|
I am currently "having a really fun time" doing personal study of line formation in HII regions, planetary nebulae, and SN remnants; so I may make a detailed comment about your very interesting and exciting project, when I can find the time.
The Crab Nebula is very different from older Supernova Remnants, in that much of the radiant energy from the Crab nebula is actually energized by the central spinning neutron star (the central pulsar).
For older SN remnants, the radiated electromagnetic radiation comes from:
(1) Shock Heating: X-rays are emitted via the thermal bremsstrahlung mechanism : compression of the gas, by shock wave, raises its temperature to 10E6 -10E7 (1 million to 10 million) degrees celsius.
(( Note: The gas in SN remnants is not all from the supernova itself: The emitting gas is a mixture of:
- ejecta from the SN itself
- a part of the surrounding ambient interstellar medium, which has been swept up by the shock wave
- circumstellar gas which was already there before the supernova explosion, as it was emitted by the supernova progenitor in its red giant phase of evolution.
))
(2) Synchrotron emission: caused by relativistic (near light speed) electrons spiralling through a magnetic field: SNR emission from the synchrotron process occurs at a very broad range of wavelengths, but it is especially prominent in the radio regime (most SN remnants are too obscured at visual wavelengths to make it worthwile to survey for them using ordinary telescopes; use a radio telescope for SN Remnant surveys!). The gas surrounding the supernova has some magnetic field lines locked into it, and this magnetic field is boosted, because it is squeezed and stretched by the compression process of the expanding shock wave. Relativistic electrons spiral through the enhanced magnetic field, causing Strong synchrotron emission.
(3) Collisional excitation: Free electrons (and protons) collide with atoms of heavy elements (e.g. Oxygen), bumping them into a higher energy state. The blast wave of a SNR leads to a high rate of collisions between the atoms in the gas, and this leads to an enhanced rate of collisional excitations.
This type of excitation leads to the radiation of several prominent emission lines in the optical and UV regimes.
A good example of collisional excitation in SNRs is the [O III] doublet which is emitted in two different lines at wavelengths of 4959 and 5007 Angstroms. The Oxygen atoms are floating around (as a relatively small impurity in the gas!), and an oxygen atom can be hit by one of the many free electrons that exist in the ionized gas, in which case the atom picks up some of the kinetic energy of the electron. The excited Oxygen ion can decay radiatively to two (out of its three existing) lowest energy levels, thereby releasing an emission-line photon.
The symbol OIII actually refers (somewhat confusingly) to an Oxygen atom which is an ion that has lost two electrons, and which therefore has two units of positive charge. A much clearer and better Symbol to represent this ion is to use the letter O with a superscript of 2+ , because this symbol clearly shows that two negative charges have been lost by the atom. These two distinct [O III] spectral lines are emitted by the transition of one of these Oxygen ions from an excited energy level to its fine-split ground level (as can be appreciated by drawing a diagram of a simple Bohr Model of an Oxygen atom nucleus and its surrounding electrons that are able to move between different energy levels)
(Another emission line that is emitted by the OIII ion is at 4363 angstroms, This comes from a different transition between electronic energy levels.)
______________________
It gets a lot more complicated than this, as a popular model for SN remnants has two shock waves;
the (as expected) expanding shock wave, but also another shock wave caused by the ejecta's impact with the surrounding gas.....and this second shock wave is an inward travelling shock wave!!
It seems counterintuitive to have two shock waves travelling in the opposite direction to each other, within a single object, but if you know anything about abstruse physical theory, you are moved to repeat the words of Obi Wan Kenobe :
"These are the truths that we must cling to, Luke"
|