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Origin of the Lyman excess in early-type stars

Journal
Date Issued
2016
Author(s)
•
Sánchez-Monge, Á.
•
•
•
•
Treviño-Morales, S. P.
DOI
Abstract
Context. 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 H II regions emit more Lyman continuum photons than expected, thus presenting a Lyman excess. Aims: 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. Methods: We observed an outflow (SiO) and an infall (HCO+) tracer toward a complete sample of 200 H II 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. Results: While the outflow tracer does not reveal any significant difference between the two subsamples of H II regions, the infall tracer indicates that the Lyman-excess sources are more associated with infall signposts than the other objects. Conclusions: 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 neighborhood. 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 H II region. Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).
Volume
588
Start page
L5
Uri
Url
Issn Identifier
0004-6361
Ads BibCode
2016A&A...588L...5C
Rights
open.access
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