Le milieu interstellaire ainsi que certaines atmosphères de corps planétaires, en particulier Titan, un des plus grands satellites du système solaire, présentent une grande diversité d'espèces chimiques. Cette chimie complexe est très différente de celle que nous connaissons sur Terre. Pour comprendre les phénomènes globaux qui s'y déroulent, une connaissance des réactions chimiques, de leur vitesse et de leurs produits est requise. A l'aide de la technique CRESU, nous sommes capables de reproduire certaines conditions des milieux les plus froids de l'espace et d'étudier ces réactions. / The interstellar medium and some atmospheres of planetary bodies, in particular Titan, one of the largest satellites of Saturn, present a large variety of chemical species. This complex chemistry is very different from the one we know on Earth. To understand the global phenomenon that happen in these environments, we need to understand the chemical reactions, their reaction rate and their products. With the CRESU technique, we are able to reproduce partially the coldest environments of space to study these reactions.

Understanding how diffuse molecular clouds at large scales (~10 pc) assemble mass into dense, star-forming cores at small scales (~ 0.1 pc) is crucial to building a holistic theory of star formation. While recent observations suggest that filaments play an important role in the mass assembly of dense cores, detailed gas kinematics studies are still lacking. My dissertation presents three innovative techniques that enable us to study star-forming filaments' complex gas kinematics in unprecedented detail: multi-component spectral fit, multi-dimensional filament identification, and membership assignment of velocity-coherent structures. Through these techniques, I analyzed star-forming filaments in the Perseus Molecular Cloud and unveiled unexpectedly complex velocity structures at scales where filaments are well resolved, to as low as the 0.01 pc scale. Moreover, the correlations I discovered between the various filament properties further suggest a scenario in which thermally supercritical filaments grow continuously via accretion from their surroundings while simultaneously forming cores through fragmentation along their lengths. / Graduate / 2022-01-08

<p>Stars form in clusters amidst complex and coupled physical phenomena. Among the most important of these is radiative feedback, which heats the surrounding gas to suppress the formation of many low-mass stars. In simulations of star formation, pre-main-sequence modeling has often been neglected and stars are assumed to have the radii and luminosities of zero-age main sequence stars. We challenge this approach by developing and integrating a one-zone protostellar evolution model for FLASH and using it to regulate the radiation output of forming stars. The impact of accurate pre-main-sequence models is less ionizing radiation and less heating during the early stages of star formation. For stars modeled in isolation, the effect of protostellar modeling resulted in ultracompact HII regions that formed slower than in the ZAMS case, but also responded to transitions in the star itself. The HII region was seen to collapse and subsequently be rebuilt as the star underwent a swelling of its radius in response to changes in stellar structure and nuclear burning. This is an important effect that has been missed in previous simulations. It implies that observed variations in HII regions may signal changes in the stars themselves, if these variation can be disentangled from other environmental effects seen in the chaotic cluster environment.</p> / Master of Science (MSc)

Interstellar extinction and reddening are inescapable sources of uncertainty in the study of astronomical objects. Many creative ways to measure its effects have been developed, two of them being evaluated in this study. I apply two recently improved methods to estimate extinction toward individual stars, one based on interstellar absorption of sodium and the other based on opacity maps of the interstellar medium. The methods are applied to 14 stars in the local neighborhood, all within a distance of 250 pc from the Sun, and their results are compared. I find that both methods have severe limitations and large uncertainties, but can still be useful under the right conditions. I also provide suggestions on how the methods can be improved. / Interstellär extinktion och rödfärgning är oundvikliga osäkerhetskällor i studiet av astronomiska objekt. Många kreativa sätt att mäta dess effekter har utvecklats, av vilka två utvärderas i denna studie. Jag tillämpar två nyligen förbättrade metoder för att uppskatta utsläckning mot enskilda stjärnor, en baserad på interstellär absorption av natrium och den andra baserad på opacitetskartor över det interstellära mediet. Metoderna tillämpas på 14 stjärnor i solens närområde, alla inom ett avstånd av 250 pc, och deras resultat jämförs. Jag finner att båda metoderna har allvarliga begränsningar och stora osäkerheter, men ändå kan vara användbara under rätt förutsättningar. Jag ger också förslag på hur metoderna kan förbättras.

We present distances and kinematics and look at the possible populations for the blue plume (BP) stars in the Canis Major Overdensity (CMa). We conducted a medium resolution spectral survey on the BP stars (N=303) in CMa (centered at l = 238° ; b = -8°) using the data from AAOmega Spectrograph. We used a modified version of the Statistics-sensitive Non-linear Iterative Peak-clipping (SNIP) algorithm to normalize our fluxed absorption spectra. After determining the radial velocities from measurements of strong absorption features for the stars we use a Bayesian analysis of spectral feature strengths and photometric colors to determine Teff, Logg and [Fe/H]. Our procedure makes use of grid for model synthetic spectra computed using SPECTRUM with Atlas9 model atmospheres and Kurucz model colors. We determine the absolute magnitude using the stellar parameters and BaSTI isochrones and compute distances and ages for the BP stars. Our analysis of the BP stars indicates Teff ranging from 6500K to 8000K, metallicity ranging from 0.0 to -1.0 with an average of -0.5. We found for this temperature range that the surface gravity of the stars could not be well constrained. From the spatial and kinematics results we found that most of the stars are thick disk stars with a small mixture of thin disk stars. The stars are most likely a mixture of thick disk blue stragglers and normal A-type stars preferentially seen to greater depths due to the low dust extinction in this location of the Galaxy.

This document consists of two very different projects but the common thread is in the interest of magnetic fields. It describes the effect of magnetic fields in two Interstellar Medium regions in the Galaxy. Electromagnetic force is one of the four fundamental forces in physics. It is not known where magnetic field has initially risen in the Universe, but what is certain is that it has significant effect in the dynamics of star formation and galaxy formation. The studies aim to better understand the effects of field in an active star forming region and in the halo of the Galaxy. We observed the HI 21 cm spectral line via the Zeeman effect in attempt to detect line-of-sight magnetic field strengths in both of the projects. For the star forming region project in Chapter 2, towards the Eagle Nebula, an upper limit of the field strength was determined. From the observational results, physical conditions of the region were modeled. For the second project in Chapter 3, we attempted to detect magnetic fields via Zeeman effect towards non galactic disk objects. All of the observed positions have radial velocities that cannot be explained by the simple galactic rotation. Hence, they are considered to be non galactic disk sources and often grouped as High Velocity Clouds. With a unique observational technique and analysis, we derived the best fit line-of-sight magnetic fields. A particular interest to us is the Smith Cloud. From the detection of magnetic field, we attempted to estimate the density of the ambient medium in the halo, which will be useful for studying the galaxy formation.

The dissertation focuses on the temperature dependent rate coefficient measurement of reactions in the interstellar medium using a coaxial molecular beam ring electrode ion trap apparatus. The first chapter introduces the previous studies of ion-molecule reactions in the ISM, the types of instruments mainly used in the reaction rate coefficient study, the former research on the ring electrode ion trap and the gas phase reaction mechanisms. Compare to other instruments, our molecular beam - ring electrode ion trap is extremely good at ion cooling and temperature control for both ions and neutral molecules. Chapter two describes each part of the instrument used in detail. Ions produced by electron impact in the ion source chamber, are mass filtered and then reach the ring electrode ion trap. In the trap, ions collide with molecules in the molecular beam where reaction takes place. When the reaction is done, all the ions remained in the trap (the reactant and product ions) come out and move to the detector. The molecular beam terminates at residual gas analyzer which is used for the number density calibration. The third chapter shows how the temperature of ions and molecules are controlled separately in order to find the reaction mechanism. Ions are cooled by the pulsed He buffer in the ring electrode trap and a chopped beam is used to make sure the ions are cooled to the desired low temperature when the reaction takes place. Chapters four to six describe the three reactions being studied using this instrument: N₂⁺ + H₂O charge transfer reaction, H₃O⁺ + C₂H₄ proton transfer reaction and H3O⁺ + (C₂H₂)₂/C₂H₂ dimer reaction. The temperature dependent rate coefficient data of these reactions are explained by the average dipole orientation theory, statistical theory and Colussi's acetylene dimer model, respectively. Two temperatures are defined and applied in the experimental rate coefficients analysis: ion-molecule center of mass collision temperature and the reaction statistical temperature which is based on the numbers of degrees of freedom of both reactants.

We present an analysis of the properties of HI holes detected in 20 galaxies that are part of “The HI Nearby Galaxy Survey” (THINGS). We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km/s, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the discs of the galaxies, out to the edge of the HI disc; 23% of the holes fall outside R25. We find that shear limits the age of holes in spirals; shear is less important in dwarf galaxies which explains why HI holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R25 are larger and older. We derive the scale height of the HI disc as a function of galactocentric radius and find that the disc flares at large radii in all galaxies. We proceed to derive the surface and volume porosity (Q2D and Q3D) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a=−2.9. Assuming that the holes are the result of massive star formation, we derive values for the supernova rate (SNR) and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that HI holes result from star formation. We use HI data from THINGS, 8μm, 24μm, 70μm and HI maps from SINGS, CO(2–1) data from HERACLES and FUV data from NGS to present a visual comparison of these maps with respect to the locations of HI holes. We find that the vast majority of HI holes are also prominent in the 8μm map and to some extent in the 24μm map. There is a lack of molecular gas from the interior of nearly all the holes, which is consistent with the idea that the latter are filled with hot gas. About 60% of young holes have FUV emission detected in their interiors highlighting the presence of the parent OB association. In addition, FUV is detected on the rims of some of the older HI holes, presumably due to the dispersion of the OB association with respect to the gas. We describe the development of a 2–D cross-correlation method to compare multi-wavelength maps in a quantitative way (quantified by Ccoef ) and give some first results from the application of this method to the nearby galaxy NGC2403. We find that the all the dust tracers are well correlated (Ccoef > 0.7) with the 8μm–24μm correlation being the highest (Ccoef > 0.88). Similarly all the star formation tracers are well linked as expected (Ccoef > 0.6). With respect to the relations between star formation and dust tracers we found that most are well matched (Ccoef > 0.7) as dust grains are heated by radiation in star forming regions. At smaller scales (15") FUV correlates poorly (Ccoef ~ 0.3) with the dust tracers, a direct consequence of the absorption of FUV photons by dust. We find that the HI is reasonably well correlated with the 8μm emission (Ccoef ~ 0.6) illustrating the fact that HI is mixed with PAH’s. Interestingly, the HI map shows some correlation with the SF map (Ccoef ~ 0.4) even though FUV and HI emissions were found to be completely uncorrelated (Ccoef ~ 0).

Basic stellar atmospheric parameters, such as effective temperature, surface gravity, and metallicity plays a vital role in the characterization of various stellar populations in the Milky Way. The Stellar parameters can be measured by adopting one or more observational techniques, such as spectroscopy, photometry, interferometry, etc. Finding new and innovative ways to combine these observational data to derive reliable stellar parameters and to use them to characterize some of the stellar populations in our galaxy is the main goal of this thesis. Our initial work, based on the spectroscopic and photometric data available in literature, had the objective of calibrating the stellar parameters from a range of available flux observations from far-UV to far-IR. Much effort has been made to estimate probability distributions of the stellar parameters using Bayesian inference, rather than point estimates. We applied these techniques to blue straggler stars (BSSs) in the galactic field, which are thought to be a product of mass transfer mechanism associated with binary stars. Using photometry available in SDSS and GALEX surveys we identified 85 stars with UV excess in their spectral energy distribution (SED) : indication of a hot white dwarf companion to BSS. To determine the parameter distributions (mass, temperature and age) of the WD companions, we developed algorithms that could fit binary model atmospheres to the observed SED. The WD mass distribution peaks at 0.4M , suggests the primary formation channel of field BSSs is Case-B mass transfer, i.e. when the donor star is in red giant phase of its evolution. Based on stellar evolutionary models, we estimate the lower limit of binary mass transfer efficiency β ~ 0.5. Next, we have focused on the Canis Major overdensity (CMO), a substructure located at low galactic latitude in the Milky Way, where the interstellar reddening (E(B-V )) due to dust is significantly high. In this study we estimated the reddening, metallicity distribution and kinematics of the CMO using a sample of red clump (RC) stars. The averageE(B-V)(~0.19)is consistent with that measured from Schlegel maps (Schlegal et.al. 1998). The overall metallicity and kinematic distribution is in agreement with the previous estimates of the disk stars. But the measured mean alpha element abundance is relatively larger with respect to the expected value for disk stars.

The study of Supernova Remnants (SNRs) is fundamental to understanding the chemical enrichment and magnetism in galaxies, including our own Milky Way. In an effort to understand the connection between the morphology of SNRs and the Galactic Magnetic Field (GMF), we have examined the radio images of all known SNRs in our Galaxy and compiled a large sample that have an axisymmetric morphology, which we define to mean SNRs with a bilateral or barrel-shaped morphology, in addition to one-sided shells. We selected the cleanest examples and model each of these at their appropriate Galactic position using two GMF models, one of which includes a vertical halo component, and another that is oriented entirely parallel to the plane. Since the magnitude and relative orientation of the magnetic field changes with distance from the Sun, we analyze a range of distances, from 0.5 to 10 kpc in each case. Using a physically motivated model of an SNR expanding into an ambient GMF that includes a vertical halo component, we find it is possible to reproduce observed morphologies of many SNRs in our sample. These results strongly support the presence of an off-plane, vertical component to the GMF, and the importance of the Galactic field on SNR morphology. Our approach also provides a potentially new method for determining distances to SNRs, or conversely, distances to features in the large-scale GMF if SNR distances are known. The mechanism for acceleration of cosmic rays in SNRs is another outstanding question in the field. To investigate this, the same sample of axisymmetric SNRs was again modelled, but this time using two competing, and physically motivated, Cosmic Ray Electron (CRE) acceleration cases: quasi-perpendicular and quasi-parallel. We find that the quasi-perpendicular CRE acceleration case is much more consistent with the data than the quasi-parallel CRE acceleration case, with G327.6+14.6 (SN1006) being a notable exception. We propose that SN1006 may be an example of a case where both quasi-parallel and quasi-perpendicular acceleration is simultaneously at play in a single SNR. / October 2016