Searching for the Ashes of Dead Stars

Abstract

Hydrogen, helium, and trace amounts of lithium were formed during the Big Bang, but most other elements originally formed deep inside stars by the process of nuclear fusion. They were later scattered across the cosmos, chiefly by supernova remnants (SNRs) - the ashes of dead stars. SNRs are formed when the material expelled in a supernova explosion interacts with its surrounding interstellar medium. The material recycled and distributed by SNRs subsequently formed most of the material we see in the present day, including in us humans. By injecting energy, momentum, and turbulence, SNRs provide 'feedback' to the process of star formation and affect the interstellar medium and the dynamics of the Galaxy in a manner that is not yet well comprehended. In order to gain a complete understanding of the nature around us, studies on a large and representative sample of SNRs are necessary. In the Milky Way, however, such a sample does not yet exist. Currently, the two catalogs of Galactic SNRs contain less than 300 confirmed SNRs, whereas statistical studies show that there should be at least 1000 SNRs. The work done for this thesis deals with attempting to correct this apparent deficiency. <br/> SNRs are brighter and hence more easily observed at radio frequencies below 1 GHz. A large-scale, low-frequency survey of the Milky Way is well suited to study and also discover SNRs. In order to understand the feasibility of conducting such a survey with the upgraded Giant Meterwave Radio Telescope (uGMRT), we carried out a 'pilot' study observing a small portion of the Galactic plane of the Milky Way. In the resulting images, we are able to recover diffuse emissions reasonably well. However, we found that a denser grid of pointings and better antenna performance, among other conditions, are necessary to achieve the target resolution and sensitivity. The recommendations from this study are being implemented for a full survey planned to be conducted in the near future. <br/> It is known that SNRs are relatively brighter in the radio regime but fainter in the mid-infrared wavelengths. Using this anti-correlation property, we searched for objects that could potentially be SNRs, which we call 'SNR candidates', in the Very Large Array images of the 4–8 GHz GLObal view on STAR formation (GLOSTAR) survey. We identified 157 candidates, of which 77 were previously detected in other studies and 80 are new detections. In addition, we found spurious entries in the catalog of Galactic SNRs; four H II regions were erroneously classified as SNRs. <br/> Confirmation of SNR candidates must be done by establishing the nonthermal nature of their radio emission, through measurements of linear polarization and spectral index. Using these properties, we showed that at least seven candidates are indeed SNRs. We justify our strategy of searching for small angular-size SNRs by conducting a Monte-Carlo simulation of the evolution of SNRs in the Milky Way, and advise future studies to continue with this strategy.