We present a carefully vetted equatorial (±30 decl.) sample of all known single (within 4'') mid M dwarfs (M2.5 V–M8.0 V) extending out to 10 pc; their proximity and low masses make them ideal targets for planet searches. For this sample of 58 stars, we provide VJ, RKC, and IKC photometry, new low-dispersion optical (6000–9000 ?) spectra from which uniform spectral types are determined, multi-epoch Hα equivalent widths, and gravity-sensitive Na i indices. For 12 of these 58 stars, strict limits are placed on the presence of stellar and substellar companions based on a pioneering program described here that utilizes precise infrared radial velocities (RVs) and optical astrometric measurements in an effort to search for Jupiter-mass, brown dwarf, and stellar-mass companions. Our infrared RV precision using CSHELL at NASA's Infrared Telescope Facility is ~90 m s?1 over timescales from 13 days to 5 yr. With our spectroscopic results the mean companion masses that we rule out of existence are 1.5 MJUP or greater in 10 day orbital periods and 7 MJUP or greater in 100 day orbital periods. We use these spectra to determine rotational velocities and absolute RVs of these 12 stars. Our mean astrometric precision using Research Consortium on Nearby Stars (RECONS; www.recons.org) data from the 0.9 m telescope at Cerro Tololo Inter-American Observatory is ~3 mas over baselines ranging from 9 to 13 yr. With our astrometric results the mean companion masses that we rule out of existence are greater than 11.5 MJUP with an orbital period of 4 yr and greater than 7.5 MJUP with an orbital period of 8 yr. Although we do not detect companions around our subsample of 12 stars, we demonstrate that our two techniques probe a regime that is commonly missed in other companion searches of late-type stars.
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