ASPENS is both an observational and archival effort that uses the single technique that directly measures companion mass: astrometry. Classic long-term astrometric studies carried out by Lippincott (1978), van de Kamp (1986), Heintz & Cantor (1994), Harrington et al. (1993) and others using photographic plates targeted dozens of stars and spanned decades. Later scrutiny has revealed that the level of reliable detection in these studies was much less than can be reached today with modern CCDs. Despite many claims, only one plate-identified companion with a perturbation signature of less than 30 mas has ever been comfirmed as real. In contrast, observing strategies and data reduction for CCD parallax programs routinely yield residuals at the few mas level, sufficient to probe for companions to M dwarf primaries (far and away the most abundant of nearby stars) that span the entire brown dwarf mass regime (13-80 MJup}).
We propose to extend the technique of a tried-and-true parallax program to include a survey for companions to nearby M dwarfs. The initial phase (1999-2002) of the Cerro Tololo Interamerican Observatory Parallax Investigation (CTIOPI based at GSU and led by T. Henry) has been responsible for the identification of more than 100 new stellar systems in the NStars volume. The telescope/instrumental setup has recently come under management of a consortium of institutions known as SMARTS (Small and Moderate Aperture Research Telescope System), which provides an opportunity for us to expand CTIOPI from 160 to 400 systems. A combined NAU/GSU allocation has allowed us to secure 3 years of telescope time at a rate of 150 nights/year. Our initial survey will study long-period companions throughout the brown-dwarf mass regime and extending into the sub-13MJup ``giant planet'' zone, even within the first duration of this program (see above figure). As a foundation for this program, we have also established an astrometry database that stores positions and calibration data as a function of time together with tools for graphic inspection and analysis. APENS observations will focus on the southern hemisphere to cover sky area that is inaccessible to a large number of planned efforts to carry out similar programs (e.g., Keck Outriggers, Pan-Stars, LSST, etc.). When the latter observations become available, we will incorporate these into the astrometry archive as well. In the meantime, observations of a smaller sample will continue in the northern hemisphere using the Lowell 31'' telescope under the auspices of National Undergraduate Research Observatory.(Left) Apparent motion of a 0.1 solar mass star at a distance of 20 pc with mean annual proper motions, mux = muy = 0.1'', and reflex motion under the influence of a 15 MJup companion in a circular orbit with semi-major axis 1.0 AU. The solid line represents 5 years of motion. Crosses represent observations in a 3-yr program. (Right) A plot of the time-varying perturbations in RA with the parallactic and proper motions taken out. Motions are due to companion with same properties as for the left panel.