Bipolar H II regions: II. Morphologies and star formation in their vicinities

Aims.We aim to identify bipolar Galactic HIIregions and to understand their parental cloud structures, morphologies, evolution, andimpact on the formation of new generations of stars. Methods.We use theSpitzer-GLIMPSE,Spitzer-MIPSGAL, andHerschel-Hi-GAL surveys to identify bipolar HIIregions and toexamine their morphologies. We search for their exciting star(s) using NIR data from the 2MASS, UKIDSS, and VISTA surveys.Massive molecular clumps are detected near these bipolar nebulae, and we estimate their temperatures, column densities, masses, anddensities. We locate Class 0/I young stellar objects (YSOs) in their vicinities using theSpitzerandHerschel-PACS emission. Results.Numerical simulations suggest bipolar HIIregions form and evolve in a two-dimensional flat- or sheet-like molecular cloud.We identified 16 bipolar nebulae in a zone of the Galactic plane between`±60◦and|b|< 1◦. This small number, when comparedwith the 1377 bubble HIIregions in the same area, suggests that most HIIregions form and evolve in a three-dimensional medium.We present the catalogue of the 16 bipolar nebulae and a detailed investigation for six of these. Our results suggest that these regionsformed in dense and flat structures that contain filaments. We find that bipolar HIIregions have massive clumps in their surroundings.The most compact and massive clumps are always located at the waist of the bipolar nebula, adjacent to the ionised gas. These massiveclumps are dense, with a mean density in the range of 105cm−3to several 106cm−3in their centres. Luminous Class 0/I sources ofseveral thousand solar luminosities, many of which have associated maser emission, are embedded inside these clumps. We suggestthat most, if not all, massive 0/I YSO formation has probably been triggered by the expansion of the central bipolar nebula, but theprocesses involved are still unknown. Modelling of such nebula is needed to understand the star formation processes at play