M Dwarfs fromHubble Space TelescopeStar Counts. III. The Groth Strip

We analyze the disk M dwarfs found in 31 new fields observed with the WideField Camera (WFC2) on the Hubble Space Telescope (HST) together with thesample previously analyzed from 22 WFC2 fields and 162 prerepair PlanetaryCamera (PC1) fields. The new observations, which include the 28 high-latitudefields comprising the Large Area Multi-Color Survey (``Groth Strip''), increasethe total sample to 337 stars, and more than double the number of late M dwarfs(M_V>13.5) from 23 to 47. The mass function changes slope at M~0.6 Msun, from anear-Salpeter power-law index of \alpha=-1.21 to \alpha=0.44. In both regimesthe mass function at the Galactic plane is given by ${d^3 N / d\log M d M_V dV} = 8.1\times 10^{-2}\pc^{-3} ({M / 0.59 M_\odot})^{\alpha}$. The correctionfor secondaries in binaries changes the low-mass index from \alpha=0.44 to\alpha\sim 0.1. If the Salpeter slope continued to the hydrogen-burning limit,we would expect 500 stars in the last four bins (14.5

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