I have found a general description of spectral line formation (for those lines found at optical and UV wavelengths) within SNRs in one of my “old standby” Reference Books that provide detailed explanations of diverse phenomena (see the last paragraph for its name and details). There follows a very general description, without any specifics about the detailed modeling and existing observations of the space distribution of the emission of various optical spectral lines in Supernova Remnants.
Here is an abstract of part of an encyclopedia entry entitled “SUPERNOVA REMNANTS, OBSERVED PROPERTIES” :
Visible & Ultraviolet wavelengths Emission from supernova remnants is largely from gas that has been excited and compressed by the expanding blast wave and by thermal instabilities. The
optically visible gas, usually visible as filamentary structures, is at near to 10,000 degrees Kelvin and a density of a few hundred atoms per cubic cm.
In contrast, the “not optically visible Gas”
surrounding the filaments is at a million degrees Kelvin or more and has a density of less than one atom per cubic centimeter; so this diffuse Very Hot gas emits in the X-ray regime, by the thermal bremsstrahlung mechanism. This Very Hot gas is heated by the passage of shock waves through supernova ejecta and through the surrounding interstellar gas.
[ denser regions of the SNR will tend to cool faster (by radiation?), and thereby become more compressed, in order to maintain pressure balance with the surrounding diffuse super-hot gas ]
The temperature and density of the optical filaments can be modeled and calculated by starting with the
observed ratios of the intensities of various observed spectral lines. For instance, the observed ratio of the intensities of the two [SII] lines (photons coming from singly ionized sulfur, at 671.7 nm and 673.1 nm) depends on the number of collisions between atoms, which depends on the density of the gas. Thus, the [SII] line ratio can be used to calculate gas density.
The H-alpha line (656.3 nanometers) is produced brightly in SNRs
(but this reference gives no comparison with H-alpha line formation in other types of nebulae, for instance Planetary nebulae and HII regions). (I am not sure how or if H-alpha line formation differs between various types of objects!)
In Supernova Remnants, the [OIII] line at 500.7 nanometers, from doubly ionized oxygen, is used as a diagnostic of shock waves, because shock waves provide the energy to remove the two electrons from the atom. The [OIII] line is very prominent in SNRs due to this ionization resulting from a strong shock wave.
In addition, emission from Supernova Remnants can be observed at visual and ultraviolet wavelengths; emanating from very highly ionized and excited gas, such as nine-times-ionized iron.
Much of the gas that is detected at visible wavelengths in SNRs is
not actually gas that originated in the supernova explosion itself (!), because most supernova remnants have swept up a large quantity of the surrounding interstellar gas. This fact makes it extremely difficult to use the observed optical spectrum of a SNR to figure out the quantity of heavy elements that the progenitor supernova is adding to the interstellar medium.
Most of the
optically visible Supernova Remnants have radiated away only a negligible fraction of their energy, so the gradual slowing down of their expansion with the passage of time is thought to be due to the accumulation of mass from the surrounding interstellar gas.
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In this post, I have abstracted information from:
“The Astronomy and Astrophysics Encyclopedia”(editor: Stephen P. Maran)(Published 1992)(ISBN: 0521417449)
This book is one of the essential references for people aspiring to obtain really detailed “physical” knowledge about the full range of astronomical phenomena, processes, and objects. The readership level is at the extreme upper end of “semi-popular”, so those mid-level amateur astronomers who are not used to a “densely technical” style will find this book to be heavy going. However, the long and very detailed entries avoid mathematics, except where absolutely necessary.
See my review of this book at
www.amazon.com, where I am called R.A. Lang
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