Interstellar bubbles in two young HII regions

Massive stars are expected to produce wind-blown bubbles in the interstella r medium; however, ring nebulae, suggesting the existence of bubbles, are r arely seen around main-sequence O stars. To search for wind-blown bubbles a round main-sequence O stars, we have obtained high-resolution Hubble Space Telescope WFPC2 images and high-dispersion echelle spectra of two pristine H II regions, N11B and N180B, in the Large Magellanic Cloud. These H II reg ions are ionized by OB associations that still contain O3 stars, suggesting that the H II regions are young and have not hosted any supernova explosio ns. Our observations show that wind-blown bubbles in these H II regions can be detected kinematically, but not morphologically, because their expansio n velocities are comparable to or only slightly higher than the isothermal sound velocity in the H II regions. Bubbles are detected around concentrati ons of massive stars, individual O stars, and even an evolved red supergian t (a fossil bubble). Comparisons between the observed bubble dynamics and m odel predictions show a large discrepancy (1-2 orders of magnitude) between the stellar wind luminosity derived from bubble observations and models an d that derived from observations of stellar winds. The number and distribut ion of bubbles in N11B differ from those in N180B, which can be explained b y the difference in the richness of stellar content between these two H II regions. Most of the bubbles observed in N11B and N180B show a blister stru cture, indicating that the stars were formed on the surfaces of dense cloud s. Numerous small dust clouds, similar to Bok globules or elephant trunks, are detected in these H II regions, and at least one of them hosts on-going star formation.