Study of the chemical and morphological evolution of molecular, clouds, using observations of high density gas tracers

Stars are formed in dense regions within molecular clouds. The different phenomena associated with the star formation processes may be characterized by studying its dust and gas content. The analysis of thermal emission of different molecular species and their transitions are a powerful tool to characterize the physical, kinematical, and chemical properties of these regions. The abundance of some of these molecules changes as the clouds evolve, and can be used as a clock to date the age of molecular clouds. In this context, the so called “early-type molecules”, like CCS, have higher abundance in the first stages of cloud evolution and therefore, are more easily detected in cold and pre-stellar dark clouds, being significantly less abundant in star forming regions. On the other hand, “late-type molecules” (like NH3) have higher abundance in active star forming regions, at later stages of cloud evolution (Suzuki et al. 1992). Due to these results, the abundance ratio [CCS]/[NH3] was proposed as an indicator of cloud evolution, with its value being higher for pre-stellar clouds and decreasing as star formation proceeds (Suzuki et al. 1992). Moreover, studies in dense cores found a CCS-NH3 spatial anticorrelation (Hirahara et al. 1992; Kuiper et al. 1996), with NH3 tracing the inner and denser regions of molecular clouds, and CCS distributed around those more central regions. This spatial anticorrelation was explained as the result of chemical and dynamical evolution of prestellar cores. Our group extended this study to active star forming regions (De Gregorio-Monsalvo et al. 2005) using the VLA, and found anticorrelation at small scales (~5 '') in the star-forming region B1-IRS, suggesting that the production of CCS takes place not only at the earlier stages of the cloud evolution, but also at later stage, after the onset of star formation. This also implied that the use of the abundance ratio [CCS]/[NH3] may not work to characterize properly the evolutionary stage of active star forming regions. In this thesis we present a follow-up of these studies, analyzing the emission of CCS and NH3 at small scales in a sample of molecular clouds at different stages of their evolution, from starless cores to active star forming regions. Our aim was to investigate if the spatial anticorrelation between CCS and NH3 previously observed in B1-IRS is present in other clouds and over a range of evolutionary stages. We also aimed at studying a possible CCS enhancement due to the interaction between the ambient dense gas and molecular outflows. Our sample contained the dense starless cores B and D of TMC-1, the very young star forming region GF9-2, and the region surrounding the protostars L1448 IRS3 and L1448C. To carry out these studies, we observed the CCS and NH3 emission in these regions at high angular resolution using the Very Large Array (VLA) interferometer. In addition, we observed other molecular tracers like CO and water masers to trace energetic mass-loss processes associated with young stellar objects in the studied regions, as well as continuum emission tracing dust (at millimeter wavelengths) and ionized gas (at centimeter wavelengths). These included data taken with the Effelsberg radio telescope and archival data from the Atacama Large Millimeter/submillimeter Array. Our data provided us with detailed information of the morphology, kinematics and physical conditions surrounding young protostars. In the dark cloud TMC-1 (an ideal region to study the physical and chemical conditions of a cloud in a very early stage of evolution, without presence of YSOs or molecular outflows), our results showed that the cores B and D present a clear chemical differentiation. We found that the CCS-NH3 anticorrelation stands even at small scales (~4'') within each core. Core B showed intense NH3 emission with filamentary structure, and seems to be in relatively advanced stage of cloud evolution. We also found that the spacing between cores is ~0.1 pc, which is in agreement with the scale of fragmentation of collapsing cores in filaments derived from models. On the other hand, Core D presents CCS emission with a more clumpy structure, suggesting that this core is very young and less evolved than core B. In GF9-2 (a cloud that contains a very young class 0 protostar which does not power an extensive molecular outflow), we found a clear CCS-NH3 anticorrelation, with no evidence of CCS enhancement due to star-forming activity. We also identified a possible “blue spot” signature (as defined by Mayén-Gijón et al. 2014), suggesting the presence of infall and rotation. This is the first time this signature is found in a low mass star forming region. The source GF9-2 is the lowest luminosity YSO in which maser emission was reported, with a single-dish telescope. However, our VLA observations could not confirm the source is associated with this type of emission. We then carried out water maser emission in a sample of low luminosity YSOs and brown dwarfs, to find the luminosity limit at which water maser emission can be produced. We did not detect any emission in our targets, which suggests that the correlation between water maser luminosities and bolometric luminosities does not hold at the lower end of the (sub)stellar mass spectrum. We present a study of the region L1448, an active star forming region that contains multiple YSOs and outflows. We observed CCS, NH3 and water maser emission and we applied successfully the cross calibration technique using the maser line to correct data from the CCS thermal line. We observed spatial anticorrelation between CCS and NH3 around the young stellar object L1448C, including some clumps showing intense CCS emission. The presence of relatively strong CCS cores close to the outflow trajectory suggests that the CCS abundance is enhanced in L1448C, like in B1-IRS. Thus, CCS molecules can be generated at later stages of evolution. In L1448 IRS3 we did not find a clear CCS enhancement. The velocity gradients of CCS and NH3 have opposite signs south of L1448C illustrating the complementarity of both molecules, and that observing only one of them would provide a biased view of the physical processes in the region. Using ALMA data we found a dust jet traced by millimeter continuum emission. This is the first time that thermal dust emission is detected in a jet, and traced up to its origin from a circumstellar disk. Our results suggested that jets can be powered by disk winds launched from a wide range of radii, as proposed in D-wind models. In this source we also detected a Keplerian disk with C18O observations, with a Keplerian Law that can be traced up to a radius ~450 au, which is larger than in other Class-0 protostars. In AFGL 437 (a high mass star forming region with a poorly collimated outflow) we suggest that the low collimation CO outflow could be the superposition of individual outflows from different sources, rather than being the interaction of the wind from a single object with an anisotropic environment, as previously suggested. One of the main general results of the thesis is the confirmation of the CCS-NH3 spatial anticorrelation is present at small scales in all the sources of our sample. This result implies that the production and destruction of CCS molecules is a continuous process that may be present in all the stages of the evolution of our sample. In particular in L1448C we found conditions that suggest a CCS enhancement due to the interaction of the ambient gas with that of the outflow in the region, similar to B1-IRS, which confirms that the generation of CCS molecules occurs not only at the early stages of cloud evolution, but also at later stages. In this case, a likely process to explain CCS enhancement is molecular desorption. These results suggest that the chemical evolution of molecular clouds is not only a time dependent process, but also depends on different physical conditions present in advanced stages of the formation of a star. The implication is that the abundance ratio CCS-NH3 may not be useful to study the stage of the evolution of active star forming regions with multiple YSOs and/or very energetic outflows, because their interactions with ambient gas may trigger the CCS regeneration, which may provide misleading conclusions if a linear evolutionary interpretation is applied.Tesis Univ. Granada