Molecular clouds in the Perseus arm toward Cassiopeia and Cepheus. (a) CO emission integrated over the velocity range −90 to −25 km s−1. The lowest contour is at 4.2 K km s−1, and each contour is 1.58 times higher than the previous, for five contours per decade. The peak intensity is 118 K km s−1. (b) Schematic of the Perseus arm clouds: the first and fifth contour levels of Fig. 1a and the location of selected Population I objects including the Cas A supernova remnant. Asterisks denote the centers of H II regions (Sharpless 1959), and circled asterisks denote H II regions at local arm velocities probably not associated with the molecular clouds in this map (except for S154; see text). Peak positions of molecular clouds are shown by filled triangles; clouds labeled with letters A–G were mapped in ﹩^{13}﹩CO as well. Dashed lines mark the approximate boundaries of the main clouds. Note that the small cloud G114.62+0.25 near S165 has a surface brightness lower than the cutoff chosen for these maps.