The process of massive star formation greatly influences its surroundings through their outflows, vast UV output and shocks from their supernova death. They form at great distances from the Earth, enshrouded by dust and gas and have relatively short lifetimes. Astrophysical masers which form in these environments may act as locators of the star forming regions. The aim of this thesis is to study massive star formation using masers to probe these regions. The three main masers used in this thesis are the Class I and Class II methanol masers and the 6035 MHz ex-OH maser. The methanol masers are divided into two groups, Class I and Class II, based on their distance from a central source. The Class I masers are separated 1-2 pc from a central source, the central source is the star forming region. The Class II masers are associated close to a star forming source. They are often associated with a 6035 MHz ex-OH maser. The 6035 MHz ex-OH masers are less common than the 6668 MHz Class I methanol masers. They are often found at sites of the 6668 MHz Class I masers and 1665/7 MHz OH masers. This thesis presents two maser surveys, the Methanol Multibeam (MMB) survey and the Class I survey. The MMB survey is currently surveying the entire Galactic Plane for the 6668 MHz Class II methanol maser and the 6035 MHz ex-OH maser. Over 60\% of the survey in the Southern hemisphere is now complete using the Parkes telescope. Over 900 6668 MHz Class I methanol masers and 110 6035 MHz ex-OH masers have been detected, with all of these masers pinpoint the location of newly forming high mass stars. Follow up observations to determine the precise locations of the 6668 MHz methanol and 6035 MHz ex-OH masers are currently underway. The first ever unbiased Class I survey has observed 1 sq degree of the Galactic Plane for the 44 GHz Class I methanol masers using the Mopra telescope in Australia. The 44 GHz Class II methanol masers are hypothesised to be associated with the outflows of high mass stellar objects. The Class I survey has detected 25 44 GHz methanol masers, with 23 being new detections. A smaller survey for 36 GHz Class I masers was also conducted using the Mopra telescope centered on the region with the highest population of 44 GHz Class I masers.

We investigate the optical and Wide-field Survey Explorer (WISE) colors of "E+A" identified post-starburst galaxies, including a deep analysis of 190 post-starbursts detected in the 2 mu m All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone. Furthermore, we find that post-starbursts occupy a distinct region of [3.4]-[4.6] versus [4.6]-[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broadband photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that the mid-infrared (4-12 mu m) properties of post-starbursts are consistent with either 11.3 mu m polycyclic aromatic hydrocarbon emission, or thermally pulsating asymptotic giant branch (TP-AGB) and post-AGB stars. The composite SED of extended post-starburst galaxies with 22 mu m emission detected with signal-to-noise ratio >= 3 requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 mu m. The composite SED of WISE. 22 mu m non-detections (S/N < 3), created by stacking 22 mu m images, is also flat, requiring a hot dust component. The most likely source of the mid-infrared emission of these E+A galaxies is a buried active galactic nucleus (AGN). The inferred upper limits to the Eddington ratios of post-starbursts are 10(-2)-10(-4), with an average of 10(-3). This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections capable of identifying AGNs as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.

In this thesis we address the problem of understanding the star formation process in giant molecular clouds in a galactic context. Most simulations of molecular clouds to date use an oversimplified set of initial conditions (turbulent spheres/boxes or colliding flows). Full galactic scale models are able to generate molecular clouds with complex morphologies and velocity fields but they fail to reproduce in detail the effects that occur at sub-pc scales (e.g. stellar feedback). Our goal is to build the bridge between these two scenarios, and to model the star formation process in molecular clouds produced in a galactic context. We extract our molecular clouds from full-scale galactic simulations, hence we need to increase the resolution by two orders of magnitude. We introduce the details of the program used to simulate molecular clouds in Chapter 2, and describe in detail the method we follow to increase the resolution of the galactic clouds. In Chapter 3 we compare our simulated galactic clouds with the more conventional approach of using turbulent spheres. We create turbulent spheres to match the virial state of three galactic clouds. We perform isothermal simulations and find that the velocity field inherited from the full-scale galactic simulations plays an important role in the star formation process. Clouds affected by strong galactic shear produce less stars compared with clouds that are compressed. We define (and test) a set of parameters to characterise the dynamical state of our clouds. To include stellar feedback in our simulations we need to introduce a cooling/heating algorithm. In Chapter 4 we analyse how the different velocity fields of our clouds change the temperature distribution even in the absence of feedback. To study the formation of molecules we need to model the chemistry of H2 in our clouds. We also add CO chemistry, and produce synthetic observations of our clouds. The effect of feedback from winds and supernovae in galactic clouds is studied in Chapter 5. We analyse the effect of winds in clouds with very different velocity fields. We find that the effect of winds is stronger in highly virialised, high star forming clouds, with clouds with weak galactic shear, compared to unbound shear-dominated clouds. The steady and continuous action of the winds appears to have a greater effect than the supernovae. In summary, the inherited properties from the galaxy have an impact on many relevant processes in star formation, influencing gravitational collapse, the formation of filamentary structures, the temperature field of the cloud, and have a considerable effect on the impact of feedback in the clouds.

The formation of stars begins with the fragmentation of molecular clouds and the formation of dense cores. This fragmentation process can either be the result of classical gravitational instabilities or triggered by some external event. The gas and dust of young protoclusters often hold the imprints of the initial conditions and triggers of that specific star forming episode. In this context, my thesis work is a study of the gas properties of young protoclus- ters within the Gould Belt. The first part of my work consists of a detailed study of the young Serpens star forming region with CO isotopologues. This study has revealed a complex temperature, column density and velocity structure. I proposed a scenario where a collision between two filamentary clouds or flows is responsible for the observed complex structure and the most recent burst of star formation in Serpens. This hypothesis was tested with SPH simulations and provides a plausible scenario. I am currently extending this work to other regions with a variety of star formation efficiencies, in search of the particular physical properties and dynamics of a molecular cloud that allow or prevent clouds to be in the verge of forming stars. As such, I have included in this manuscript my study of the gas in the B59 star forming region, the only active clump in the Pipe Nebula. The results from this study have shown it to be very different from Serpens, even though further studies are needed to provide a complete picture of the region. B59 was taken as the starting point for a larger study of the entire Pipe Nebula, driven by the peculiarly low star formation efficiency in the cloud and a test to the physical properties of cores prior to star formation.

Through the study of accretion onto the young, low-mass stars known as T Tauri Stars (TTS), we can better understand the formation of our solar system. Gas is funneled along stellar magnetic field lines into magnetospheric accretion columns where it reaches free-fall velocities and shocks at the stellar surface, generating emission that carries information about the inner regions of the protoplanetary disk. Accretion is a variable process, with characteristic timescales ranging from minutes to years. In this dissertation, I use simulations, models, and observations to provide insight into the driving forces of mass accretion rate variability on timescales of minutes to weeks and the structure of the inner disk. Using hydrodynamic simulations, I find that steady-state, transonic accretion occurs naturally in the absence of any other source of variability. If the density in the inner disk varies smoothly in time with roughly day-long time-scales (e.g., due to turbulence), traveling shocks develop within the accretion column, which lead to rapid increases in the accretion luminosity followed by slower declines. I present the largest Hubble Space Telescope (HST) spectral variability study of TTS to date. I infer mass accretion rates and accretion column surface coverage using newly updated accretion shock models. I find typical changes in the mass accretion rate of order 10% and moderate changes in the surface coverage for most objects in the sample on week timescales. Individual peculiar epochs are further discussed. I find that the inner disk is inhomogeneous and that dust may survive near the magnetic truncation radius. Next, I link 2-minute cadence light curves from the Transiting Exoplanet Survey Satellite (TESS) to accretion using ground-based U-band photometry. Additional HST observations for one target enable more detailed connections between TESS light curves and accretion. I also use the TESS light curves to identify rotation periods and patterns of quasi-periodicity. Finally, I connect hydrodynamic simulations, accretion shock models, and stellar rotation to predict signatures of a turbulent inner disk. I generate light curves from these models to make comparisons to previous month-long photometric monitoring surveys of TTS using metrics of light curve symmetry and periodicity.

Philosophiae Doctor - PhD / Observations of galaxies at di↵erent wavelengths have shaped our understanding of their formation and evolution through time. The commonly derived parameters, such as stellar mass and star formation rate (SFR), rely on the assumption that the radiation received is exclusively generated by the stars within the galaxy. This assumption is true for pure star-forming (SF) galaxies, but not in the presence of an active galactic nucleus (AGN). AGNs are structures that also radiate in the full electromagnetic spectrum, inducing additional flux to that emitted by stars. Their small sizes in comparison to the host galaxy (⌧1 %) generally make them invisible in galaxy images. AGNs come in many variations making the most powerful (e.g., quasi-stellar objects) easily identifiable, whereas others with much weaker signatures can be hidden in the total emission from the host. Therefore it is imperative to find accurate methods to separate and study the properties of AGNs versus pure SF galaxies.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18065号 / 理博第3943号 / 新制||理||1568(附属図書館) / 30923 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 中村 卓史, 教授 鶴 剛, 教授 畑 浩之 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18790号 / 理博第4048号 / 新制||理||1582(附属図書館) / 31741 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 中村 卓史, 教授 鶴 剛, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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Four regions of massive star formation in the Large Magellanic Cloud (LMC) were observed for water and methanol maser emission and radio continuum emission. A total of 42 radio detections were made including 27 new radio sources, four water masers, and eight compact HII regions. The lobes of a radio galaxy were resolved for the first time, and the host galaxy identified. Seven sources were associated with known massive young stellar objects (YSOs). A multi-wavelength analysis using both the infrared and radio spectrum was used to characterize the sources. Mid-infrared color-magnitude selection criteria for ultracompact HII (UCHII) regions in the LMC are presented, yielding 136 UCHII region candidates throughout that galaxy. New maser detections identified two previously unknown massive YSOs. No methanol masers were detected, consistent with previous studies and supporting the hypothesis that the LMC may be deficient in these molecules. These discoveries contribute to the history of star formation in the LMC, which will lead to a better understanding of star formation in the Milky Way and throughout the universe.