The multiphase interstellar medium: A study of various physical and kinematical properties

Abstract

Interstellar medium (ISM) is the relatively empty space in between the stars. It has different phases, like, neutral, molecular, and ionized. Neutral interstellar medium, which is pervaded all over the galaxy, is sometimes compressed by supernova explosions or feedback from the massive stars and forms molecular clouds. From these molecular clouds, eventually, star formation happens. These stars influence the ambient medium during their life cycle, and again from this medium, star formation may occur. In this way, all these phases are interre- lated in a circle. Thus, studying the different physical and kinematic properties of these gas components is essential for understanding the complex processes inside a galaxy. Principal constituent of the ISM is hydrogen. In addition to that, helium and other heavy metals are also present. These gas components have different physical and kinematic properties (e.g., kinetic temperatures, length scales, velocities, turbulence, frozen magnetic fields, etc.). Measurement of these properties is essential for better understanding the evolutionary processes of a galaxy. Thermal stability analysis shows that neutral ISM is bistable in nature. However, various numerical simulations, including turbulence in the ISM dynamic, show that the multiphase nature of ISM may exist. These different phases are characterized by the kinetic temperature of the gas components. Several methods exist in the literature, from which neutral medium’s kinetic temperature measurement is possible. Here we have studied its different physical and kinematical properties through H I 21 cm transition. We have first discussed various difficulties in the measurement of joint emission-absorption studies, mainly in the emission spectra. After that, we have given an iterative method to study all its physical and kinematical properties only through absorption spectra. From this iterative method, we have obtained all the gas clouds properties self-consistently. Results are broadly in agreement with the theoretical expectations. Next, we have discussed about the molecular cloud, where we have studied the magnetic field through Zeeman splitting measurement. Molecular clouds are believed to be formed due to the compression of neutral gas by supernova explosions or feedback from massive stars. These gas clouds then evolve and eventually form stars. Due to the turbulence and magnetic field, the evolutionary process is very complex. Suppose the magnetic field is high at the preliminary stage of the cloud evolution. In that case, it reduces through the ambipolar diffusion process. Likewise, if turbulence is high at the initial stage, it is dissipated at a smaller dissipative scale. Here, we have studed the Zeeman splitting measurement of CCS radical in the TMC-1C prestellar core. We have found that in the initial stage of the cloud, magnetic field was dynamically important, but later on, during the evolutionary process, magnetic flux is gradually reduced through the ambipolar diffusion process. This type of prestellar core eventually comes to a stage of gravitational collapse after reducing the opposing forces like turbulence and magnetic field. Thus, apart from the magnetic field, measurement of turbulence and the nature of gravitational infall is also essential. Here, we have measured the level of turbulence and modelled the infall velocity of the dense core nucleus. In addition to this core, we have also studied TMC-1 and L1544 cores with CCS radical and NH 3 molecule. We have compared the fluxes of these species with single-dish observations to understand the complex and clumpy structures of the cores. Finally, we have studied the H II region and the associated photodissociation region (PDR) of DR 21. Photodissociation region is believed as a nursery of star formation. Due to the feedback of the existing stars, PDR is compressed and frequently comes to a stage of gravitational collapse. Thus measurement of magnetic field and turbulence is essential in the PDR. H I , OH, C^{+} , etc., are the prime tracers of PDR. We have studied the magnetic field using the Zeeman splitting measurement of OH radical. We have found that even if the magnetic field is dynamically important in the cloud fragmentation process, it is not so much important here in this evolutionary stage of the PDR. Apart from the magnetic field, we have also studied the kinematic of these regions with additional [C II ] 158 μm and H 72α lines. We have found the spectral asymmetry in the OH absorption profile because it only traces the front side of the PDR. In summary, this thesis develops a formalism to consistently estimate thermal and non-thermal broadening from observed H I absorption spectra and unquestionably establish the presence of so-called unstable phase gas in the diffuse ISM and the fact that the turbulence is sub- or transonic for this phase; the thesis also reports one of the first magnetic field measurement from the thermal OH transition in PDR to show that the magnetic field is not dynamically very important. Further, the thesis presents the first detection of Zeeman splitting of the CCS transition using high-resolution interferometric data showing that ambipolar diffusion is a viable mechanism to aid the gravitational collapse. The thesis also demonstrates the possibility of learning the details of the gas-star-gas cycle of the galaxy through detailed modeling of common tracers like H I and OH, but also opens up the possibility of future deep observations of tracers like CCS for kinematics study, including magnetic field measurement, of dense molecular cloud cores.