A white dwarf is the remnant of a star like the Sun that has exhausted its fuel and, after expelling its outer layers, does not undergo fusion processes in its core and gradually cools down. There are different types of white dwarfs, depending on the composition of their atmosphere. Those containing heavy elements such as calcium, magnesium or iron are called DZ. These metals reveal that they have acquired material from their surroundings very recently, otherwise they would have already passed into their core due to the star's enormous density, and are key to the study of exoplanetary geology, i.e. the composition of those planets that have already been destroyed.

DZ white dwarfs have so far been discovered by chance using spectroscopic data and are not detectable using broadband imaging. Therefore, we do not know how representative the detected DZs are of their total population. In this context, the J-PLUS survey shows another of its applications: its ability to detect DZs thanks to the information from the J0395 narrow filter, which is sensitive to the presence of calcium in the atmosphere of white dwarfs.

J-PLUS is the main project carried out with the T80Cam camera installed on the JAST80 telescope and has allowed to analyse the presence of calcium in a complete and representative population of about 4500 white dwarfs. It has been estimated what fraction of dwarfs show calcium at different temperatures and a sustained increase has been found between 13,500 and 5,500 degrees. This increase is not found in the largest spectroscopic sample of DZs available, which shows no increase for temperatures below 10 000 degrees. This suggests that they have lost half of the existing DZs due to their observing strategy.

In addition, J-PLUS has been used to select 39 white dwarfs with a high probability of having calcium in their atmosphere. Twenty of them already had spectra, and time was requested from the Gran Telescopio Canarias to observe another six. In all cases, the presence of metals was confirmed, supporting the methodology used. Thanks to the J-PLUS mapping, it has been established how the detection of white dwarfs with calcium varies as the temperature changes. This result will serve as a quality test for the massive spectroscopic mapping that is underway or planned for the coming years.