2018A&A...617A..67S

Aims. We aim to identify bipolar Galactic HII regions and to understand their parental cloud structures, morphologies, evolution, and impact on the formation of new generations of stars.
Methods. We use the Spitzer-GLIMPSE, Spitzer-MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar HII regions and to examine 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, and densities. We locate Class 0/I young stellar objects (YSOs) in their vicinities using the Spitzer and Herschel-PACS emission.
Results. Numerical simulations suggest bipolar HII regions 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 l±60° and |b|<1°. This small number, when compared with the 1377 bubble HII regions in the same area, suggests that most HII regions 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 regions formed in dense and flat structures that contain filaments. We find that bipolar HII regions 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 massive clumps are dense, with a mean density in the range of 10⁵cm^–3 to several 10⁶cm^–3 in their centres. Luminous Class 0/I sources of several thousand solar luminosities, many of which have associated maser emission, are embedded inside these clumps. We suggest that most, if not all, massive 0/I YSO formation has probably been triggered by the expansion of the central bipolar nebula, but the processes involved are still unknown. Modelling of such nebula is needed to understand the star formation processes at play.