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

TL;DR

This study establishes infrared period–luminosity relations for Galactic Miras by combining multi-epoch DIRBE and unTimely/WISE photometry with Gaia distances and contemporaneous optical periods. It derives nine near-infrared PL-relations, shows Galactic Miras are fainter than their LMC counterparts and that slopes steepen with wavelength, and uses synthetic SEDs to extract period–temperature, period–bolometric-luminosity, and period–radius relationships. The work also scrutinizes Gaia parallax uncertainties using PL scatter, VLBI comparisons, and 47 Tuc AGB stars, finding inflation factors typically in the range 1.0–1.8 (best around 1.3) rather than the large factors reported in prior studies, thereby validating Gaia distances for extended AGB stars. It further demonstrates that Miras with changing periods largely adhere to the fundamental-mode PL relations when contemporaneous measurements are used, and that OH/IR Miras with long periods can be accommodated within the near-IR PL framework, offering reliable distance indicators for Galactic LPVs.

Abstract

Miras and other long-period variable (LPV) stars on the 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. We aim to establish the PL relations of solar-neighbourhood Miras for several near-IR photometric bands. To this end, we used multi-epoch photometry from the DIRBE and unTimely/WISE catalogues, Gaia parallax distances, and contemporary pulsation periods obtained from optical observations of a well-selected sample of solar-neighbourhood Miras. We show that clearly defined PL relations in the nine investigated near-IR bands emerge from our data, and we report the slopes and zero-point magnitudes. We find that Galactic Miras are fainter in the near-IR than their Large Magellanic Cloud siblings. We derive average period-temperature, period-bolometric-luminosity, and period-radius relations from fits to synthetic SEDs 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. Furthermore, we performed such tests based on a comparison with parallaxes obtained with the VLBI and with a sample of LPVs in the globular cluster 47 Tuc. 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 1.7, and most likely by $\sim1.3$. For more uncertain parallaxes, we find evidence that the distances (parallaxes) are generally overestimated (underestimated). Nevertheless, we find strong evidence that the large error-inflation factors reported for AGB stars in the literature are unrealistic. Our results lend confidence to the parallax measurements of these highly extended, variable stars.

Figures (10)

• Figure 1: PL diagram using the dereddened, absolute magnitude in the WISE $W1$ band, $M_{{\rm W1},0}$. Stars of different chemical spectral types are represented by different colours and symbol shapes; see the legend. The red dashed line is the linear least-square fit reported in Table \ref{['tab:PL-relations']}.
• Figure 2: Same as Fig. \ref{['fig:unTimely_PLW1']}, but for the DIRBE [2.2] band, $M_{[2.2],0}$.
• Figure 3: DIRBE Wesenheit index $W_{[2.2],[1.25]-[2.2]}$ vs $\log P$ of the DIRBE gold sample. Stars of different chemical spectral types are represented by different colours and symbol shapes, see the legend. The dashed black, blue, and red lines show linear fits to the overall DIRBE gold sample, the S stars, and the C stars, respectively; see Table \ref{['tab:Wesenheit']}. The same slope as for the overall sample was adopted for the S and C stars.
• Figure 4: Synthetic SEDs constructed with the PL-relations given in Table \ref{['tab:PL-relations']}. The symbols connected by solid lines represent the fluxes derived for $\log(P) = 2.2$, 2.3, 2.4, 2.5, 2.6, and 2.7 (from bottom to top). The WISE bands are plotted as open symbols, while all other bands are plotted as filled symbols. The dotted lines are blackbody fits to each of the six synthetic SEDs, excluding the WISE bands.
• Figure 5: Relations derived from the blackbody fits to the synthetic SEDs in Fig. \ref{['fig:synthSEDs']}. Top panel: blackbody temperature $T_{\rm bb}$; middle panel: logarithm of the stellar luminosity in solar units; bottom panel: stellar radius in solar units as a function of $\log(P)$. The grey lines show the relations given by Eq. \ref{['Eq:Tbb']}, \ref{['Eq:Lstar']}, and \ref{['Eq:Rstar']}.