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We used a combination of public RR Lyrae star catalogs and a Bayesian methodology to derive robust structural parameters of the inner Halo (<25 kpc) and Thick Disc of the Milky Way. RR Lyrae stars are an unequivocal tracer of old metal-poor populations, for which accurate distances and extinctions can be individually estimated and so, are a reliable independent means of tracing the population of the old high-/Fe disc usually associated to the Thick Disc. In particular, the chosen RR Lyrae sample spans regions at low galactic latitude toward the anti-center direction, allowing to probe the outermost parts of the disc. Our results favour a Thick Disc with short scale height and short scale length, hᵦ=0. 65-₀. ₀₅^+0. 09 kpc, hR=2. 1-₀. ₂₅^+0. 82 kpc, for a model in which the inner Halo has a constant flattening of q=0. 90-₀. ₀₃^+0. 05 and a power law index of n=-2. 78-₀. ₀₅^+0. 05. Similar short scales for the Thick Disc are also found when considering an inner Halo with flattening dependent on radius. We also explored a model in which the Thick Disc has a flare and, although this is only mildly constrained by our data, a flare onset in the inner 11 kpc is highly disfavoured.
Mateu et al. (Thu,) studied this question.