An accurate distance to high-velocity cloud Complex C

We report an accurate distance of d = 10 +/- 2.5 kpc to the high-velocity cloud Complex C. Using high signal-to-noise ratio Keck HIRES spectra of two horizontal-branch stars, we have detected Ca II K absorption lines from the cloud. Significant nondetections toward a further three stars yield robust lower distance limits. The resulting H I mass of Complex C is M-HI = 4.9(-2.2)(+2.9); 10(6) M-circle dot; a total mass of M-tot = 8.2(-2.6)(+4.6); 10(6) M-circle dot is implied, after corrections for helium and ionization. At 10 kpc, Complex C has physical dimensions 3; 15 kpc, and if it is as thick as it is wide, then the average density is log < n > similar or equal to -2.5. We estimate the contribution of Complex C to the mass influx may be as high as similar to 0.14 M-circle dot yr(-1).