Abstract
HII regions are ionized regions of interstellar gas, which ionized by intensive ultraviolet radiation from nearby hot stars. Radio recombination line is one of the main tools for investigating the HII region, in particular, the distribution of HII is obtained and the main physical conditions of the interstellar matter are determined from hydrogen lines. This study was carried out based on the analysis of the H110a recombination emission line toward the W40 HII region, which is one of the most active star-forming regions in the Aquila Molecular Cloud. We used archival observational data of the H110a radio recombination line in the Aquila Molecular Cloud, obtained during February in 2015 using the 26m NanShan radiotelescope in the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences. During the period of the study, an integrated intensity map of H110a toward the W40 HII region and the corresponding spectra were constructed. The electron density and temperature of the HII region, the emission measure, the optical thickness, the Lyman continuum flux, the excitation parameter, the radius of the Strömgren sphere, and the mass of ionized hydrogen inside the sphere were calculated. The values of the emission measure and optical thickness indicate that the H110a recombination radio line is optically thin and traces a very dense region (<6462 AU) with a high electron temperature. The numbers of photons of the Lyman continuum indicates the presence of a massive O9.5 star, that equivalent to a zero-age main-sequence star located within the HII region. The obtained values of the physical parameters indicate that the region of ionized hydrogen under study is an ultracompact region. The results of this article lay the foundation for further research aimed at studying the evolution of HII regions, as well as their role in the processes of starformation.