New neighbours: II. An M9 dwarf at 4 pc, DENIS-P J104814.7-395606.1

A&A New neighbours: II. An M9 dwarf at d DENIS-P J104814.7−395606.1? 4 pc, X. Delfosse 0 2 4 T. Forveille 0 1 2 E. L. Mart n 2 8 J. Guibert 2 6 7 J. Borsenberger 2 5 F. Crifo 2 6 C. Alard 2 6 N. Epchtein 2 10 P. Fouque 2 3 9 G. Simon 2 6 F. Tajahmady 2 6 7 0 LAOG , Observatoire de Grenoble, BP 53, 38041 Grenoble , France 1 Canada-France-Hawaii Telescope Corporation , 65-1238 Mamaloha Highway, Kamuela, HI 96743 , USA 2 Send o print requests to: X. Delfosse 3 European Southern Observatory , Casilla 19001, Santiago 19 , Chile 4 Instituto de Astrof sica de Canarias , 38200 La Laguna, Tenerife, Canary Islands , Spain 5 Institut d'Astrophysique de Paris , 98bis boulevard Arago, 75014 Paris , France 6 Observatoire de Paris (DASGAL/UMR-8633) , 75014 Paris , France 7 Centre d'Analyse des Images de l'INSU , 61 avenue de l'Observatoire, 75014 Paris , France 8 Institute for Astronomy, University of Hawaii at Manoa , 2680 Woodlawn Drive, Honolulu, HI 96822 , USA 9 DESPA, Observatoire de Paris , 5 place J. Janssen, 92195 Meudon Cedex , France 10 Observatoire de Nice , BP 4229, 06304 Nice Cedex 4 , France We present the discovery of a previously unknown member of the immediate solar neighbourhood, DENIS-P J104814.7−395606.1 (hereafter DENIS 1048−39), identi ed while mining the DENIS database for new nearby stars. A HIRES echelle spectrum obtained with the 10-m Keck telescope shows that it is an M9 dwarf. DENIS 1048−39 has a very bright apparent magnitude (I = 12:67) for its spectral type and colour (I − J = 3:07), and is therefore very nearby. If it is single its distance is only 4:1 0:6 pc, ranking it as between our 12th and 40th closest neighbour. It is also the closest star or brown dwarf with a spectral type later than M7V. Its proper motion was determined through comparison of Sky atlas Schmidt plates, scanned by the MAMA microdensitometer, with the DENIS images. At 1.5200 yr−1 it primarily attests the closeness of DENIS 1048−39 and hence its dwarf status. These characteristics make it an obvious target for further detailed studies. astronomical data base; surveys { astrometry and celestial mechanism; astrometry { stars; low mass; brown dwarfs { stars; late-type 1. Introduction Much of our understanding of stellar astronomy rests upon the nearest stars. As individual objects they are the brightest and hence best studied examples of their spectral type, and they have distances that can usually be measured directly from an accurate trigonometric parallax. These stars are also the source of some of the most accurate stellar masses: for nearer multiple systems the same physical separation translates into a wider angular separation, and hence into a better characterized orbit. As a population, the solar neighbourhood sample therefore provides deep insight into the nature of our Galaxy's components, through studies of its stellar luminosities and mass functions, its kinematics, its chemical composition, and its multiplicity statistics. For the moderately bright (MV < 7) G and K dwarfs, our current census of the solar neigbourhood is complete out to at least 25 pc (Jahreiss 1994) . Intrinsically fainter stars however dominate the galactic population budget, and represent a large fraction of its mass budget: M dwarfs account for at least 70% of all stars, and about 50% of the mass of the galactic disk. For this fainter population our present census unfortunately becomes incomplete at a rather small distance: from a comparison of observed star densities within 5 and 10 pc, Henry et al. (1997) demonstrate that over 100 systems are missing in the larger volume, even under the purposedly optimistic assumption that the current inventory is complete for the 5 pc sphere. In addition to these systems which are altogether missing, a further source of incompleteness is that some of the identi ed nearby stellar systems have unrecognized components, as illustrated by the sustained discovery rate of new companions to nearby M dwarfs (e.g. Henry et al. 1999; Delfosse et al. 1999a; Beuzit et al. in preparation) . To address both of these incompleteness sources, some of us are conducting a systematic search for companions to nearby M dwarfs (Delfosse et al. 1999a, for a presentation of that programme) , and we use the DENIS near infrared sky survey to complete the identi cation of the southern solar neighbourhood M dwarfs. Here we present an initial result of that second programme, the discovery and initial characterization of a previously unknown very nearby (d 4 pc) M9 dwarf. 2. Observational data The DEep Near-Infrared Survey (DENIS) is a southern Fig. 1. 8521 A CsI line of very late M dwarf templates and sky survey (Epchtein 1997) , which will provide full cover- DENIS 1048−39 age of the southern hemisphere in two near-infrared bands (J and Ks) and one optical band (I). DENIS observations are carried out on the ESO 1 m telescope at La Silla. viously unknown red object (I − J = 3:07), DENISDichroic beam splitters sepa (...truncated)