The Spectral Type of the Ionizing Stars and the Infrared Fluxes of HII Regions

Abstract

The 20 cm radio continuum fluxes of 91 HII regions in a previously compiled catalog have been determined. The spectral types of the ionizing stars in 42 regions with known distances are estimated. These spectral types range from B0.5 to O7, corresponding to effective temperatures of 29 000-37 000 K. The dependences of the infrared (IR) fluxes at 8, 24, and 160 µm on the 20 cm flux are considered. The IR fluxes are used as a diagnostic of heating of the matter, and the radio fluxes as measurements of the number of ionizing photons. It is established that the IR fluxes grow approximately linearly with the radio flux. This growth of the IR fluxes probably indicates a growth of the mass of heated material in the envelope surrounding the HII region with increasing effective temperature of the star. * Electronic address: ATopchieva@inasan.ru arXiv:1811.09093v2 [astro-ph.GA] 26 Nov 2018photodissociation region (PDR). The hydrogen in this region is predominantly neutral, but the degree of ionization of the gas is about 0.01% due to the ionization of carbon. Since the density of the heated gas and dust in a PDR is usually high (see, e.g., the review D. J.Hollenbach и A. G. G. M. Tielens [5]), PDRs are luminous in the infrared (IR). Results from the "Spitzer" IR telescope reveal the presence of thousands of nebulae in the Galactic disk whose shapes are ring-like at 8 µm [6][7][8]. Deharveng et al. [9] showed that most of these ring nebulae are IR reflections of HII regions. Polycyclic aromatic hydrocarbons (PAHs) radiate at 8 µm in PDRs heated by the UV radiation of massive stars. Longer-wavelength IR emission in HII regions is radiated by larger heated dust grains [10,11]. It is interesting that 24 µm emission (the photometric range of the "Spitzer" telescope) is observed inside the 8 µm rings, while longer-wavelength emission (70, 160, 250, and sometimes 350 µm) is coincident with the ring observed at 8 µm [8]. Studies of the spatial distribution of the IR emission in HII regions can provide information about the dust dynamics and the destruction of dust by UV radiation from massive stars (e.g., E. Kruegel [12], J. S. Mathis [13], Pavlychenkov et al. [14], Akimkin et al. [15, 16], Murga et al. [17]). The spectral type and spectral energy distributions of the ionizing stars are key parameters in any model for the dynamics and destruction of dust and PAHs in the ISM [10, 17].Unfortunately, such data are available for only a relatively small fraction of the IR nebulae observed with the "Spitzer" telescope, which were studied before the launch of Spitzer.However, the spectral types of young, massive stars in HII regions can be estimated based on archival data on the radio emission of the heated ionized gas (e.g., [18]). This makes it possible to obtain the parameters of the ionizing sources in large numbers of HII regions.During the expansion of an HII region, there is an increase in the mass of the surrounding neutral, dense envelope due to the action of shocks [19]. However, no analytical and numerical studies of time vari...

Molecular envelope around the HII region RCW 120

“…Analysing a large set of objects from a catalogue of IR ring-like nebulae around H ii regions embedded into molecular clouds (Topchieva et al 2017), and also large sample of AKARI data of IR bubbles (Hanaoka et al 2019), we conclude that the images of the H ii regions at 8 µm appear as quite uniform bubbles due to the significant optical depths, while their shape at longer wavelengths represents a number of clumps around massive stars. Topchieva et al (2018b) constructed an evolutionary sequences of H ii regions and checked how fluxes at 8, 24, and 160 µm change during their expansion. While they found that the electron number density decreases along with increasing size of the H ii regions in agreement with theoretical expectations, there was not any particular trend for the IR fluxes.…”

The Distribution of Dense Cores near HII Regions

“…However, when averaged over much of their surface area, the extinction to these HII regions is typically only a few magnitudes as seen in many studies including from measurements of the Blamer decrement over large apertures (Fich and Silkey, 1991). Even deeply embedded, presumably younger HII regions show the neutral material in very clumpy structures with spherical shells in 3D (Topchieva et al, 2018(Topchieva et al, ,2019. This is further complicated by the observation that many HII regions that are seen in the visible are on the near side of large molecular clouds and emerging towards the observer, with much less neutral material on the near side than on the far side.…”