From link above.
> Hubble images of the same spot of sky, which contains around 240,000 stars, taken nine years apart.
I fully approve of taking images of the same part of the sky many years apart with the same space telescope – it happens so rarely.
> They then carefully analyzed the movement of the stars, picking out those in the bulge thanks to their slower speed. Having picked out the bulge stars, of which there were some 70,000, the team then set to work identifying the white dwarfs. They did so by studying the colors of the stars and comparing them to theoretical models that suggested that hot white dwarfs should appear with a blue tinge relative to younger stars. The team found 70 white dwarfs.
Using speed as a proxy for distance doesn’t always work. For instance, bright disk stars beyond the galactic bulge will pretend to be nearer fainter stars. But it should be good enough in this case. Stars this blue are either going to be white dwarfs, blue stragglers or blue supergiants. Luminosity should allow the astronomers to distinguish between the three types. Only 1 in 1000 were found to be white dwarfs, eh.
The paper itself, http://arxiv.org/pdf/1505.07128.pdf has a different science slant, as can be seen from the title “NEW INSIGHTS ON the Galactic bulge initial mass function”. The question here is “how big is a star?”. That’s not something that can be determined by simulation because the size and dynamics of the clouds of gas and dust that collapse to form stars can’t be predicted accurately. The Galactic bulge is an excellent place to look at the IMF because a) there are lots of stars there to look at (in this case 70,000), and b) they’re all roughly the same distance away so we can actually get a reliable luminosity.
“We found that the Galactic bulge IMF can be fitted with two power laws with a break at $M\sim $ 0.56 ${M}_{\odot }$, the slope being steeper ($\alpha =-2.41\pm 0.50$) for the higher masses, and shallower ($\alpha =-1.25\pm 0.20$) for the lower masses.”
Let me see how they separated out galactic bulge stars from disk stars.
“we considered stars with µ ≤ −2 mas yr −1 to belong to the bulge”
I hope that’s a misprint, because it directly contradicts their Figure 3. In Figure 3 it looks as if decent criteria are |µ| ≤ 0.2 mas yr −1 . That’s a difference of a factor of ten and a sign change.