SEQUENTIAL STAR FORMATION IN THE Sh 254-258 MOLECULAR CLOUD: HEINRICH HERTZ TELESCOPE MAPS OF COJ= 2-1 AND 3-2 EMISSION

The molecular cloud associated with the Sh 254–258 group of five small H ii regions appears to be forming a (late)-OB association. We have mapped the associated molecular cloud in the J = 2–1 line of the CO and 13 CO molecules over 0 75 × 1°, and the CO J = 3–2 line toward the two main peaks, with the University of Arizona Heinrich Hertz Submillimeter Telescope. The data are analyzed with a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with images at optical, IR, and radio wavelengths. From this analysis, we propose a scenario for sequential formation of the stars exciting the H ii regions, triggered by the compression of the molecular gas as a consequence of the expansion of the adjacent H ii regions.