How to find massive stars…with bubbles!

In the “about” page of the Milky Way Project you can read in the describtion of bubbles:
Bubbles are interesting places — they mark the locations where radiation from newly formed massive stars is impacting on the surrounding cloud material.

But how can you find those stars if one bubble has sometimes hundreds or thousands of stars? Usually you look for bright and blue stars, but you can only take a few spectra for the brightest stars.
I did look at B-Type stars in the last post for Reflection Nebula that are also bubbles. There I did see that the B-Type stars are often bright and blue, but also have a envelope of heated and shocked dust at 24 μm (red in the right image, brightest star in the left image):

B2V stock 16

Other B-Type stars don’t have heated dust around them or they have formed bowshocks, if they are runaway stars. So I invented this method, maybe some researchers did already invent a similar method, but I don’t know of any.

First I looked at known bubbles (Simpson+, 2012) and matched the position with known Spectra of stars (Skiff, 2009-2016). Now I could match the sources with MIPSGAL point sources (Gutermuth+, 2015). I did get a list of 18 known B-Type stars that have this envelope: known B-Type stars.

This did help me to create the method. I did need different values to make the list as short as possible:


Important is radius from the centre of a bubble, set to 0.6 arcmin. This way only sources are choosen that are possible origin of the bubbles. Next are the both magnitude limits, so I don’t get a endless list. Instead of K-mag you can also choose another band/wavelength in the MIPSGAL catalog. Important is only that there should be a star connected to the 24 μm source. The last important step is the Full Width Half Maximum (FWHM). For point sources this is also smaller than 7, so I choose this limit even if there are also some B-Type stars with smaller values. With this list I did again look for the MIPSGAL sources. The last step is to choose bright close NOMAD sources (Zacharias+ 2005). After the elimination of duplicates and looking for the spectral-Type of known sources I did get the final list: New candidate B-type stars.

As you can see there are only B-Type or OB-Type stars. No other spectral types appear in the list. Number 8 does have two MIPSGAL sources. I am very confident that this list does contain a lot of less bright B-Type stars.
O-Type stars need a different method. They have stronger stellar winds that can form larger and brighter envelopes or remove the dust close to them. Multiple O-Type stars can even heat the dust of an entire bubble, like in the Omega Nebula.

This method is in a very early stage and can be improved. For example the maximum distance form the bubble centre can be modeled after the size of the bubble. If the size of the bubble is 100%, the majority of the B-Type stars with dust evelopes are inside an area with the same shape as the bubble, but with maybe 40% of the size of the bubble, but some might be also close to the rim of a bubble. I did a mistake for the 24 μm magnitude and the other magnitudes can be choosen different, but there should be not too much restrictions, the bubble+MIPSGAL+NOMAD method is already one good method.
With Data Release 2 of the Milky Way Project this opens new possibilities for the search of massive stars, just like the new catalog of bowshocks.


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