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The interstellar medium – the space between the stars in our Galaxy – is multiphase, turbulent, and magnetic. Magnetism in the interstellar medium is difficult to observe and to simulate, and the study of interstellar magnetic fields is riddled with open questions. In this Thesis we make progress in several important areas. We use analytic theory, simulations, and observations to advance our understanding of an important plasma instability, of the diffuse neutral medium, and of prospects for uncovering cosmic inflation. We take an unusual approach to the study of the magnetorotational instability, the mechanism thought to be the primary driver of turbulence and angular momentum transport in astrophysical accretion disks. We conduct a weakly nonlinear analysis of the instability in several important geometries, and derive an envelope equation that governs the evolution of the system on long length- and timescales. We show that the saturated state of the magnetorotational instability may itself be unstable on these large spatial and temporal scales, and we demonstrate that the character of these instabilities will depend on the geometry of the background magnetic field. We posit a possible new saturation mechanism for the magnetorotational instability in a local geometry, when a particular nonideal effect is considered. We derive new insights into the diffuse interstellar medium, where we present the discovery that thin, linear neutral hydrogen structures are ubiquitous in the cold neutral medium. We demonstrate that these linear features are extremely well aligned with the interstellar magnetic field, as traced by both starlight polarization and polarized dust emission. We discuss the implications of this discovery for cosmological studies. A major goal of modern cosmology is the detection of a particular signature in the polarized cosmic microwave background that would be direct evidence for inflation. This goal has thus far been thwarted by the polarized foreground emission from magnetically aligned interstellar dust grains. We demonstrate that the alignment of neutral hydrogen with the interstellar magnetic field can be used to produce higher-fidelity maps of the foreground polarization field, and we present and test a new Bayesian method for constructing improved foreground maps.

Thesis (Doctoral)--Rijksuniversiteit Groningen, 2004. / Includes bibliographical references (p. [131]-137).

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Solid hydrogen coated graphite particles may be expelled from regions of star -formation into the general interstellar medium. The solid para- hydrogen mantles, which contain a small proportion of orthomolecules are stable against evaporation in the general interstellar radiation field. They are also stable against physical and chemical sputtering in HI regions. Extinction efficiencies are calculated for solid hydrogen particles and for graphite particles with solid hydrogen mantles. Graphite core -solid hydrogen grains are capable of producing excellent agreement with the interstellar extinction observations from 2u - 1100 A. The graphite core radius may be in the range ro = 0.04 - 0.06u and the solid hydrogen mantle radius in the range r = 0.15 - 0.25u. The albedo and phase function of these particles are consistent with the requirements imposed by the diffuse galactic light. Solid hydrogen is strongly anisotropic in its crystal structure and optical properties. Approximate models yield good agreement with the observed trends of the wavelength dependence of interstellar polarisation.

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The lifetime of a Giant Molecular Cloud (GMC) and the total mass of stars that form within it are crucial to the understanding of star formation rates across a whole galaxy. In particular, the stars within a GMC may dictate its disruption and the quenching of further star formation. Indeed, observations show that the Milky Way contains GMCs with extensive expanding bubbles while the most massive stars are still alive. Simulating entire GMCs is challenging, due to the large variety of physics that needs to be included, and the computational power required to accurately simulate a GMC over tens of millions of years. Using the radiative-magneto-hydrodynamic code Enzo, I have run many simulations of GMCs. I obtain robust results for the fraction of gas converted into stars and the lifetimes of the GMCs: (A) In simulations with no stellar outputs (or ``feedback''), clusters form at a rate of 30% of GMC mass per free fall time; the GMCs were not disrupted but contained forming stars. (B) Including ionization gas pressure or radiation pressure into the simulations, both separately and together, the star formation was quenched at between 5% and 21% of the original GMC mass. The clouds were fully disrupted within two dynamical times after the first cluster formed. The radiation pressure contributed the most to the disruption of the GMC and fully quenched star formation even without ionization. (C) Simulations that included supernovae showed that they are not dynamically important to GMC disruption and have only minor effects on subsequent star formation. (D) The inclusion of a few micro Gauss magnetic field across the cloud slightly reduced the star formation rate but accelerated GMC disruption by reducing bubble shell disruption and leaking. These simulations show that new born stars quench further star formation and completely disrupt the parent GMC. The low star formation rate and the short lifetimes of GMCs shown here can explain the low star formation rate across the whole galaxy.

The lifetime of a Giant Molecular Cloud (GMC) and the total mass of stars that form within it are crucial to the understanding of star formation rates across a whole galaxy. In particular, the stars within a GMC may dictate its disruption and the quenching of further star formation. Indeed, observations show that the Milky Way contains GMCs with extensive expanding bubbles while the most massive stars are still alive. Simulating entire GMCs is challenging, due to the large variety of physics that needs to be included, and the computational power required to accurately simulate a GMC over tens of millions of years. Using the radiative-magneto-hydrodynamic code Enzo, I have run many simulations of GMCs. I obtain robust results for the fraction of gas converted into stars and the lifetimes of the GMCs: (A) In simulations with no stellar outputs (or ``feedback''), clusters form at a rate of 30% of GMC mass per free fall time; the GMCs were not disrupted but contained forming stars. (B) Including ionization gas pressure or radiation pressure into the simulations, both separately and together, the star formation was quenched at between 5% and 21% of the original GMC mass. The clouds were fully disrupted within two dynamical times after the first cluster formed. The radiation pressure contributed the most to the disruption of the GMC and fully quenched star formation even without ionization. (C) Simulations that included supernovae showed that they are not dynamically important to GMC disruption and have only minor effects on subsequent star formation. (D) The inclusion of a few micro Gauss magnetic field across the cloud slightly reduced the star formation rate but accelerated GMC disruption by reducing bubble shell disruption and leaking. These simulations show that new born stars quench further star formation and completely disrupt the parent GMC. The low star formation rate and the short lifetimes of GMCs shown here can explain the low star formation rate across the whole galaxy.