Physical parameters of the Orion Bar photodissociation region from radio recombination line observations at 8 mm

Observations of carbon (C), hydrogen and helium (H, He) radio recombination lines (RRLs) at four positions in the Orion Bar photodissociation region (PDR) and toward the center of Orion A have been performed with the RT-22 radio telescope (Pushchino) at 8 mm. The physical parameters of the PDR at these points have been estimated by comparing the carbon RRLs and infrared CII and OI lines. A hydrogen number density in the range 1.2–3.1 × 10⁵ cm^?3 and a mean size of the region along the line of sight (L) in the range 0.006–0.04 pc have been derived. The PDR temperature decreases with increasing distance from the exciting star (θ ¹ C Ori) from 210–230 to 140–150 K (a distance of ≈5′). The data obtained confirm the increase in the PDR size along the line of sight toward the Orion Bar, where, however, L has turned out to be less than the available values in the literature, which can be explained by the presence of clumps in the PDR. A density jump is evident in the Orion Bar region. The PDR zone encompasses the core of the HII region by a thin layer and extends farther, delineating the boundary and the ionization front of the core of the HII region in the Orion Bar and further out the boundary between the halo of the HII region and the molecular cloud. The derived emission measure (EM) toward the Orion Bar has been compared with other C RRL observations. The EM measured from carbon RRLs is EM ≈ 100(±50%) pc cm^?6, imposing constraints on the possible two-component PDR structure. Estimates show that the star θ ¹ C Ori is quite sufficient as a carbon ionization source in the Orion Bar PDR. Some of the data on the ionized hot gas (HII) in this direction have been obtained from H and He RRLs. In particular, the radial velocities (V _lsr) of the HII region are blueshifted with respect to V _lsr of the PDR by 10–17 km s^?1, while the relative ionized helium abundance decreases with increasing distance from the star, indicating that the helium ionization zone is smaller than the ionized hydrogen one.     

Determination of the primordial helium abundance from radio recombination line observations

We describe a method for determining the primordial helium abundance (Y _p) from radio recombination lines and present results of the first stage of our measurements. We analyze the observational data for hydrogen and helium radio recombination lines from six Galactic H II regions obtained at different times. We have found Y _p = 25.2−25.5% with an error of ±0.9% from four sources.     

Detection and application of carbon recombination lines in millimeter range

We carried out millimeter-range observations of carbon radiolines from Photo-Dissociation Regions (PDRs). Comparison with observations of these objects in infrared fine structure lines of CII and OI allows one to determine the physical conditions in the PDRs (density, temperature, and thickness of the PDR layer) that were done for several regions connected to HII regions. It is shown that recombination radiolines also allow the kinematics of the complexes including HII regions and PDRs to be traced.     

Molecular emission in dense massive clumps from the star-forming regions S231–S235

The paper is concerned with the study of the star-forming regions S231–S235 in radio lines of molecules of the interstellar medium—carbon monoxide (CO), ammonia (NH₃), cyanoacetylene (HC₃N), in maser lines—methanol (CH₃OH) and water vapor (H₂O). The regions S231–S235 belong to the giant molecular cloudG174+2.5. The goal of this paper is to search for new sources of emission toward molecular clumps and to estimate their physical parameters from CO and NH₃ molecular lines. We obtained new detections ofNH₃ andHC₃Nlines in the sources WB89673 and WB89 668 which indicates the presence of high-density gas. From the CO line, we derived sizes, column densities, and masses of molecular clumps. From the NH₃ line, we derived gas kinetic temperatures and number densities in molecular clumps. We determined that kinetic temperatures and number densities of molecular gas are within the limits 16–30 K and 2.8–7.2 × 10³ cm^?3 respectively. The shock-tracing line of CH₃OH molecule at a frequency of 36.2 GHz was detected in WB89 673 for the first time.     

Search for Bright Masers in the Water Vapor Line towards the Dust Clumps of the Galaxy

Astronomy Reports - The paper is devoted to the analysis of the observation results with the single-mirror radio telescope RT-22 (PRAO, LPI) in the water vapor maser line at 22 GHz in the direction...     

Effective temperature of the star <Emphasis Type="Italic">ϑ</Emphasis>1 Ori C

We analyze the calibrations (the dependence of the spectral type of a star on its effective temperature) by comparing the calculated ionization structure of an H II region with that obtained from observations for the source Orion A in radio recombination H and He lines. The effective temperature of the star ?1 Ori C (the ionization source) is most likely ≈37 000 K.     

Physical conditions in photodissociation regions from observations of carbon radio recombination lines and IR fine-structure lines of CII and OI

Observations of the C56α line—the first carbon radio recombination line in the millimeter waveband—in the directions of the Orion Bar, NGC 2024, and W3 are reported. The results are analyzed together with data for the IR fine-structure lines of CII and OI, enabling unambiguous determination of the temperature and density in the photodissociation regions separating the HII regions and parent molecular clouds.