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Distances and parallax bias in Gaia DR2

Schönrich, Ralph ; McMillan, Paul LU orcid and Eyer, Laurent (2019) In Monthly Notices of the Royal Astronomical Society 487(3). p.3568-3580
Abstract

We derive Bayesian distances for all stars in the radial velocity sample of Gaia DR2, and use the statistical method of Schönrich, Binney & Asplund 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 δp. On average, we find δp=-0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the Gaia set (compared to δp=-0.048 mas on the raw parallax errors), with... (More)

We derive Bayesian distances for all stars in the radial velocity sample of Gaia DR2, and use the statistical method of Schönrich, Binney & Asplund 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 δp. On average, we find δp=-0.054 mas (parallaxes in Gaia DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the Gaia set (compared to δp=-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. Lastly, we find that for this sample δp appears to strongly depend on σp (when including the additional 0.043 mas) with a statistical confidence far in excess of 10σ and a proportionality factor close to 1, though the dependence varies somewhat with σp. Correcting for the σp dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods nvis and ecliptic latitude. Every study using Gaia DR2 parallaxes/distances should investigate the sensitivity of its results on the parallax biases described here and-for fainter samples-in the DR2 astrometry paper.

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author
; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
astrometry, Galaxy: kinematics and dynamics, parallaxes, solar neighbourhood, stars: distances, kinematics and dynamics
in
Monthly Notices of the Royal Astronomical Society
volume
487
issue
3
pages
13 pages
publisher
Oxford University Press
external identifiers
  • scopus:85069507573
ISSN
0035-8711
DOI
10.1093/mnras/stz1451
language
English
LU publication?
yes
id
3ac5b7b5-63de-4e3d-a23d-f92bca09dc06
date added to LUP
2021-03-10 09:09:44
date last changed
2023-01-01 04:48:13
@article{3ac5b7b5-63de-4e3d-a23d-f92bca09dc06,
  abstract     = {{<p>We derive Bayesian distances for all stars in the radial velocity sample of <i>Gaia </i>DR2, and use the statistical method of Schönrich, Binney &amp; Asplund 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 <i>δ</i><sub>p</sub>. On average, we find <i>δ</i><sub>p</sub>=-0.054 mas (parallaxes in <i>Gaia </i>DR2 need to be increased) when accounting for the parallax uncertainty underestimate in the <i>Gaia </i>set (compared to <i>δ</i><sub>p</sub>=-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. Lastly, we find that for this sample<i> δ</i><sub>p</sub> appears to strongly depend on <i>σ</i><sub>p</sub> (when including the additional 0.043 mas) with a statistical confidence far in excess of 10<i>σ</i> and a proportionality factor close to 1, though the dependence varies somewhat with <i>σ</i><sub>p</sub>. Correcting for the <i>σ</i><sub>p</sub> dependence also resolves otherwise unexplained correlations of the offset with the number of observation periods <i>n</i><sub>vis</sub> and ecliptic latitude. Every study using <i>Gaia </i>DR2 parallaxes/distances should investigate the sensitivity of its results on the parallax biases described here and-for fainter samples-in the DR2 astrometry paper.</p>}},
  author       = {{Schönrich, Ralph and McMillan, Paul and Eyer, Laurent}},
  issn         = {{0035-8711}},
  keywords     = {{astrometry; Galaxy: kinematics and dynamics; parallaxes; solar neighbourhood; stars: distances, kinematics and dynamics}},
  language     = {{eng}},
  number       = {{3}},
  pages        = {{3568--3580}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{Distances and parallax bias in Gaia DR2}},
  url          = {{http://dx.doi.org/10.1093/mnras/stz1451}},
  doi          = {{10.1093/mnras/stz1451}},
  volume       = {{487}},
  year         = {{2019}},
}