Reflection Nebulae, Bubbles and B-Type Stars

A reflection nebula is a cloud of interstellar dust that scatters the light of one star or many stars. The light of a reflection nebula does have similar spectra like the light it scatters. Because the stars are often blue, many reflection nebulae are blue and they are different from emission nebula, which have different spectra.

Only stars that are bright enough can light the dust, so that we can see the reflection nebula. On the other hand, the star should not be too hot or it would ionize the dust, which would then be an emission nebula.

I thought that some bubbles could have the right conditions for reflection nebula. So I used the Merged catalogue of reflection nebulae (Magakian, 2003) and reduced the 913 nebulae to 125, which are located in the same area like the Milky Way Project Bubble catalogue DR1 to search for matching sources. 22 sources do have a good match.

The 22 reflection+bubble nebulae sometimes do have recorded B-Type stars in SIMBAD and one does have a G0-Type star in its centre. The reflection+bubble nebulae with B0- and B1-type stars are always also an emission nebula.

emission nebula
B0- and B1-type stars: left DSS2, middle IRAC, right IRAC+MIPS

Nebulae around B2- to B5-type stars are less good visible and all of them are listed as reflection nebulae but not as emission nebulae.

not emission nebula
B2- to B5-type stars: left DSS2, middle IRAC, right IRAC+MIPS

Five of the reflection+bubble nebulae are inverted-bubbles and two are even donut-bubbles. Inverted bubbles and donut-bubbles are thought to host lower spectral type of stars. This is the same condition for a reflection nebula. So it is not unusual to see some of those rare bubbles as reflection nebula.
Here are the two donut-bubbles:

donut-bubbles
Donut-bubbles: left DSS2, middle IRAC, right IRAC+MIPS

The higher spectral type stars in reflection+bubble nebulae do create sometimes a blue image of the infrared bubble. The most impressive reflection+bubble nebula is around the B2V-type star, called “CPD-61 3587”, which is located on the top of a pillar. The blue reflection nebula and the pillar in the background are almost the same in the infrared. The pillar is part of multiple pillars, pointing into the direction of the most massive star of the cluster Stock 16: HD 115455.

B2V stock 16
Reflection nebula around a B2V-type star, called CPD-61 3587

Only a few of the bubbles are reflection nebulae (about 0.4%). But a large fraction of the reflection nebulae are bubbles (about 17%). The possibility of a bubble being a reflection nebula is low, because the reflection nebula needs the right star or stars to form and the right amount of dust around it. Many stars just ionize the dust in the bubble or they are too weak to light the dust. Additionally there should be no dark clouds between the bubble and our telescopes, so we can see the blue reflection nebula, while infrared bubbles are also seen if there is a dark cloud in front of it.

The list of 22 reflection nebulae that are also bubbles can be downloaded here as a pdf-file:
Bubbles with reflection nebula nature

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