In-depth study of the hypercompact HII region G24.78+0.08 A1 - Kölner UniversitätsPublikationsServer

Cesaroni, R., Beltran, M. T., Moscadelli, L., Sanchez-Monge, A. and Neri, R. (2019). In-depth study of the hypercompact HII region G24.78+0.08 A1. Astron. Astrophys., 624. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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DOI:10.1051/0004-6361/201834700

Abstract

Context. The earliest phases of the evolution of a massive star are closely related to the developement of an HII region. Hypercompact HII regions are the most interesting in this respect because they are very young, and hence best suited to study the beginning of the expansion of the ionised gas inside the parental core. Aims. We have analysed the geometrical and physical structure of the hypercompact HII region G24.78+0.08 A1, making use of new continuum and hydrogen recombination line data (H41 alpha, H63 alpha, H66 alpha, H68 alpha) and data from the literature (H30 alpha, H35 alpha). Methods. We fit the continuum spectrum with a homogenous, isothermal shell of ionised gas at 10 4 K and derive the size of the HII region and the Lyman continuum luminosity of the ionising star. We also fit the recombination line spectra emitted from the same shell with a model taking into account expansion at constant speed. Results. The best fits to the continuum and line spectra allow the derivation of the Lyman continuum luminosity of the ionising star, HII region size, geometrical thickness of the shell, and expansion velocity. Comparison between the 5 cm and 7 mm brightness temperature distributions demonstrates that a thin layer of ionised gas of a few 1000 K at the surface of the HII region is necessary to reproduce the morphology of the continuum emission at both wavelengths. Conclusions. We confirm that the G24 A1 hypercompact HII region consists of a thin shell ionised by an 09.5 star. The shell is expanding at a speed comparable to the sound speed in the ionised gas. The radius of the HII region exceeds the critical value needed to trap the ionised gas by the gravitational field of the star, consistent with the observed expansion.

Item Type:

Journal Article

Creators:

Creators	Email	ORCID	ORCID Put Code
Cesaroni, R.	UNSPECIFIED	UNSPECIFIED	UNSPECIFIED
Beltran, M. T.	UNSPECIFIED	UNSPECIFIED	UNSPECIFIED
Moscadelli, L.	UNSPECIFIED	UNSPECIFIED	UNSPECIFIED
Sanchez-Monge, A.	UNSPECIFIED	UNSPECIFIED	UNSPECIFIED
Neri, R.	UNSPECIFIED	UNSPECIFIED	UNSPECIFIED