Using Galactic Cepheids to verify Gaia parallaxes

TL;DR

This study explores using Galactic Cepheids as independent standard candles to verify Gaia satellite parallax measurements, aiming to detect and correct potential systematic errors in Gaia's parallax zero point.

Contribution

It demonstrates the feasibility and limitations of using Galactic Cepheids for Gaia parallax validation through simulations, highlighting the method's sensitivity to extinction errors.

Findings

Approximately 20,000 Galactic Cepheids are expected, with about half observable by Gaia.

Under ideal conditions, the parallax zero point can be determined with 0.2 microarcsec accuracy.

Realistic assumptions increase uncertainty and highlight the importance of accurate extinction corrections.

Abstract

Context. The Gaia satellite will measure highly accurate absolute parallaxes of hundreds of millions of stars by comparing the parallactic displacements in the two fields of view of the optical instrument. The requirements on the stability of the 'basic angle' between the two fields are correspondingly strict, and possible variations (on the microarcsec level) are therefore monitored by an on-board metrology system. Nevertheless, since even very small periodic variations of the basic angle might cause a global offset of the measured parallaxes, it is important to find independent verification methods. Aims. We investigate the potential use of Galactic Cepheids as standard candles for verifying the Gaia parallax zero point. Methods. We simulate the complete population of Galactic Cepheids and their observations by Gaia. Using the simulated data, simultaneous fits are made of the parameters of the period-luminosity relation and a global parallax zero point. Results. The total number of Galactic Cepheids is estimated at about 20 000, of which nearly half could be observed by Gaia. In the most favourable circumstances, including negligible intrinsic scatter and extinction errors, the determined parallax zero point has an uncertainty of 0.2 microarcsec. With more realistic assumptions the uncertainty is several times larger, and the result is very sensitive to errors in the applied extinction corrections. Conclusions. The use of Galactic Cepheids alone will not be sufficient to determine a possible parallax zero-point error to the full potential systematic accuracy of Gaia. The global verification of Gaia parallaxes will most likely depend on a combination of many different methods, including this one.