Maser Region Research Articles

The Fe Kα emission line in water megamaser galaxies

Water maser emissions at around 22 GHz from extragalactic sources are significant in astrophysics, offering a unique means to measure cosmological distances and thus constrain the Hubble parameter with high precision. They also provide the most precise method for measuring black hole masses beyond the Milky Way. The challenge of their limited detection has spurred researchers to seek clues for better characterisation. Despite these efforts, the physical environments in which these masers form still need to be clarified. Consequently, statistical studies have been extensively employed to enhance our understanding of these environments. A sample of 32 Active Galactic Nuclei (AGN)-powered water megamaser galaxies and their Fe Kα emission line at 6.4 keV data was compiled and statistically analysed to investigate any correlation between the two parameters for the first time. A Spearman correlation analysis shows a statistically significant inverse correlation between the maser luminosity (L22 GHz) and the Fe Kα luminosity (LFe Kα). Specifically, Spearman’s coefficient ρ was −0.48, with a p-value = 5 × 10−3. This anti-correlation suggests that in luminous X-ray galaxies, AGN feedback may heat the circumnuclear clouds, leading to more substantial maser emissions, as it favours warm gas while weakening Fe Kα, which is associated with cold, dense gas environments. Additionally, the obscuration and orientation of the AGN’s circumnuclear material play a significant role, with denser, edge-on tori potentially favouring maser production while diluting Fe Kα emission. The lack of significant correlations between L22 GHz and other AGN indicators, such as absorption-corrected L2−10 keV and L12 μ, combined with the previously found anti-correlation between LFe Kα and these indicators suggest that the observed L22 GHz-LFe Kα anti-correlation is driven by localised effects within the masing region rather than broader AGN properties. These findings highlight the need for further research with larger samples and more detailed modelling to disentangle the various factors influencing maser activity in AGN environments and validate the anticorrelation’s significance, as sources of uncertainty might still have affected the results.

The Discovery of Polarized Water Vapor Megamaser Emission in a Molecular Accretion Disk

For the first time in an extragalactic source, we detect linearly polarized H2O maser emission associated with the molecular accretion disk of NGC 1068. The position angles of the electric polarization vectors are perpendicular to the axes of filamentary structures in the molecular accretion disk. The inferred magnetic field threading the molecular disk must lie within ∼35° of the sky plane. The orientation of the magnetic fields relative to the disk plane implies that the maser region is unstable to hydromagnetically powered outflow; we speculate that the maser region may be the source of the larger-scale molecular outflow found in Atacama Large Millimeter/submillimeter Array studies. The new very long baseline interferometry observations also reveal a compact radio continuum source, NGC 1068*, aligned with the near-systemic maser spots. The molecular accretion disk must be viewed nearly edge on, and the revised central mass is M = (16.6 ± 0.1) × 106 M ☉.

Investigating Asymmetry Development from SiO to H2O Maser Regions in VX Sagittarii

Simultaneous very-long-baseline interferometry monitoring observations of H2O and SiO masers toward VX Sagittarii were conducted from 2014 February to 2019 January. Thirty epochs of observations revealed that the H2O and SiO masers had asymmetric and ring-like structures, respectively. However, from 2017 September to 2018 March, the SiO maser transformed from a ring-like structure to a northeast–southwest (NE–SW) extension, and the 43.1 and 86.2 GHz SiO maser components had velocities of 39.48 and 10.65 km s−1 in the NE–SW direction, suggesting a possible localized strong shock wave. The H2O maser had a double-sided structure oriented in the NE–SW direction with near-stellar velocity components, which aligned with the extended direction of the SiO maser. The nonregular optical brightness and maser intensity variations were speculated to be related to the morphological evolution of the SiO maser. During the stable states attained by regular pulsations, the SiO maser region was presumed to experience radial acceleration, which reverted the SiO maser to a ring-like structure. However, the H2O maser region, where the acceleration almost terminates, retained its asymmetric morphology due to the prior influence of external forces. The results suggest that substantial energy transfer can alter the dynamics of the SiO maser and surrounding atmosphere, leading to an asymmetric distribution in the H2O maser region.

Maser polarization through anisotropic pumping

Context. Polarized emission from masers is an excellent tool to study magnetic fields in maser sources. The linear polarization of the majority of masers is understood as an interplay of maser saturation and anisotropic pumping. However, for the latter mechanism, no quantitative modeling has been presented yet. Aims. We aim to construct a comprehensive model of maser polarization, including quantitative modeling of both anisotropic pumping and the effects of maser saturation on the polarization of masers. Methods. We extended regular (isotropic) maser excitation modeling with a dimension that describes the molecular population alignments, as well as including the linear polarization dimension to the radiative transfer. The results of the excitation analysis yielded the anisotropic pumping and decay parameters, which were subsequently used in one-dimensional proper maser polarization radiative transfer modeling. Results. We present the anisotropic pumping parameters for a variety of transitions from class I CH3OH masers, H2O masers, and SiO masers. SiO masers are highly anisotropically pumped due to them occurring in the vicinity of a late-type star, which irradiates the maser region with a strong directional radiation field. Class I CH3OH masers and H2O masers occur in association with shocks, and they are modestly anisotropically pumped due to the anisotropy of the excitation region. Conclusions. Our modeling constitutes the first quantitative constraints on the anisotropic pumping of masers. We find that anisotropic pumping can explain the high polarization yields of SiO masers, as well as the modest polarization of unsaturated class I CH3OH masers. The common 22 GHz H2O maser has a relatively weak anisotropic pumping; in contrast, we predict that the 183 GHz H2O maser is strongly anisotropically pumped. Finally, we outline a mechanism through which non-Zeeman circular polarization is produced, when the magnetic field changes direction along the propagation through an anisotropically pumped maser.

Astrometric VLBI observations of H2O masers in an extreme OH/IR star candidate NSV 17351

Abstract The results of astrometric very long baseline interferometry (VLBI) observations toward an extreme OH/IR star candidate NSV 17351 are presented. Using the VERA (VLBI Exploration of Radio Astrometry) VLBI array , we observed 22 GHz H2O masers of NSV 17351 and derived an annual parallax of 0.247 ± 0.035 mas, which corresponds to a distance of 4.05 ± 0.59 kpc, from the observation. By averaging proper motions of 15 maser spots further, we determined the systemic proper motion of NSV 17351 to be (μαcos δ, μδ)avg. = (−1.19 ± 0.11, 1.30 ± 0.19) mas yr−1. The maser spots spread out over a region of 20 × 30 mas, which can be converted to a spatial distribution of ∼80 × 120 au at the source distance. Internal motions of the maser spots suggest an outward-moving maser region with respect to the estimated position of the central star. From single-dish monitoring of the H2O maser emission, we estimate the pulsation period of NSV 17351 to be 1122 ± 24 d. This is the first report of the periodic activity of NSV 17351, indicating that NSV 17351 could have a mass of ∼4 M⊙. We confirmed that the time variation of H2O masers can be used as a period estimator of variable OH/IR stars. Furthermore, by inspecting dozens of double-peaked H2O maser spectra for the last 40 years, we discovered the long-term acceleration in the radial velocity of the circumstellar matter to be 0.17 ± 0.03 km s−1 yr−1. We finally determined the position and kinematics in the Milky Way Galaxy and found that NSV 17351 is located in an interarm region between the Outer and Perseus arms. We note that the astrometric VLBI observation toward extreme OH/IR stars shows us a useful sample of the Galactic dynamics.

Polarisation of molecular lines in the circumstellar envelope of the post-asymptotic giant branch star OH 17.7–2.0

Context. The role of magnetic field in shaping planetary nebulae (PNe), either directly or indirectly after being enhanced by binary interaction, has long been a topic of debate. Large-scale magnetic fields around pre-PNe have been inferred from polarisation observations of masers. However, because masers probe very specific regions, it is still unclear if the maser results are representative of the intrinsic magnetic field in the circumstellar envelope (CSE). Aims. Molecular line polarisation of non-maser lines can provide important information about the magnetic field. A comparison between the magnetic field morphology determined from maser observations and that observed in the more diffuse CO gas can reveal if the two tracers probe the same magnetic field. Methods. We compared observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) of molecular line polarisation around the post-asymptotic giant branch (post-AGB) or pre-PN star OH 17.7−2.0 with previous observations of polarisation in the 1612 MHz OH maser region. Earlier mid-infrared observations indicate that OH 17.7−2.0 is a young bipolar pre-PN, with both a torus and bipolar outflow cavities embedded in a remnant AGB envelope. Results. We detect CO J = 2 − 1 molecular line polarisation at a level of ∼4% that displays an ordered linear polarisation structure. We find that, correcting for Faraday rotation of the OH maser linear polarisation vectors, the OH and CO linearly polarised emission trace the same large-scale magnetic field. A structure function analysis of the CO linear polarisation reveals a plane-of-the-sky magnetic field strength of B⊥ ∼ 1 mG in the CO region, consistent with previous OH Zeeman observations. Conclusions. The consistency of the ALMA CO molecular line polarisation observation with maser observations indicate that both can be used to determine the magnetic field strength and morphology in CSEs. The new observations indicate that the magnetic field has a strong toroidal field component projected on the torus structure and a poloidal field component along the outflow cavity. The existence of a strong, ordered, magnetic-field around OH 17.7−2.0 indicates that the magnetic field is likely involved in the formation of this bipolar pre-PN.

Multi-frequency VLBI observations of maser lines during the 6.7 GHz maser flare in the high-mass young stellar object G24.33+0.14

Context. Recent studies have shown that 6.7 GHz methanol maser flares can be a powerful tool for verifying the mechanisms of maser production and even the specific signatures of accretion rate changes in the early stages of high-mass star formation. Aims. We characterize the spatial structure and evolution of methanol and water masers during a flare of methanol maser emission at 6.7 GHz in the high-mass young stellar object (HMYSO) G24.33+0.14. Methods. Very Long Baseline Array (VLBA) was used to image the 6.7 and 12.2 GHz methanol and 22.2 GHz water vapor masers at three epochs guided by monitoring the methanol line with the Torun 32m telescope. The 6.7 GHz maser maps were also obtained with the European VLBI Network (EVN) and Long Baseline Array (LBA) during the flare. The Wide-field Infrared Survey Explorer (WISE) data were used to find correlations between the 6.7 GHz maser and infrared (IR) fluxes. Results. The 6.7 GHz methanol maser cloudlets are distributed over ~3500 au, and the morphology of most of them is stable although their brightness varies following the course of the total flux density on a timescale of two months. The 12.2 GHz methanol maser cloudlets cover an area an order of magnitude smaller than that of 6.7 GHz emission, and both transitions emerge from the same masing gas. The 22.2 GHz maser cloudlets lie in the central region and show a systematic increase in brightness and moderate changes in size and orientation, together with the velocity drift of the strongest cloudlet during two months of the Very Long Baseline Interferometry (VLBI) observing period. Time lag estimates imply the propagation of changes in the physical conditions of the masing region with a subluminal speed (~0.3c). A tight correlation of IR (4.6 μm) and 6.7 GHz flux densities is found, supporting the radiative pumping model. Proper motion analysis does not reveal any signs of expansion or inflow of the methanol cloudlets within ~6 mas over ~10 yr. Comparison with the 230 GHz Atacama Large Millimeter Array (ALMA) data indicates that the methanol masers are distributed in the inner part of the rotating disk, whereas the 22.2 GHz emission traces the compact inner component of the bipolar outflow or a jet structure. Conclusions. The maser morphology in the target is remarkably stable over the course of the flare and is similar to the quiescent state, possibly due to less energetic accretion events that can repeat on a timescale of ~8 yr.

Simultaneous Time-monitoring Observations of H2O and SiO Masers toward the Supergiant VX Sagittarii

We performed simultaneous monitoring observations of the 22.2 GHz H2O and 43.1/42.8/86.2/129.3 GHz SiO masers toward the red supergiant VX Sagittarii using the Korean VLBI Network single-dish telescopes. The observations were conducted about every 2 months from 2013 May to 2019 January (30 epochs in total). They included four optical maxima in the active phase of the optical pulsation cycles. The line profile of a H2O maser always comprised various velocity components with a wider velocity range and varied from highly redshifted to blueshifted velocities with respect to the stellar velocity, in contrast to those of the SiO masers. We examined the relation between peak intensities and velocities of 11 detailed components in the line profile of the H2O maser and the pulsation phases. The peak intensity of each component generally exhibited a better correlation with the pulsation phases than that of total intensity. The peak velocities of several components gradually decreased or increased with respect to the stellar velocity, implying an accelerating motion and the development of asymmetries in the H2O maser region. The characteristics of four transition SiO maser properties were compared according to the stellar pulsation phases. The intensity and velocity variation trend of the 43.1 GHz SiO maser was similar to that of the 42.8 GHz SiO maser. However, the variation trend of the 43.1 and 42.8 GHz SiO masers was different from that of the 86.2 and 129.3 GHz SiO masers. This difference stems from the different location of each maser reflecting a different excitation condition.

Discovery of recurrent flares of 6.7 GHz methanol maser emission in Cepheus A HW2

Context.Class II methanol masers at 6.7 GHz originate close to high-mass young stellar objects (HMYSOs). Their high sensitivity to local condition variations makes them a useful marker of the activity of the emerging massive stars.Aims.We aim to closely examine the variability of the 6.7 GHz CH3OH masers in Cep A HW2 using the new and archival single-dish and high-resolution Very-Long-Baseline Interferometry (VLBI) datasets.Methods.We monitored 6.7 GHz methanol masers towards the target between 2009 and 2021 using the Torun 32m radio telescope, and analysed nine datasets of VLBI observations taken over 16 yr.Results.Faint, extremely redshifted maser emission located close to the presumed central star position and coincident with the radio jet shows flaring activity with a period of ~5 yr. Flares have an asymmetric profile with a rise-to-decay time ratio of 0.18 and relative amplitude higher than 10. The velocity of the flaring cloudlets drifts at a rate of 3–4 × 10−5km s−1d−1for about 11.5 yr of the monitoring. The time-lag between the peaks of the two flaring features implies a propagation speed of the exciting factor of ~1000 km s−1. Synchronised and anticorrelated variations of the flux density of blue- and redshifted features begin ~2.5 yr after flares of the extremely redshifted emission.Conclusions.Our observations suggest that the recurrent flares are the response of low-gain unsaturated maser regions to a relatively low increase in luminosity, which has little effect on the behaviour of most maser regions of higher gain.

A Multitransition Methanol Maser Study of the Accretion Burst Source G358.93–0.03-MM1

Abstract 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, near-IR/(sub)millimeter-dark and far-IR-loud massive young stellar object 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 ∼0.″2 around the bursting source and shows a clear velocity gradient in the north–south direction, with redshifted features to the north and blueshifted 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 Very Large Array, very long baseline interferometry, Submillimeter Array, and Atacama Large Millimeter/submillimeter Array 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.

OH maser towards IRAS 06056+2131: polarization parameters and evolution status

ABSTRACT We present high-angular resolution observations of OH maser emission towards the high-mass star-forming region IRAS 06056+2131. The observations were carried out using the UK radio interferometer array, Multi-Element Radio Linked Interferometer Network (MERLIN) in the OH main lines at 1665 and 1667 MHz, in addition to the OH satellite line at 1720 MHz. The results of this study reveal the small upper limit to the size of emission in the 1665-MHz line with an estimated total intensity of ∼4 Jy. We did not detect any emission from the 1667 and 1720-MHz lines. The full polarization mode of MERLIN enables us to investigate the magnetic field in the OH maser region. In this transition, a Zeeman pair is identified from which a magnetic strength of ∼−1.5 mG is inferred. Our results show that IRAS 06056+2131 is highly polarized, with ∼ 96 ${{\ \rm per\ cent}}$ circular polarization and ∼6 ${{\ \rm per\ cent}}$ linear polarization. The linear polarization angle is ∼29°, implying a magnetic field which could be aligned with the outflow direction detected towards this region, but the actual magnetic field direction has an uncertainty of up to 110° due to the possible effects of Faraday rotation. The star-forming evolutionary status of the embedded protostellar object is discussed.

Review of Zeeman Effect Observations of Regions of Star Formation

(Edited for length) The Zeeman effect is the only observational technique available to measure directly the strength of magnetic fields in regions of star formation. We review the physics of the Zeeman effect and its practical use in both extended gas and in masers. We discuss observational results for the five species for which the Zeeman effect has been detected in the ISM -- H~I, OH, and CN in extended gas and OH, CH$_3$OH, and H$_2$O in masers. These species span densities from $\sim10$ cm$^{-3}$ to $\sim10^{10}$ cm $^{-3}$, which allows magnetic fields to be measured over the full range of cloud densities. However, there are significant limitations, including that only the line-of-sight component of the magnetic field strength can usually be measured and that there are often significant uncertainties about the physical conditions being sampled, particularly for masers. We discuss statistical methods to partially overcome these limitations. The results of Zeeman observations are that the mass to magnetic flux ratio is subcritical (gravity dominates magnetic support) at lower densities but supercritical for $N_H \gtrsim 10^{22}$ cm$^{-2}$. Above $n_H\sim 300$ cm$^{-3}$, which is roughly the density at which clouds typically become self-gravitating, the strength of magnetic fields increases approximately as $B \propto n^{2/3}$, which suggest that magnetic fields do not provide significant support at high densities. This is consistent with high-density clouds being supercritical. However, magnetic fields have a large range in strengths at any given density, so the role of magnetic fields should differ significantly from one cloud to another. And for maser regions the dependence of field strength on density may have a slightly lower slope. Turbulent reconnection theory seems to best match the Zeeman observational results.

Time variations of H2O and SiO masers in the proto-Planetary Nebula OH 231.8+4.2

Abstract H2O (22 GHz) and SiO masers (43, 86, 129 GHz) in the bipolar proto-planetary nebula OH 231.8+4.2 were simultaneously monitored using the 21-m antennas of the Korean VLBI Network in 2009–2015. Both species exhibit periodic flux variations that correlate with the central star’s optical light curve, with a phase delay of up to 0.15 for the maser flux variations with respect to the optical light curve. The flux densities of SiO v = 2, J = 1→0 and H2O masers decrease with time, implying that they may disappear in 10–20 years. However, there seems to have been a transient episode of intense H2O maser emission around 2010. We also found a systematic behaviour in the velocity profiles of these masers. The velocities of the H2O maser components appear to be remarkably constant, suggesting ballistic motion for the bipolar outflow in this nebula. On the other hand, those of the SiO maser clumps show a systematic radial acceleration of the individual clumps, converging to the outflow velocity of the H2O maser clumps. Measuring the full widths at zero power of the detected lines, we estimated the expansion velocities of the compact bipolar outflow traced by H2O maser and SiO thermal line, and discussed the possibility of the expanding SiO maser region in the equatorial direction. All of our analyses support that the central host star of OH231.8 is close to the tip of the AGB phase, and that the mass-loss rate recently started to decrease because of incipient post-AGB evolution.

Discovery of a New Class I Methanol Maser Transition at 266.8 GHz

Abstract We report the detection of a new class I methanol maser candidate from the 52−41 E transition (266.8 GHz). This methanol transition has been detected toward a nearby high-mass star-forming region G352.630-1.067 (distance ∼0.7 kpc), in Submillimeter Array (SMA) observations. The new candidate transition has a similar spatial distribution as the 42−31 E (218.4 GHz) and 8−1−70 E (229.7 GHz) transitions, which are known class I maser transitions. Thermal methanol emission in this source is confined to a central hot core, while the three class I maser transitions are detected in two additional regions. These two maser-only emission regions are clearly associated with shocked gas traced by 2 μm Ks-band and thermal v = 0, J = 5−4 SiO molecular emission. In contrast to the thermal methanol emission from the hot core, the three class I maser transitions show an positive trend in the rotation diagram for the two maser regions. Large velocity gradient modeling of the 266.8, 218.4, and 229.7 GHz transitions shows that the 266.8 GHz transition can be a maser for a wide range of conditions. The intensity ratios for the three methanol transitions detected in maser regions can be reproduced under conditions that are typical for class I methanol maser sites. These facts all support the hypothesis that the detected emission from the 266.8 GHz methanol (52−41 E) transition is masing.

Periodic variability of the mainline hydroxyl masers in G9.62+0.20E

We present the results of a monitoring campaign using the KAT-7 and HartRAO 26m telescopes, of hydroxyl, methanol and water vapour masers associated with the high-mass star forming region G9.62+0.20E. Periodic flaring of the main line hydroxyl masers were found, similar to that seen in the 6.7 and 12.2 GHz methanol masers. The 1667 MHz flares are characterized by a rapid decrease in flux density which is coincident with the start of the 12.2 GHz methanol maser flare. The decrease in the OH maser flux density is followed by a slow increase till a maximum is reached after which the maser decays to its pre-flare level. A possible interpretation of the rapid decrease in the maser flux density is presented. Considering the projected separation between the periodic methanol and OH masers, we conclude that the periodic 12.2 methanol masing region is located about 1600 AU deeper into the molecular envelope compared to the location of the periodic OH masers. A single water maser flare was also detected which seems not to be associated with the same event that gives rise to the periodic methanol and OH maser flares.

Study of variable stars associated with maser sources: G025.65+1.05

We report variation of K-band infrared (IR) emission in the vicinity of the G025.65+1.05 water and methanol maser source. New observational data were obtained with the 2.5 m telescope at the Caucasian Mountain Observatory (CMO) of Moscow State University on 2017–09–21 during a strong water maser flare. We found that the IR source situated close to the maser position had decreased brightness in comparison to archive data. This source is associated with a massive young stellar object (MYSO) corresponding to the compact IR source IRAS 18316–0602 (RAFGL 7009S). A similar decrease in K-brightness of the IR source close to the maser position was observed in March 2011 when the water maser experienced a period of increased activity. The dips in MYSO brightness can be related to the maser flare phases. Maser flares that are concurrent with dips in the IR emission can be explained if the lower IR radiation field enables a more efficient sink for the pumping cycle by allowing IR photons to escape the maser region.

Rapidly evolving episodic outflow in IRAS 18113−2503: clues to the ejection mechanism of the fastest water fountain

ABSTRACT Water fountains are evolved stars showing early stages of collimated mass-loss during transition from the asymptotic giant branch, providing valuable insight into the formation of asymmetric planetary nebulae. We report the results of multi-epoch VLBI observations, which determine the spatial and three-dimensional kinematic structure of H2O masers associated with the water fountain IRAS 18113−2503. The masers trace three pairs of high-velocity (∼150–300 km s−1) bipolar bow shocks on a scale of 0${^{\prime\prime}_{.}}$18 (∼2000 au). The expansion velocities of the bow shocks exhibit an exponential decrease as a function of distance from the central star, which can be explained by an episodic, jet-driven outflow decelerating due to drag forces in a circumstellar envelope. Using our model, we estimate an initial ejection velocity ∼840 km s−1, a period for the ejections ∼10 yr, with the youngest being ∼12 yr old, and an average envelope density within the H2O maser region $n_{\text{H}_2}{\approx }10^{6}$ cm−3. We hypothesize that IRAS 18113−2503 hosts a binary central star with a separation of ∼10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.

Zeeman Effect Observations in Class I Methanol Masers

AbstractWe report the detection of the Zeeman effect in the 44 GHz Class I methanol maser line toward the star forming region DR21W. The 44 GHz methanol masers in this source occur in a ∼3” linear structure that runs from northwest to southeast, with the two dominant components at each end, and several weaker maser components in between. Toward a 93 Jy maser in the dominant northwestern component, we find a significant Zeeman detection of −23.4 ± 3.2 Hz. If we use the recently published result of Lankhaar et al. (2018) that the F=5-4 hyperfine transition is responsible for the 44 GHz methanol maser line, then their value of z = −0.92 Hz mG−1 yields a line-of-sight magnetic field of Blos =25.4 ± 3.5 mG. If Class I methanol masers are pumped in high density regions with n∼107–8 cm−3, then magnetic fields in these maser regions should be a few to several tens of mG. Therefore, our result in DR21W is certainly consistent with the expected values.Using the above noted splitting factor in past Zeeman effect detections in Class I methanol masers reported by Sarma & Momjian (2011) and Momjian & Sarma (2017) in the star forming regions OMC-2 and DR21(OH) result in Blos values of 20.0 ± 1.2 mG and 58.2 ± 2.9 mG, respectively. These are also consistent with the expected values.

Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732

Context. The importance of the magnetic field in high-mass-star formation is not yet fully clear and there are still many open questions concerning its role in the accretion processes and generation of jets and outflows. In the past few years, masers have been successfully used to probe the magnetic field morphology and strength at scales of a few au around massive protostars, by measuring linear polarisation angles and Zeeman splitting. The massive protostar IRAS 18089-1732 is a well studied high-mass-star forming region, showing a hot core chemistry and a disc-outflow system. Previous SMA observations of polarised dust revealed an ordered magnetic field oriented around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic field in the dense region probed by 6.7 GHz methanol maser observations and compare it with observations in dust continuum polarisation, to investigate how the magnetic field in the compact maser region relates to the large-scale field around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz of IRAS 18089-1732 and we analysed the polarised emission by methanol masers. Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz methanol maser region is consistent with the magnetic field constrained by the SMA dust polarisation observations. A tentative detection of circularly polarised line emission is also presented. Conclusions. We found that the magnetic field in the maser region has the same orientation as in the disk. Thus the large-scale field component, even at the au scale of the masers, dominates over any small-scale field fluctuations. We obtained, from the circular polarisation tentative detection, a field strength along the line of sight of 5.5 mG which appeared to be consistent with the previous estimates.

Long-term Variability of H2CO Masers in Star-forming Regions

Abstract We present results of a multi-epoch monitoring program on variability of 6 cm formaldehyde (H2CO) masers in the massive star-forming region NGC 7538 IRS 1 from 2008 to 2015, conducted with the Green Bank Telescope, the Westerbork Radio Telescope , and the Very Large Array. We found that the similar variability behaviors of the two formaldehyde maser velocity components in NGC 7538 IRS 1 (which was pointed out by Araya and collaborators in 2007) have continued. The possibility that the variability is caused by changes in the maser amplification path in regions with similar morphology and kinematics is discussed. We also observed 12.2 GHz methanol and 22.2 GHz water masers toward NGC 7538 IRS 1. The brightest maser components of CH3OH and H2O species show a decrease in flux density as a function of time. The brightest H2CO maser component also shows a decrease in flux density and has a similar LSR velocity to the brightest H2O and 12.2 GHz CH3OH masers. The line parameters of radio recombination lines and the 20.17 and 20.97 GHz CH3OH transitions in NGC 7538 IRS 1 are also reported. In addition, we observed five other 6 cm formaldehyde maser regions. We found no evidence of significant variability of the 6 cm masers in these regions with respect to previous observations, the only possible exception being the maser in G29.96−0.02. All six sources were also observed in the isotopologue transition of the 6 cm H2CO line; absorption was detected in five of the sources. Estimated column density ratios [ ]/[ ] are reported.