Infrared period–luminosity relations of Galactic Miras based on multi-epoch photometry and the Gaia parallax uncertainty

Abstract

Context. Miras and other long-period variable (LPV) stars on the asymptotic giant branch (AGB) follow period–luminosity (PL) relations. These relations have been difficult to study for Galactic LPVs because their distances were poorly known in the past. Aims. We aimed to establish the PL relations of solar neighbourhood Miras for several near-IR photometric bands. Methods. To this end, we used multi-epoch photometry from the DIRBE and unTimely/WISE catalogues, Gaia parallax distances, and contemporary pulsation periods obtained optical observations of a well-selected sample of solar neighbourhood Miras. Results. We show that clearly defined PL relations in the nine investigated near-IR bands emerge from our data, for which we report slopes and zero-point magnitudes. We find that the Galactic Miras are fainter in the K band than their siblings in the Large Magellanic Cloud. We derived the average period–temperature, period–bolometric-luminosity, and period–radius relations from fits to synthetic spectral energy distributions constructed from the PL relations. By applying AGB evolutionary models, the scatter of stars around the PL sequences can also be used to test whether the parallax uncertainties quoted in the Gaia catalogue are realistic or underestimated. Furthermore, we performed such tests based on a comparison with VLBI parallaxes and with a sample of LPVs in the globular cluster 47 Tuc. Conclusions. We conclude that for Galactic Miras with a fractional parallax uncertainty of ≤0.1 in the Gaia catalogue, the parallax uncertainty is underestimated by factors between 1.0 and 1.8. For more uncertain parallaxes, we find evidence that distances (par- allaxes) are generally overestimated (underestimated). Furthermore, we find strong evidence that the large error inflation factors of AGB stars reported in the literature are unrealistic. Our results lend confidence to the Gaia parallax measurements of these highly extended variable stars.