Methanol Masers Research Articles

Variabilities of Methanol Maser and Thermal Molecular Lines from an Accretion Burst Source G358.93-0.03

From 2019 January to June, the high-mass young stellar object G358.93-0.03 underwent a remarkable accretion burst. Detecting variations in the physical and kinematic environments during episodic accretion is essential for exploring such events. Searching for new maser or molecular lines and monitoring their variability during the accretion burst and postburst stages is feasible for such a study. Using the Purple Mountain Observatory 13.7 m telescope, we carried out monitoring of methanol masers and molecular lines toward G358.93-0.03 in the 3 mm band during the burst (2019 March to June) and postburst stages (2022 October). In addition to the previously abundant detection of the methanol maser lines in the burst stage, eight new methanol maser lines were detected, identified from transitions at 85.57, 93.20, 94.54, 94.82, 99.77, 102.96, 104.35, and 104.41 GHz. Their integrated intensity exhibited an exponential decline during the burst stage, likely associated with the decay process of the accretion burst. None of the eight methanol maser lines were detected in the postburst stage. Besides maser lines, 12 thermal molecular lines were detected in both the burst and postburst stages. Variability of some of them was observed in both stages, but most notably, the ratio of HCO+/N2H+ increased significantly in the burst stage compared to the postburst stage. It is likely caused by the changes of both molecular lines during the burst and postburst stages due to the heating process induced by episodic accretion. Therefore, the comparison of their line intensity ratios might offer a new method for tracing episodic accretion bursts.

Protostellar Outflows at the EarliesT Stages (POETS)

Context. Understanding the launching mechanism of winds and jets remains one of the fundamental challenges in astrophysics. The Protostellar Outflows at the EarliesT Stages (POETS) survey has recently mapped the 3D velocity field of the protostellar winds in a sample (37) of luminous young stellar objects (YSOs) at scales of 10–100 au via very long baseline interferometry (VLBI) observations of the 22 GHz water masers. In most of the targets, the distribution of the 3D maser velocities can be explained in terms of a magnetohydrodynamic (MHD) disk wind (DW). Aims. Our goal is to assess the launching mechanism of the protostellar wind in the YSO IRAS 21078+5211, the most promising MHD DW candidate from the POETS survey. Methods. We have performed multi-epoch Very Long Baseline Array (VLBA) observations of the 22 GHz water masers in IRAS 21078+5211 to determine the 3D velocities of the gas flowing along several wind streamlines previously identified at a linear resolution of ~1 au. Results. Near the YSO at small separations along (xl ≤ 150 au) and across (R ≤ 40 au) the jet axis, water masers trace three individual DW streamlines. By exploiting the 3D kinematic information of the masers, we determined the launch radii of these streamlines with an accuracy of ~1 au, and they lie in the range of 10–50 au. At increasingly greater distances along the jet (110 au ≤ xl ≤ 220 au), the outflowing gas speeds up while it collimates close to the jet axis. Magneto-centrifugal launching in a radially extended MHD DW appears to be the only viable process to explain the fast (up to 60 km s−1) and collimated (down to 10°) velocities of the wind in correspondence with launch radii ranging between 10 and 50 au. At larger separations from the jet axis (R ≥ 100 au), the water masers trace a slow (≤20 km s−1), radially expanding arched shock-front with kinematics inconsistent with magneto-centrifugal launching. Our resistive-magnetohydrodynamical simulations indicate that this shock-front could be driven by magnetic pressure. Conclusions. The results obtained in IRAS 21078+5211 demonstrate that VLBI observations of the 22 GHz water masers can reliably determine the launching mechanism of protostellar winds.

A global view on star formation: The GLOSTAR Galactic plane survey

Studies of Galactic H II regions are of crucial importance for studying star formation and the evolution of the interstellar medium. Gaining an insight into their physical characteristics contributes to a more comprehensive understanding of these phenomena. The GLOSTAR project aims to provide a GLObal view on STAR formation in the Milky Way by performing an unbiased and sensitive survey. This is achieved by using the extremely wideband (4–8 GHz) C-band receiver of the Karl G. Jansky Very Large Array and the Effelsberg 100 m telescope. Using radio recombination lines observed in the GLOSTAR survey with the VLA in D-configuration with a typical line sensitivity of 1 σ ~ 3.0 mJy beam−1 at ~5 km s−1 and an angular resolution of 25″, we cataloged 244 individual Galactic H II regions (−2° ≤ ℓ ≤ 60° and |b| ≤ 1°, and 76° ≤ ℓ ≤ 83° and −1° ≤ b ≤ 2°) and derived their physical properties. We examined the mid-infrared (MIR) morphology of these H II regions and find that a significant portion of them exhibit a bubble-like morphology in the GLIMPSE 8 μm emission. We also searched for associations with the dust continuum and sources of methanol maser emission, other tracers of young stellar objects, and find that 48% and 14% of our H II regions, respectively, are coextensive with those. We measured the electron temperature for a large sample of H II regions within Galactocentric distances spanning from 1.6 to 13.1 kpc and derived the Galactic electron temperature gradient as ~372 ± 28 K kpc−1 with an intercept of 4248 ± 161 K, which is consistent with previous studies.

A Network Approach for the Accurate Characterization of Water Lines Observable in Astronomical Masers and Extragalactic Environments.

The water molecule, crucial to the chemical composition and dynamics of the universe, is typically identified in its gas phase via radio and submillimeter transitions, with frequencies up to a few THz. To understand the physicochemical behavior of astronomical objects, accurate transition frequencies are required for these lines. From a set of 26 new and 564 previous Lamb dip measurements, utilizing our ultrasensitive laser-based spectrometers in the near-infrared region, ultrahigh-precision spectroscopic networks were set up for H2 16O and H2 18O, augmented with 40 extremely accurate frequencies taken from the literature. Based on kHz-accuracy paths of these networks, considerably improved line-center frequencies have been obtained for 35 observed or predicted maser lines of H2 16O, as well as for 14 transitions of astronomical significance of H2 18O. These reference frequencies, attached with 5-25 kHz uncertainties, may help future studies in various fields of astrochemistry and astrophysics, in particular when precise information is demanded about Doppler-velocity components, including the gas flows of galactic cores, the kinematics of planetary nebulae, or the motion in exoplanetary atmospheres.

First detection of the J−1 → (J − 1)0 − E methanol maser transitions at J = 7 and 10

Context. Class I methanol masers provide sensitive information about the shocked environment around star-forming regions. Among the brightest Class I methanol masers, we have those in the J−1 → (J − 1)0 − E line series, currently reported for the J = 4 − 9 transitions, with the only exception being the J = 7 one at 181.295 GHz, and never expanded to higher J transitions. Aims. We aim to search for population inversion in the 7−1 → 60 − E and 10−1 → 90 − E methanol transition lines at 181.295 and 326.961 GHz, respectively, and also extend the number of known low-mass star-forming sources harboring Class I methanol masers. Methods. We employed the Atacama Pathfinder Experiment (APEX) 12 m telescope to survey low-mass Galactic sources, focusing on methanol emission lines. We built rotation diagrams for all sources with detected J = 7 methanol line transitions, while employing radiative transfer modeling (both in and out of local thermodynamic equilibrium) to characterize methanol excitation conditions in detail for one specific source with detected masers. Results. We detected the 7−1 → 60 − E and 10−1 → 90 − E methanol transitions in 6 out of 19 sources. Among them, we firmly determined the 10−1 → 90 − E maser nature in CARMA 7, L1641N, NGC 2024, and Serpens FIRS, and we show evidence for the presence of inverted population emission in the 7−1 → 60 − E line toward CARMA 7 and L1641N. This represents the first report of methanol maser emission in these particular transitions. Our study supports previous works indicating that conditions for Class I methanol maser emission are satisfied in low-mass star-forming regions and expands the range of detectable frequencies toward higher values.

The accretion burst of the massive young stellar object G323.46−0.08

Context. Accretion bursts from low-mass young stellar objects (YSOs) have been known for many decades. In recent years, the first accretion bursts of massive YSOs (MYSOs) have been observed. These phases of intense protostellar growth are of particular importance for studying massive star formation. Bursts of MYSOs are accompanied by flares of Class II methanol masers (hereafter masers), which are caused by an increase in exciting mid-infrared (MIR) emission. They can lead to long-lasting thermal afterglows of the dust continuum radiation visible at infrared (IR) and (sub)millimeter (hereafter (sub)mm) wavelengths. Furthermore, they might cause a scattered light echo. The G323.46−0.08 (hereafter G323) event, which shows all these features, extends the small sample of known MYSO bursts. Aims. Maser observations of the MYSO G323 show evidence of a flare, which was presumed to be caused by an accretion burst. This should be verified with IR data. We used time-dependent radiative transfer (TDRT) to characterize the heating and cooling timescales for eruptive MYSOs and to infer the main burst parameters. Methods. Burst light curves, as well as the pre-burst spectral energy distribution (SED) were established from archival IR data. The properties of the MYSO, including its circumstellar disk and envelope, were derived by using static radiative transfer modeling of pre-burst data. For the first time, TDRT was used to predict the temporal evolution of the SED. Observations with SOFIA/HAWC+ were performed to constrain the burst energy from the strength of the thermal afterglow. Image subtraction and ratioing were applied to reveal the light echo. Results. The G323 accretion burst is confirmed. It reached its peak in late 2013/early 2014 with a Ks-band increase of ∼2.5 mag. Both Ks-band and integrated maser flux densities follow an exponential decay. TDRT indicates that the duration of the thermal afterglow in the far-infrared (FIR) can exceed the burst duration by years. The latter was proved by SOFIA observations, which indicate a flux increase of (14.2 ± 4.6)% at 70 μm and (8.5 ± 6.1)% at 160 μm in 2022 (2 yr after the burst ended). A one-sided light echo emerged that was propagating into the interstellar medium. Conclusions. The burst origin of the G323 maser flare has been verified. TDRT simulations revealed the strong influence of the burst energetics and the local dust distribution on the strength and duration of the afterglow. The G323 burst is probably the most energetic MYSO burst that has been observed so far. Within 8.4 yr, an energy of (0.9−0.8+2.5) × 1047 erg was released. The short timescale points to the accretion of a compact body, while the burst energy corresponds to an accumulated mass of at least (7−6+20) MJup and possibly even more if the protostar is bloated. In this case, the accretion event might have triggered protostellar pulsations, which give rise to the observed maser periodicity. The associated IR light echo is the second observed from a MYSO burst.

Hyperfine structure of the methanol molecule as traced by Class I methanol masers

Hyperfine structure of the methanol molecule as traced by Class I methanol masers

An Outflow-driven Water Maser Associated with Positive Black Hole Feedback in the Dwarf Galaxy Henize 2–10

Henize 2–10 is a dwarf galaxy experiencing positive black hole (BH) feedback from a radio-detected low-luminosity active galactic nucleus. Previous Green Bank Telescope (GBT) observations detected a H2O “kilomaser” in Henize 2–10, but the low angular resolution (33″) left the location and origin of the maser ambiguous. We present new Karl G. Jansky Very Large Array observations of the H2O maser line at 22.23508 GHz in Henize 2–10 with ∼2″ resolution. These observations reveal two maser sources distinct in position and velocity. The first maser source is spatially coincident with the known BH outflow and the region of triggered star formation ∼70 pc to the east. Combined with the broad width of the maser (W 50 ∼ 66 km s−1), this confirms our hypothesis that part of the maser detected with the GBT is produced by the impact of the BH outflow shocking the dense molecular gas along the flow and at the interface of the eastern star-forming region. The second maser source lies to the southeast, far from the central BH, and has a narrow width (W 50 ∼ 8 km s−1), suggesting a star formation–related origin. This work has revealed the nature of the H2O kilomaser in Henize 2–10 and illustrates the first known connection between outflow-driven H2O masers and positive BH feedback.

Masers and Star Formation Activities in W51A

Concurrently with the maser flare observed in W51-North during the 3 month monitoring of H2O, NH3, and CH3OH maser variability from 2020 January to April using the Tianma 65 m Radio Telescope, we conducted Very Large Array mappings for these three maser species across the entirety of W51A region. After finding the ring-shaped H2O maser which might trace the disk surrounding the protostar residing in W51-North, the NH3 (9,6) maser delineated a jet which might be illuminated in the luminosity outburst possibly caused by the infalling streamer’s interaction with the protostar’s disk. An analysis of the comprehensive distribution of maser spots allowed us to affirm that W51N4 or ALMAmm31 serves as the primary source of the Lacy jet. Furthermore, we observed that class I methanol maser spots may extend beyond the locations of the H2O and NH3 (9,6) masers within the outflow. Additionally, emissions from other NH3 maser transitions coincided with specific 1.3 mm continuum sources. The arrangement of H2O maser spots in the vicinity of W51e2-E potentially indicates episodic accretions in this source. Combining the data from the Atacama Large Millimeter/submillimeter Array data archive for W51-North, W51e2, and W51e8, we have discovered that the H2O, NH3, and CH3OH masers, as well as the HC3N and SiO emissions are found to be good tools for tracing outflow in this work for W51A.

Proper motion study of the 6.7 GHz methanol maser rings

Context. Methanol masers at 6.7 GHz are well-known signposts of high-mass star-forming regions. Due to their high brightness, they enable us to derive the three-dimensional gas kinematics near protostars and young stars. Aims. We aim to understand the origin of the ring-like structures outlined by methanol maser emission in a number of sources. This emission could be, a priori, spatially associated with an outflow and/or disc around a high-mass protostar. In cases of expansion or rotation, maser proper motions should be, for instance, diverging from the ring centre or perpendicular to the ring radius. Methods. Using sensitive, three-epoch observations spanning over 8 yr with the European VLBI Network, we have started the most direct investigations of maser rings using very accurate proper motion measurements with uncertainties below ~1 km s−1. Results. We present full results for the five targets of our sample, G23.207−00.377, G23.389+00.185, G28.817+00.365, G31.047+00.356, and G31.581+00.077, where proper motions show similar characteristics; maser cloudlets do not move inwards towards the centre of the rings but rather outwards. We also include the most circular source, G23.657−00.127, in the discussion as a reference. The magnitude of maser proper motions ranges from a maximum of about 13 km s−1 to 0.5 km s−1. In two of the five sources with a high number of maser spots (>100), namely G23.207−00.377 and G23.389+00.185, we show that the size of the best elliptical model, fitted to the distribution of persistent masers, increases in time in a manner similar to the case of G23.657−00.127. Moreover, we checked the separations between the pairs of spots from distinct regions, and we were able to assess that G28.817+00.365 and G31.047+00.356 can be interpreted as showing expanding motions. We analysed the profiles of single maser cloudlets and studied their variability. Contrary to single-dish studies, the interferometric data indicate variability of the emission of single-masing cloudlets. Conclusions. In five of the six targets, namely, G23.207−00.377, G23.389+00.185, G23.657-00.127, G28.817+00.365, and G31.047+00.356, expansion motions prevail. Only in the case of G31.581+00.077 can a scenario of disc-like rotation not be excluded. Complementary observations of thermal tracers as well as searching for ultra-compact H II regions in the same sources are needed. Although the overall morphology of the maser emission has remained stable, the intensities of individual maser cloudlets varied from epoch to epoch, suggesting internal instabilities.

Water vapour masers in long-period variable stars

Context. Water maser emission is often found in the circumstellar envelopes of evolved stars, that is, asymptotic giant branch stars and red supergiants with oxygen-rich chemistry. The H2O emission shows strong variability in evolved stars of both of these types. Aims. We wish to understand the reasons for the strong variability of water masers emitted at 22 GHz. In this paper, we study U Her and RR Aql as representatives of Mira variable stars. Methods. We monitored U Her and RR Aql in the 22 GHz maser line of water vapour with single-dish telescopes. The monitoring period covered about two decades between 1990 and 2011, with a gap between 1997 and 2000 in the case of RR Aql. Observations were also made in 1987 and 2015 before and after the period of contiguous monitoring. In addition, maps of U Her were obtained in the period 1990–1992 with the Very Large Array. Results. We find that the strongest emission in U Her is located in a shell with boundaries of 11–25 AU. The gas-crossing time is 8.5 yr. We derive lifetimes for individual maser clouds of ≤4 yr based on the absence of detectable line-of-sight velocity drifts of the maser emission. The shell is not evenly filled, and its structure is maintained over much longer timescales than those of individual maser clouds. Both stars show brightness variability on several timescales. The prevalent variation is periodic, following the optical variability of the stars with a lag of 2–3 months. Superposed are irregular fluctuations of a few months in duration, with increased or decreased excitation at particular locations, and long-term systematic variations on timescales of a decade or more. Conclusions. The properties of the maser emission are governed by those of the stellar wind while traversing the H2O maser shell. Inhomogeneities in the wind affecting the excitation conditions and prevalent beaming directions likely cause the variations seen on timescales of longer than the stellar pulsation period. We propose the existence of long-living regions in the shells, which maintain favourable excitation conditions on timescales of the wind-crossing times through the shells or orbital periods of (sub)stellar companions. The H2O maser properties in these two Mira variables are remarkably similar to those in the semiregular variables studied in our previous papers regarding shell location, outflow velocity, and lifetime. The only difference is the regular brightness variations of the Mira variables caused by the periodic pulsation of the stars.

Maser Activity of Organic Molecules toward Sgr B2(N)

At centimeter wavelengths, single-dish observations have suggested that the Sagittarius (Sgr) B2 molecular cloud at the Galactic Center hosts weak maser emission from several organic molecules, including CH2NH, HNCNH, and HCOOCH3. However, the lack of spatial distribution information on these new maser species has prevented us from assessing the excitation conditions of the maser emission as well as their pumping mechanisms. Here, we present a mapping study toward Sgr B2 north (N) to locate the region where the complex maser emission originates. We report the first detection of the Class I methanol (CH3OH) maser at 84 GHz and the first interferometric map of the methanimine (CH2NH) maser at 5.29 GHz toward this region. In addition, we present a tool for modeling and fitting the unsaturated molecular maser signals with non-LTE radiative transfer models and Bayesian analysis using the Markov Chain Monte Carlo approach. These enable us to quantitatively assess the observed spectral profiles. The results suggest a two-chain-clump model for explaining the intense CH3OH Class I maser emission toward a region with low continuum background radiation. By comparing the spatial origin and extent of maser emission from several molecular species, we find that the 5.29 GHz CH2NH maser has a close spatial relationship with the 84 GHz CH3OH Class I masers. This relationship serves as observational evidence to suggest a similar collisional pumping mechanism for these maser transitions.

On the Structure of the Sagittarius Spiral Arm in the Inner Milky Way

We report measurements of trigonometric parallax and proper motion for two 6.7 GHz methanol and two 22 GHz water masers located in the far portion of the Sagittarius spiral arm as part of the BeSSeL Survey. Distances for these sources are estimated from parallax measurements combined with three-dimensional kinematic distances. The distances of G033.64−00.22, G035.57−00.03, G041.15−00.20, and G043.89−00.78 are 9.9 ± 0.5, 10.2 ± 0.6, 7.6 ± 0.5, and 7.5 ± 0.3 kpc, respectively. Based on these measurements, we suggest that the Sagittarius arm segment beyond about 8 kpc from the Sun in the first Galactic quadrant should be adjusted radially outward relative to previous models. This supports the suggestion of Xu et al. that the Sagittarius and Perseus spiral arms might merge in the first quadrant before spiraling inward to the far end of the Galactic bar.

MeerKAT and ALMA view of the AGAL045.804−0.356 clump

Abstract This study presents a detailed analysis of the GAL045.804−0.356 massive star-forming clump. A high-angular resolution and sensitivity observations were conducted using MeerKAT at 1.28 GHz and ALMA interferometer at 1.3 mm. Two distinct centimetre radio continuum emissions (source A and source B) were identified within the clump. A comprehensive investigation was carried out on source A, the G45.804−0.355 star-forming region (SFR) due to its association with Extended Green Object (EGO), 6.7 GHz methanol maser and the spatial coincidence with the peak of the dust continuum emission at 870 μm. The ALMA observations revealed seven dense dust condensations (MM1 to MM7) in source A. The brightest (Sν ∼ 87 mJy) and massive main dense core, MM1, was co-located with the 6.7 GHz methanol maser. Explorations into the kinematics revealed gas motions characterised by a velocity gradient across the MM1 core. Furthermore, molecular line emission showed the presence of an extended arm-like structure, with a physical size of 0.25 pc × 0.18 pc (∼ 50000 au × 30000 au) at a distance of 7.3 kpc. Amongst these arms, two arms were prominently identified in both the dust continuum and some of the molecular lines. A blue-shifted absorption P-Cygni profile was seen in the H2CO line spectrum. The findings of this study are both intriguing and new, utilising data from MeerKAT and ALMA to investigate the characteristics of the AGAL45 clump. The evidence of spiral arms, the compact nature of the EGO and < 2 km s−1 velocity gradient are all indicative of G45.804−0.355 being oriented face-on.

The candidates of long-periodic variable sources in 6.7 GHz methanol masers associated with four high-mass star-forming regions

Abstract Results of the long-term monitoring observations by the Hitachi 32 m radio telescope of the 6.7 GHz Class II methanol masers associated with four high-mass star-forming regions are presented. We detected periodic flux variability in G06.795−0.257, G10.472+0.027, G12.209−0.102, and G13.657−0.599 with the periods of 968, 1624, 1272, and 1266 d, respectively, although the detected period is tentative due to the short monitoring term relative to the estimated period. The facts that the flux variation patterns show the symmetric sine curves and that the luminosities of the central protostar and periods of maser flux variation are consistent with the expected period–luminosity (PL) relation suggest that the mechanisms of maser flux variability of G10.472+0.027 and G12.209−0.102 can be explained by protostellar pulsation instability. From the PL relation, the central stars of these two sources are expected to be very high-mass protostars with a mass of $\sim 40\, M_{\odot }$ and to have a mass accretion rate of $\sim 2 \times 10^{-2}\, M_{\odot }\:$yr−1. On the other hand, G06.795−0.257 and G13.657−0.599 have intermittent variation patterns and have luminosities that are an order of magnitude smaller than those expected from the PL relation, suggesting that the variation mechanisms of these sources originated from a binary system. Since almost all the maser features vary with the same period regardless of the geometry, periodic accretion models may be appropriate mechanisms for flux variability in G06.795−0.257 and G13.657−0.599.

Discovery of widespread non-metastable ammonia masers in the Milky Way

We present the results of a search for ammonia maser emission in 119 Galactic high-mass star-forming regions (HMSFRs) known to host 22\,GHz H$_2$O maser emission. Our survey has led to the discovery of non-metastable NH$_3$ inversion line masers toward 14 of these sources. This doubles the number of known non-metastable ammonia masers in our Galaxy, including nine new very high-excitation ($J,K$) = (9,6) maser sources. These maser lines, including NH$_3$ (5,4), (6,4), (6,5), (7,6), (8,6), (9,6), (9,8), (10,8), and (11,9), arise from energy levels of 342 K, 513 K, 465 K, 606 K, 834 K, 1090 K, 942 K, 1226 K, and 1449 K above the ground state. Additionally, we tentatively report a new metastable NH$_3$ (3,3) maser in G048.49 and an NH$_3$ (7,7) maser in G029.95. Our observations reveal that all of the newly detected NH$_3$ maser lines exhibit either blueshifted or redshifted velocities with respect to the source systemic velocities. Among the non-metastable ammonia maser lines, larger velocity distributions, offset from the source systemic velocities, are found in the ortho-NH$_3$ ($K=3n$) transitions than in the para-NH$_3$ ($K ones.

Star formation in G11.497-1.485: Two-epoch VLA study of a 6.7 GHz methanol maser flare

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.

Distribution Properties of the 6.7 GHz Methanol Masers and Their Surrounding Gases in the Milky Way

An updated catalog consisting of 1092 6.7 GHz methanol maser sources is reported in this work. Additionally, the NH3 (1, 1), NH3 (2, 2), and NH3 (3, 3) transitions were observed toward 214 star-forming regions using the Shanghai Tianma radio telescope in order to examine the differences in physical environments, such as the excitation temperature and column density of molecular clouds associated with methanol masers on the Galactic scale. Statistical results reveal that the number of 6.7 GHz methanol masers in the Perseus arm is significantly lower than that in the other three main spiral arms. In addition, the Perseus arm also has the lowest gas column density among the main spiral arms traced by the NH3 observations. Both findings suggest that the Perseus arm has the lowest rate of high-mass star formation compared to the other three main spiral arms. We also observed a trend in which both the luminosity of the 6.7 GHz methanol masers and the ammonia gas column density decreased with the galactocentric distance. This finding indicates that the density of material in the inner Milky Way is generally higher than that in the outer Milky Way. This further suggests that high-mass stars are more easily formed at the head of the spiral arms. Furthermore, we found that the column density of ammonia gas is higher in the regions on the arms than in the inter-arm regions, supporting that the former is more likely to be the birthplace of high-mass stars.

The influence of accretion bursts on methanol and water in massive young stellar objects

Context. The effect of accretion bursts on massive young stellar objects (MYSOs) represents a new research field in the study of young stars and their environment. The impact of such bursts on the disk and envelope has been observed and plays the role of a “smoking gun” providing information about the properties of the burst itself. Aims. We aim to investigate the impact of an accretion burst on massive disks with different types of envelopes and to study the effects of an accretion burst on the temperature structure and the chemistry of the disk. We focus on water and methanol as chemical species for this paper. Methods. The thermochemical code of ProDiMo (PROtoplanetary DIsk MOdel) is used to perform simulations of high-mass protoplanetary-disk models with different types of envelopes in the presence of an accretion burst. The models in question represent different evolutionary stages of protostellar objects. We calculated and show the chemical abundances in three phases of the simulation (pre-burst, burst, and post-burst). Results. More heavily embedded disks show higher temperatures. The impact of the accretion burst is mainly characterized by the desorption of chemical species present in the disk and envelope from the dust grains to the gas phase. When the post-burst phase starts, the sublimated species freeze out again. The degree of sublimation depends strongly on the type of envelope the disk is embedded in. An accretion burst in more massive envelopes produces stronger desorption of the chemical species. However, our models show that the timescale for the chemistry to reach the pre-burst state is independent of the type of envelope. Conclusions. The study shows that the disk’s temperature increases with a more massive envelope enclosing it. Thus, the chemistry of MYSOs in earlier stages of their evolution reacts stronger to an accretion burst than at later stages where the envelope has lost most of its mass or has been dissipated. The study of the impact of accretion bursts could also provide helpful theoretical context to the observation of methanol masers in massive disks.

Sub-parsec-scale jet-driven water maser with possible gravitational acceleration in the radio galaxy NGC 1052

Abstract We report sub-parsec-scale observations of the 321 GHz H2O emission line in the radio galaxy NGC 1052. The H2O line emitter size is constrained in <0.6 mas distributed on the continuum core component. The brightness temperature exceeding 106 K and the intensity variation indicate certain evidence for maser emission. The maser spectrum consists of redshifted and blueshifted velocity components spanning ∼400 km s−1, separated by a local minimum around the systemic velocity of the galaxy. The spatial distribution of maser components shows a velocity gradient along the jet direction, implying that the population-inverted gas is driven by the jets interacting with the molecular torus. We identified a significant change of the maser spectra between two sessions separated by 14 days. The maser profile showed a radial velocity drift of 127 ± 13 km s−1 yr−1 implying inward gravitational acceleration at 5000 Schwarzschild radii. The results demonstrate the feasibility of future very long baseline interferometry observations to resolve the jet–torus interacting region.