J-PLUS: Systematic impact of metallicity on photometric calibration with
  the stellar locus
  We present the updated photometric calibration of the twelve optical
passbands for the Javalambre Photometric Local Universe Survey (J-PLUS) second
data release (DR2), comprising 1088 pointings of two square degrees, and study
the systematic impact of metallicity in the stellar locus technique. The [Fe/H]
metallicity from LAMOST DR5 for 146184 high-quality calibration stars, defined
with S/N > 10 in J-PLUS passbands and S/N > 3 in Gaia parallax, was used to
compute the metallicity-dependent stellar locus (ZSL). The initial
homogenization of J-PLUS photometry, performed with a unique stellar locus, was
refined by including the metallicity effect in colours via the ZSL. The
variation of the average metallicity along the Milky Way produces a systematic
offset in J-PLUS calibration. This effect is well above 1% for the bluer
passbands and amounts 0.07, 0.07, 0.05, 0.03, and 0.02 mag in u, J0378, J0395,
J0410, and J0430, respectively. We modelled this effect with the Milky Way
location of the J-PLUS pointing, providing also an updated calibration for
those observations without LAMOST information. The estimated accuracy in the
calibration after including the metallicity effect is at 1% level for the bluer
J-PLUS passbands and below for the rest. We conclude that photometric
calibration with the stellar locus technique is prone to significant systematic
bias along the Milky Way location for passbands bluer than lambda = 4500 A. The
updated calibration method for J-PLUS DR2 reaches 1-2% precision and 1%
accuracy for twelve optical filters within an area of 2176 square degrees.
