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gaia data release 3 documentation

10.8 Long period variables

10.8.5 Quality assessment and validation


The observational Hertzsprung–Russell diagram of all LPV candidates with relative parallax uncertainties better than 15 % is shown in Figure 10.17. Almost all LPV candidates are found at the expected location in the diagram where asymptotic giant branch (AGB) stars stand. A small group of objects is also found at the blue side of the diagram below the AGB stars, close to the red clump brightness. These are likely red giant branch (RGB) stars.

Figure 10.17: Observational Hertzsprung–Russell, diagram of the Gaia DR3 LPV candidates with relative parallax uncertainties better than 15 %. The background points in grey represent the distribution of a random sample of about one million Gaia DR3 sources with parallax uncertainties better than 10 %.
Figure 10.18: Left panel: Gaia DR3 period distribution (in green) compared to the period distribution published in the Gaia DR2 catalogue of LPV candidates (in red). The periods are expressed in days. Right panel: 5–95 % trimmed G range versus Gaia DR3 periods.

The distribution of the periods published in Gaia DR3 is shown by the green histogram in the left panel of Figure 10.18. It shows an overall distribution similar to what was published in the Gaia DR2 catalogue of LPV candidates, shown in red in the panel. Compared to the Gaia DR2 sample with published periods, the Gaia DR3 sample (a) contains many more sources and with amplitudes down to 0.1 mag (the lower limit was 0.2 mag in Gaia DR2), (b) contains periods down to 35 d (60 d in Gaia DR2), and (c) is based on observation durations of 34 months (22 months in Gaia DR2), thereby allowing many more detections with periods longer than 500 d.

The period is shown against the 5–95 % trimmed range of G in the right panel of Figure 10.18. The well-known trend of amplitude increasing with period is clearly visible on the short-period side of the diagram, while long secondary periods are visible at P300 d. The group of points at the largest amplitudes is due to Mira variables. The figure also shows the presence of period aliases at short periods, visible from their vertical distributions at fixed values of the period.

Figure 10.19: Model amplitudes published in Gaia DR3 versus half of the 5–95 % trimmed range of the G light curves of LPV candidates. A 1:1 diagonal line is drawn in orange to guide the eyes.

The model amplitudes published in the catalogue are shown versus half the 5–95 % trimmed range QR5(G) of the G light curves in Figure 10.19. For Miras (QR5(G)1), we see that the one-period model generally grasps well the full light curve amplitude, with model amplitudes about equal to half QR5(G). This is expected from the fact that Miras pulsate in one mode. At smaller amplitudes, where semi-regular red giants are found, the one-period model only grasps part of the variability, as expected from the multi-periodic nature of semi-regulars. We therefore generally have the amplitude of the one-period model smaller than QR5(G).

Figure 10.20: Distribution of the pseudo-wavelength separation ΔλRP between the two main peaks in the low-resolution RP spectra. From top to bottom panels: the sample of Galactic candidates with relative parallax uncertainties better than 10 %, candidates in the LMC, and candidates in the SMC, respectively. A vertical red dashed line is drawn at ΔλRP=7. The ordinate range is kept identical in all three panels to ease comparison.

The histogram of the pseudo-wavelength separation ΔλRP between the two highest peaks in the RP spectra is shown in Figure 10.20 for three samples: Galactic candidates with relative parallax uncertainties better than 10 % (top panel), candidates in the Large Magellanic Cloud (LMC; middle panel), and candidates in the Small Magellanic Cloud (SMC; bottom panel). The LMC and SMC regions are defined with the same conditions on right ascension, declination, proper motion on the sky, and parallax as in Mowlavi et al. (2019). A good distinction is observed between O-rich candidates (ΔλRP<7) and C-rich candidates (ΔλRP>7). The distribution is a little more complex for candidates in the Galactic disc and bulge, probably due to the effect of extinction and/or higher metallicities (Lebzelter et al. 2023). In Figure 10.20, we also note the increased efficiency to produce C-stars in lower metallicity environments: the fraction of C-stars is the lowest in the solar-metallicity Galactic sample shown in the top panel and largest in the SMC shown in the bottom panel.


Figure 10.21: Gaia DR3 period compared with the closest period (among primary, secondary, and tertiary) in the best-match OGLE-III source.

The Gaia DR3 periods are compared to the periods obtained by the Optical Gravitational Lensing Experiment (OGLE) for LPVs in the Magellanic Clouds and Galactic bulge. We cross-match our candidates with the OGLE-III catalogues (Soszyński et al. 2009a, 2011, 2013) within 2 arcsec. Figure 10.21 compares, for each matched source, the Gaia DR3 period with the closest OGLE-III period among the primary, secondary, and tertiary periods. The Gaia DR3 periods are within 10 % of the closest OGLE-III period for about 55 % of the matched sources. In about half of the cases, the period detected by Gaia corresponds to the primary period from OGLE-III, while the remaining cases are roughly evenly split between secondary and tertiary periods.

Figure 10.22: Gaia- 2MASS diagram showing the density distribution of Gaia LPV candidates in the LMC classified as O-rich (in blue in the left panel) and C-rich (in red in the right panel). The diagram plots the 2MASS Ks magnitude versus the difference between the GRP Wesenheit index and the WKs,J-Ks Wesenheit index (see text). The distributions in grey in the background of both plots represent all Gaia DR3 LPV candidates in the LMC.

The distinction between C-rich and O-rich stars has been confirmed on a statistical basis by comparison with M- and C-stars classified from the ground. S-type stars (O-rich giants with intermediate properties between M- and C-types, and showing spectral signatures of s-process elements) and hot C-stars cannot be safely identified with the help of this criterion. In addition, sources with ΔλRP values larger than 10 (see Figure 10.20) are most probably O-rich stars, and therefore misclassified as C-rich stars in the Gaia DR3 catalogue. For more details and an improved criterion (including colour-dependence), we refer to Lebzelter et al. (2023).

An additional validation of the C-rich classification is presented in Figure 10.22, which combines Gaia and 2MASS data (Skrutskie et al. 2006). The figure shows the Gaia-2MASS diagram of the LPV candidates in the LMC, plotting the 2MASS Ks magnitude versus the difference between the GRP Wesenheit index (WRP,BP-RP=GRP-1.3(GBP-GRP)) and the WKs,J-Ks Wesenheit index (WKs,J-Ks=Ks-0.686(J-Ks)). O-rich LPVs (in blue in the left panel) are distributed in the left part of the diagram, while C-rich LPVs (in red in the right panel) are distributed in the right part of the diagram, as expected from the study of Lebzelter et al. (2018).