Meeting the Cool Neighbours I: Nearby stars in the NLTT Catalogue - Defining the sample

Abstract

We are currently undertaking a program aimed at identifying previously-unrecognised late-type dwarfs within 20 parsecs of the Sun. As a first step, we have cross-referenced Luyten's NLTT proper motion catalogue against the second incremental release of the 2MASS Point Source Catalogue, and use optical/infrared colours, derived by combining Luytens's m(r) estimates with 2MASS data, to identify candidate nearby stars. This paper describes the definition of a reference sample of 1245 stars, and presents a compilation of literature data for over one-third of the sample. Only 274 stars have trigonometric parallax measurements, but we have used data for nearby stars with well-determined trigonometric parallaxes to compute colour-magnitude relations in the (M(V), (V-K)), (M(V), (V-I)) and (M(I), (I-J)) planes, and use those relations to determine photometric parallaxes for NLTT stars with optical photometry. Based on the 2MASS JHK data alone, we have identified a further 42 ultracool dwarfs ((J-K > 0.99) and use (J-K) colours to estimate photometric parallaxes. Combining these various techniques, we identify 308 stars with formal distances of less than 20 parsecs, while a further 46 have distance estimates within 1-sigma of our survey limit. Of these 354 stars, 75, including 39 of the ultracool dwarfs. are new to nearby star catalogues. Two stars with both optical and near-infrared photometry are potential additions to the immediate Solar Neighbourhood, with formal distance estimates of less than 10 parsecs.