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We have completed a high-resolution (R=60,000) optical spectroscopic surveyof 185 nearby M dwarfs identified using ROSAT data to select active, youngobjects with fractional X-ray luminosities comparable to or greater thanPleiades members. Our targets are drawn from the NStars 20-pc census and theMoving-M sample with distances determined from parallaxes or spectrophotometricrelations. Nearly half of the resulting M dwarfs are not present in the Gliesecatalog and have no previously published spectral types. We identified 30spectroscopic binaries (SBs) from the sample, which have strong X-ray emissiondue to tidal spin-up rather than youth. This is equivalent to a 16%spectroscopic binary fraction, with at most a handful of undiscovered SBs. Weestimate upper limits on the age of the remaining M dwarfs using spectroscopicyouth indicators such as surface gravity-sensitive indices (CaH and K I). Wefind that for a sample of field stars with no metallicity measurements, asingle CaH gravity index may not be sufficient, as higher metallicities mimiclower gravity. This is demonstrated in a sub-sample of metal-rich RV standards,which appear to have low surface gravity as measured by the CaH index, yet showno other evidence of youth. We also use additional youth diagnostics such aslithium absorption and strong H-alpha emission to set more stringent agelimits. Eleven M dwarfs with no H-alpha emission or absorption are likely old(>400 Myr) and were caught during an X-ray flare. We estimate that our finalsample of the 144 youngest and nearest low-mass objects in the field is lessthan 300 Myr old, with 30% of them being younger than 150 Myr and 4 very young(<10 Myr), representing a generally untapped and well-characterized resource ofM dwarfs for intensive planet and disk searches.

Identifying the Young Low-mass Stars within 25 pc. I. Spectroscopic Observations