I had a closer look at the continuum in Buil's spectrum of NGC604 I posted above.
ngc604_20130810_098_cbuil_continuum.png
Although weak and noisy, it is effectively flat and certainly not the thermal continuum of a hot star so it does appear that the continuum is indeed produced by non thermal processes within the nebula. I wonder why is appears confined to the inner part of the nebula ? (Distance from the ionising source, temperature, density?)
Cheers
Robin
That's a very interesting spectrum. I assume it has been corrected for instrumental response, so the fact that the continuum looks even more weak and noisy in the extreme blue and extreme red truly reflects some underlying wavelength dependence.
I have a hunch that I would love if anyone could comment on:
A) more absorption in the blue might be due to the very general trend of a transition probability being inversely related to the wavelength. Shorter wavelength transitions tend to have stronger absorptions.
B) more absorption in the red might be due to transitions from very high quantum states. The classic example of this are radio recombination lines, which have very high transition probabilities but I suspect that some of the reasons for these high probabilities (see chatGPT for good explanation) might generally apply even when the transitions begin at high quantum states. These high quantum states are not normal in stars but could be found in ionizing environments like or near HII regions. I tried to ask ChatGPT to calculate the Einstein A coefficients for the n=200 to n=3 transition. I got a range of values, but when I asked to carefully consider the large dipole moments of the higher quantum states, I generally got relatively higher Einstein A values compared to transitions from lower quantum states.
C) based on A, when I asked ChatGPT to calculate and compare the Einstein A coefficients for n=200 to n=3 vs n=200 to n=2 the latter had higher values. So perhaps the higher quantum states explains greater absorption in the blue and red (near the Balmer and Paschen limits respectively)
The continuum that we see is a result of photons that evade the increased chance of absorption by the above two mechanisms.
Edited by Organic Astrochemist, 20 January 2025 - 04:43 PM.