Origin of the Lyman excess in early-type stars^⋆

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: cesa@arcetri.astro.it
² I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
³ INAF, Istituto di Astrofisica e Planetologia Spaziale, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ IRAM, Avenida Divina Pastora, 7, Núcleo Central, 18012 Granada, Spain

Received: 26 November 2015
Accepted: 16 February 2016

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.