Optical Properties of Metal-poor T Dwarf Candidates

Context. Metal-poor brown dwarfs are poorly understood because they are extremely faint and rare. Only a few candidates have been identified as T-type subdwarfs in infrared surveys and their optical properties remain unconstrained. Aims. We aim to improve the knowledge of the optical properties of T subdwarf candidates to break the degeneracy between metallicity and temperature and to investigate their atmospheric properties. Methods. Deep \(z\)-band images of 10 known T subdwarf candidates were collected with the 10.4-m Gran Telescopio Canarias. Low-resolution optical spectra for two of them were obtained with the same telescope. Photometric measurements of the \(z\)-band flux were performed for all the targets and they were combined with infrared photometry in \(J, H, K, W1\) and \(W2\)-bands from the literature to obtain the colours. The spectra were compared with solar-metallicity T dwarf templates and with laboratory spectra. Results. We found that the targets segregate into three distinct groups in the \(W1 - W2\) vs. \(z - W1\) colour-colour diagram. Group I objects are mixed with solar-metallicity T dwarfs. Group III objects have \(W1 - W2\) colours similar to T dwarfs but very red \(z - W1\) colours. Group II objects lie between Group I and III. The two targets for which we obtained spectra are located in Group I and their spectroscopic properties resemble normal T dwarfs but with water features that are deeper and have a shape akin to pure water. Conclusions. We conclude that the \(W1 - W2\) vs. \(z - W1\) colour-colour diagram is excellent to break the metallicity-temperature degeneracy for objects cooler than L-type. A revision of the spectral classification of T subdwarf might be needed in the future, according to the photometric and spectroscopic properties of WISE1810 and WISE0414 in Group III discussed in this work.