Nebulosity of an O Star (Homework Question)

[Jade45]

1. Question
Suppose you examined the spectrum of some nebulosity surrounding a main-sequence spectral-type O star and found that it contained no emission lines, only the continuous spectrum of the star. What conclusions could you draw about the nature of the interstellar material around that star?

2. Relevant Information
Was suggested to look at
HI regions
HII regions
dust
and how these appear.

3. Attempt at Problem
I know O stars are usually hot and produce ionized hydrogen making it an HII region but that also makes strong emission lines from the hydrogen. So it's probably not an HII region.
So my next thought is the cloud isn't close enough to be ionized so it's neutral hydrogen and an HI region. Another thought was that it's still an HII region but a reflection nebula and this would keep the continuous spectrum. My biggest issue is that reflection nebulas are usually around cold stars so it may not work out for an O-type star.

I'm not sure if I'm missing a key component or if the answer is very obvious and I'm missing it.

 

[Ken G]

Ask yourself if there can be much gas in the vicinity of the O star. If there were, what would the O starlight do to that gas, and what would that look like in the spectrum? So it sounds like your analysis is on the right track-- there can't be any clouds very close to the star. But if you see nebulosity, it must be from a more distant background cloud that contains dust, so I think you are right that reflection nebula is what you want to think about.

By the way, I'm wondering if the questioner has overlooked the fact that O stars have strong winds, so the star makes its own nebulosity. Perhaps they mean you are looking in a region of the spectrum that does not contain wind lines, but could contain interstellar lines if there was an HII region there. I think they might have just forgotten about the stellar wind.