Distances and parallax bias in Gaia DR2. (arXiv:1902.02355v1 [astro-ph.GA])

<a href="http://arxiv.org/find/astro-ph/1/au:+Schonrich_R/0/1/0/all/0/1">Ralph Schönrich</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+McMillan_P/0/1/0/all/0/1">Paul McMillan</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eyer_L/0/1/0/all/0/1">Laurent Eyer</a>

We derive Bayesian distances for all stars in the RV sample of Gaia DR2, and

use the statistical method of Schoenrich, Binney & Asplund(2012) to validate

the distances and test the Gaia parallaxes. In contrast to other methods, which

rely on special sources, our method directly tests the distances to all stars

in our sample. We find clear evidence for a near-linear trend of distance bias

f with distance s, proving a parallax offset delta p. On average, we find delta

p = 0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for

the parallax uncertainty under-estimate in the Gaia set (delta p = 0.048 mas on

the raw parallax errors) with negligible formal error and a systematic

uncertainty of about 0.006 mas. The value is in concordance with results from

asteroseismic measurements, but differs from the much lower bias found on

quasar samples. We further use our method to compile a comprehensive set of

quality cuts in colour, apparent magnitude, and astrometric parameters. Last,

we find that for this sample delta p appears to strongly depend on the parallax

uncertainty sigma_p (when including the additional 0.043 mas) with a

statistical confidence far in excess of 10sigma and a proportionality factor

close to 1, though the dependence varies somewhat with sigma_p. Correcting for

the sigma_p dependence also resolves otherwise unexplained correlations of the

offset with the number of observation periods n_{vis} and ecliptic latitude.

We derive Bayesian distances for all stars in the RV sample of Gaia DR2, and

use the statistical method of Schoenrich, Binney & Asplund(2012) to validate

the distances and test the Gaia parallaxes. In contrast to other methods, which

rely on special sources, our method directly tests the distances to all stars

in our sample. We find clear evidence for a near-linear trend of distance bias

f with distance s, proving a parallax offset delta p. On average, we find delta

p = 0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for

the parallax uncertainty under-estimate in the Gaia set (delta p = 0.048 mas on

the raw parallax errors) with negligible formal error and a systematic

uncertainty of about 0.006 mas. The value is in concordance with results from

asteroseismic measurements, but differs from the much lower bias found on

quasar samples. We further use our method to compile a comprehensive set of

quality cuts in colour, apparent magnitude, and astrometric parameters. Last,

we find that for this sample delta p appears to strongly depend on the parallax

uncertainty sigma_p (when including the additional 0.043 mas) with a

statistical confidence far in excess of 10sigma and a proportionality factor

close to 1, though the dependence varies somewhat with sigma_p. Correcting for

the sigma_p dependence also resolves otherwise unexplained correlations of the

offset with the number of observation periods n_{vis} and ecliptic latitude.

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