In the paper, archival data of the RadioAstron ground–space interferometer project concerning observations of radiation in the OH hydroxyl lines (1665 and 1667 MHz transitions) in the Cep A source have been analyzed. The observations have been carried out on January 7, 2013. The duration of the observation session was about one hour. Ground-based radio telescopes at the Zelenchuk Observatory (Special Astrophysical Observatory of the Russian Academy of Sciences, North Caucasus, Russian Federation), in Yevpatoria (Crimea, Russian Federation), and at the Torun Astronomy Center (Republic of Poland) have participated in the observations. No interferometric response has been detected at the ground–space bases. Several compact sources with angular sizes from 10 to 20 milliarcseconds have been detected at the base between the radio telescopes in Torun and at the Zelenchuk Observatory. These angular sizes correspond to linear sizes of 7.5–15 astronomical units (AU). It has been shown that the detected compact sources are distributed over a region of 750–1500 AU.

At the early stage of the protostar evolution, gas is mostly in the molecular form, while the process of protostars development causes a variety of surrounding matter emission in molecular lines at many frequencies of any ranges. This work is aimed at measuring the parameters of molecular radio lines in the vicinity of some young protostellar objects to expand our understanding of their evolutionary trends. Observations have been carried out with the RT-22 radio telescope at the Pushchino Radio Astronomical Observatory, Lebedev Physical Institute from July 2022 until May 2023. Two standard transistor receivers, which operated at frequencies of 20–24 GHz (13.5-mm range) and 36.2–37.7 GHz (8-mm range), have been used. Emission in the lines of four molecules, each one being a “tracer” of the environmental physical state: water ( H_2O ), methanol ( CH_3OH ), ammonia ( NH_3 ), and cyanoacetylene ( HC_3N ) has been studied. Observational results for three IRDC objects, for two CORE-type objects, and for the star forming region Onsala-1 have been obtained. Radio emission in the water vapor line has been detected in all studied objects, except for IRDC G027.94–00.47. For three objects: IRDC G028.37+0.07b, IRDC G024.33+0011, and Onsala-1, the rotational and kinetic temperatures in the radiation region have been determined from the emission spectra of the ammonia molecule, and the concentration of ammonia and the density of molecular hydrogen have been estimated. Emission in lines of cyanoacetylene in the direction of to sources IRDC G028.37+0.07b, and Onsala-1 has been detected, and the column density of this molecule has been determined.

The paper describes a Space-VLBI observation of the 22 GHz H2O masers in the massive star-forming region G25.65+1.05, using the 10-m space antenna of Radioastron together with the ground-based VLBA array. Two observing epochs at the pre-flare and post-flare state of the maser source are presented. Leveraging the exceptional angular resolution provided by space-ground baselines along with the broad UV coverage from VLBA baselines, we gained a detailed perspective on the area associated with the maser flare events.

Context. Maser flares are particularly significant in the study of massive star formation as they not only signal but also provide unique insights into transient phenomena such as accretion bursts. Aims. With this project, we aim to investigate the context of the ongoing 6.7 GHz methanol maser flare in the little-known massive star-forming region G11.497-1.485. Methods We carried out two epochs of the Karl G. Jansky Very Large Array (VLA) observation for 6.7 GHz and 12 GHz class II methanol, 22 GHz water masers, and continuum in the C , Ku , and K bands. Results. The VLA overview revealed the presence of five distinct radio-continuum sources (CM1-4 and N) in G11.497-1.485. The central source, CM1, is found to show signs of accretion disc fragmentation, highlighted by the centimetre-continuum-traced fragments, and is found to drive a high-energy jet, the ends of which are marked by non-thermal knots CM2 and CM3. CM1 showed a gradual flaring of methanol masers and a fading of a 22 GHz water maser, which might be signalling an accretion burst. The two remaining sources of the region, CM4 and N, make up one of the most compact jet and disc–jet systems found to date. Conclusions. The obtained data reveal, for the first time, the structure of the G11.497-1.485 region. The change in fluxes of the maser and the continuum emission confirm a transient event and reveal its impact on multiple sources in the region.

Giant molecular clouds (GMC) in our and other galaxies and small dense molecular clouds inside the Galaxy (IRDC) form cores due to gravitational instability, in which massive stars and clusters of low-mass stars arise. The high background of infrared radiation inside the Galaxy creates advantages in favor of IRDCs in the study of star formation processes and accompanying phenomena such as accretion, the appearance of HII regions, bipolar outflows, and others that cause various responses in their molecular composition. As a part of studying the evolutionary state in the cloud IRDC MSXDCG24.33+011 (the alternative name is G24.33+014), observations of the water vapor maser were made. On November 28 (2022) during the observations using the RT-22 of the Pushchino Radio Astronomy Observatory, the H2O maser detail at the velocity of V_LSR = 103.15 km/s with the linewidth of 0.52 km/s was detected. Peak flux of 49.5( ± 6 ) Jy was recorded. This detail was not detected on RT-22 at the PRAO on July 5 (2022) and has not been seen before by other researchers.

ABSTRACT We present results on simultaneous observations of Class I methanol masers at 25, 36, and 44 GHz towards 22 Galactic targets carried out with the Effelsberg 100-m telescope. The study investigates relations between the hyperfine (HF) structure of the torsion–rotation transitions in CH$_3$OH and maser activity. By analysing the radial velocity shifts between different maser lines together with the patterns of the HF structure based on laboratory measurements and quantum-chemical calculations, we find that in any source only one specific HF transition forms the maser emission and that this transition changes from source to source. The physical conditions leading to this selective behaviour are still unclear. Using accurate laboratory rest frequencies for the 25 GHz transitions, we have refined the centre frequencies for the HF multiplets at 36, 44, and 95 GHz: $f_{\scriptscriptstyle 36} = (36169.2488\pm 0.0002_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$ MHz. $f_{\scriptscriptstyle 44} = (44069.4176\pm 0.0002_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$ MHz, and $f_{\scriptscriptstyle 95} = (95169.4414\pm 0.0003_{\scriptscriptstyle \rm stat} \pm 0.0004_{\scriptscriptstyle \rm sys})$ MHz. Comparison with previous observations of 44 GHz masers performed 6–10 yr ago with a Korean 21-m Korean Very Long Baseline Interferometry Network telescope towards the same targets confirms the kinematic stability of Class I maser line profiles during this time interval and reveals a systematic radial velocity shift of $0.013\pm 0.005$ km s$^{-1}$ between the two telescopes.

Context. The G25.65+1.04 source is one of the few known ‘super-flare’ water masers in the Milky Way, but in contrast to other super-flare sources, it remains the least studied. In fact, even the nature of the source driving the water maser is still unclear. Aims. With this project, we aim to clarify the composition and properties of the G25.65+1.04 region by means of examining the parameters of the continuum sources and establishing their association with masers of different types. Methods. Our previous VLA (Karl G. Jansky Very Large Array) observation detected four continuum peaks (VLA 1-4), three of which (VLA 1-3) were closely spaced and presented a linear orientation. However, the observation, which used the VLA B configuration, lacked the spatial resolution to resolve individual sources. A higher-resolution (A configuration) VLA observation of the continuum and spectral lines was conducted in 2019 using the L, S, C, and Ku bands. Results. For the first time, the continuum source VLA 1 – associated with the flaring water maser – is resolved into two components: VLA 1A and 1B. The component VLA 1A and the water maser are found to spatially coincide and are thought to be powered by the same source, a protostar at an early stage of evolution showing active ejection. We argue that VLA 2 pinpoints an actively ejecting high-mass protostar, as it is associated with a 6.7 GHz methanol maser and a magnetised jet traced by a 22 GHz H2O maser. Highly polarised OH maser emission is detected in the vicinity of VLA 1-2, with the brightest OH maser found in VLA 2. The magnetic field, identified from the OH maser emission, ranges from ∼ + 0.4 mG in VLA 1A to ∼ − 8 mG in VLA 2. Conclusions. The G25.65+1.04 region is found to consist of at least two young stellar objects: VLA 1A and VLA 2. Both sources are found to be at an active accretion and ejection stage of evolution.

ABSTRACT Infrared dark clouds (IRDCs) represent the earliest stage of high-mass star formation and host molecular cores at different states of activity from quiet state without any IR-signature of star formation to active state with IR-signatures and pronounced maser activity. Many IRDCs show indications of shocked gas associated with protostar outflows and can contain class I methanol masers (cIMMs). The aim of this study is to probe different types of cores in IRDCs with cIMMs and thermal molecular emission and to investigate the relationship between presence of cIMMs and physical conditions in IRDCs. For a sample of 37 molecular cores, using the 20-m Onsala radio telescope, we collected molecular line data at 44, 85, and 97 GHz for more than 15 species including CH3OH, CH3CCH, and CS. Kinetic temperature of the gas and molecular column densities were obtained. Methanol emission at 44 GHz was detected in 29 sources, with 4 sources are being new discoveries. None of the cores in quiescent state show emission at 44 GHz. Our results testify that cIMM emission is a reliable marker of advanced state of molecular cores. The higher detection rate for intermediate and IR-quiet sources suggests that cIMMs most readily trace the early stages of star formation characterized by moderate IR-signatures. We found that masers with higher flux densities tend to be associated with emission in the CH3CCH lines with higher integrated intensities. Sources undetected at 44 GHz have 4 times lower integrated intensities of CH3CCH and exhibit poorer molecular spectra than the most sources with cIMMs.

We present the most complete to date interferometric study of the centimeter wavelength methanol masers detected in G358.93-0.03 at the burst and post-burst epochs. A unique, NIR/(sub)mm-dark and FIR-loud MYSO accretion burst was recently discovered in G358.93-0.03. The event was accompanied by flares of an unprecedented number of rare methanol maser transitions. The first images of three of the newly-discovered methanol masers at 6.18, 12.23, and 20.97 GHz are presented in this work. The spatial structure evolution of the methanol masers at 6.67, 12.18, and 23.12 GHz is studied at two epochs. The maser emission in all detected transitions resides in a region of $\sim$0.2$^{\prime\prime}$ around the bursting source and shows a clear velocity gradient in the north-south direction, with red-shifted features to the north and blue-shifted features to the south. A drastic change in the spatial morphology of the masing region is found: a dense and compact "spiral" cluster detected at epoch I evolved into a disperse, "round" structure at epoch II. During the transition from the first epoch to the second, the region traced by masers expanded. The comparison of our results with the complementary VLA, VLBI, SMA, and ALMA maser data is conducted. The obtained methanol maser data support the hypothesis of the presence of spiral-arm structures within the accretion disk, which was suggested in previous studies of the source.

The massive young stellar object (MYSO) G358.93-0.03-MM1 showed an extraordinary near-infrared- to (sub-)millimetre-dark and far-infrared-loud accretion burst, which is closely associated with flares of several class II methanol maser transitions, and, later, a 22 GHz water maser flare. Water maser flares provide an invaluable insight into ejection events associated with accretion bursts. Although the short timescale of the 22 GHz water maser flare made it impossible to carry out a very long baseline interferometry observation, we could track it with the Karl G. Jansky Very Large Array (VLA). The evolution of the spatial structure of the 22 GHz water masers and their association with the continuum sources in the region is studied with the VLA during two epochs, pre- and post-H2O maser flare. A drastic change in the distribution of the water masers is revealed: in contrast to the four maser groups detected during epoch I, only two newly formed clusters are detected during epoch II. The 22 GHz water masers associated with the bursting source MM1 changed in morphology and emission velocity extent. Clear evidence of the influence of the accretion burst on the ejection from G358.93-0.03-MM1 is presented. The accretion event has also potentially affected a region with a radius of ~2'' (~13 500 AU at 6.75 kpc), suppressing water masers associated with other point sources in this region.

Class I methanol masers (cIMMs) and 1720 MHz OH masers are believed to arise from a common collisional pumping mechanism, and both trace shocked gas regions in supernova remnants (SNRs). However, their coexistence in star formation regions (SFRs) and their association with a front of bipolar outflow (a source of shock-stimulated collisional pumping) remain unclear. To search for collisionally pumped OH(1720) masers, we conducted a Very Large Array survey of the 18 cm OH masers and continuum emission toward a sample of 80 SFRs associated with 44 GHz cIMMs. Main-line OH maser emission was detected in 50% of the sample, and OH(1720) maser emission was detected in 20%. Continuum emission was detected in 28% of sources. A catalog of the detected OH masers is presented. Individual OH masers are found in close proximity, and the regions of masers of different transitions are often overlapping. A typical linear projected offset between the OH(1720) and OH(1665) masers is ∼0.04 pc. A remarkable number of ∼81% of the OH(1720) masers are associated with the continuum emission. The median separation between the targeted 44 GHz cIMMs and OH(1720) masers is ∼0.2 pc, which is similar to reported distances between cIMMs and H ii regions. The observed properties of the detected OH(1720) masers are different from those in SNRs and indicate that a nonlocal line overlap mechanism is responsible for their excitation. Thus, while both 44 GHz cIMMs and OH(1720) masers trace shocked gas in SNRs, physical conditions favorable for excitation of cIMMs, but not OH(1720) masers, are present in SFRs.

The observational data of an H2О maser at a frequency of 22.2280 GHz in the dark reflection nebula NGC 2071 in the direction of the infrared object IRS 1 at the target coordinates RA(2000) = $${{05}^{{\text{h}}}}{{47}^{{\text{m}}}}04_{.}^{{\text{s}}}758$$ , DEC(2000) = $$00{}^\circ 21' 42_{.}^{'' }700$$ have been processed as part of the scientific program of the ground-space interferometer RadioAstron. The duration of the session on January 11, 2014 was 70 min. The space radio telescope (SRT-10) and three radio telescopes of the ground-based network took part in the observations: RT-32 (Medicina, Italy), RT-32 (Toruń, Poland), and RT-64 (Kalyazin, Russia). The following parameters were implemented: angular resolution 70 μs at the ground-space baseline with maximum baseline projections 3.1 ED ( $$ \sim {\kern 1pt} 40{\kern 1pt} {\kern 1pt} 000$$ km); synthesized beam of the ground-based part of the interferometer $$0.006'' \times 0.0006'' $$ (PA = –23°); spectral resolution 7.81 kHz (i.e., 0.11 km/s). A map of the distribution of maser spots with 13 spatial components was obtained. The map has a size $$ \sim {\kern 1pt} 100 \times 100$$ mas (milliarcseconds), which corresponds to $$ \sim 40 \times 40$$ AU at a distance of 390 pc to the nebula. The range of the line-of-sight velocities of the components is 4.7–20.5 km/s with a width of spectral features 0.2–0.6 km/s; the density of the correlated flux at the line maximum varies from $$ \sim {\kern 1pt} 4$$ to $$ \sim {\kern 1pt} 29$$ Jy. There was recorded one spatial component (with a radial velocity of 14.3 km/s) for which there was a correlation at the ground-space baselines SRT–Tr and SRT–Mc at an ultrahigh angular resolution with a reliability level above 6σ. Based on the analysis of the dependence of the visibility function on the baseline projections, a two-component model is proposed for the spatial structure of this component with the angular dimensions of the extended and compact constituents 4 mas and 0.06 mas, i.e., 1.56 AU (with an uncertainty of 10%) and 0.023 AU (with an uncertainty of 50%), respectively.

ABSTRACT We present a study of 18 cm OH maser emission toward 20 high-mass young stellar object outflow candidates (Extended Green Objects, EGOs) identified from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). All four OH ground state lines at 1612, 1665, 1667, and 1720 MHz, together with 20-cm continuum emission, were observed with the Karl G. Jansky Very Large Array C-configuration. Follow-up polarimetric single-dish observations with the Nançay radio telescope were performed for the same OH transitions, except 1612 MHz. OH maser emission is found to be an uncommon feature of the sample, with a 50 per cent detection rate for the entire sample and a ∼44 per cent detection rate for the ‘likely’ EGOs. No 20-cm continuum emission is detected toward any of the sources. In most cases, the detected OH maser emission arises in vicinity to compact central sources; OH masers coexist with the 6.7 GHz methanol masers, but are found in more diffuse and extended halo-shaped regions of several thousand astronomical units in size. Comparing EGO samples with OH maser detection and non-detection, EGOs showing OH maser emission tend to have lower dust clump masses, but higher 24 and 4.5 μm flux densities. Thus, OH maser emission might be an indicator of more evolved EGOs, since strong compact mid-infrared emission in the absence of compact radio continuum emission is thought to be associated with the later stage of massive star formation.

In 2017-18 the source of outstanding H2O maser bursts G25.65+1.05 have been intensively studied with a wide range of baselines - from compact array (JVLA) and ground VLBI (EVN, VLBA, KaVA) to space VLBI (RadioAstron mission supported by VLBA and EVN telescopes) - the report gives a brief summary of these observations.

The source G25.65+1.05 (RAFGL7009S, IRAS 18316-0602) is the least studied of the three regions of massive star formation known to show exceptionally powerful H2O maser bursts. We report spectral line observations of the H2O maser at 22 GHz, the methanol maser transitions at 6.7, 12.2, and 44 GHz, and the continuum in these same frequency bands with The Karl G. Jansky Very Large Array at the post-burst epoch of 2017. For the first time, maps of 22 GHz H2O and 44 GHz CH3OH maser spots are obtained and the absolute position of the 22 GHz H2O bursting feature is determined with milliarcsecond precision. We detected four continuum components, three of which are closely spaced in a linear orientation, suggesting a physical link between them.

Water masers are well-known to be variable on a variety of time scales, but only three Galactic H2O masers are known to flare to the level of 10(5)-10(6) Jy (T-B similar to 10(17) K): Orion KL, W49N, and the recently discovered G25.65+1.05. Recently detected flaring activity of H2O maser in the massive star-forming region G25.65+1.05 gave us a unique opportunity to study the fine structure of H2O maser emission in the bursting state with extremely high space VLBI angular resolution. Observation of the source was carried out with similar to 9 Earth diameter space-ground baseline within the framework of the RadioAstron project. H2O maser emission from two spectral features, including the bursting one, was detected in the experiment. Only similar to 1% of the bursting H2O maser emission was detected on the space-ground baselines: it indicates the presence of a very compact spatial structure with a size of 25 gas, which corresponds to 0.05 AU or solar diameters at the distance to the source of 2.08 kpc, and the brightness temperature of similar to 3 x 10(16) K. Analysis of the flux density as a function of the baseline length for the bursting H2O maser feature in the source shows that most of the emission comes from an extended "halo" structure, while the core of emission is very compact and has an extreme brightness temperature. These results are in agreement with the model of interacting maser clouds considered as the likely explanation of the nature of the burst in the source. Under the assumption of such a model, the beam size of maser emission is reduced while the brightness temperatures similar to the highest observed values are produced. (C) 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.

A comparative analysis is presented for the physical parameters of gas in various types of molecular clouds, derived from observations and theoretical calculations. Cool, dense fragments of infrared dense clouds (IRDCs)—rare star-forming regions with OH radical emission in the 1720-MHz satellite line, extended green objects (EGOs)— diffuse bipolar outflows discovered by the Spitzer mission at short IR wavelengths, and gas-dust condensations hosting class I and II methanol masers (class I MM and class II MM) are considered. The magnetic fields of these regions are calculated using the empirical criterion of Crutcher (ApJ, 520, 706, 1999) and the equation for numerical modeling of cold turbulent clouds of Ostriker et al. (ApJ, 546, 980, 2001). This modeling is done for three values of the parameter β = ( c s / c A ) 2 relating the sound and Alfvén speeds, which describes the effect of the magnetic field: strong ( β = 0.01), intermediate ( β = 0.1), and weak ( β = 1). In 69 EGOs with densities of 10 4 –10 5.6 cm −3 , derived earlier from observations, and with an acceptable temperature range, from T k ≈ 30 K to T k ≈ 100 K, the results of the calculations are most consistent with β = 0.1 (| B̄ |= 0.26 mG) and β = 1 (| B̄ |= 0.15 mG) for intermediate and weak magnetic fields, respectively. For class I methanol masers, with gas densities not exceeding 10 6 cm −3 and a temperature T k = 80 K, the most plausible value is β = 1 (| B < 0.4 mG), i.e., the expected effect of the magnetic field is weak. Comparison of the sound and Alfvén speeds suggests that very faint, low-velocity C-type shocks may exist in these objcts, with the magnetic field exerting little control over the chaotic motions of the material. The shocks are continuous, and do not affect the parameters in the maser condensations. The magnetic intensities found for class I MMs and OH (1720 MHz) regions differ only slightly; according to this parameter, which is related to the density of the medium and the kinetic temperature of the gas, they may be observed in either different or the same gas-dust condensations.