AVAO wrote: ↑Fri Mar 18, 2022 7:37 am
What is the reason why IC 447 does not also glow in red like Mon 15?
There are enough young, hot stars ...
No, there aren't. I believe that the hottest star in IC 447 is of spectral class B3. That is not enough to ionize an emission nebula.
By contrast, the Rosette Nebula contains at least three stars of spectral class O, and NGC 2264 contains O7-type star 15 Mon. That is more than enough to ionize a nebula.
Okay, edit. According the the otherwise extremely non-informative Simbad page on IC 447 (IC 2169)
this nebula is in fact an HII (ionized) region. That means there is
a red Hα component in the nebula. The reason why there is any Hα emission there at all, even though the hottest star is "only" a B3 star, could be that there are several B-type stars in the nebula. The combined amount of ultraviolet light from several B-type stars could be enough to ionize a nebula, even if none of the individual stars are capable of doing it on their own.
However, the red component in IC 447 is weak and completely overwhelmed by the blue reflection nebulosity.
Check out the picture of the Iris Nebula and its central star, B2V-type star HD 200775.
The central star of the Iris Nebula, HD 200775, is of spectral class B2 and is therefore hotter than any star in IC 447. You can see two small and faint pink regions very close to the star in Göran Nilsson's image. These regions glow pink from Hα emission. But as you can see, these pink patches are small and faint, and most pictures of the Iris Nebula do not, in fact, show them.
See the full resolution of Göran Nilsson's image here