Distance constraints to the anticenter high-velocity clouds

High-resolution 21 cm column density maps of neutral hydrogen obtained with the 305 m Arecibo telescope are used to select target stars in a search for optical interstellar absorption lines at high velocities. Four stars are observed toward the high-velocity knot (AC 0) in the anticenter shell. Three stars are observed toward the anticenter chain high-velocity cloud AC I. And five stars are observed toward the high-velocity cloud in Cetus called Cohen's stream. Two additional stars are observed toward the ''hook'' seen at the southeastern edge of the anticenter shell, even though no high-resolution H I map was obtained for this region. Optical spectra of the Na I D and Ca II H and K lines obtained with the Hamilton spectrograph on the Lick 3 m telescope show no high-velocity interstellar absorption. The high-resolution H I maps allow the results of an absorption-line nondetection to be characterized more accurately. A target star lying toward the dense small-scale clumps and filaments in the high-velocity gas, but for which there is no optical signature, can be confidently declared to lie in the foreground of the high-velocity gas. The case in which the line of sight to the star intersects the high-velocity cloud, but unfortunately passes through a region with a column density too low to detect, is ruled out by the use of these high-resolution maps, unless there is high-contrast spatial structure on scales below 3'. The AC 0 and the anticenter shell are strongly constrained to be more than 610 pc from the Sun, with a weaker distance lower limit of 950 pc. If the intermediate-velocity interstellar absorption detected toward LS V+30 31 can be associated with the anticenter shell, then the supershell is closer than 6500 pc. The anticenter chain cloud AC I is almost certainly at least 650 pc from the Sun. Narrow H I emission lines indicative of cold gas are observed at high velocities in the AC 0 and AC I maps. Cohen's stream is similarly constrained to be at least 350 pc and possibly more than 390 pc from the Sun. The anticenter shell ''hook'' is further than 350 pc.