Origin of the Lyman excess in early-type stars

Abstract

Ionized regions around early-type stars are believed to be well-known objects, but until recently, our knowledge of the relation between the free-free radio emission and the IR emission has been observationally hindered by the limited angular resolution in the far-IR. The advent of Herschel has now made it possible to obtain a more precise comparison between the two regimes, and it has been found that about a third of the young HII regions emit more Lyman continuum photons than expected, thus presenting a Lyman excess. With the present study we wish to distinguish between two scenarios that have been proposed to explain the existence of the Lyman excess: (i) underestimation of the bolometric luminosity, or (ii) additional emission of Lyman-continuum photons from an accretion shock. We observed an outflow (SiO) and an infall (HCO+) tracer toward a complete sample of 200 HII regions, 67 of which present the Lyman excess. Our goal was to search for any systematic difference between sources with Lyman excess and those without. While the outflow tracer does not reveal any significant difference between the two subsamples of HII regions, the infall tracer indicates that the Lyman-excess sources are more associated with infall signposts than the other objects. Our findings indicate that the most plausible explanation for the Lyman excess is that in addition to the Lyman continuum emission from the early-type star, UV photons are emitted from accretion shocks in the stellar neighbourhood. This result suggests that high-mass stars and/or stellar clusters containing young massive stars may continue to accrete for a long time, even after the development of a compact HII region.

arXiv commentLetter 4 pages, 3 figures and 1 table. Accepted for publication in A&A Letters

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• The physical and chemical structure of Sagittarius B2, I. Three-dimensional thermal dust and free-free continuum modeling on 100 au to 45 pc scales
• The Coordinated Radio and Infrared Survey for High-Mass Star Formation III. A catalogue of northern ultra-compact H II regions
• Weak and Compact Radio Emission in Early High-Mass Star Forming Regions: II. The Nature of the Radio Sources
• The accretion burst of the massive young stellar object G323.46 -0.08
• A search for radio jets from massive young stellar objects. Association of radio jets with H2O and CH3OH masers
• Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions
• ATLASGAL-selected high-mass clumps in the inner Galaxy. VII. Characterisation of mid-J CO emission
• SCOTCH -- Search for Clandestine Optically Thick Compact HIIs
• Multiwavelength and astrometric study of the DBS-89-90-91 embedded clusters region
• The GUAPOS project -- VII: Physical structure and molecular environment of the G31.41+0.31 HII region