J-PLUS: The fraction of calcium white dwarfs along the cooling sequence

We used the Javalambre Photometric Local Universe Survey (J-PLUS) second data release (DR2) photometry in 12 optical bands over $2\,176$ deg$^2$ to estimate the fraction of white dwarfs with the presence of Ca ii H+K absorption along the cooling sequence. We compared the J-PLUS photometry against metal-free theoretical models to estimate the equivalent width in the $J0395$ passband of $10$ nm centered at $395$ nm (ewca ), a proxy to detect calcium absorption. A total of $4\,399$ white dwarfs with effective temperatures within $30\,000 > eff > 5\,500$ K and masses of $M > 0.45$ odot $ were analyzed. Their ewca distribution was modeled using two populations, corresponding to polluted and non-polluted systems, to estimate the fraction of calcium white dwarfs ($f_ Ca $) as a function of $T_ eff $. The probability of each individual white dwarf presenting calcium absorption, pca , was also computed. The comparison of ewca with both the measured Ca/He abundance and the identification of metal pollution from spectroscopy shows that ewca correlates with the presence of Ca ii H+K absorption. The fraction of calcium white dwarfs changes along the cooling sequence, increasing from $f_ Ca 0$ at eff = 13\,500$ K to Ca 0.15$ at eff = 5\,500$ K. This trend reflects the selection function of calcium white dwarfs in the optical. We compare our results with the fractions derived from the 40 pc spectroscopic sample and from Sloan Digital Sky Survey (SDSS) spectra. The trend found in J-PLUS observations is also present in the 40 pc sample; however, SDSS shows a deficit of metal-polluted objects at eff < 12\,000$ K. Finally, we found $39$ white dwarfs with $ > 0.99$. Twenty of them have spectra presented in previous studies, whereas we obtained follow-up spectroscopic observations for six additional targets. These $26$ objects were all confirmed as metal-polluted systems. The J-PLUS optical data provide a robust statistical measurement for the presence of Ca ii H+K absorption in white dwarfs. We find a $15 3$ increase in the fraction of calcium white dwarfs from eff = 13\,500$ K to $5\,500$ K, which reflects their selection function in the optical from the total population of metal-polluted systems.