Heat pump systems with vertical ground heat exchangers and uncovered solar thermal collectors

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The Role of Turbulence in the Process of Star Formation

The aim of this thesis is to study the role of interstellar turbulence in the process of star formation. We demonstrate that supersonic turbulent motions significantly affect various properties of the interstellar medium (ISM). Therefore, we run numerical simulations of molecular clouds in different environments. In particular, we study typical clouds located in the Milky Way disk as well as clouds which can be found in more extreme regions in our Galaxy, e.g. in the Central Molecular Zone (CMZ) near the Galactic Center. In addition, we perform radiative transfer calculations of numerous diagnostic fine structure lines and compare our results with observational measurements. Furthermore, we analyze the influence of the turbulence on different observables, e.g. on the structure functions, the ∆-variance, the power spectra as well as the star formation efficiencies. We also study the impact of turbulent motions on the chemistry and the different phases of the ISM. Our studies about Milky Way disk clouds show that the statistical properties of the turbulence are significantly influenced by the individual gas tracers. Moreover, our investigations about CMZ-like clouds show that high levels of turbulence can significantly suppress, but never inhibit star formation, owing to local compression of gas by turbulent shocks. Finally, we show that various atomic tracers accurately reflect most of the physical properties of both the H2 and the total gas of the cloud and that they provide a very useful alternative to common molecular lines when we study the ISM in the CMZ

Principal Component Analysis of Molecular Clouds: Can CO reveal the dynamics?

We use Principal Component Analysis (PCA) to study the gas dynamics in numerical simulations of typical MCs. Our simulations account for the non-isothermal nature of the gas and include a simplified treatment of the time-dependent gas chemistry. We model the CO line emission in a post-processing step using a 3D radiative transfer code. We consider mean number densities n_0 = 30, 100, 300 cm^{-3} that span the range of values typical for MCs in the solar neighbourhood and investigate the slope \alpha_{PCA} of the pseudo structure function computed by PCA for several components: the total density, H2 density, 12CO density, 12CO J = 1 -> 0 intensity and 13CO J = 1 -> 0 intensity. We estimate power-law indices \alpha_{PCA} for different chemical species that range from 0.5 to 0.9, in good agreement with observations, and demonstrate that optical depth effects can influence the PCA. We show that when the PCA succeeds, the combination of chemical inhomogeneity and radiative transfer effects can influence the observed PCA slopes by as much as ~ +/- 0.1. The method can fail if the CO distribution is very intermittent, e.g. in low-density clouds where CO is confined to small fragments.Comment: 12 pages, 8 figures, accepted for publication in MNRA

Synthetic observations of molecular clouds in a galactic centre environment - I. Studying maps of column density and integrated intensity

We run numerical simulations of molecular clouds, adopting properties similar to those found in the central molecular zone (CMZ) of the Milky Way. For this, we employ the moving mesh code AREPO and perform simulations which account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of gas and dust. We perform simulations using an initial density of n0 = 103 cm-3 and a mass of 1.3 × 105 M⊙. Furthermore, we vary the virial parameter, defined as the ratio of kinetic and potential energy, α = Ekin/|Epot|, by adjusting the velocity dispersion. We set it to α = 0.5, 2.0 and 8.0, in order to analyse the impact of the kinetic energy on our results. We account for the extreme conditions in the CMZ and increase both the interstellar radiation field (ISRF) and the cosmic ray flux (CRF) by a factor of 1000 compared to the values found in the solar neighbourhood. We use the radiative transfer code RADMC-3D to compute synthetic images in various diagnostic lines. These are [C II] at 158 μm, [O I] (145 μm), [O I] (63 μm), 12CO (J = 1 → 0) and 13CO (J = 1 → 0) at 2600 and 2720 μm, respectively. When α is large, the turbulence disperses much of the gas in the cloud, reducing its mean density and allowing the ISRF to penetrate more deeply into the cloud's interior. This significantly alters the chemical composition of the cloud, leading to the dissociation of a significant amount of the molecular gas. On the other hand, when α is small, the cloud remains compact, allowing more of the molecular gas to survive. We show that in each case the atomic tracers accurately reflect most of the physical properties of both the H2 and the total gas of the cloud and that they provide a useful alternative to molecular lines when studying the interstellar medium in the CMZ

Centroid Velocity Statistics of Molecular Clouds

We compute structure functions and Fourier spectra of 2D centroid velocity (CV) maps in order to study the gas dynamics of typical molecular clouds (MCs) in numerical simulations. We account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of the gas and use a 3D radiative transfer tool to model the CO line emission in a post-processing step. We perform simulations using three different initial mean number densities of n_0 = 30, 100 and 300 cm^{-3} to span a range of typical values for dense gas clouds in the solar neighbourhood. We compute slopes of the centroid velocity increment structure functions (CVISF) and of Fourier spectra for different chemical components: the total density, H2 number density, 12CO number density as well as the integrated intensity of 12CO (J=1-0) and 13CO (J=1-0). We show that optical depth effects can significantly affect the slopes derived for the CVISF, which also leads to different scaling properties for the Fourier spectra. The slopes of CVISF and Fourier spectra for H2 are significantly steeper than those for the different CO tracers, independent of the density and the numerical resolution. This is due to the larger space-filling factor of H2 as it is better able to self-shield in diffuse regions, leading to a larger fractal co-dimension compared to CO.Comment: 12 pages, 6 figures, submitted to MNRA

Principal component analysis of molecular clouds: Can CO reveal the dynamics?

We use principal component analysis (PCA) to study the gas dynamics in numerical simulations of typical molecular clouds (MCs). Our simulations account for the non-isothermal nature of the gas and include a simplified treatment of the time-dependent gas chemistry. We model the CO line emission in a post-processing step using a 3D radiative transfer code. We consider mean number densities n0 = 30, 100, 300 cm−3 that span the range of values typical for MCs in the solar neighbourhood and investigate the slope αPCA of the pseudo-structure function computed by PCA for several components: the total density, H2 density, 12CO density, 12CO J = 1 → 0 intensity and 13CO J = 1 → 0 intensity. We estimate power-law indices αPCA for different chemical species that range from 0.5 to 0.9, in good agreement with observations, and demonstrate that optical depth effects can influence the PCA. We show that when the PCA succeeds, the combination of chemical inhomogeneity and radiative transfer effects can influence the observed PCA slopes by as much as ≈±0.1. The method can fail if the CO distribution is very intermittent, e.g. in low-density clouds where CO is confined to small fragments