Physical and Chemical Properties of Protocluster Clumps and Massive Young Stellar Objects Associated to Infrared Dark Clouds

Abstract

The study of high-mass stars is important not only because of the effects they produce in their environment through outflows, expanding HII regions, stellar winds, and eventually supernova shock waves, but also because they play a crucial role in estimating star formation rates in other galaxies.<br /> Although we have an accepted evolutionary scenario that explains (isolated) low-mass star formation, the processes that produce massive stars (M_star > 8 M_sol) and star clusters, especially their earliest stages, are not well understood. <br /> The newly discovered class of interstellar clouds now termed infrared dark clouds (IRDCs) represent excellent laboratories to study the earliest stages of high-mass star formation given that some of the clumps within them are known to have high masses (~100's M_sol), high densities (n > 10^5 cm^-3), and low temperatures (10-20 K) as expected for the birthplaces of high-mass stars. Some questions remain unanswered: Do IRDCs harbor the very early stages of high-mass star formation, i.e., the pre-protocluster phase? If so, how do they compare with low-mass star formation sites? Is there chemical differentiation in IRDC clumps? What is the mass distribution of IRDCs? <br /> In this dissertation and for the first time, a catalog of 12 529 IRDC candidates at 24 um has been created using archival data from the MIPSGAL/Spitzer survey, as a first step in searching for the massive pre-protocluster clumps. From this catalog, a sample of ~60 clumps has been selected in order to perform single-pointing observations with the IRAM 30m, Effelsberg 100m, and APEX 12m telescopes. One IRDC clump seems to be a promising candidate for being in the pre-protocluster phase. <br /> In addition, molecular line mapping observations have been performed on three clumps within IRDCs and a detailed chemical study of 10 molecular lines has been carried out. A larger difference in column densities and abundances has been found between these clumps and high-mass protostellar objects than between these clumps and low-mass pre-stellar cores and protostellar objects. A non-LTE Monte Carlo code was used to model the N_2H^+ (1-0) and (3-2) lines in order to constrain the physical properties of two clumps. <br /> Six IRDC complexes have been mapped in the 870 um dust continuum emission with the LABOCA instrument on the APEX 12m telescope. Line observations have been carried out in order to obtain temperature and kinematic distances of selected clumps. Physical properties such as masses, effective radii, and column densities have been obtained. The mass spectrum of these clumps has been fitted with a power-law whose best-fitting index is alpha =-1.60. This value is consistent with the CO clump mass function reported in the literature. A relation between the dust emission at 870 um and the degree of extinction (contrast) at 24 um has been obtained by combining dust emission observations and extinction studies. <br /> A study with the Plateau de Bure Interferometer of a core in an archetypal filamentary IRDC at few arcsecond resolution has been carried out to determine its physical and chemical structure. Extended 4.5 um emission, "wings" in the CH_3OH 2_k -> 1_k spectra, and a CH_3OH abundance enhancement provide evidence of an outflow in the East-West direction. In addition, a gradient of ~4 km/s in the same direction has been found, which is interpreted as being produced by an outflow(s)-cloud interaction. <br /> Finally, Very Large Array interferometric observations of the 7_0-6_1 A^+ (class I) methanol maser transition at 44 GHz toward three high-mass star-forming regions have been carried out in order to provide accurate maser positions and parameters. For all three sources, the masers were well-separated from the HII region, with projected distances ranging from 0.1 to 0.3 pc.