Ultra-Compact H II Regions and the Early Lives of Massive StarsM. G. HoareUniversity of LeedsS. E. Kurtz and S. LizanoUniversidad Nacional Aut´onoma de M´exico - MoreliaE. KetoHarvard UniversityP. HofnerNew Mexico Institute of Technology and National Radio Astronomy ObservatoryWe review the phenomenon of ultra-compact H II regions (UCHIIs) as a key phase in theearly lives of massive stars. This most visible manifestation of massive star formation beginswhen the Lyman continuum output from the massive young stellar object becomes sufficientto ionize the surroundings from which it was born. Knowledge of this environment is gainedthrough an understanding of the morphologies of UCHII regions and we examine the latestdevelopments in deep radio and mid-IR imaging. SPITZER data from the GLIMPSE surveyare an important new resource in which PAH emission and the ionizing stars can be seen.These data provide good indications as to whether extended radio continuum emission aroundUCHII regions is part of the same structure or due to separate sources in close proximity. Wereview the role played by strong stellar winds from the central stars in sweeping out centralcavities and causing the limb-brightened appearance. New clues to the wind properties fromstellar spectroscopy and hard X-ray emission are discussed. A range of evidence from velocitystructure, proper motions, the molecular environment and recent hydrodynamical modelingindicates that cometary UCHII regions require a combination of champagne flow and bow shockmotion. The frequent appearance of hot cores, maser activity and massive young stellar objects(YSOs) ahead of cometary regions is noted. Finally, we discuss the class of hyper-compactH II regions or broad recombination line objects. They are likely to mark the transition soonafter the breakout of the Lyman continuum radiation from the young star. Models for theseobjects are presented, including photo-evaporating disks and ionized accretion flows that aregravitationally trapped. Evolutionary scenarios tracing young massive stars passage throughthese ionized phases are discussed.1. INTRODUCTIONThe time when newly formed massive stars begin to ion-ize their surroundings is one of the energetic events thatunderlines their important role in astrophysics. As theyevolve, the copious amounts of UV radiation and power-ful stellar winds they produce have a profound effect on thesurrounding interstellar medium. Their early lives are spentdeeply embedded within dense molecular cores whose highcolumn densities absorb the optical and near-IR light fromthe young stars, shielding them from view. One of the firstobservablemanifestations of a newly formed massive star isthe radio free-free emission of the H II region surroundingthe star. Since only the most massive stars produce sig-nificant radiation beyond the Lyman limit, embedded H IIregions are a unique identifier of high mass star formation.The absorption of the UV and optical radiation by dust,both in and outside the newly formed nebula, heats thegrains to temperatures that range from the sublimation tem-perature close to the star to interstellar temperatures in thesurrounding molecular cloud. Owing to the high luminosityof the massive stars, H II regions are some of the strongestinfrared sources in the galaxy. Similar to the radio emis-sion, the thermal IR radiation is little affected by extinction.Thus the combination of the radio and IR wave bands al-lows us to peer deep into the star forming clouds to studythe processes of star formation within.The youngest massive stars are associated with thesmallest H II regions. These are the ultra-compact H IIregions (UCHII) and the newly identified class of hyper-compact (HCHII) regions. UCHII regions were first dis-tinguished from merely “compact” H II regions around25 years ago, and came to be defined observationally(Wood and Churchwell, 1989a) as those regions with sizes≤ 0.1 pc, densities ≥ 104cm−3, and emission measures≥ 107pc cm−6. Since then, hundreds of UCHII regions1with these general properties have been found. Whilst thedivision of H II regions into different degrees of compact-ness may be a convenient label, for the larger objects atleast, it is likely to have little physical significance. Oncein the expansion phase, the physics of their dynamics prob-ably stays the same until the molecular material is clearedaway and the OB star joins the field population. Of greaterinterest are the smallest H II regions, as they more to tellus about the process of massive star formation. The ionizedgas within the UCHII and HCHII regions not only revealsproperties of the stars themselves, but also lights up theimmediate surroundings to allow investigations of the den-sity distribution and environment into which the massivestars are born. The external environment has a profoundinfluence on the evolution of the H II regions.The properties of UCHII regions and their immediateprecursors have been reviewed previously by Churchwell(2002) and Kurtz et al. (2000). In this review we concen-trate on more recent developments in the field of UCHIIand HCHII regions. These include new theories of massivestar forming accretion flows and the new views of UCHIIregions opened up by infrared studies on large ground-based telescopes and the SPITZER satellite. The SPITZERGLIMPSE survey has covered a large part of the innergalactic plane at unprecedented spatial resolution and depthin the 4-8 µm region where there is a local minimum inthe extinction curve. High resolution X-ray studies withChandra are also beginning to bear on the problem. To-gether these promise great new insights into how OB starsare formed and interact with their environment.2. ULTRA-COMPACT H II REGIONS2.1 MorphologiesThe morphologies of UCHII regions are important sincethey yield clues to the state of the surrounding medium rel-atively soon after a massive star has formed. The commonappearance of a regular morphology indicates that there areordered physical processes occurring during massive starformation rather than just stochastic ones.As part of their pioneering high resolution radio sur-veys of massive star forming regions Wood and Churchwell(1989a) developed a morphological classification schemefor UCHII regions. Together with Kurtz et al. (1994),they found that 28% of UCHII regions are spherical, 26%cometary, 26% irregular, 17% core-halo and 3% shell. Thesignificant numbers of unresolved sources have been omit-ted here, since
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