Gaia's White Dwarf catalog
Size: 73.221 White Dwarfs
It has been a while since I said I would start working on Gaia's DR2 implementation in Space Engine (and I haven't show anything until now). A lot of decissions are been made and we are still probing how well behaved is the astrometry and the photometry of this second data release to try generate realistic estimates for the spectral types of each source and how far we can assume the inverse of the parallax to be a good estimator of the actual distance.
A lot of valid information is coming out at this point and I don't want you to wait indefinitely so I've implemented Gaia's DR2 White Dwarf catalog in SE. This is the first Gaia DR2 population to be reduced to a catalog since White Dwarfs are easily identifiable in Hertzsprung-Russell diagrams as a detached region.
The recently published White Dwarf catalogue consists of 73.221 White Dwarfs that I've grouped, using Space Engine's classification scheme;
- 62.847 WD (Generic White Dwarfs)
- 7.921 DA (Hydrogen Balmer lines are dominant in the optical spectrum)
- 901 DB (Helium I lines are dominant in the optical spectrum)
- 827 DC (Highly continuous featureless spectrum)
- 304 DZ (Metals present in the atmosphere with atomic metal lines all over the optical spectrum)
- 286 DQ (Atomic and molecular Carbon lines are dominant in the optical spectrum)
- 66 DAZ (Hydrogen and Metal lines are visible in the optical spectrum)
- 34 DAB (Hydrogen and Helium I lines are both strong in the optical spectrum)
- 26 DBZ (Helium I and metal lines are visible in the optical spectrum)
- 5 DAO (Hydrogen and Helium II lines are both strong in the optical spectrum)
- 3 DX (Exotic spectrum with significant peculiarities)
- 1 DO (Helium II lines are dominant in the optical spectrum)
The temperatures used for Space Engine do not exactly coincide with those of the actual catalog because I randomized them using gaussians centered on the real measured values and standard deviation equal to the uncertainties in temperature. This was done to avoid unrealistic round values (instead of 7000 K it is more natural to use for example 7137 K, which is still consistent with the uncertainties).
How to install this addon?
Download and save the "White_Dwarfs_GaiaDR2.csv" file inside your SpaceEngine/addons/catalogs/stars folder (create the necessary folders if they aren't there).
The size of the file is just 7.54 MB. If you see some catalog error please report it in this thread so I can fix it.
Why this catalog is so important?
The astrometric limiting criteria used here is to have parallax errors below the 10% of the actual parallax values. In that way we can still assume the inverse of the parallax to be a good unbiased estimator of the distance. For larger errors several corrections will come before the third Gaia data release (DR3) which will account for Lutz-Kelker bias (and some artifacts in the data created by Gaia's scanning law). But for now 73.221 White Dwarfs with very accurate 3D positions are available in what is currently the largest White Dwarf catalogue ever made by far. As you can see in the video, the stars are concentrated in a spheroid of roughly 3.000 ly in diameter. Gaia can see a lot farther than that but the cut in 10% of relative error for the parallaxes and the fact that White Dwarfs have low luminosity (because of their small radius) limit the size of the sample to that volume of space.
Another important feat is that for the first 100 pc around the Solar System we have virtually reached completeness. This means that in the central ~650 ly diameter sphere we believe that those are all the White Dwarfs that actually exist in real life. If there is a White Dwarf inside this volume Gaia DR2 has probably detected it by now. For Space Engine this means that in future versions procedural generation for White Dwarfs in this region should be lowered to zero.
Another interesting thing is that we now have the most accurate density distribution value for White Dwarfs (that will tweek procedural generation parameters in the Milky Way); 4,9 White Dwarfs per cubic parsec (with a 0,4 uncertainty).
White Dwarfs can easily be separated from other stellar types by using accurate HR diagrams like the above. But for that we need good absolute magnitude values, which are calculated on apparent magnitude and distance. So it all depends on the accuracy of the parallax measurements in the end and that's why Gaia yields the best so far. There are probably many more White Dwarfs in DR2 waiting to be discovered but since their parallax measurements have higher than 10% errors it can be a tricky thing for now. In 2021 we will reach DR3 and with that many thousand more will be available.