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On the recently released H-R diagram of data from Gaia DR2, there is a clump of hot but not very luminous stars around 30,000K and less than 10 solar luminosity. I can't find this clump on any other H-R diagram. What are these objects? My first thought is they may be neutron stars or another stellar remnant, but that doesn't seem likely.

Gaia H-R diagram

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  • $\begingroup$ Just a guess: maybe they're young O stars that are obscured by a dense gas cloud (e.g. in a star-forming region)? $\endgroup$ Commented Apr 26, 2018 at 19:52

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My best guess is that these are subdwarfs - well, really, O-type subdwarfs and B-type subdwarfs, which are distinct from the other subdwarf populations (which retain their hydrogen). They're largely composed of helium, with a thin shell of hydrogen at their surface. While the exact mechanism of their formation is unknown, it's likely that they've lost mass to a companion, donating much of their hydrogen envelopes and retaining helium and the carbon/oxygen/etc. in their cores. There are, however, other proposed formation mechanisms, including a late helium flash of sorts in a red giant or the merger of two white dwarfs, both with low amounts of hydrogen.

The HR diagram given in Heber (2009) shows these populations, to the far left (Figure 1). They fall to the left of the main sequence, hotter than many white dwarfs and main sequence O- and B- type stars. While these subdwarfs are hot, they are much less luminous than these main sequence stars because of their size (perhaps 0.1-0.2 solar radii).

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