Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association

¹ Centro de Astrobiologia (CAB-CSIC), Ctra. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain e-mail: ege@cab.inta-csic.es
² University of Central Florida, Dept. of Physics, PO Box 162385, Orlando, FL 32816-2385, USA e-mail: tata@physics.ucf.edu
³ Department of Physics, HCMIU, Vietnam National University Administrative Building, Block 6, Linh Trung Ward, Thu Duc District, HCM, Vietnam e-mail: pbngoc@hcmiu.edu.vn
⁴ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan e-mail: pbngoc@asiaa.sinica.edu.tw
⁵ Research School of Astronomy and Astrophysics, College of Science, Australian National University, ACT 2611, Australia e-mail: bessell@mso.anu.edu.au
⁶ Laboratoire d'Astrophysique de Grenoble, Université J. Fourier, BP 53, 38041 Grenoble, France e-mail: [forveille;delfosse]@obs.ujf-grenoble.fr
⁷ Telescopio Nazionale Galileo, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, Spain e-mail: magazzu@tng.iac.es
⁸ Observatoire de Besançon, Institut UTINAM, University of Franche-Comté, CNRS-UMR 6213, BP 1615, 25010 Besançon Cedex, France e-mail: celine@obs-besancon.fr

Received: 5 February 2010
Accepted: 8 April 2010

Abstract

Aims. We search for new ultracool dwarfs and report here our observations of 78 ultracool dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. We analyze low-resolution optical spectroscopic observations to estimate spectral types of all candidates.

Methods. We derive spectral types for each object using measurements of the PC3 spectral index as defined in Martín et al. They range from M6 to L3. The Hα emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra to identify young stellar objects. Spectroscopic indices of TiO, VO, CrH, and FeH molecular features are also reported.

Results. A rule-of-thumb criterion for selecting young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 42 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them being new L dwarfs within 20 pc).