In this thesis, we focus on obtaining and interpreting observational information on (i) the role of multiplicity on the properties of young stars; (ii) the early evolutionary influence of circumstellar disks; and (iii) the nature of accretion in young systems. To facilitate this research, we conducted an extensive multi-epoch high-resolution spectroscopic survey at optical wavelengths (3,200-10,000 A) of ~200 T Tauri stars in the ~2 Myr old Chamaeleon I, and Taurus-Auriga star-forming regions with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan Clay 6.5 m telescope. From the spectroscopic data, we identify eight close binaries and four close triples, of which three and two, respectively, are new discoveries. We find that the multiplicity fraction for Cha I and Tau-Aur are similar to each other, and to the results of field star surveys. The frequency of systems with close companions in our sample is not seen to depend on primary mass or accretion. We probed for evidence of disk braking. We did not see a statistically significant difference between the distribution of rotational velocities with the presence of an inner disk. Also, our findings show that F-K stars in our sample have larger rotational velocities and specific angular momentum than M stars. We also analyzed accretion variability in our sample using the H\alpha 10% width and the CaII-8662 line flux as accretion diagnostics. We find that the maximum extent of accretion variability in our sample was reached on timescale of a few days, indicating that rotation could significantly contribute to the variability.

We demonstrate the application of CMB techniques to measure the clustering of infrared emitting star-forming galaxies. We detect correlations in the cosmic far-infrared background due to the clustering of star-forming galaxies in observations made with the Balloon-borne Large Aperture Submillimeter Telescope, BLAST, at 250, 350, and 500μm. We perform jackknife and other tests to confirm the reality of the signal. The measured correlations are well fit by a power law over scales of 5–25 arcminutes, with ∆I/I = 15.1 ± 1.7%. We adopt a specific model for submillimeter sources in which the contribution to clustering comes from sources in the redshift ranges 1.3≤z≤2.2, 1.5≤z≤2.7,and1.7≤z≤3.2,at 250, 350 and 500 μm, respectively. With these distributions, our measurement of the power spectrum, P(kθ), corresponds to linear bias parameters, b = 3.8±0.6,3.9±0.6 and 4.4±0.7, respectively. We further interpret the results in terms of the halo model, and find that at the smaller scales, the simplest halo model fails to fit our results. One way to improve the fit is to increase the radius at which dark matter halos are artificially truncated in the model, which is equivalent to having some star-forming galaxies at z ≥ 1 located in the outskirts of groups and clusters. In the context of this model we find a minimum halo mass required to host a galaxy is log(Mmin/M⊙) = 11.5+0.4, and we derive effective biases beff = 2.2 ± 0.2, 2.4 ± 0.2, −0.1 and 2.6 ± 0.2, and effective masses log(Meff/M⊙) = 12.9 ± 0.3, 12.8 ± 0.2, and 12.7 ± 0.2 , at 250, 350 and 500 μm, corresponding to spatial correlation lengths of r0 = 4.9, 5.0, and 5.2 ±0.7 h−1 Mpc, respectively. Finally, we discuss implications for clustering measurement strategies with Herschel and Planck.

This thesis encompasses the study of the mid-infrared (IR) dust properties in diffuse high latitude cirrus and in the dense environments of supernova remnants (SNRs) in the plane of our Galaxy. Unlike the well known emission properties of dust grains in the diffuse ISM in the far-IR and submillimeter, the mid-IR spectrum is still relatively unconstrained. We extend the correlation of dust emission with H I column densities to mid-IR wavelengths and look for evidence of variations in the emissivity of dust associated with local and halo gas. This is accomplished by spatially correlating the IR maps from the IRIS/IRAS survey at 12, 25, 60 and 100 μm with H I column density maps inferred from 21-cm line emission observations obtained with the GBT (at a 9′ resolution). We find that IVCs (halo clouds thought to be part of the Galactic fountain) show color ratios consistent with a dust evolution scenario in which large dust grains are shattered into smaller ones (VSGs). The low 12 μm emission found suggests a reduced abundance of PAHs in IVCs. We also address the IR extragalactic emission seen in our residual maps and quantify its power spectrum behaviour. Continuing with the mid-IR theme, we conducted a comprehensive study of the morphology and energetics of SNRs in the plane of our Galaxy. We make use of the Spitzer MIPSGAL (at 24 and 70 μm) and GLIMPSE (at 8 μm) surveys to detected infrared counterparts to SNR candidates in Green’s catalog. We find that a third of the sample shows IR emission and calculate the corresponding fluxes. We explore the relation between IR colors to place constraints on the different IR SNRs emission mechanisms. Aided by archival radio data, we find that most candidates detected show IR-to-radio ratios consistent with SNRs with a few exceptions displaying ratios seen in H II regions. Finally, we explore the connection between the IR and the high-energy X-ray emission of SNRs and find a good morphological association between the 24 μm emission and the X-ray features in younger remnants. The IR power is often greater.

This thesis encompasses the study of the mid-infrared (IR) dust properties in diffuse high latitude cirrus and in the dense environments of supernova remnants (SNRs) in the plane of our Galaxy. Unlike the well known emission properties of dust grains in the diffuse ISM in the far-IR and submillimeter, the mid-IR spectrum is still relatively unconstrained. We extend the correlation of dust emission with H I column densities to mid-IR wavelengths and look for evidence of variations in the emissivity of dust associated with local and halo gas. This is accomplished by spatially correlating the IR maps from the IRIS/IRAS survey at 12, 25, 60 and 100 μm with H I column density maps inferred from 21-cm line emission observations obtained with the GBT (at a 9′ resolution). We find that IVCs (halo clouds thought to be part of the Galactic fountain) show color ratios consistent with a dust evolution scenario in which large dust grains are shattered into smaller ones (VSGs). The low 12 μm emission found suggests a reduced abundance of PAHs in IVCs. We also address the IR extragalactic emission seen in our residual maps and quantify its power spectrum behaviour. Continuing with the mid-IR theme, we conducted a comprehensive study of the morphology and energetics of SNRs in the plane of our Galaxy. We make use of the Spitzer MIPSGAL (at 24 and 70 μm) and GLIMPSE (at 8 μm) surveys to detected infrared counterparts to SNR candidates in Green’s catalog. We find that a third of the sample shows IR emission and calculate the corresponding fluxes. We explore the relation between IR colors to place constraints on the different IR SNRs emission mechanisms. Aided by archival radio data, we find that most candidates detected show IR-to-radio ratios consistent with SNRs with a few exceptions displaying ratios seen in H II regions. Finally, we explore the connection between the IR and the high-energy X-ray emission of SNRs and find a good morphological association between the 24 μm emission and the X-ray features in younger remnants. The IR power is often greater.

A study of clouds and variability in brown dwarf atmospheres is presented, with a focus on understanding the mechanism by which condensate clouds are dissipated at the transition between L and T spectral types. The results of a large J-band variability survey of over 60 mid-L to T dwarfs, and related observations are presented. We find statistically significant evidence (at the 95% confidence level) for an increase in large amplitude variability (peak to peak variations larger than 2%) within the L/T transition (L9-T3.5 spectral types), suggesting that the disruption of dust clouds by weather phenomena may contribute to the rapid decline in condensate opacity and J-band brightening observed to occur in this regime. Alternatively, the large amplitude variability observed could be the result of increasing contrast between discrete cloud features and the underlying atmosphere, rather than the development of cloud holes. We highlight the discovery of a T dwarf with 26% variability, and provide a detailed comparison with atmosphere models in order to infer the nature of cloud features responsible. Results are consistent with both the presence of thick storm features or cloud holes in our target's atmosphere, but are inconsistent with the presence of magnetic spots. Supporting work related to clouds in L dwarf atmospheres, and the L/T transition binary fraction is presented. From studying an unusually blue L-dwarf companion to a nearby M-dwarf star we infer that thin or patchy condensate clouds, rather than low metallicity and/or high surface gravity, are most likely responsible for its peculiar colors. The discovery of significant variability for a different blue L dwarf in our large variability survey provides further evidence for this hypothesis. Finally, combining our own high contrast imaging observations for 8 L/T transition targets with previous results we infer a resolved L/T transition binary fraction of 14(+7-6)%, consistent with that inferred for the general brown dwarf population. This latter result suggests that the L/T transition sample is not highly contaminated by unresolved multiples.

In this thesis we investigate the two cosmic epochs of inflation and recombination, through their imprints on the temperature and polarization anisotropies of the cosmic microwave background radiation. To probe the early universe we develop a map-based maximum-likelihood estimator to measure the amplitude of inflation-induced gravity waves, parametrized by $r$, from the cosmic microwave background (CMB) polarization maps. Being optimal by construction, the estimator avoids $E$-$B$ mixing, a possible source of contamination in the tiny $B$-mode detection, the target of many current and near future CMB experiments. For various observational cases, we probe the dependence of $r$ measurement on the signal from different scales of $E$ and $B$ polarization. We make forecasts for Spider-like and Planck-like experimental specifications and to investigate the sky-coverage optimization of the Spider-like case. We also use a novel information-based framework to compare how different generations of CMB experiments reveal information about the early universe, through their measurements of $r$. We also probe the epoch of recombination by investigating possible fluctuations in the free electron fraction $\Xe$ around the fiducial model of the standard recombination scenario. Though theoretically well studied, the detailed assumptions in the recombination history, based on standard atomic physics, have never been directly tested. However, for our CMB-based cosmological inferences to be reliable, the recombination scenario needs to be observationally verified. We approach this problem in a model-independent way and construct rank-ordered parameter eigen-modes with the highest power to probe $\Xe$. We develop an information-based criterion to truncate the eigen-mode hierarchy, which can be used in similar hierarchical model selections as well. The method is applied to simulations of Planck+ACTPol and a cosmic variance limited survey with differing simulated recombination histories and recovered $\Xe$ trajectories are constructed. We apply the method to currently available CMB datasets, WMAP9+ACT/SPT. The first constructed eigen-mode turns out to be a direct measure of the damping envelope. Its current measurement with SPT slightly indicates a damping tail anomaly, while ACT data agree well with the standard scenario. High resolution Planck data will resolve this tension with high significance.

The explosion of massive stars involves the formation of a shock wave. In stars that develop iron cores, this shock wave stalls on its way out due to neutrino emission and the breakup of heavy nuclei flowing through the shock. For the explosion to succeed, a fraction of the gravitational binding energy of the collapsed core that is radiated in neutrinos needs to be absorbed by the material below the shock. How much energy is needed depends on the interplay between non-spherical hydrodynamic instabilities, neutrino heating, and nuclear dissociation. This thesis seeks to understand this interplay through numerical experiments that model the key physical components of the system and separate them out to examine their individual effects. Specifically, one- and two-dimensional time-dependent hydrodynamic simulations are performed to study the effects of non-spherical shock oscillations, neutrino-driven convection, and alpha particle recombination on the dynamics of the system and the critical heating rate for explosion. We find that nuclear dissociation has a significant effect on the linear stability and saturation amplitude of shock oscillations. At the critical neutrino heating rate for an explosion, convection due to a negative entropy gradient plays a major role in driving dipolar shock motions. One dimensional explosions are due to a global instability involving the advection of entropy perturbations from the shock to the region where the accretion flow cools due to neutrino emission. Large scale shock expansions in two-dimensions are due to a finite amplitude instability involving the balance between buoyancy forces and the ram pressure of the flow upstream of the shock. During these expansions, a significant amount of energy is released when nucleons recombine into alpha particles, constituting a significant last step in the transition to explosion. The critical neutrino heating rate for an explosion depends sensitively on the starting radius of the shock relative to the radius at which the binding energy of an alpha particle is comparable to the gravitational binding energy.

Our universe is inherently inhomogeneous yet it is common in the study of cosmology to model our universe after the homogeneous and isotropic Friedmann-Lemaıtre-Roberson-Walker (FLRW) model. In this thesis spacetime matchings are applied to investigate more general inhomogeneous cosmologies. The Cheese Slice universe, constructed from matching together FLRW and Kasner regions satisfying the Darmois matching conditions, is used as a prime example of an inhomogeneous cosmology. Some observational consequences of this model are presented. The lookback time verses redshift relation is calculated using a numerical algorithm and it is shown that the relative thickness of the Kasner regions have the greatest impact on anisotropies an observer would see. The number of layers and distribution of layers play a smaller role in this regard. The relative thickness of the Kasner slice should be on the order of one ten thousandth the thickness of the FLRW regions to have the anisotropies fall within the observed CMB limit. The approach to the singularity of a spacetime matching is examined. A criterion is presented for a matched spacetime to be considered Asymptotically Velocity Term Dominated (AVTD). Both sides of the matching must be AVTD and each leaf of the respective foliations mush match as well. It is demonstrated that the open and flat Cheese Slice universe are both AVTD and the singularity is also of AVTD type. The Cheese Slice model is then examined as a braneworld construction. The possibility of a Cheese Slice brane satisfying all the energy conditions is shown. However, the embedding of such a brane into a symmetric bulk is non-trivial. The general embedding of a matched spacetime into a bulk is investigated using a Taylor series approximation of the bulk. It is found that the energy-momentum tensor of such a brane cannot have discrete jumps if the embedding does not have a corner. A 3+1+1 decomposition of the bulk spacetime is then carried out. With the spacetime being deconstructed along two preferred timelike hypersurfaces, this becomes a natural environment to discuss the matching of branes. We find that there are conditions on the matter content of the branes to be matched if an observer on the brane is to see the matching surface as a boundary surface with no additional stress energy. Matching more than two bulks is also examined and shown to allow for more general brane configurations.

In this thesis we investigate the two cosmic epochs of inflation and recombination, through their imprints on the temperature and polarization anisotropies of the cosmic microwave background radiation. To probe the early universe we develop a map-based maximum-likelihood estimator to measure the amplitude of inflation-induced gravity waves, parametrized by $r$, from the cosmic microwave background (CMB) polarization maps. Being optimal by construction, the estimator avoids $E$-$B$ mixing, a possible source of contamination in the tiny $B$-mode detection, the target of many current and near future CMB experiments. For various observational cases, we probe the dependence of $r$ measurement on the signal from different scales of $E$ and $B$ polarization. We make forecasts for Spider-like and Planck-like experimental specifications and to investigate the sky-coverage optimization of the Spider-like case. We also use a novel information-based framework to compare how different generations of CMB experiments reveal information about the early universe, through their measurements of $r$. We also probe the epoch of recombination by investigating possible fluctuations in the free electron fraction $\Xe$ around the fiducial model of the standard recombination scenario. Though theoretically well studied, the detailed assumptions in the recombination history, based on standard atomic physics, have never been directly tested. However, for our CMB-based cosmological inferences to be reliable, the recombination scenario needs to be observationally verified. We approach this problem in a model-independent way and construct rank-ordered parameter eigen-modes with the highest power to probe $\Xe$. We develop an information-based criterion to truncate the eigen-mode hierarchy, which can be used in similar hierarchical model selections as well. The method is applied to simulations of Planck+ACTPol and a cosmic variance limited survey with differing simulated recombination histories and recovered $\Xe$ trajectories are constructed. We apply the method to currently available CMB datasets, WMAP9+ACT/SPT. The first constructed eigen-mode turns out to be a direct measure of the damping envelope. Its current measurement with SPT slightly indicates a damping tail anomaly, while ACT data agree well with the standard scenario. High resolution Planck data will resolve this tension with high significance.

A study of clouds and variability in brown dwarf atmospheres is presented, with a focus on understanding the mechanism by which condensate clouds are dissipated at the transition between L and T spectral types. The results of a large J-band variability survey of over 60 mid-L to T dwarfs, and related observations are presented. We find statistically significant evidence (at the 95% confidence level) for an increase in large amplitude variability (peak to peak variations larger than 2%) within the L/T transition (L9-T3.5 spectral types), suggesting that the disruption of dust clouds by weather phenomena may contribute to the rapid decline in condensate opacity and J-band brightening observed to occur in this regime. Alternatively, the large amplitude variability observed could be the result of increasing contrast between discrete cloud features and the underlying atmosphere, rather than the development of cloud holes. We highlight the discovery of a T dwarf with 26% variability, and provide a detailed comparison with atmosphere models in order to infer the nature of cloud features responsible. Results are consistent with both the presence of thick storm features or cloud holes in our target's atmosphere, but are inconsistent with the presence of magnetic spots. Supporting work related to clouds in L dwarf atmospheres, and the L/T transition binary fraction is presented. From studying an unusually blue L-dwarf companion to a nearby M-dwarf star we infer that thin or patchy condensate clouds, rather than low metallicity and/or high surface gravity, are most likely responsible for its peculiar colors. The discovery of significant variability for a different blue L dwarf in our large variability survey provides further evidence for this hypothesis. Finally, combining our own high contrast imaging observations for 8 L/T transition targets with previous results we infer a resolved L/T transition binary fraction of 14(+7-6)%, consistent with that inferred for the general brown dwarf population. This latter result suggests that the L/T transition sample is not highly contaminated by unresolved multiples.