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标题：In-depth study of the hypercompact H  II region G24.78+0.08 A1 ★ ★★
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作者：R. Cesaroni ; M. T. Beltrán ; L. Moscadelli 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2019
卷号：624
DOI：10.1051/0004-6361/201834700
语种：English
出版社：EDP Sciences
摘要：Context.The earliest phases of the evolution of a massive star are closely related to the developement of an H IIregion. Hypercompact H IIregions 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 H IIregion G24.78+0.08 A1, making use of new continuum and hydrogen recombination line data (H41α, H63α, H66α, H68α) and data from the literature (H30α, H35α).Methods.We fit the continuum spectrum with a homogenous, isothermal shell of ionised gas at 104K and derive the size of the H IIregion 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, H IIregion 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 H IIregion is necessary to reproduce the morphology of the continuum emission at both wavelengths.Conclusions.We confirm that the G24 A1 hypercompact H IIregion consists of a thin shell ionised by an O9.5 star. The shell is expanding at a speed comparable to the sound speed in the ionised gas. The radius of the H IIregion exceeds the critical value needed to trap the ionised gas by the gravitational field of the star, consistent with the observed expansion.
关键词：Key wordsenstars: early-typeHII regionsISM: individual objects: G24.78+0.08stars: winds, outflows