Sequential and spontaneous star formation around the mid-infrared halo H ii region KR 140

Abstract

We use Two-Micron All-Sky Survey and Midcourse Space Experiment infrared observations, along with new molecular line (CO) observations, to examine the distribution of young stellar objects (YSOs) in the molecular cloud surrounding the halo H ii region KR 140 in order to determine if the ongoing star formation activity in this region is dominated by sequential star formation within the photodissociation region (PDR) surrounding the H ii region. We find that KR 140 has an extensive population of YSOs that have ‘spontaneously’ formed due to processes not related to the expansion of the H ii region. Much of the YSO population in the molecular cloud is concentrated along a dense filamentary molecular structure, traced by C18O, that has not been erased by the formation of the exciting O star. Some of the previously observed submillimetre clumps surrounding the H ii region are shown to be sites of recent intermediate- and low-mass star formation while other massive starless clumps clearly associated with the PDR may be the next sites of sequential star formation.