An IPHAS-based search for accreting very low-mass objects using VO tools

Context. The main goal of this paper is to prove that accreting very low-mass stars and brown dwarfs can be identified in IPHAS, an H emission survey of the northern Milky Way. Full exploitation of the IPHAS database and a future extension of it in the southern hemisphere will be useful in identifying very low-mass accreting objects near to and far from well-known star-forming regions. Aims. We have used Virtual Observatory tools to cross-match the IPHAS catalogue with the 2MASS catalogue. We defined photometric criteria to identify H emission sources with near-infrared colours similar to those of known young very low-mass stars and brown dwarfs. 4000 candidates were identified that met our criteria over an area of 1600 square degrees. We present low-resolution optical spectra of 113 candidates. Spectral types have been derived for the 33 candidates that have spectroscopically confirmed H emission, negligible reddening and spectral class M. We have also measured H emission and investigated the NaI doublet (818.3 nm, 819.5 nm) in these 33 objects. Methods. We confirm that 33 IPHAS candidates have strong H emission indicative of disc accretion for their spectral type. Twenty-three of them have spectral class M4 or later, of which ten have classes in the range M5.5–M7.0 and could thus be very young brown dwarfs. Many objects also have a weak NaI doublet, an indication of low surface gravity. Results. IPHAS provides a very valuable database for identifying accreting very low-mass stars and brown dwarfs. Virtual Observatory tools provide an efficient method for identifying these objects over large areas of the sky. Based on our success rate of 23 H emission objects with spectral type in the range M4–M7 out of 113 candidates with spectroscopic follow-up, we estimate that there could be hundreds of such objects in the full IPHAS survey.