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

¹ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³ Department of Astronomy, 6th floor, MongManWai Building, Tsinghua University, Beijing 100084, PR of China
⁴ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
⁵ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Spain
⁶ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès 1, 08028 Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Esteve Terradas, 1, Edifici RDIT, Campus PMT-UPC, 08860 Castelldefels (Barcelona), Spain
⁸ Department of Physics, Universita’ degli Studi di Trieste, Via A. Valerio 2, 34127 Trieste, Italy
⁹ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
¹⁰ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
¹¹ Departament de Física, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
¹² European Southern Observatory, Casilla 19001, Santiago 19, Chile
¹³ Donostia International Physics Centre (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain
¹⁴ IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain
¹⁵ Observatório Nacional – MCTI (ON), Rua Gal. José Cristino 77, São Cristóvão, 20921-400 Rio de Janeiro, Brazil
¹⁶ University of Michigan, Department of Astronomy, 1085 South University Ave., Ann Arbor, MI 48109, USA
¹⁷ University of Alabama, Department of Physics and Astronomy, Gallalee Hall, Tuscaloosa, AL 35401, USA
¹⁸ Instituto de Astrofísica de Canarias, La Laguna, 38205, Tenerife, Spain
¹⁹ Departamento de Astrofísica, Universidad de La Laguna, 38206, Tenerife, Spain
²⁰ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05508-090 São Paulo, Brazil

^★ Corresponding author; clsj@cefca.es

Received: 23 June 2024
Accepted: 21 September 2024

Abstract

Aims. We used the Javalambre Photometric Local Universe Survey (J-PLUS) second data release (DR2) photometry in 12 optical bands over 2176 deg² to estimate the fraction of white dwarfs with the presence of Ca II H+K absorption along the cooling sequence.

Methods. 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 (EW_J0395), a proxy to detect calcium absorption. A total of 4399 white dwarfs with effective temperatures within 30 000 > T_eff > 5500 K and masses of M > 0.45 M_⊙ were analyzed. Their EW_J0395 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, p_Ca, was also computed.

Results. The comparison of EW_J0395 with both the measured Ca/He abundance and the identification of metal pollution from spectroscopy shows that EW_J0395 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 T_eff = 13 500 K to f_Ca ≈ 0.15 at T_eff = 5500 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 T_eff < 12 000 K. Finally, we found 39 white dwarfs with pCa > 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.

Conclusions. 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 T_eff = 13 500 K to 5500 K, which reflects their selection function in the optical from the total population of metal-polluted systems.