Two stellar clocks, one history: A Gaia colour-magnitude-diagram reconstruction of the solar-neighbourhood star formation history tested against white dwarfs

Context . The solar-neighbourhood star formation history (SFH) can be constrained from the colour-magnitude diagram (CMD) of non-degenerate stars and tested independently with the white-dwarf luminosity function (WDLF). These two tracers sample complementary evolutionary phases and provide a consistency check on both the recovered SFH and the adopted white-dwarf (WD) cooling models. Aims . We reconstruct the local SFH from the Gaia DR3 CMD and test whether the resulting history reproduces the observed 40 pc WDLF. Methods . We fitted non-degenerate stars in a cylinder of radius 200 pc and vertical extent ± 40 pc using the SFERA synthetic-population code. The fit was performed with the PISA and PARSEC stellar libraries. Each CMD-derived solution was then used to generate a synthetic WD population for comparison with the 40 pc WDLF, adopting the same initial–final mass relation and four WD cooling model sets: BaSTI, MIST, La Plata, and Montréal. Results . The two CMD-based solutions agree on the main features of the local SFH: low activity at the oldest ages, an early episode around 9–11 Gyr ago, reduced activity between about 4 and 9 Gyr, and enhanced star formation over the last few gigayears. The predicted WDLFs reproduce the bright and intermediate part of the observed distribution for all cooling grids, but differ substantially around the WDLF maximum and along the faint tail. MIST and BaSTI shift too much weight to magnitudes fainter than the observed maximum, whereas La Plata gives the closest match to the faint tail but leaves larger residuals around the maximum. Montréal provides the best overall compromise. On this cooling scale, progressively removing the oldest synthetic WDs shows that the faint tail disfavours a substantial contribution from WDs with total ages older than about 11 Gyr.