Radio and Infrared Studies of Supernova Remnants Interacting with Molecular Clouds

Abstract

Supernova (SN) explosion is one of the most energetic events in a galaxy, which injects enormous energy (10^51 erg) into the interstellar medium and supplies heavy elements and possibly dust grains. Because massive stars are formed by the gravitational collapse of molecular clouds (MCs), there is a good possibility that a supernova remnant (SNR) interacting with its parental MC. The evolution and physical characteristics of its remnant, i.e., the SNRs, are strongly governed by the environment. Also, the SNR shock wave propagating into the MC compresses, accelerates, heats, and even destroys the cloud. It can also change the chemical evolution of the cloud, and possibly can trigger star formation. In this thesis, I have carried out radio and infrared studies to understand the characteristics of the interaction between SNRs and MCs.

In the first part, I present the results of 12CO J = 1??0 line observations of eleven Galactic SNRs obtained using the Seoul Radio Astronomy Observatory (SRAO) 6-m radio telescope. The observation is made as a part of the SRAO CO survey of SNRs between l = 70 and 190 degree, which is intended to identify SNRs interacting with MCs. The mapping areas for the individual SNRs are determined to cover their full extent in the radio continuum. I use half-beam grid spacing (60'') for 9 SNRs and full-beam grid spacing (120'') for the rest. I detect CO emission towards most of the remnants. In six SNRs, MCs showed a good spatial relation with their radio morphology, although no direct evidence for the interaction was detected. Two SNRs are particularly interesting: G85.4+0.7, where there is a lamentary molecular cloud along the radio shell, and 3C434.1, where a large molecular cloud appears to block the western half of the remnant.

In the second part, I present a detailed follow-up study of the semicircular Galactic SNR 3C434.1 (G94.0+1.0). I observe an area covering the whole SNR in the 12CO J = 1-0 emission line using the SRAO 6-m telescope, and find a large molecular cloud superposed on the faint western part of the SNR. The cloud is elongated along the north-south direction and shows a very good spatial correlation with the radio features of the SNR. I carry out 12CO J = 2-1 line observations of this cloud using the K olner Observatorium f ur Sub-Millimeter Astronomie (KOSMA) 3-m telescope and detect a region in which the 12CO J = 2-1 to J = 1-0 ratio was high (1.6). This higher excitation together with the morphological relation, strongly suggest that the molecular cloud have interacted with the SNR. Based on the systematic velocity of the molecular cloud (??-13 km/s), I estimate a kinematic distance of 3.0 kpc to the SNR-molecular cloud system and derive the physical parameters of the SNR. I examine the variation of the radio spectral index over the remnant and find that it is flatter in the western part, wherein the SNR is interacting with the molecular cloud. I therefore propose that 3C434.1 is the remnant of a supernova explosion that occur just outside the boundary of a relatively thin, sheet-like molecular cloud. I present a hydrodynamic model showing that its asymmetric radio morphology can result from its interaction with this blocking molecular cloud.

In the third part, I present far-infrared (far-IR) studies of Galactic SNRs. The new high-resolution data from AKARI FIS (version 2.0) and Herschel PACS/SPIRE reveals detailed far-IR features that we have not seen before. Comparing the AKARI and Herschel images with the radio continuum and X-ray images, I have found that 59 out of 294 known SNRs have far-IR counterpart by visual inspection method. Most far-IR detected SNRs show shell, clump, or filament structures. I derive the far-IR fluxes of the detected SNRs and carry out statistical studies of the far-IR SNRs.