Physical conditions in the circumstellar gas surrounding supernova 1987A
Abstract of thesis entitled
PHYSICAL CONDITIONS IN THE
CIRCUMSTELLAR GAS SURROUNDING
Woo Sui Chi
for the degree of Master of Philosophy
at The University of Hong Kong
in January 2005
Supernovae are stellar explosions. They come from massive stars which cannot support their gravitational core-collapse. Heavy elements are ejected and shock waves are produced from supernova explosions. The shock waves ignite the circumstellar gas lost by these massive stars before the explosion thereby making them luminous. Studying the radiation from the circumstellar medium can enhance our understanding of the stellar evolution.
Supernova (SN) 1987A is a Type II supernova. Because of its proximity and brightness, it provides an excellent opportunity to observe the birth and development of a supernova remnant. Its circumstellar matter has been observed as a three-ring system. The inner ring is believed to be formed by a slow, dense red giant wind swept up by a later, faster, blue supergiant wind. Spectroscopy and direct imaging provide the clues to the origin of the inner ring.
The physical conditions of the inner ring of SN 1987A were investigated. Spectral imaging of the SN 1987A circumstellar ring was obtained on December 1987 with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. In this project, the azimuthal flux profiles of the H, He, N, Ne, O, and S emission line images were examined, thus leading to line ratios at each position angle. The radial azimuthal profiles of each emission line were also adopted to examine the size and inclination of the inner ring. The photoionization code provided by P. Lundqvist was used to model the emission line fluxes from the inner ring around SN 1987A. The code follows the ionization, recombination and cooling of the gas with time. The emissivities of the observed lines were modelled by adjusting a variety of hydrogen densities and the ratios of elemental abundances of H, He, N, C, and O. The observed emission lines showed that high ionized lines were formed at low density regions whereas low ionized lines were formed at high density regions, which is consistent with the model. The observed flux profiles show a good fit of the hydrogen densities at ~ (2 - 7) x 10-3cm-3 with the elemental abundances of He/H~ 0.15, N/C~ 4.5 and N/O~ 2.5. Such high N/O ratio indicates the gas of the inner ring originates from an evolved star which has processed the CNO cycle.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:supernovae plasma ionized gases
Date of Publication:01/01/2005