Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia’s internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error σ(π0) will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for π0. Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.

Abstract
1. INTRODUCTION
2. RR LYRAE STAR BASED ZERO POINT: NAIVE“CIRCULAR” ARGUMENT
3. MATHEMATICAL DESCRIPTION
4. NUMERICAL ESTIMATES
5. RR LYRAE DISTANCE SCALE
6. CONCLUSION
REFERENCES

• Andrew Gould(Max-Planck-Institute for Astronomy/Korea Astronomy and Space Science Institute/Department of Astronomy) Corresponding author
• Juna A. Kollmeier(Observatories of the Carnegie Institution of Washington)