Investigation of the evolution of dark clouds

Abstract

Stars form in the densest regions of molecular clouds which are actually gravitationally unstable cores that resides at the junction of filaments. Before the onset of the gravitational collapse in cores, the build up of filaments and further substructures takes place. The formation of dense cores and the filaments is a crucial step in the star formation process. The question involving how the diffuse gas transforms to dense regions decides the star formation rate and the mass distribution of stars in molecular clouds. The association of cores with the filaments which turn into cores indicates that the filaments play a central role in the process of forming stars. However, even with a plethora of observational and theoretical information available, the role of turbulence, self-gravity and magnetic fields in the formation of filamentary structures is still a debated topic. Understanding the connection of the molecular cloud structure with its formation mechanism is crucial in knowing the intricacies of star formation process. In this thesis, we investigated the characteristics, dynamics, and the global evolution of filamentary structures formed in two different scenarios either by isolated star formation or triggered star formation. In order to characterise observationally the process of filament and dense core formation with different physical effects in molecular clouds, we studied two regions in detail, L1172/L1174 and L1157/L1158. These regions are located in Cepheus constellation at higher galactic latitude. The striking difference between the ongoing star formation activity in both the regions makes them the favourable testbeds for studying the filaments formation in isolated or clustered sites. We studied each of these regions by means of large-scale molecular line observations in the range of millimeter or sub-millimetre wavelengths by choosing tracers sensitive to different density structures in clouds. The results were complimented with the dust continuum emissions. We first extracted the filaments and cores using and estimated their geometric properties and further estimated the detailed velocity structure of all the filaments and cores. The large scale magnetic field morphology was inferred either from ground based optical polarization or from the Planck polarization maps. One part of the regions studied, L1172/L1174 resembles the hub-filament morphology. The presence of intermediate mass star can significantly influence the surrounding gas by interaction of stellar winds with the neutral molecular cloud. Our detailed characterization of the gas kinematics of filaments and cores around massive star showed the presence of high velocity gas is affecting the high density gas. The gas show spectral signatures of expansion or contraction through blue or red-skewed profiles. We further explored the origin behind the hub-filament morphology of L1172/L1174 complex by studying the physical conditions of high density gas and its comparison with the dust emission in the long filament associated to the complex. Our results reveal the presence of velocity gradients which may be responsible for the accretion of gas towards the high massive core. Based on the correlation of magnetic field lines with the filament orientation, the same is found to be dynamically important for the morphology of the filament. To understand the physical state of gas in the filaments and the cores in L1147/L1158 complex which is in close proximity to L1172/L1174 and share similar radial velocity and distances but low star formation efficiency, we studied the molecular line mapping of the whole region at 0.1 km s−1 spectral resolution. The velocity structure of each of the filament was investigated in detail. The kinematical information on both the complexes will help in understanding the difference in star formation activity and testing various theories of turbulent magnetized medium. The large scale gas dynamics in both the complexes suggest that the filaments indeed preserve the initial condition of star formation. The next part of the thesis explores the importance of motion of the cloud with the respect the magnetic fields. We also studied the distribution of core orientation over a sample of molecular clouds which builds up a link between cloud-scale magnetic field and core orientation. We established the distribution of offsets between core orientation and motion of the complexes in the galaxy. We also studied a set of nearby clouds with asymmetric dust emission using magnetic fields using starlight polarization and the thermal dust emission using Planck and the relative orientation of cores with the magnetic fields.