Survey of Catalogued Proper Motion Stars

The NLTT (New Luyten Two Tenths) Catalogue gives information on tens of thousands of proper motion stars discovered during Luyten's survey of POSS-I plates. These objects include nearby dwarfs of solar metallicity and generally more distant, Population II stars of high space motion. There are 311 NLTT objects falling within the 2MASS protocam survey area. To date, we have recovered 247 at their predicted 1993 positions. The other 64 include a few fainter, bluer stars below the limit of the 2MASS survey, but most of the remaining objects have been temporarily lost simply because of poorly recorded 1950 positions. We are in the process of comparing the POSS-I images in these areas with the later-epoch 2MASS images to recover these ``lost'' objects.

The 247 stars already identified include objects with motions between 0.2 and 1.5 arcseconds per year and with colors spanning the width of the H-R diagram, from the bright B dwarf Regulus down to M dwarfs at the survey limit in K. Most of these stars, particularly those with smaller motions, have not been followed up due to the scant data previously available (mainly just POSS-I blue and red magnitudes). Data from the 2MASS survey will provide much needed additional information.

Figure 1 shows the POSS-I R magnitudes (taken from the digitized APM Northern Sky Catalogue) combined with 2MASS protocam magnitudes for these objects. If we assume momentarily that all of the stars shown in the plot are dwarfs of solar metallicity, we can assign a unique for each R-K color, thus deriving a photometric parallax for each object. Objects nearest the Sun fall along the upper envelope; distances are labelled for the three NLTT objects in Figure 1 which are already listed in the Third Edition of the Catalogue of Nearby Stars. The plot shows a number of other objects presumably within 25 parsecs of the Sun which are not included in the latest Edition. These are prime targets for further study. The coolest NLTT objects shown here are of spectral type M5 V. For comparison, the positions of several ultra-cool dwarfs (types of M7 V or later) are also plotted (where the R magnitude is from the APM survey and the K magnitude is from the literature). The coolest M dwarf so far identified in the 2MASS protocamera data is also shown, its position confirming an estimated type of M6-7 V.

Not all of the stars plotted here, of course, will have solar metallicity, or, for that matter, be stars lying along a hydrogen-burning main sequence. Easily distinguished from the other objects, at R-K = 0.36 and R = 15.42, is the previously catalogued DA6 white dwarf LP 322-267.

From the standpoint of this author, there are two driving scientific investigations which a follow-up of the NLTT objects, particularly by the full-blown 2MASS survey, can address:

1. By providing more accurate estimates of their photometric parallaxes, a much more complete census of objects near the Sun can be achieved. The luminosity and mass functions based on the nearby sample are the benchmarks against which all other stellar LFs and MFs are compared, so it is imperative that we make this sample as complete as possible. Based on the number of systems known within 5 pc of the Sun (and assuming that it is complete), we can conclude that the census of systems out to 8 pc is 26% incomplete in the north alone. As an example of an overlooked object, the star LHS 292 (which lies at a distance of only 4.6 pc) was not recognized as a nearby dwarf until 1986 and only then because it was part of a systematic survey of the highest motion (/yr) NLTT objects.
2. Recent data is beginning to suggest that the ultra-cool dwarfs (types M7 V and later) are kinematically different from their early- to mid-M counterparts. Specifically, it appears that these objects have smaller motions on average than warmer M dwarfs. If this is true, these objects near the main sequence terminus will be found among those NLTT objects with smaller proper motions, i.e., among the objects which have received no follow-up. Taking this one step further, if there is a sprinkling of brown dwarfs contained within the NLTT Catalogue, they have to be young objects (or else they would have cooled below detection limits), and young objects have smaller space motions on average. Hence, the list of uninvestigated, low-motion objects may yet contain a few buried treasures which 2MASS data will help to unearth.

 
Figure 1: Color-magnitude diagram of the NLTT objects (solid points) falling within the area surveyed by the 2MASS protocamera. For comparison are shown the locations of ultra-cool dwarfs (open squares) and the coolest dwarf detected to date in the 2MASS protocamera data (cross). In all cases, the R magnitudes come from the APM survey of POSS-I plates and the K magnitudes are from 2MASS (except for the ultra-cool dwarfs, whose K magnitudes are taken from the literature). Distances for those NLTT objects appearing in the latest edition of the Catalogue of Nearby Stars are also labelled.