-phase were detected by a quadrupole mass spectrometer, providing a measurement of the outgoing H2 or D2 flux. By comparing the experimental measurements with the expected flux from a one-dimensional diffusion model, a diffusion coefficient D could be derived for experiments carried out at different temperatures. Dependance on the diffusion coefficient D with the temperature followed an Arrhenius-type equation. The activation energy for the diffusion process was estimated (ED(H2)=1660+-110 K, ED(D2)=2090+-90 K), as well as the pre-exponential factor (D0(H2)=0.0007+0.0013-0.0004 cm2 s-1, D0(D2)=0.0045+0.005-0.0023 cm2 s-1) The strong decrease of the diffusion coefficient at low dust particle temperatures exponentially increases the diffusion times in astrophysical environments. Therefore, transient dust heating by cosmic rays needs to be invoked for the release of the photo- produced H2 molecules in cold PDR regions, where destruction of the aliphatic component in hydrogenated amorphous carbons most probably takes place

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&

Desorption Kinetics and Binding Energies of Small Hydrocarbons

Small hydrocarbons are an important organic reservoir in protostellar and protoplanetary environments. Constraints on desorption temperatures and binding energies of such hydrocarbons are needed for accurate predictions of where these molecules exist in the ice versus gas phase during the different stages of star and planet formation. Through a series of temperature programmed desorption experiments, we constrain the binding energies of 2- and 3-carbon hydrocarbons (C_2H_2—acetylene, C_2H_4—ethylene, C_2H_6—ethane, C_3H_4—propyne, C_3H_6—propene, and C_3H_8—propane) to 2200–4200 K in the case of pure amorphous ices, to 2400–4400 K on compact amorphous H_2O, and to 2800–4700 K on porous amorphous H_2O. The 3-carbon hydrocarbon binding energies are always larger than the 2-carbon hydrocarbon binding energies. Within the 2- and 3-carbon hydrocarbon families, the alkynes (i.e., least-saturated) hydrocarbons exhibit the largest binding energies, while the alkane and alkene binding energies are comparable. Binding energies are ~5%–20% higher on water ice substrates compared to pure ices, which is a small increase compared to what has been measured for other volatile molecules such as CO and N_2. Thus in the case of hydrocarbons, H_2O has a less pronounced effect on sublimation front locations (i.e., snowlines) in protoplanetary disks