Annotation of the results obtained in 2020
In the third year of the project, we conducted complex model fitting for 15 massive young stellar objects and Herbig Ae/Be stars using radiative transfer models and observations from VISIR, MSX and WISE. This approach facilitates choosing models which fully describe the available observational data. The modeling was conducted using software developed for the project, which takes the spatial intensity distribution of the observations into account. This software is freely available at https://github.com/avdyachenko/RADPROFfitter. Using observations of the massive young stellar object AFGL 4176 obtained with VLTI/MATISSE, we reconstructed images using the IRBis, MiRA and SQUEEZE algorithms. We also conducted a component-based analysis using the PMOIRED package, which is capable of including the spectral form and arbitrary radial intensity distributions in the fitting process. Changes in the visibility amplitude at different position angles with similar projected baselines show clear deviations from spherical symmetry; hence, we used a two-dimensional Gaussian function, inclined to the line of sight. For the radial temperature gradient of the disk, described by a power law, we found a power index of -0.4. We conducted an analysis of the classical T Tauri star Th 28 using observations with the integral field spectrograph VLT/SINFONI. Emission in spectral lines of H, H2 and [Fe II] arises primarily from three components: the jet ([Fe II]), circumstellar disk (H2), and a spherical region around the central star (H). From the ratio of H2 line intensities, we determined that these lines are excited by the stellar wind, and not from shocks or fluorescence. In the [Fe II] lines, besides the jet, we also find compact emission knots in both jet components at distances of about 1 arcsec from the central star. The extended emission from the jet in the [Fe II] lines is correlated with photocenter shifts obtained from a spectroastrometric analysis. The form of the extracted spectroastrometric signal implies that the large-scale asymmetry of the jet is most likely connected with the jet launching mechanism. The size of the jet launching region was constrained to be less than 17 au, and the initial opening angle of the jet is about 28 deg. This makes the jet of Th 28 significantly less collimated than the majority of classical T Tauri stars. We determined the column densities of CO and HCO+ and C+ in the compact H II regions S235A and S235C. We showed that the column densities of CO and HCO+ are in agreement with the results of a one-dimensional model, but the column density of C+ is an order of magnitude higher than predicted. In order to obtain the observed C+ column densities in the model, we had to either increase the abundance of C by at least two, or use a model with dense clumps embedded in rarefied gas. We also show that in strong ultraviolet radiation fields C can be located not only in the gas phase in molecules of CO and ions of C+, but also on dust grains. In the less dense photo-dissociation region of S235C this effect is not seen. The analysis of the spectral line profiles of CO, HCO+ and C+ allowed us to determine the expansion rate of the H II regions and the molecular cloud surrounding them. We showed that while the [C II] lines are affected by self absorption, and don’t show signs typical of an expanding envelope, the peak velocities of the [13C II] and HCO+ (3-2) lines are different. We attributed this to the expansion of the photo-dissociation region (emitting in the [C II] lines) into the surrounding molecular gas in the direction of the observer. Using an analysis of position-velocity diagrams, we showed that the best tracer for the expansion of photo-dissociation regions around H II regions is the [O I] line at 63 µm. The differences between the peaks in the profile of this line corresponds to twice the expansion velocity. Optically thin lines of compact photo-dissociation regions synthetic position-velocity diagrams do not produce doubly peaked line profiles. We also showed that in real objects analyses using position-velocity diagrams can be complicated by self absorption in the cold, rarefied envelope of the surrounding molecular cloud. The results of this study were presented online to the community during a SOFIA tele-talk (https://www.sofia.usra.edu/science/meetings-and-events/events/pdr-structure-and-kinematics-around-compact-hii-regions-s235-and). In the star-forming region RCW 120, we showed that massive, young protostars are characterized by unusually small molecular abundances, which can be an order of magnitude smaller than those seen in hot cores in other star-forming regions. The observed relative abundance of CH3CN/CH3OH = 0.01 is near the lower limit of values seen in hot cores. We conclude that such values are characteristic of an early warming phase in massive protostars. Finally, as part of the project, we expanded the functionality of the oifits Python module for working with OIFITS files, used for storing reduced observations from optical interferometers. The module, available at https://github.com/pboley/oifits/, now includes full support for version 2 of the OIFITS format. Besides supporting all the requirements of the OIFITS2 standard, the module also manages all cross references in loaded files, and can be used to combine and edit observations from different sources with different instrument and interferometer combinations.

 

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Annotation of the results obtained in 2018
During the first stage of project #18-72-10132 “Studies of massive star-forming regions and massive young stellar objects with spatially resolved observations at optical and infrared wavelengths” observations were conducted using Russian and international optical and infrared telescopes. Spectral observations of young massive stars in the S235 star-forming region were conducted on Russian telescopes belonging to the Special Astrophysical Observatory of the Russian Academy of Sciences and the Kourovka Astronomical Observatory from November 2018 to January 2019. This region is bright in the optical and infrared, making it a good target for observations. There, stars over a wide range of masses are found, as well as a variety of stages of the interstellar medium, from cold and dense clumps to hot ionized regions (HII) around massive stars. For the first time, using high-resolution spectra, we determined the spectral and luminosity class of the ionizing star of S235: a main-sequence O9.7 star, which corresponds to a mass of about 15 times the mass of the Sun. Using archive photometric observations we determined the amount of interstellar absorption to the star to be 4 magnitudes at optical wavelengths. Analysis of spectral energy distribution of the star shows that it is surrounded by an extended structuring emitting at far-infrared wavelengths, although this is generally a characteristic of younger stars, still in the process of formation. Using photometric observations with the MaNGaL instrument on the Zeiss-1000 telescope of SAO RAS, we constructed interstellar extinction and electron density maps for the entire ionized region of S235, and also showed the location of shock fronts located at the interface between the HII region and surrounding molecular gas, currently in the process of forming new stars. We also investigated the bright infrared sources S235 IRS1 and IRS2: using long-slit spectra, we classified these objects as Herbig Be stars, with a mass lower than the ionizing star of the HII region. In S235 IRS2, we identified a high-speed jet. Using observations with international telescopes, we studied massive young stellar objects, which emit primarily at infrared wavelengths. In the near-infrared (2.0-2.4 µm), using the Large Binocular Telescope LBT (USA), we studied G192.16-3.82, which is surrounded by a circumstellar disk and several “knots” of emission in molecular hydrogen lines from a forming massive star. We measured radial velocities (approximately 100 km/s) and gas temperature (around 2600 K) of the knots, as well as the time between bursts of gas accretion onto the central object (bursts occur approximately every 200 years). Using the VISIR instrument on the VLT telescope of the European Southern Observatory we studied images of Herbig Ae/Be stars and massive young stellar objects at a wavelength of 20 µm. We observed a large bipolar nebula around the B[e] star V921 Sco, which, according to previous studies, may be connected with episodic ejections of material from the central source. In the image of the massive young stellar object IRAS 13481-6124 we resolved two companions (at distances of 1.2 and 2.6 arcseconds), which were detected for the first time, and could influence the structure of the accretion disk around this massive forming star. Using the PIONIER instrument, also on the VLT, which combines the light of 4 separate telescopes into a “virtual” telescope with a diameter of up to 130 m, we studied observations of the massive young stellar objects PDS 27 and PDS 37 with a resolution of 1.3 milliarcseconds. A joint study with an international team of scientists showed that both systems contain protostar companions at distances of 30-50 astronomical units (10-30 milliarcseconds). These are the closest multiple systems of massive young stellar objects resolved to date. Currently, together with colleagues from the University of Leeds (England) and the Max Planck Institute for Astronomy (Germany), we are studying the massive young stellar object IRAS 13481-6124 using the second-generation interferometric instrument MATISSE on the VLT telescopes at infrared wavelengths, as well as the ALMA radio interferometer at wavelengths of 0.8-1.1 mm. These observations allow us to achieve a resolution of up to 5 milliarcseconds (about 10 astronomical units), and resolve the asymmetric structure of the disk around this forming massive star, helping us to determine the role of disk fragmentation and accretion mechanisms onto the central object.

 

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Annotation of the results obtained in 2019
In the second year of the project, we constructed maps of the physical parameters of the HII region S235: electron density, optical extinction, and the depth along the line of sight. We showed that the electron density varies from 10-20 cm(-3) to more than 300-400 cm(-3) along a transition from the north-east to the south-west of the region, where the central star BD+35 1201 is located. The more dense part of the HII region, together with the central star, are more deeply embedded in neutral gas. The size of the dense area of the HII region is about 2 pc in the image plane and along the line of sight, while the depth along the line of sight extends up to 10-20 pc in the more rarefied regions. We conducted a study of the physical parameters of the gas and dust clouds surrounding massive young stellar objects in the star-formation region RCW 120. In this region, a young generation of stars in the molecular clumps Core 1 and Core 2, including massive stars, is located along the periphery of the extended HII region. In order to estimate the physical parameters, we used observations at frequencies of 220-241 GHz: molecular lines of C34S(5-4), CH3OH(8-7), and the series of lines of CH3OH(5-4) and CH3CN(12-11). We determined the molecular abundances relative to hydrogen in Core 2: x(CS)=4.5*10(-10), x(SO2)=6.6*10(-12), x(CH3OH)=3.2*10(-9), x(CH3CN)=2.5*10(-11). We showed that the density of gas in RCW 120 Core 2 is higher than 10(6) cm(-3). The gas temperature in RCW 120 Core 1 is only 8 K, while in Core 2 it is 22 K. Although highly excited lines of CH3CN(12-11) are seen in Core 2, this object has not yet moved to the more evolved status of a hot core, and is still at an early evolutionary stage. We constructed maps of the excitation temperature, column density of molecular hydrogen, and interstellar extinction around the young stellar object Th 28. The excitation temperatures lie in the range of 1000-3000 K, and the extinction ranges from 1-3 mag. We see a range of radial velocities, spanning from 100-200 km/s in the maps. We also show a horizontal gradient in iron lines near the central source, which attests to the presence of a circumstellar disk. In an international collaboration with scientists from the UK and Germany using observations from the radio interferometer ALMA, we showed the spiral structure of the gas disk around the massive young stellar object AFGL 4176. The spiral structure is asymmetric, which could arise due to interactions with surrounding companions. We evaluated the gravitational stability of the disk using the Safronov-Toomre criterion and lines of CH3CN. We show that gravitational instabilities exist in both the eastern and western spiral arms. These gravitational instabilities most likely indicate that the disk around this massive star is undergoing fragmentation. Using the ALMA observations, we also reveal the rich chemistry around AFGL 4176, showing more than 200 spectral lines from 25 different molecules. From the channel maps, we show that lines of CH3CN and HC(O)NH2 reliably trace the disk, while the outflows and cavity are traced by lines of C34S, H2CS and CH3CCN. The line profiles in the disk suggest the presence of shocks, which coincide with a bar structure in the spiral. Large-scale motion of the gas is seen in the form of a rotating tor, spanning up to 0.5 pc, which is well traced by emission in the lines of NH3. The mass of the torus is estimated at several thousand solar masses.

 

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