Interstellar Bubbles in Two Young H [CSC]ii[/CSC] Regions

Massive stars are expected to produce wind-blown bubbles in the interstellarmedium; however, ring nebulae, suggesting the existence of bubbles, are rarelyseen around main-sequence O stars. To search for wind-blown bubbles aroundmain-sequence O stars, we have obtained high-resolution Hubble Space TelescopeWFPC2 images and high-dispersion echelle spectra of two pristine HII regions,N11B and N180B, in the Large Magellanic Cloud. These HII regions are ionized byOB associations that still contain O3 stars, suggesting that the HII regionsare young and have not hosted any supernova explosions. Our observations showthat wind-blown bubbles in these HII regions can be detected kinematically butnot morphologically because their expansion velocities are comparable to oronly slightly higher than the isothermal sound velocity in the HII regions.Bubbles are detected around concentrations of massive stars, individual Ostars, and even an evolved red supergiant (a fossil bubble). Comparisonsbetween the observed bubble dynamics and model predictions show a largediscrepancy (1--2 orders of magnitude) between the stellar wind luminosityderived from bubble observations and models and that derived from observationsof stellar winds. The number and distribution of bubbles in N11B differ fromthose in N180B, which can be explained by the difference in the richness ofstellar content between these two HII regions. Most of the bubbles observed inN11B and N180B show a blister-structure, indicating that the stars were formedon the surfaces of dense clouds. Numerous small dust clouds, similar to Bokglobules or elephant trunks, are detected in these HII regions and at least oneof them hosts on-going star formation.Comment: 24 pages and 16 figures (JPG), to be published in the AJ. A postscript version of the paper and the figures is avalaible at ftp://ftp.astro.uiuc.edu/pub/chu/naze/N1