Radio recombination lines comprise a powerful tool for studying the interstellar medium. One of the important tasks is to measure the primordial abundance of helium formed during the primordial nucleosynthesis of the Universe, which, in turn, allows us to verify the conclusions of the Standard Cosmological Model. The Orion A nebula is an interesting object for studying this problem. Previously, we found that, in this HII region, the region of ionized helium is smaller than the region of ionized hydrogen. Therefore, the actual helium abundance, n (He)/ n (H), is not less than the maximum measured value of y + = n (He + )/ n( H + ). This makes it possible to obtain restrictions on the primordial helium abundance. This article presents new observations of radio recombination lines in Orion A at a wavelength of 13 mm. It was found that the maximum value of y + is in the range 10.03–11.55%. Therefore, it can be expected that the primordial helium abundance ( Y p , He/H mass ratio) can be not less than ≈24.93–29.40%, which allows deviations from the conclusions of the Standard Model; for instance, it allows the presence of unknown light particles during primordial nucleosynthesis. Observations of doubly ionized helium in Orion A and the planetary nebula NGC 7027 were also made. It was found that the contribution of doubly ionized helium, y ++ = n (He ++ )/ n (H + ), is <7 × 10 –4 in Orion A and 2.7(±1.3)% for NGC 7027. Estimates of the electron temperature are also made. In particular, for NGC7027, it was found that Т e ≈ 11 900–12 300 K, which is higher than for the HII regions.

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 of dust clumps from the ATLASGAL and Bolocam large-scale blind surveys in the millimeter continuum. The aim of this paper is to search for masers in the water vapor line in previously unexplored sources and to study the variability of known masers. Observations of 140 sources resulted in the detection of maser emission at 22 GHz in six sources, and in four sources (ATLASGAL 016.987+00.981, 017.216+00.821, 018.701–00.229, and Bolocam 081.174–00.100) masers were detected for the first time. In two sources (ATLASGAL 034.195–0.592 and Bolocam 081.174–0.1), the water vapor line masers showed significant variability during the 8-month observation and follow-up program. Comparison of RT-22 observational data with archival data from the GBT‑100 telescope (US National Radio Astronomy Observatory) showed that 9 out of 140 sources show a significant change in the radiation flux density of masers: two sources changed from the category of masers with low brightness ( $$\sim {\kern 1pt} 0.5$$ Jy) to the category bright (more than 10 Jy), and in seven relatively bright earlier (more than 3 Jy) masers, no radiation was detected. Significant changes in the characteristics of the maser emission may indicate changes in the environmental conditions in which the maser emission was produced, which is of interest for further study of maser-related processes.

Recombination radio lines (RRLs) of hydrogen (H), helium (He-4), and carbon (C) are a powerful tool for studying the interstellar medium (ISM) in space. The RRLs observations allow to obtain the physical parameters of regions of ionized hydrogen (HII regions) as well as of the photo-dissociation regions; and to estimate the effective temperature of stars that ionize the HII region. There is also an important cosmological task for RRL – measuring Primordial helium abundance produced at the stage of Primordial nucleosynthesis of the Universe. It turned out that the Orion A nebula is an interesting object for the latter task. At the time, the recombination radio lines observations of hydrogen, helium (H, He), and carbon (C) at a number of positions in the Orion A HII region were carry out with the RT22 radio telescope (Pushchino) at wavelengths of 8 and 13 mm. The relative helium abundance, y = n(He)/n(H), in these positions was obtained. The behavior of this value over the nebula showed that the helium ionization zone is smaller than the hydrogen one with different ratios for the core and halo. The location where the maximum y value is expected was also determined. For the established ionization structure, it means that the actual helium abundance in Orion A, n(He)/n(H), will not be less than the maximum yvalue. This allows to estimate the limitations on the Primordial helium abundance. In this work, new H and He RRL observations were made at 13 mm in the direction of the expected maximum of y. RRLs were observed in two transitions 65α and 66α. It was found that the maximum y value is in the range of 10 − 12%. Hence, we can expect that the Primordial helium abundance (Yp, the ratio He/H by mass) lies in the range of ≈ 26.4 − 29%, and the number of light neutrino-type particles during Primordial nucleosynthesis may exceed the standard value. To refine the result the work will be continue.

The radio recombination lines (RRLs) of hydrogen, helium (H, He) and carbon (C) have been observed at several positions of the HII region Orion A with the RT-22 radio telescope (Pushchino) at 8 and 13 mm. Information about the ionization structure of the HII region has been obtained. The behavior of y+ = n(He+)/n(H+) over the nebula and model calculations suggest that the effective temperature (Teff) of the star θ1 C Ori is in the range 35 000–37 500 K, corresponding to a spectral type ≈O6.5 V, which is important for the calibration of hot O-B stars. The electron temperatures (Te) of this HII region have been measured by taking into account the departures from local thermodynamic equilibrium (LTE); their distribution over the nebula up to distances of 300 arcsec from the center has been derived. The inferred temperatures are in the range 6600–8400 K, strictly decreasing in the eastward directionwith distance from the center, also tend to drop toward the periphery in the southward and westward directions. The turbulent velocities (Vt) of the ionized gas and their distribution over the nebula have been determined. The values of Vt inferred from H RRLs are in the range 9–13 km s−1.

New results of the Primordial Helium abundance (Yp) measurement by radio recombination lines (RRL) observations from galactic HII regions are presented. The RRL observations were carried out with two Parabolas: RT32(λ = 13.5 mm and 3.5 cm, Medicina, Italy) and RT22 (λ = 8 and 13.5 mm, Pushchino, Russia). The current value was obtained on the base of five source observations with the accurate correction for both the ionization structure and dust influence. At present, the obtained Yp = 25.74(±1.0)value seems to be higher than that suggested by optical data, allowing the presence of unknown light particles, and is in agreement with the result from cosmic microwave background asymmetry.

Observations of recombination radio lines (RRL) of hydrogen, helium (H, He) and carbon (C) were carried out in several positions of the HII region Orion A with RT22 radio telescope (Pushchino, Russia) at the wavelengths of 8 and 13 mm. The information about the ionization structure of the HII region was received. It is obtained that the measured helium abundance increases in the directions to "North" and "West” with a maximum at angular distances of 100-150˝ after which it declines. The maximum measured relative helium abundance, y+ = n(He+)/n(H+), is in the range of 9.4 – 11.0 %, therefore the actual He abundance n(He)/n(H)) is ≥ 9.4(±0.5) %. With these estimates, the lower limit of the primordial helium abundance Yp ≥ 25.19 (±1.15) % should be expected. This limit is still not strong enough to assert the excess over Yp predicted by the standard cosmological model (≈ 24.8%), but it admits the existence of unknown light particles. The y+ behavior and model calculations indicate that Teff (effectivetemperature) of q1 Ori C star is 35000 – 37500 K, corresponding to the star spectral type of ≈ O6.5 V, which is important for the calibration of hot OB-stars. Measured electron temperatures (Te) of the HII region, taking into account the deviations from the LTE, are in the range of 6600 – 8400 K and are strictly decreasing in direction to the “East”. Alas information on the turbulent velocities of the ionized gas and its electron density was obtained.

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.

Physical parameters of HII/PDR complex in Ori Bar were estimated using observations of recombination radio lines (RRL) of carbon (C), hydrogen (H) and helium (He) with RT-22 radio telescope in Pushchino. Possible structure of the complex is discussed. Effective temperature (Teff) of the star ionizing Orion nebula was estimated. Obtained estimate provides additional constraint to existing “spectral class – Teff“ calibrations for O-stars indicating that the q1 С Ori star with ≈ O6.5 V spectral class has Teff in the range 36 000–37 500 K

На радиотелескопе РТ-22 (Пущино) на волне 8 мм были проведены наблюдения рекомбинационных радиолиний (РРЛ) углерода (С), водорода и гелия (Н, Не) в четырех позициях области фотодиссоциации (ОФД ) Orion Bar, а также на центр Орион А. Из сравнения полученных РРЛ углерода и инфракрасных линий СII и ОI проведены оценки физических параметров ОФД в этих точках. Получены концентрация водорода в интервале 1.2– см и средний размер области по лучу зрения ( в интервале 0.006–0.04 пк. Температура ОФД уменьшается с удалением от возбуждающей звезды ( С Ori) от 210–230 К до 140–150 К (расстояние ). Полученные данные подтверждают увеличение размеров ОФД по лучу зрения в направлении на Orion Bar, где, однако, величина оказалась меньше, чем имеющиеся значения в литературе, что можно объяснить наличием сгустков в ОФД. В районе Orion Bar намечается скачок плотности. Зона ОФД тонким слоем охватывает ядро области HII и продолжается дальше, обозначая в районе Orion Bar границу и ионизационный фронт ядра HII-области, а далее - границу между гало HII области и молекулярным облаком. В направлении на Orion Bar проведено сравнение полученной меры эмиссии (МЕ) с другими наблюдениями РРЛ С. Измеренная по РРЛ углерода пк см , что накладывает ограничения на возможную двухкомпонентную структуру ОФД. Оценки показывают, что в качестве источника ионизации углерода в ОФД Orion Bar вполне достаточно звезды С Ori. По РРЛ Н и Не получены некоторые данные об ионизованном горячем газе (HII) в этом направлении. В частности, лучевые скорости ( ) HII-области смещены в голубую сторону по отношению к ОФД на 10–17 км/с, а относительное содержание ионизованного гелия уменьшается с удалением от звезды, указывая, что зона ионизации гелия меньше зоны ионизованного водорода.

Gas density and temperature in star forming regions around Sh2-235 are derived from ammonia line observations. This information is used to evaluate formation scenarios and to determine evolutionary stages of the young embedded clusters S235 East1, S235 East2, and S235 Central. We also estimate the gas mass in the embedded clusters and its ratio to the stellar mass. S235 East1 appears to be less evolved than S235 East2 and S235 Central. In S235 East1 the molecular gas mass exceeds that in the other clusters. Also, this cluster is more embedded in the parent gas cloud than the other two. Comparison with a theoretical model shows that the formation of these three clusters could have been stimulated by the expansion of the Sh2-235 HII region (hereafter S235) via a collect-and-collapse process, provided the density in the surrounding gas exceeds $3\cdot10^3$ cm$^{-3}$, or via collapse of pre-existing clumps. The expansion of S235 cannot be responsible for star formation in the southern S235 A-B region. However, formation of the massive stars in this region might have been triggered by a large-scale supernova shock. Thus, triggered star formation in the studied region may come in three varieties, namely collect-and-collapse and collapse of pre-existing clumps, both initiated by expansion of the local HII regions, and triggering by an external large-scale shock. We argue that the C235 A HII region expands into a highly non-uniform medium with increasing density. It is too young to trigger star formation in its vicinity by a collect-and-collapse process. There is an age spread inside the S235 A-B region. Massive stars in the S235 A-B region are considerably younger than lower mass stars in the same area. This follows from the estimates of their ages and the ages of associated HII regions.

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.

Observations of H and He radio recombination lines in the source W51 have been performed with the RT-22 radio telescope (Pushchino) in two transitions: 56 α (8 mm) and 65 α (13 mm). We have estimated the spectral line parameters and determined the relative abundance of ionized helium, y + = (9.3 ± 0.35)%. We have carried out a model study of the correction ( R ) for the ionization structure of HII regions (when passing from the observed y + = N (He + )/ N (H + ) to the actual y = N (He)/ N (H)) as a function of the spectral type of the ionizing star. Hence it follows that it is desirable to choose the sources excited by hot stars of spectral types no later than O6 V to estimate the helium abundance. In this case, the correction is expected to be small and essentially constant, R in the range 1.0–1.05. We have analyzed the correction for the ionization structure of W51, obtained an actual abundance of helium in the range y = (8.9–9.7)%, and determined its primordial abundance Y p (produced during primordial nucleosynthesis in the Universe) in this source. We have made a new estimate of the primordial helium abundance from six Galactic HII regions, where we observed H and He radio recombination lines at different times. The weighted mean Y p = 25.64(±0.70)% has been obtained. On the one hand, this value of Y p does not yet disagree strongly with the conclusions of the standard cosmologicalmodel, but, on the other hand, it admits the existence of at least one unknown light particle in the period of primordial nucleosynthesis outside the scope of the standard cosmological model. One should continue to refine Y p for more reliable conclusions to be reached.

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.

Results of the Ori A HII region mapping based on hydrogen ( H), helium ( He) and carbon ( C) Radio Recombination lines ( RRL) are presented. Observations were made with the same angular resolution ( 2') using the 32 m VLBI dish of Medicina ( Italy, 22.4 GHz) and the Pushchino RT-22 dish ( Russia, 36.5 GHz). The behaviour of the ionized helium abundance, y(+), with distance from the center shows that the He+ zone size is smaller than that of H+. Such a behaviour is different for the core and for the envelope, as well as for different directions from the center. The helium abundance, N( He)/ N( H) = 10.0( +/- 0.8)%, is measured. Derived line radial velocities, their widths and y(+) data support the well- known " blister- type" structure of this HII region. LTE electron temperatures ( 7800- 9600 K) are also measured.

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 ϑ1 Ori C (the ionization source) is most likely ≈37 000 K.