Verifying Gaia Astrometric Results of Stars
with VLBI AstrometryShuangjing Xu(SHAO)

The12th East Asian VLBI Workshop @Ibaraki University
2019.9.26
1•Gaia: a giant leapfor astronomy!

Gaia DR2
Based on observationsof 22 months>1.3 billion sources:
positions, parallaxes
and proper motions>0.5 million quasars:
Gaia-CRF (CelestialReference Frame)••21, Systematic errors in Gaia DR2••Parallax zero-point

Spin of Gaia-CRF at bright end

Stars G < 13HIPPARCOS-29 μasQSOs without Galactic plane
Basic Angle Variations
ALong-scan centroiding errors

QSOs G > 15
ICRF(Lindegren et al. 2018)3

1, Systematic errors in Gaia DR2•Formal uncertainty•Extinction

4(Lindegren et al. IAU 2018)

2,VLBI for Parallax zero-point
Parallax: Gaia DR2 VS VLBI
Sample: > 100 individual stars
Median uncertainty:
Gaia DR2(123 μas) VLBI (74 μas)••

10Binary• Disagreement:

ϖGaia (mas)NO

AGB stars, Binaries

YES

TypeAGB
Other Star

5PulsarYSO• The Parallax zero-point−75±29 μas (with only 34/108 stars)
005
ϖVLBI (mas)10(Xu et al. 2019)
5AGB stars: angular diameters, magnitude, and color.
•This shows that redder AGB stars give larger parallax uncertainties;
possible reasons include(1)they tend to be larger,(2)they probably have more surface brightness variations, and(3)they will likely have more circumstellar dust to corrupt their images.
One should be cautious when using the Gaia parallaxes for AGB stars.

(Xu et al. 2019)

VLBI Maser astrometry of AGB /Long Period Variables (Nakagawa et al2016; Zhang et al 2017)

6

•

2,VLBI for Parallax zero-point
QSO:GaiaZinn et al. (2018)
Xu et al. (2019)
Stassun & Torres (2018)
Schonrich et al. (2019)
Riess et al. (2018)
Muraveva et al. (2018)
Lindegren et al. (2018)
Leung & Bovy (2019)
Khan et al. (2019)
Hall et al. (2019)•Star: Gaia&VLBI(Lindegren et al. 2018)

Groenewegen (2018)
Graczyk et al. (2019)−0.125−0.100−0.075−0.050−0.025Gaia DR2 zero−point offsets (mas)The zero point:~ −0.03 mas for the faintquasars(without Galactic plane)
more negative for the bright stars(most in the Galactic plane)••(Xu et al. 2019)70.0003,VLBI for Gaia-CRF Spin
The Gaia reference frame for brightsources examined using VLBI
observations of radio stars(Lindegren 2019)

Solution at T = J2015.5 with 26/41 radio stars
Orientation (mas):Spin (mas yr−1 ):εX(T): −0.347 ± 0.137
ωX : −0.077 ± 0.051
εY(T): +0.358 ± 0.245
ωY : −0.096 ± 0.042
εZ(T): +0.050 ± 0.045
ωZ : −0.002 ± 0.028
8

3,VLBI for Gaia-CRF Spin

41(26) radio stars (Lindegren 2019)108 radio stars•• Rehandling archive data or Reobserving•Star position
phase calibration improved, closer calibrators

9(Xu et al. 2019)

4, Improvement in the future
Gaia will achieve < 10 μas astrometry•Formal uncertainty in parallax•Formal uncertainty in proper motion(Lindegren et al. IAU 2018)

For VLBI, we need•High astrometry accuracy•During the Gaia period104, Improvement in the future
Verifying Gaia Astrometric Results of Stars with New VLBI Astrometry
Stars observed by VLA+PT•46 Radio
Quiescent: 1~10 mJy; Flare: >100 mJy. (David et al. 2007)•Catalogue of Radio Stars > 3000 (Wendker, 2001)VLASS SummaryFrequency

2-4GHzResolution

2.5 arcsec

Sky coverage

All Sky North of Dec. -40 deg

Sensitivity per/3 epoch

120 µJy/69 µJy RMS

Expected sources~5 million

epoch 12017.9 - 2019.7~ 200 point-like candidates need VLBI confirmation11

4, Improvement in the future

Pilot observations:••

to find candidate radio stars suitable for VLBI astrometry
EAVN 2019B (K band)
EVN 2019A (X band)

Follow up astrometry observations:• ~20 radio stars with ~20 μas astrometry(Gu ̈del M. 2002 )12

5, Summary

VLBI astrometry of radio stars in Gaia’s era

Gaia-CRF and ICRF with stars at bright end•Link
Orientation: ~ ± 0.020 (mas)Spin:~ ± 0.005 (mas/yr)

the astrometry results of Gaia
•Verify
Parallax zero-point: < ± 0.010 (mas)AGB stars and binaries•Physical property for each radio star
Cooperation is Welcome!

13