The circumgalactic medium (CGM) is thought to contain the massive reservoir of gas exchanged over the course of galactic evolution, including the fuel for future star formation and the remnants of a galaxy’s merger history. Models and observations suggest that the CGM has a very low density, and faint optical or UV emission from this gas is exceedingly difficult to detect. This thesis is a combination of simulations, instrumentation and observations aimed at ultimately understanding the distribution and kinematics of ionized gas in the CGM. We present a suite of small-box hydrodynamic simulations created to study the interaction between smooth gas inflow and supernovae-driven outflow at the disk-halo interface where the galactic disk transitions into the CGM. They track the fate and kinematic evolution of gas accreting onto the galactic disk and find evidence of partial mixing with the enriched outflow. We use equilibrium photoionization models to create mock surface brightness maps of Ha and OVI emission. These observables motivate the need for new instrumentation and in suit, we present the newly commissioned Circumgalactic Ha Spectrograph (CHaS): a custom integral field unit (IFU) spectrograph tailored to detect low-surface brightness optical emission in the low-redshift universe. CHaS is deployed in the focal plane of the MDM Observatory Hiltner 2.4-meter telescope, conducting wide-field (10' x 10') spectral imaging with a competitive survey speed proportional to the high instrument grasp. A microlens array segments the field of view into > 60,000 spectra with a spatial resolution of 2.6'' and a resolving power of R ~ 10,000. Accordingly, CHaS is capable of resolving structure on scales less than 1 kpc (at 10 Mpc) and distinguishing emission lines separated by less than 40 km/s. As designed, a 50-100h exposure with CHaS would be the deepest H-alpha image and velocity field ever obtained, reaching a surface brightness of a few mR on scales of a few arcmin. Shorter, hour-long integrations with CHaS reveal a detailed map of the denser interstellar medium and bright emission at the disk-halo interface. We present results for three early commissioning targets: NGC 4631, NGC 7331 and NGC 1068, including high-resolution velocity maps and detections of new extended emission line regions far into the halo. We report a previously unnoted ribbon of ionized gas around NGC 1068, extending tens of kpc from the galactic disk beyond the known outer filamentary structure. Ongoing observations will provide a deeper probe of ionized gas far into the CGM of many nearby galaxy targets, detecting faint extended emission and mapping the velocity of ionized gas beyond the disk.

The astrophysical origins of the rapid neutron-capture process (r-process), which gives rise to roughly half of the elements heavier than iron, has remained a mystery for almost 70 years. The likely violent events, which seed the r-process abundances in our solar system and galaxy, remain uncertain to this day. This is in part due to nuclear physics uncertainties associated with the r-process itself, but mainly due to uncertainties in astrophysics modeling. The discovery of the radioactively-powered kilonova emission from the neutron star merger event GW170817 confirmed the violent deaths of neutron stars as one key site of the r-process in the universe. However, other evidence appears to favor an additional r-process channel that more promptly follows star formation in the universe, such as core-collapse supernovae (CCSNe), i.e. the brilliant births of neutron stars. The two viable sites for the r-process are (1) core-collapse supernovae (CCSNe), which are explosions of massive stars at the end of their lives and (2) compact object mergers, which are violent collisions of stellar remnants formed at the endpoints of stellar evolution. Chapters 2 and 3 of this dissertation present general relativistic magnetohydrodynamic simulations of one potential r-process site associated with CCSNe: the neutrino-driven wind. These outflows are launched from the hot proto-neutron star (PNS) remnant by neutrino-heating above their surfaces, within seconds after the collapse of a massive star. However, previous work has shown that spherically symmetric winds from non-rotating PNS fail to achieve the requisite conditions for a robust r-process. Chapter 2 explores for the first time the combined effects of rapid rotation and strong gravity of the PNS on the wind properties. Chapter 3 explores the impact of a dynamically strong ordered magnetic field on the properties of non-rotating PNS winds. The wind in both cases is simulated in a controlled environment rather than as a part of a self-consistent global CCSNe simulation, to assess the viability of r-process nucleosynthesis as a function of PNS properties (neutrino energies/luminosities, rotation rate, magnetization). We find that rapid rotation allows for outflows that are ~10% more neutron-rich in the equatorial region, where the mass loss rate is roughly an order of magnitude higher than that of otherwise equivalent non-rotating models. The birth of very rapidly spinning neutron stars may thus be a site for the production of light r-process nuclei (38 < Z < 47). For PNSs with sufficiently strong magnetic fields (such that magnetic pressure exceeds gas pressure above the PNS surface), we find that equatorial outflows are trapped by the magnetic field in a region near the surface, and therefore receive additional neutrino heating relative to a freely-expanding unmagnetized wind. This allows a modest fraction of the wind material to achieves entropies high enough to synthesize 2nd peak r-process elements via an alpha-rich freeze-out mechanism. The final chapter explores the interplay between the r-process and the dynamics of compact object merger ejecta. Gravitational wave observatories are expected to detect several additional binary neutron star (BNS) and black hole-neutron star (BHNS) mergers in current and future observing runs, some of which may be accompanied by electromagnetic counterparts such as kilonovae. However, distinguishing more distant BNS from BHNS mergers based on their associated gamma-ray bursts (GRB), has proven tricky. This chapter presents a calculation of the effects of r-process heating on the dynamics of tidal ejecta from BNS and BHNS mergers. In particular we explore whether late-time fall-back of weakly bound debris created during the merger to the central black hole remnant, can explain the temporally extended X-ray emission observed following several merger GRB on timescales of several seconds to minutes. As a result of the different impact that r-process heating has depending on the composition of the ejecta and the mass of the black hole, a method to differentiate BHNS from BNS mergers, based on their extended X-ray emission, is proposed.

In the last few decades, the emergence of large-scale infrared surveys has led to a revolution in the study of star formation. In particular, NASA’s Spitzer Space Telescope has recently carried out mid- and far-infrared observations of numerous star formation regions with unprecedented resolution and sensitivity, and has uncovered thousands of forming stars. In combination with present and future large-scale near-infrared and sub-mm surveys, spectral energy distributions from near-infrared to mm wavelengths will be available for these thousands of young stars. Never before has there been such a wealth of multi-wavelength data for so many young stars. Traditional techniques for studying the physical properties of young stars through their spectral energy distributions have usually focused either on the analysis of many sources using simple observational diagnostics such as colours or spectral indices, or on the analysis of a few sources through the detailed modelling of their full spectral energy distributions. The work presented in the first part of this thesis aims to bridge these two techniques through the efficient modelling of the spectral energy distributions of many young stars. In particular, the technique developed for this work makes it straightforward to find out how well different physical parameters are constrained, whether any parameters are degenerate, and whether additional data would resolve the degeneracies. In the second part of this thesis, a census of intrinsically red sources observed by Spitzer in the Galactic plane is presented, including a catalogue of over 11,000 likely young stellar objects. This sample of sources is the largest uniformly selected sample of young stars to date, and effectively provides a map of the sites of star formation in the mid-plane of the Milky-Way. In parallel, this census has uncovered over 7,000 candidate asymptotic giant branch stars, of which over 1,000 are variable at 4.5 or 8.0 microns.

Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The similar to 3.5mag brightening in the V band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1mag for 6. days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from similar to 10,000K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km s(-1). The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitation of H-2 is detected in emission from vibrational levels as high as v = 11, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, though the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.

In this thesis a critical assessment of the ages derived using theoretical pre-main-sequence (pre-MS) stellar evolutionary models is presented by comparing the predictions to the low-mass pre-MS population of 14 young star-forming regions (SFRs) in colour-magnitude diagrams (CMDs). Deriving pre-MS ages requires precise distances and estimates of the reddening. Therefore, the main-sequence (MS) members of the SFRs have been used to derive a self-consistent set of statistically robust ages, distances and reddenings with associated uncertainties using a maximum-likelihood fitting statistic and MS evolutionary models. A photometric method (known as the Q-method) for de-reddening individual stars in regions where the extinction is spatially variable has been updated and is presented. The effects of both the model dependency and the SFR composition on these derived parameters are also discussed. The problem of calibrating photometric observations of red pre-MS stars is examined and it is shown that using observations of MS stars to transform the data into a standard photometric system can introduce significant errors in the position of the pre-MS locus in CMD space. Hence, it is crucial that precise photometric studies (especially of pre- MS objects) be carried out in the natural photometric system of the observations. This therefore requires a robust model of the system responses for the instrument used, and thus the calculated responses for the Wide-Field Camera on the Isaac Newton Telescope are presented. These system responses have been tested using standard star observations and have been shown to be a good representation of the photometric system. A benchmark test for the pre-MS evolutionary models is performed by comparing them to a set of well-calibrated CMDs of the Pleiades in the wavelength regime 0.4−2.5 μm. The masses predicted by these models are also tested against dynamical masses using a sample of MS binaries by calculating the system magnitude in a given photometric band- pass. This analysis shows that for Teff ≤ 4000 K the models systematically overestimate the flux by a factor of 2 at 0.5 μm, though this decreases with wavelength, becoming negligible at 2.2 μm. Thus before the pre-MS models are used to derive ages, a recalibration of the models is performed by incorporating an empirical colour-Teff relation and bolometric corrections based on the Ks-band luminosity of Pleiades members, with theoretical corrections for the dependence on the surface gravity (log g). The recalibrated pre-MS model isochrones are used to derive ages from the pre-MS populations of the SFRs. These ages are then compared with the MS derivations, thus providing a powerful diagnostic tool with which to discriminate between the different pre- MS age scales that arise from a much stronger model dependency in the pre-MS regime. The revised ages assigned to each of the 14 SFRs are up to a factor two older than previous derivations, a result with wide-ranging implications, including that circumstellar discs survive longer and that the average Class II lifetime is greater than currently believed.

The kinematics and dynamics of young stellar populations enable us to test theories of star formation. With this aim, we continue our analysis of the SDSS-III/APOGEE IN-SYNC survey, a high-resolution near-infrared spectroscopic survey of young clusters. We focus on the Orion A star-forming region, for which IN-SYNC obtained spectra of similar to 2700 stars. In Paper IV we used these data to study the young stellar population. Here we study the kinematic properties through radial velocities (vr). The young stellar population remains kinematically associated with the molecular gas, following a similar to 10 km s(-1) gradient along the filament. However, near the center of the region, the vr distribution is slightly blueshifted and asymmetric; we suggest that this population, which is older, is slightly in the foreground. We find evidence for kinematic subclustering, detecting statistically significant groupings of colocated stars with coherent motions. These are mostly in the lower-density regions of the cloud, while the ONC radial velocities are smoothly distributed, consistent with it being an older, more dynamically evolved cluster. The velocity dispersion sigma(v) varies along the filament. The ONC appears virialized, or just slightly supervirial, consistent with an old dynamical age. Here there is also some evidence for ongoing expansion, from a v(r)-extinction correlation. In the southern filament, sigma(v) is similar to 2-3 times larger than virial in the L1641N region, where we infer a superposition along the line of sight of stellar subpopulations, detached from the gas. In contrast, sv decreases toward L1641S, where the population is again in agreement with a virial state.

No description available.

Stars form via the gravitational collapse of molecular clouds during which time the protostellar object contracts by over seven orders of magnitude. If all the angular momentum present in the natal cloud was conserved during collapse, stars would approach rotational velocities rapid enough to tear themselves apart within just a few Myr. In contrast to this, observations of pre-main sequence rotation rates are relatively slow (∼ 1 − 15 days) indicating that significant quantities of angular momentum must be removed from the star. I use observations of fully convective pre-main sequence stars in two well-studied, nearby regions of star formation (namely the Orion Nebula Cluster and Taurus-Auriga) to determine the removal rate of stellar angular momentum. I find the accretion disc-hosting stars to be rotating at a slower rate and contain less specific angular momentum than the disc-less stars. I interpret this as indicating a period of accretion disc-regulated angular momentum evolution followed by near-constant rotational evolution following disc dispersal. Furthermore, assuming that the age spread inferred from the Hertzsprung-Russell diagram constructed for the star forming region is real, I find that the removal rate of angular momentum during the accretion-disc hosting phase to be more rapid than that expected from simple disc-locking theory whereby contraction occurs at a fixed rotation period. This indicates a more efficient process of angular momentum removal must operate, most likely in the form of an accretion-driven stellar wind or outflow emanating from the star-disc interaction. The initial circumstellar envelope that surrounds a protostellar object during the earliest stages of star formation is rotationally flattened into a disc as the star contracts. An effective viscosity, present within the disc, enables the disc to evolve: mass accretes inwards through the disc and onto the star while momentum migrates outwards, forcing the outer regions of the disc to expand. I used spatially resolved submillimetre detections of the dust and gas components of protoplanetary discs, gathered from the literature, to measure the radial extent of discs around low-mass pre-main sequence stars of ∼ 1−10 Myr and probe their viscous evolution. I find no clear observational evidence for the radial expansion of the dust component. However, I find tentative evidence for the expansion ofthe gas component. This suggests that the evolution of the gas and dust components of protoplanetary discs are likely governed by different astrophysical processes. Observations of jets and outflows emanating from protostars and pre-main sequence stars highlight that it may also be possible to remove angular momentum from the circumstellar material. Using the sample of spatially resolved protoplanetary discs, I find no evidence for angular momentum removal during disc evolution. I also use the spatially resolved debris discs from the Submillimetre Common-User Bolometer Array-2 Observations of Nearby Stars survey to constrain the amount of angular momentum retained within planetary systems. This sample is compared to the protoplanetary disc angular momenta and to the angular momentum contained within pre-stellar cores. I find that significant quantities of angular momentum must be removed during disc formation and disc dispersal. This likely occurs via magnetic braking during the formation of the disc, via the launching of a disc or photo-evaporative wind, and/or via ejection of planetary material following dynamical interactions.

Ionising radiation is present in a variety of astrophysical problems, and it is particularly important for shaping the process of star formation in molecular clouds, containing hot, high-mass stars. In order to account for the effects of ionising radiation within numerical models of star formation, we need to combine a hydrodynamics method with a radiative transfer method and obtain a radiation hydrodynamics scheme (RHD). In this thesis I achieve live radiation hydrodynamics by coupling the Smoothed Particle Hydrodynamics (SPH) code Phantom with the Monte Carlo Radiative Transfer (MCRT) code CMacIonize. Since SPH is particle-based and MCRT is grid-based, I construct an unstructured, Voronoi grid in order to establish a link between the two codes. In areas with large density gradients, a Voronoi grid based purely on the SPH particle positions achieves insufficient resolution, and therefore I propose a novel algorithm for inserting a small number of additional grid cells to improve the local resolution. Furthermore, the MCRT calculations require the knowledge of an average density for each Voronoi cell. To address this, I develop an analytic density mapping from SPH to a Voronoi grid, by deriving an expression for the integrals of a series of kernel functions over the volume of a random polyhedron. Finally, I demonstrate the validity of the live RHD through the benchmark test of D-type expansion of an H II region, where good agreement is shown with the existing literature. The RHD implementation is then used to perform a proof-of-concept simulation of a collapsing cloud, which produces high-mass stars and is subsequently partially ionised by them. The presented code is a valuable tool for future star formation studies, and it can be used for modelling a broad range of additional astronomical problems involving ionising radiation and hydrodynamics.

We present an analysis of observations using the Spitzer Space Telescope and the James Clerk Maxwell Telescope of deeply embedded protostars in the Perseus Giant Molecular Cloud. Building on the results of Jørgensen et al. (2007), we attempt to characterize the physical properties of these deeply embedded protostars, discovered due to their extremely red near infrared colours and their proximity to protostellar cores detected at 850 μm. Using a grid of radiative transfer models by Robitaille et al. (2006), we fit the observed fluxes of each source, and build statistical descriptions of the best fits. We also use simple one dimensional analytic approximations to the protostars in order to determine the physical size and mass of the protostellar envelope, and use these 1D models to provide a goodness-of-fit criterion when considering the model grid fits to the Perseus sources. We find that it is possible to create red [3.6]-[4.5] and [8.0]-[24] colours by inflating the inner envelope radius, as well as by observing embedded protostars through the bipolar outflows. The majority of the deeply embedded protostars, however, are well fit by models seen at intermediate inclinations, with outflow cavity opening angles < 30o, and scattering of photons off of the cavity walls produces the red colours. We also discuss other results of the SED fitting.