J-PLUS: Discovery and characterisation of ultracool dwarfs using Virtual Observatory tools

II. Second data release and machine learning methodology

¹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Canada, Madrid, Spain
e-mail: pmas@cab.inta-csic.es
² Spanish Virtual Observatory, Spain
³ Departamento de Ingenieria Mecánica, Universidad de la Rioja, San José de Calasanz 31, 26004 Logroño, La Rioja, Spain
⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁶ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
⁷ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
⁸ Departamento de Ingeniería de Organización, Administración de Empresas y Estadística, Universidad Politécnica de Madrid, c/ José Gutiérrez Abascal 2, 28006 Madrid, Spain
⁹ Departamento de Construcción e Ingeniería de Fabricación, Universidad de Oviedo, Pedro Puig Adam, Sede Departamental Oeste, Modulo 7, 1 a planta, 33203 Gijón, Spain
¹⁰ Observatório Nacional - MCTI (ON), Rua Gal, José Cristino 77, Sào Cristóvão, 20921-400 Rio de Janeiro, Brazil
¹¹ University of Michigan, Department of Astronomy, 1085 South University Ave., Ann Arbor, MI, USA
¹² University of Alabama, Department of Physics and Astronomy, Gallalee Hall, Tuscaloosa, AL 35401, USA
¹³ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain
¹⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05508-090 São Paulo, Brazil
¹⁵ Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal, 4, 20018 Donostia-San Sebastián, Guipuzkoa, Spain
¹⁶ IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

Received: 28 April 2022
Accepted: 14 July 2022

Abstract

Context. Ultracool dwarfs (UCDs) comprise the lowest mass members of the stellar population and brown dwarfs, from M7 V to cooler objects with L, T, and Y spectral types. Most of them have been discovered using wide-field imaging surveys, for which the Virtual Observatory (VO) has proven to be of great utility.

Aims. We aim to perform a search for UCDs in the entire Javalambre Photometric Local Universe Survey (J-PLUS) second data release (2176 deg²) following a VO methodology. We also explore the ability to reproduce this search with a purely machine learning (ML)-based methodology that relies solely on J-PLUS photometry.

Methods. We followed three different approaches based on parallaxes, proper motions, and colours, respectively, using the VOSA tool to estimate the effective temperatures and complement J-PLUS photometry with other catalogues in the optical and infrared. For the ML methodology, we built a two-step method based on principal component analysis and support vector machine algorithms.

Results. We identified a total of 7827 new candidate UCDs, which represents an increase of about 135% in the number of UCDs reported in the sky coverage of the J-PLUS second data release. Among the candidate UCDs, we found 122 possible unresolved binary systems, 78 wide multiple systems, and 48 objects with a high Bayesian probability of belonging to a young association. We also identified four objects with strong excess in the filter corresponding to the Ca ii H and K emission lines and four other objects with excess emission in the Hα filter. Follow-up spectroscopic observations of two of them indicate they are normal late-M dwarfs. With the ML approach, we obtained a recall score of 92% and 91% in the 20 × 20 deg² regions used for testing and blind testing, respectively.

Conclusions. We consolidated the proposed search methodology for UCDs, which will be used in deeper and larger upcoming surveys such as J-PAS and Euclid. We concluded that the ML methodology is more efficient in the sense that it allows for a larger number of true negatives to be discarded prior to analysis with VOSA, although it is more photometrically restrictive.