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Observational studies of a dense cloud core on the verge of star formation
Observational studies of a dense cloud core on the verge of star formation
Pre-stellar cores represent the first identifiable stages of star formation, and are therefore perfect laboratories for astronomers to study how our own Solar system was formed. This thesis focuses on L1544, a pre-stellar core situated in the nearby Taurus Molecular Cloud, at a distance of 140 pc from us. The proximity allows us to resolve the inner structure of the core with currently available telescopes, and to zoom in on the inner, very dense regions. The core shows clear signs of contraction and chemical evolution, and its nucleus is very dense (n(H2) > 10^7 cm−3) and cold (T < 7 K). The physical structure of L1544 has been recently modeled, and this model agrees well with the observed molecular line and continuum emission. Moreover, the model predicts an increase in the dust opacity by a factor of 4 towards the central regions of L1544, which suggests grain coagulation towards the center. This theory can only be tested studying the continuum emission at millimeter wavelengths, as this is the spectral window expected to be affected by emission from large grains. Currently it is not known if grain coagulation is effective already during the prestellar phase, before the formation of a protostar. Studying this effect is important, as dust coagulation can affect the formation and evolution of protoplanetary disks. In this thesis, I present studies of dust emission properties and physical and chemical conditions in the pre-stellar core L1544. These studies are based on continuum and spectral line observations at millimeter wavelengths. The continuum observations with the IRAM 30 m telescope, presented in the first part of this thesis, can be modeled with a constant value of the dust opacity and spectral index, with no indication of grain growth in this core. It is shown, however, that the spatial resolution and the sensitivity of the observations are not sufficient for detecting the predicted effect, but interferometers, such as ALMA, should be able to find evidence for grain growth in the inner 2000 au of the core. The second part of this thesis discusses deuterated methanol (CH2DOH), a molecule which is solely formed on the dust grain surfaces, and its relation to CO, H2CO, and their isotopologues. The formation of CH2DOH requires large amounts of CO on grains, so it is useful to study both molecules simultaneously. Comparison with H2CO, which can also form in the gas, helps to understand the different conditions governing gas-phase and solid-phase reactions. As expected, we find that CH2DOH and methanol (CH3OH) thrive in similar conditions, and that both species form where CO is sufficiently depleted. On the other hand, H2CO and its deuterated species show different distributions than those of CH3OH, CH2DOH, and C17O, indicating that gas-phase reactions may play an important role. Comparison between two different chemical models shows that theoretical calculations should take into account reactive desorption, quantum tunneling, and time evolution. A need for further laboratory work is also pointed out. The last part of the thesis focuses on the dust continuum emission from L1544 at 1.1 and 3.3 mm. These new maps, obtained with two of the newest mm-continuum facilities (AzTEC at the LMT and MUSTANG-2 at the GBO) show gradients in the dust opacity across the cloud, which is consistent with variations in the thickness of the ice mantles on the dust grains surfaces. Our results also show that the current physical description of the cloud needs to be revised.
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Chacón Tanarro, Ana
2018
Englisch
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Chacón Tanarro, Ana (2018): Observational studies of a dense cloud core on the verge of star formation. Dissertation, LMU München: Fakultät für Physik
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Abstract

Pre-stellar cores represent the first identifiable stages of star formation, and are therefore perfect laboratories for astronomers to study how our own Solar system was formed. This thesis focuses on L1544, a pre-stellar core situated in the nearby Taurus Molecular Cloud, at a distance of 140 pc from us. The proximity allows us to resolve the inner structure of the core with currently available telescopes, and to zoom in on the inner, very dense regions. The core shows clear signs of contraction and chemical evolution, and its nucleus is very dense (n(H2) > 10^7 cm−3) and cold (T < 7 K). The physical structure of L1544 has been recently modeled, and this model agrees well with the observed molecular line and continuum emission. Moreover, the model predicts an increase in the dust opacity by a factor of 4 towards the central regions of L1544, which suggests grain coagulation towards the center. This theory can only be tested studying the continuum emission at millimeter wavelengths, as this is the spectral window expected to be affected by emission from large grains. Currently it is not known if grain coagulation is effective already during the prestellar phase, before the formation of a protostar. Studying this effect is important, as dust coagulation can affect the formation and evolution of protoplanetary disks. In this thesis, I present studies of dust emission properties and physical and chemical conditions in the pre-stellar core L1544. These studies are based on continuum and spectral line observations at millimeter wavelengths. The continuum observations with the IRAM 30 m telescope, presented in the first part of this thesis, can be modeled with a constant value of the dust opacity and spectral index, with no indication of grain growth in this core. It is shown, however, that the spatial resolution and the sensitivity of the observations are not sufficient for detecting the predicted effect, but interferometers, such as ALMA, should be able to find evidence for grain growth in the inner 2000 au of the core. The second part of this thesis discusses deuterated methanol (CH2DOH), a molecule which is solely formed on the dust grain surfaces, and its relation to CO, H2CO, and their isotopologues. The formation of CH2DOH requires large amounts of CO on grains, so it is useful to study both molecules simultaneously. Comparison with H2CO, which can also form in the gas, helps to understand the different conditions governing gas-phase and solid-phase reactions. As expected, we find that CH2DOH and methanol (CH3OH) thrive in similar conditions, and that both species form where CO is sufficiently depleted. On the other hand, H2CO and its deuterated species show different distributions than those of CH3OH, CH2DOH, and C17O, indicating that gas-phase reactions may play an important role. Comparison between two different chemical models shows that theoretical calculations should take into account reactive desorption, quantum tunneling, and time evolution. A need for further laboratory work is also pointed out. The last part of the thesis focuses on the dust continuum emission from L1544 at 1.1 and 3.3 mm. These new maps, obtained with two of the newest mm-continuum facilities (AzTEC at the LMT and MUSTANG-2 at the GBO) show gradients in the dust opacity across the cloud, which is consistent with variations in the thickness of the ice mantles on the dust grains surfaces. Our results also show that the current physical description of the cloud needs to be revised.