Cepheids and RR Lyrae stars are primary standard candles of the cosmological distance ladder because they follow canonical relations linking their intrinsic luminosity to the pulsation period and/or the metal abundance. Specifically, for Cepheids the intrinsic luminosity (L) at any passband depends on the period (P) of light variation. This is traditionally referred to as the Cepheid period–luminosity relation or Leavitt law, after its discoverer Mrs Henrietta Swan Leavitt (Leavitt & Pickering 1912). Modern realisations of the Cepheid PL relations from optical to infrared passbands include, among others, the ground-based studies of Madore & Freedman (1991), Ripepi et al. (2012), and Gieren et al. (2013), works based on Hubble Space Telescope (HST) data such as those by Freedman et al. (2001), Saha et al. (2006), and Riess et al. (2011), and theoretical investigations such as those by Marconi et al. (2005). Among the most recent studies of the Cepheid period–luminosity relations are those based on Gaia trigonometric parallaxes of Galactic Cepheids (e.g. Clementini 2017; hereafter Paper I and references therein; Riess et al. 2016, 2018). For RR Lyrae stars the intrinsic luminosity (L) in the infrared passbands depends on P and possibly stellar metallicity (Z; PL – metallicity relation – PL(Z)), as first shown by Longmore et al. (1986) and later confirmed by (i) empirical studies of field and cluster RR Lyrae stars (e.g. Sollima et al. 2006, 2008; Borissova et al. 2009), (ii) theoretical models by Marconi et al. (2015) and Neeley et al. (2017), and (iii) the Gaia parallax-calibrated relations of Sesar et al. (2017), Paper I and references therein, and Muraveva et al. (2018a,b). In the visual passband, the luminosity L depends on Z in the form of the so-called RR Lyrae luminosity–metallicity relation (see e.g. Cacciari & Clementini 2003; Clementini et al. 2003; the pulsation models by Bono et al. 2003; the theoretical calibration by Catelan et al. 2004; or the Gaia-based relations in Paper I; Muraveva et al. 2018a and references therein). The predicted precision of the Gaia end-of-mission parallaxes for local Cepheids and RR Lyrae stars1 will allow us to determine the slope and zero-point of these fundamental relations with unprecedented accuracy, thus setting the basis for a global reassessment of the whole cosmic distance ladder. As a first anticipation of the Gaia potential in this field of the cosmic distance ladder and a first assessment of improved precision with respect to previous astrometric missions such as HIPPARCOS, and the dramatic increase in statistics compared to what is achievable, for instance, through measuring parallaxes with the HST, Gaia DR1 published parallaxes for more than 700 Galactic Cepheids and RR Lyrae stars, computed as part of the Tycho-Gaia Astrometric Solution (TGAS; Lindegren et al. 2016). A number of papers after Gaia intermediate data releases in 2016 and 2018 (Gaia Data Release 1 – DR1 and DR2, respectively) have discussed Gaia Cepheids and RR Lyrae stars, specifically presenting the released samples (Clementini et al. 2016, 2019), their parallaxes (e.g. Lindegren et al. 2016) and possible offsets affecting them (Arenou et al. 2017, 2018); and addressing in particular their use as standard candles (Casertano et al. 2017 and Paper I for Gaia DR1 and Riess et al. 2018; Muraveva et al. 2018a for Gaia DR2). In Paper I we have used TGAS parallaxes, along with literature photometry and spectroscopy, to calibrate the zero-point of the PL relations of classical and type II Cepheids, and the near-infrared PL and PL(Z) relations of RR Lyrae stars by fitting these relations through adopting different techniques that operate either in parallax or absolute magnitude space. In that paper, different sources of biases affecting the TGAS samples of Cepheids and RR Lyrae stars were discussed at some length, and the possible systematic errors caused in the inferred luminosity calibrations were analysed in detail.
Citation: Cacciari & Clementini 2003
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