Why do only Type 1a supernovae have Silicon absorption lines in their spectra?

[John Coffin]

I’ve acquired an Alpy 200 and thanks to the expert advice of Robin Leadbeater, I’ve been preparing to collect a spectrum of my first supernova. I’ve been learning about the physics of these events, and haven’t been able to find an explanation of why only the Type 1a supernova has absorption lines of Silicon in its spectrum. I know that Type 1a supernova occur when a carbon-oxygen White Dwarf gains mass near the Chandrasehkar limit so that electron degeneracy pressure can no longer counteract gravity and the star collapses with a runaway fusion reaction. Presumably the silicon comes from oxygen burning. But in the other types of supernovae due to core collapse, there are outer shells of silicon, so why don’t they show up in the spectra?

[Robin Leadbeater]

Very “hand waving” but as I understand it, it is because of the difference in the mechanism. With a core collapse type II supernova the core remains, compressed into a neutron star and the visible spectrum results from the heating of the outer layers of the star, mainly hydrogen, hence the strong Balmer lines. With a type Ia, the white dwarf is completely consumed by the thermonuclear explosion resulting from the increase in pressure just prior to the impending total collapse, converting all the material into heavier elements including silicon, the Si II (and Calcium) lines showing strongly in the visible part of the spectrum.

[Author]

Posts