Examinando por Autor "Wakelam, V."

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Acceso Abierto
Evolutionary view through the starless cores in Taurus Deuteration in TMC 1-C and TMC 1-CP
(EDP Sciences, 2021-06-15) Navarro Almaida, D.; Fuente, A.; Majumdar, L.; Wakelam, V.; Caselli, P.; Rivière Marichalar, P.; Treviño Morales, S. P.; Cazaux, S.; Jiménez Serra, I.; Kramer, C.; Chacón Tanarro, A.; Kirk, J. M.; Ward Thompson, D.; Tafalla, M.; Centre National D'Etudes Spatiales (CNES); Agencia Estatal de Investigación (AEI); European Research Council (ERC); Navarro Almaida, D. [0000-0002-8499-7447]; Fuente, A. [0000-0001-6317-6343]; Wakelam, V. [0000-0001-9676-2605]; Caselli, P. [0000-0003-1481-7911]; Rivière Marichalar, P. [0000-0003-0969-8137]; Treviño Morales, S. P. [0000-0002-4033-2881]; Ward Thompson, D. [0000-0003-1140-2761]; Jiménez Serra, I. [0000-0003-4493-8714]; Tafalla, M. [0000-0002-2569-1253]
Context. The chemical and physical evolution of starless and pre-stellar cores are of paramount importance to understanding the process of star formation. The Taurus Molecular Cloud cores TMC 1-C and TMC 1-CP share similar initial conditions and provide an excellent opportunity to understand the evolution of the pre-stellar core phase. Aims. We investigated the evolutionary stage of starless cores based on observations towards the prototypical dark cores TMC 1-C and TMC 1-CP. Methods. We mapped the prototypical dark cores TMC 1-C and TMC 1-CP in the CS 3 → 2, C34S 3 → 2, 13CS 2 → 1, DCN 1 → 0, DCN 2 → 1, DNC 1 → 0, DNC 2 → 1, DN13C 1 → 0, DN13C 2 → 1, N2H+ 1 → 0, and N2D+ 1 → 0 transitions. We performed a multi-transitional study of CS and its isotopologs, DCN, and DNC lines to characterize the physical and chemical properties of these cores. We studied their chemistry using the state-of-the-art gas-grain chemical code NAUTILUS and pseudo time-dependent models to determine their evolutionary stage. Results. The central nH volume density, the N2H+ column density, and the abundances of deuterated species are higher in TMC 1-C than in TMC 1-CP, yielding a higher N2H+ deuterium fraction in TMC 1-C, thus indicating a later evolutionary stage for TMC 1-C. The chemical modeling with pseudo time-dependent models and their radiative transfer are in agreement with this statement, allowing us to estimate a collapse timescale of ~1 Myr for TMC 1-C. Models with a younger collapse scenario or a collapse slowed down by a magnetic support are found to more closely reproduce the observations towards TMC 1-CP. Conclusions. Observational diagnostics seem to indicate that TMC 1-C is in a later evolutionary stage than TMC 1-CP, with a chemical age ~1 Myr. TMC 1-C shows signs of being an evolved core at the onset of star formation, while TMC 1-CP appears to be in an earlier evolutionary stage due to a more recent formation or, alternatively, a collapse slowed down by a magnetic support.
Acceso Abierto
Gas phase Elemental abundances in Molecular cloudS (GEMS) II. On the quest for the sulphur reservoir in molecular clouds: the H2S case
(EDP Sciences, 2020-05-12) Navarro Almaida, D.; Le Gal, R.; Fuente, A.; Rivière Marichalar, P.; Wakelam, V.; Cazaux, S.; Caselli, P.; Laas, J. C.; Alonso Albi, T.; Loison, J. C.; Gerin, M.; Kramer, C.; Roueff, E.; Bachiller, R.; Commerçon, B.; Friesen, R.; García Burillo, S.; Goicoechea, J. R.; Giuliano, B. M.; Jiménez Serra, I.; Kirk, J. M.; Lattanzi, V.; Malinen, J.; Marcelino, N.; Martín Doménech, R.; Muñoz Caro, G. M.; Pineda, J.; Tercero, B.; Treviño Morales, S. P.; Roncero, O.; Tafalla, M.; Ward Thompson, D.; European Research Council (ERC); European Commission (EC); Agencia Estatal de Investigación (AEI); Navarro Almaida, D. [0000-0002-8499-7447]; Unidad de Excelencia Científica María de Maeztu Centro de Astrobiología del Instituto Nacional de Técnica Aeroespacial y CSIC, MDM-2017-0737
Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question. Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance. Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when nH > 2 × 104. This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5-10. Along the three cores, atomic S is predicted to be the main sulphur reservoir. Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.
Acceso Abierto
Gas phase Elemental abundances in Molecular cloudS (GEMS) III. Unlocking the CS chemistry: the CS+O reaction
(EDP Sciences, 2021-02-02) Bulut, N.; Roncero, O.; Aguado, A.; Loison, J. C.; Navarro Almaida, D.; Wakelam, V.; Fuente, A.; Roueff, E.; Le Gal, R.; Caselli, P.; Gerin, M.; Hickson, K. M.; Spezzano, S.; Riviére Marichalar, P.; Alonso Albi, T.; Bachiller, R.; Jiménez Serra, I.; Kramer, C.; Tercero, B.; Rodríguez Baras, M.; García Burillo, S.; Goicoechea, J. R.; Treviño Morales, S. P.; Esplugues, G.; Cazaux, S.; Commercon, B.; Laas, J. C.; Kirk, J.; Lattanzi, V.; Martín Doménech, R.; Muñoz Caro, G. M.; Pineda, J. E.; Ward Thompson, D.; Tafalla, M.; Marcelino, N.; Malinen, J.; Friesen, R.; Giuliano, B. M.; Agúndez, Marcelino; Hacar, A.; Agencia Estatal de Investigación (AEI); Marcelino, N. [0000-0001-7236-4047]; Roncero, O. [0000-0002-8871-4846]; Pineda, J. [0000-0002-3972-1978]; Agundez, M. [0000-0003-3248-3564]; Tafalla, M. [0000-0002-2569-1253]
Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10−15 cm3 s−1, which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.