GHz CH3OH Research Articles

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.

ALMA observations of the Extended Green Object G19.01–0.03 – ii. A massive protostar with typical chemical abundances surrounded by four low-mass pre-stellar core candidates

ABSTRACT We present a study of the physical and chemical properties of the Extended Green Object (EGO) G19.01−0.03 using sub-arcsecond angular resolution Atacama Large Millimetre/submillimetre Array (ALMA) 1.05 mm and Karl G. Jansky Very Large Array (VLA) 1.21 cm data. G19.01−0.03 MM1, the millimetre source associated with the central massive young stellar object (MYSO), appeared isolated and potentially chemically young in previous Submillimetre Array observations. In our ∼0.4 arcsec-resolution ALMA data, MM1 has four low-mass millimetre companions within 0.12 pc, all lacking maser or outflow emission, indicating they may be pre-stellar cores. With a rich ALMA spectrum full of complex organic molecules, MM1 does not appear chemically young, but has molecular abundances typical of high-mass hot cores in the literature. At the 1.05 mm continuum peak of MM1, N(CH3OH) = (2.22 ± 0.01) × 1018 cm−2 and $T_{\mathrm{ex}} = 162.7\substack{+0.3 \\ -0.5}$ K based on pixel-by-pixel Bayesian analysis of LTE synthetic methanol spectra across MM1. Intriguingly, the peak CH3OH Tex = 165.5 ± 0.6 K is offset from MM1’s millimetre continuum peak by 0.22 arcsec ∼ 880 au, and a region of elevated CH3OH Tex coincides with free–free VLA 5.01 cm continuum, adding to the tentative evidence for a possible unresolved high-mass binary in MM1. In our VLA 1.21 cm data, we report the first NH3(3,3) maser detections towards G19.01−0.03, along with candidate 25 GHz CH3OH 5(2, 3) − 5(1, 4) maser emission; both are spatially and kinematically coincident with 44 GHz Class I CH3OH masers in the MM1 outflow. We also report the ALMA detection of candidate 278.3 GHz Class I CH3OH maser emission towards this outflow, strengthening the connection of these three maser types to MYSO outflows.

Broadband VLA Spectral-line Survey of a Sample of Ionized Jet Candidates

The study of the interaction between ionized jets, molecular outflows, and their environments is critical to understanding high-mass star formation, especially because jets and outflows are thought to be key in the transfer of angular momentum outward from accretion disks. We report a low spectral resolution Karl G. Jansky Very Large Array (VLA) survey for OH, NH3, CH3OH, and hydrogen radio recombination lines, toward a sample of 58 high-mass star-forming regions that contain numerous ionized jet candidates. The observations are from a survey designed to detect radio continuum; the novel aspect of this work is to search for spectral lines in broadband VLA data (we provide the script developed in this work to facilitate exploration of other data sets). We report detection of 25 GHz CH3OH transitions toward 10 sources; 5 of them also show NH3 emission. We found that most of the sources detected in CH3OH and NH3 have been classified as ionized jets or jet candidates and that the emission lines are coincident with, or very near (≲0.1 pc), these sources; hence, these molecular lines could be used as probes of the environment near the launching site of jets/outflows. No radio recombination lines were detected, but we found that the rms noise of stacked spectra decreases following the radiometer equation. Therefore, detecting radio recombination lines in a sample of brighter free–free continuum sources should be possible. This work demonstrates the potential of broadband VLA continuum observations as low resolution spectral-line scans.

A Multiwavelengh study towards Galactic HII region G10.32-0.26

AbstractWe present the results of high-resolution continuum and molecular line observations towards the Galactic HII region, G10.32-0.26. The continuum map with ALMA reveals the five cores with masses ranging from 2.5–9.2 Me. The results show that the brightest peak, Peak 1, is an HCHII region with an excitation temperature of : 12000 K, an electron density of 3.4×107 cm3, and a radius of 14 au. The central object is estimated to be a B0.5 star. The class II 6.7 GHz CH3OH maser coincides with Peak 1, implying class II CH3OH maser is associated with a later evolutionary stage of star formation. Using KaVA and KVN observation, we detect the class I 44 and 95 GHz CH3OH masers near the weakest peak, Peak 5. We successfully imaged class I 95 GHz CH3OH masers with VLBI for the first time. In Peak 5, the high-velocity SiO emission also exists. The continuum emission can be modelled with grey-body dust emission with Td∼30 K, and the molecular species are poor near Peak 5, suggesting Peak 5 is in an early stage of star formation.

The water and methanol masers in the face-on accretion system around the high-mass protostar G353.273+0.641

AbstractWe report on a direct comparison of VLBI maser data and ALMA thermal-emission data for the high-mass protostar G353.273+0.641. We detected a gravitationally-unstable disk by dust and a high-velocity jet traced by a thermal CO line by ALMA long-baselines (LB). 6.7 GHz CH3OH masers trace infalling streamlines inside the disk. The innermost maser ring indicates another compact accretion disk of 30 au. Such a nested system could be caused by angular momentum transfer by the spiral arms. 22 GHz H2O masers trace the jet-accelerating region, which are directly connecting the CO jet and the protostar. The recurrent maser flares imply episodic jet ejections per 1–2 yr, while typical separation of CO knots indicates a variation of outflow rate per 100 yr. Our study demonstrates that VLBI maser observations are still a powerful tool to explore detailed structures nearby high-mass protostars by combining ALMA LB.

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

Class II methanol (CH3OH) masers are amongst the clearest signposts of recent high-mass star formation (HMSF). A complete catalogue outlines the distribution of star formation in the Galaxy, the number of young star-forming cores, and the physical conditions of their environment, made possible by the Global View on Star Formation (GLOSTAR) survey. This blind survey of the radio regime of 4–8 GHz maps the Galactic mid-plane in the radio continuum, 6.7 GHz methanol line, the 4.8 GHz formaldehyde line, and several radio recombination lines. We present an analysis of the observations of the 6.7 GHz CH3OH maser transition using data from the D-configuration of the Very Large Array (VLA). We analysed data covering Galactic longitudes from –2° < l < 60° and Galactic latitudes of |b| < 1°. We detected a total of 554 methanol masers, out of which 84 are new, and we catalogued their positions, velocity components, and integrated fluxes. With a typical noise level of ~18 mJy beam−1, this is the most sensitive unbiased methanol survey for methanol masers to date. We also searched for dust continuum and radio continuum associations, finding that 97% of the sources are associated with dust and 12% are associated with radio continuum emission.

Evidence for an Interaction between the Galactic Center Clouds M0.10–0.08 and M0.11–0.11

We present high-resolution (∼2–3″; ∼0.1 pc) radio observations of the Galactic center cloud M0.10−0.08 using the Very Large Array at K and Ka band (∼25 and 36 GHz). The M0.10−0.08 cloud is located in a complex environment near the Galactic center Radio Arc and the adjacent M0.11−0.11 molecular cloud. From our data, M0.10−0.08 appears to be a compact molecular cloud (∼3 pc) that contains multiple compact molecular cores (5+; <0.4 pc). In this study, we detect a total of 15 molecular transitions in M0.10−0.08 from the following molecules: NH3, HC3N, CH3OH, HC5N, CH3CN, and OCS. We have identified more than sixty 36 GHz CH3OH masers in M0.10−0.08 with brightness temperatures above 400 K and 31 maser candidates with temperatures between 100 and 400 K. We conduct a kinematic analysis of the gas using NH3 and detect multiple velocity components toward this region of the Galactic center. The bulk of the gas in this region has a velocity of 51.5 km s−1 (M0.10−0.08) with a lower-velocity wing at 37.6 km s−1. We also detect a relatively faint velocity component at 10.6 km s−1 that we attribute to being an extension of the M0.11−0.11 cloud. Analysis of the gas kinematics, combined with past X-ray fluorescence observations, suggests M0.10−0.08 and M0.11−0.11 are located in the same vicinity of the Galactic center and could be physically interacting.

Cyanopolyyne line survey towards high-mass star-forming regions with TMRT

Context. Cyanopolyynes (HC2n+1 N, n = 1,2,3), which are the linear carbon chain molecules, are precursors for the prebiotic synthesis of simple amino acids. They are important for understanding prebiotic chemistry and may be good tracers of the star formation sequence. Aims. We aim to search for cyanopolyynes in high-mass star-forming regions (HMSFRs) at possibly different evolutionary stages, investigate the evolution of HC3N and its relation with shock tracers, and detect the existence of HC5N and HC7N in HMSFRs with a formed protostar. Methods. We carried out a cyanopolyyne line survey towards a large sample of HMSFRs using the Shanghai Tian Ma 65 m Radio Telescope (TMRT). Our sample consisted of 123 targets taken from the TMRT C band line survey. It included three kinds of sources, namely those with detection of the 6.7 GHz CH3OH maser alone, with detection of the radio recombination line (RRL) alone, and with detection of both (hereafter referred to as Maser-only, RRL-only, and Maser-RRL sources, respectively). For our sample with detection of cyanopolyynes, their column densities were derived using the rotational temperature measured from the NH3 lines. We constructed and fitted the far-infrared (FIR) spectral energy distributions (SED; obtained from the Herschel FIR data and the Atacama Pathfinder Experiment data at 870 µm) of our HC3N sources. Moreover, by analysing the relation between HC3N and other shock tracers, we also investigate whether HC3N is a good tracer of shocks. Results. We detected HC3N in 38 sources, HC5N in 11 sources, and HC7N in G24.790+0.084, with the highest detection rate being found for Maser-RRL sources and a very low detection rate found for RRL-only sources. The mean column density of HC3N was found to be (1.75 ± 0.42) × 1013, (2.84 ± 0.47) × 1013, and (0.82 ± 0.15) × 1013 cm−2 for Maser-only, Maser-RRL, and RRL-only sources, respectively. Based on a fit of the FIR SED, we derive their dust temperatures, H2 column densities, and abundances of cyanopolyynes relative to H2. The mean relative abundance of HC3N was found to be (1.22 ± 0.52) × 10−10 for Maser-only, (5.40 ± 1.45) × 10−10 for Maser-RRL, and (1.65 ± 1.50) × 10−10 for RRL-only sources, respectively. Conclusions. The detection rate, the column density, and the relative abundance of HC3N increase from Maser-only to Maser-RRL sources and decrease from Maser-RRL to RRL-only sources. This trend is consistent with the proposed evolutionary trend of HC3N under the assumption that our Maser-only, Maser-RRL, and RRL-only sources correspond to massive young stellar objects, ultracompact H ii regions, and normal classical H ii regions, respectively. Our detections enlarge the sample of HC3N in HMSFRs and support the idea that unsaturated complex organic molecules can exist in HMSFRs with a formed protostar. Furthermore, a statistical analysis of the integrated line intensity and column density of HC3N and shock-tracing molecules (SiO, H2CO) enabled us to find positive correlations between them. This suggests that HC3N may be another tracer of shocks, and should therefore be the subject of further observations and corresponding chemical simulations. Our results indirectly support the idea that the neutral-neutral reaction between C2H2 and CN is the dominant formation pathway of HC3N.

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.

An Excited-state OH Maser Survey toward WISE Point Sources

We report a survey of excited-state hydroxyl (ex-OH) masers at the 4.7 and 6.0 GHz transitions toward a sample consisting of 3348 massive star-forming region candidates selected from the all-sky Wide-field Infrared Survey Explorer point-source catalog. The survey was conducted with the Shanghai Tianma Radio Telescope. In total, 6, 9, and 30 sources were detected with the ex-OH masers at the 4766, 6031, and 6035 MHz transitions, respectively. Among them, one 4766 MHz, one 6031 MHz, and five 6035 MHz ex-OH maser sources are newly identified. A series of statistical analyses derived that the ex-OH masers were detected efficiently toward the subsamples associated with both the 6.7 GHz CH3OH maser and radio recombination lines (RRLs), whereas compared to the CH3OH masers, the ex-OH masers are more likely to be produced toward the sources with stronger 22 μm band emission. A significant luminosity correlation is found between the ex-OH masers and RRLs. In addition, we found that the magnetic field strength of most sources with the 6.7 GHz CH3OH maser was stronger relative to that of sources without the CH3OH maser. Combined with these, it demonstrates that the ex-OH maser is associated with more evolved star-forming regions, likely associated with thicker dust envelopes. Meanwhile, the ex-OH masers might be a potential tracer for measuring the Galactic magnetic field information on the large-scale views using their Zeeman pairs.

EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions

Context. Although the role of magnetic fields in launching molecular outflows in massive young stellar objects has been convincingly demonstrated by theoretical arguments, observationally, the alignment of the magnetic field lines with the molecular outflows is still under debate. Aims. We aim to complete the measurements of the direction of the magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether the magnetic field and outflows are aligned. Methods. In 2012, we started a large very long baseline interferometry campaign with the European VLBI Network to measure the magnetic field orientation and strength toward a sample of 31 massive star-forming regions (called the flux-limited sample) by analyzing the polarized emission of 6.7 GHz CH3OH masers. In the previous papers of the series, we have presented 80% of the sample. Here, we report the linearly and circularly polarized emission of 6.7 GHz CH3OH masers toward the last five massive star-forming regions of the flux-limited sample. The sources are G30.70-0.07, G30.76-0.05, G31.28+0.06, G32.03+0.06, and G69.52-0.97. Results. We detected a total of 209 CH3OH maser cloudlets, 15% of which show linearly polarized emission (0.07–16.7%), and 2% of which show circularly polarized emission (0.2–4.2%). As reported in previous papers, in the last five sources of the flux-limited sample, we also measured well-ordered linear polarization vectors. Zeeman splitting was measured toward G30.70-0.07, G32.03+0.06, and G69.52-0.97. Conclusions. The statistical analysis of the entire flux-limited sample shows that the observations are consistent with a bimodal distribution in the difference between the 3D magnetic field direction and the outflow axis, with half the magnetic field directions being perpendicular and the other half being parallel to the outflow. In addition, we determined that typical values of the linear and circular polarization fractions for 6.7 GHz CH3OH masers are Pl = 1.0–2.5% and PV = 0.5–0.75%, respectively. From the circularly polarized spectra of the CH3OH maser features, we found that a typical Zeeman splitting is in the range between 0.5 m s−1 and 2.0 m s−1. This would correspond to 9 mG &lt; |B||| &lt; 40 mG if F = 3 → 4 is the most favored of the eight hyperfine transitions that might contribute to the maser emission.

VLA Observations of Nine Extended Green Objects in the Milky Way: Ubiquitous Weak, Compact Continuum Emission, and Multi-epoch Emission from Methanol, Water, and Ammonia Masers

Abstract We have observed a sample of nine Extended Green Objects (EGOs) at 1.3 and 5 cm with the Very Large Array (VLA) with subarcsecond resolution and ∼7–14 μJy beam−1-sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find an EGO-associated continu um emission—within 1″ of the extended 4.5 μm emission—in every field, which is typically faint (order 101–102 μJy) and compact (unresolved at 0″.3–0″.5). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal (19% of detections) and thermal free–free processes (e.g., ionized jets and compact H ii regions, 78% of sample) and warm dust (1 source). We also find an EGO-associated 6.7 GHz CH3OH and 22 GHz H2O maser emission in 100% of the sample and a NH3 (3,3) masers in ∼45%; we do not detect any NH3 (6,6) masers at ∼5.6 mJy beam−1 sensitivity. We find statistically-significant correlations between L radio and L bol at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between L H 2 O and L radio for our sample. From these data, we conclude that EGOs likely host multiple different centimeter continuum-producing processes simultaneously. Additionally, at our ∼1000 au resolution, we find that all EGOs except G18.89−0.47 contain 1 ∼ 2 massive sources based on the presence of CH3OH maser groups, which is consistent with our previous work suggesting that these are typical massive protoclusters, in which only one to a few of the young stellar objects are massive.

Massive protocluster of a periodic maser source G188.95+0.89

ABSTRACT We report the results of ongoing monitoring of the 6.7 GHz CH3OH masers associated with G188.95+0.89. In these observations, five features are periodically varying and at least two exhibit evidence of velocity drifts. It is not clear the cause of these velocity drifts. The spectra have varied significantly since detection in 1991. The 11.45 km s−1 feature has decreased exponentially from 2003. Complementary ALMA 1.3-mm continuum and line observational results are also presented. Eight continuum cores (MM1–MM8) were detected in G188.95+0.89. We derived the masses of the detected cores. G188.95+0.89 MM2 was resolved into two continuum cores (separated by 0.1 arcsec) in ALMA band 7 observations. Also, CH3OH (4(2, 2)-3(1, 2)) thermal emission associated with MM2 is double peaked. We propose the presence of multiple (at least binary) young stellar objects in MM2. SiO emission exhibits a bow-shock morphology in MM2 while strong emission of 12CO at the east and west of MM2 suggests the presence of an east–west bipolar outflow.

Infrared observations of the flaring maser source G358.93−0.03

Context. Class II methanol masers are signposts of massive young stellar objects (MYSOs). Recent evidence shows that flares of these masers are driven by MYSO accretion bursts. Thus, maser monitoring can be used to identify such bursts which are hard to discover otherwise. Infrared observations reveal burst-induced changes in the spectral energy distribution (first and foremost a luminosity increase), which provide valuable information on a very intense phase of high-mass star formation. Aims. In mid-January 2019, flaring of the 6.7 GHz CH3OH maser (hereafter maser) of the MYSO G358.93-0.03 (hereafter G358) was reported. The international maser community initiated an extensive observational campaign which revealed extraordinary maser activity and yielded the detection of numerous new masering transitions. Interferometric imaging with the Atacama Large Millimeter/submillimeter Array and the Submillimeter Array resolved the maser emitting core of the star forming region and proved the association of the masers with the brightest continuum source (MM1), which hosts a hot molecular core. These observations, however, failed to detect a significant rise in the (sub)millimeter dust continuum emission. Therefore, we performed near-infrared (NIR) and far-infrared (FIR) observations to prove or disprove whether the CH3OH flare was driven by an accretion burst. Methods. NIR imaging with the Gamma-Ray Burst Optical/Near-infrared Detector has been acquired and integral-field spectroscopy with the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) was carried out on two occasions to detect possible counterparts to the (sub)millimeter sources and compare their photometry to archival measurements. The comparison of pre-burst and burst spectral energy distributions is of crucial importance to judge whether a substantial luminosity increase, caused by an accretion burst, is present and if it triggered the maser flare. Radiative transfer modeling of the spectral energy distribution (SED) of the dust continuum emission at multiple epochs provides valuable information on the bursting MYSO. Results. The FIR fluxes of MM1 measured with FIFI-LS exceed those from Herschel significantly, which clearly confirms the presence of an accretion burst. The second epoch data, taken about 16 months later, still show increased fluxes. Our radiative transfer modeling yielded major burst parameters and suggests that the MYSO features a circumstellar disk which might be transient. From the pre-burst, burst, and post-burst SEDs, conclusions on heating and cooling time-scales could be drawn. Circumstances of the burst-induced maser relocation have been explored. Conclusions. The verification of the accretion burst from G358 is another confirmation that Class II methanol maser flares represent an alert for such events. Thus, monitoring of these masers greatly enhances the chances of identifying MYSOs during periods of intense growth. The few events known to date already indicate that there is a broad range in burst strength and duration as well as environmental characteristics. The G358 event is the shortest and least luminous accretion burst known to date. According to models, bursts of this kind occur most often.

Search for radio jets from massive young stellar objects

Context.Recent theoretical and observational studies debate the similarities of the formation process of high- (&gt;8M⊙) and low-mass stars. The formation of low-mass stars is directly associated with the presence of disks and jets. Theoretical models predict that stars with masses up to 140M⊙can be formed through disk-mediated accretion in disk-jet systems. According to this scenario, radio jets are expected to be common in high-mass star-forming regions.Aims.We aim to increase the number of known radio jets in high-mass star-forming regions by searching for radio-jet candidates at radio continuum wavelengths.Methods.We used theKarl G. JanskyVery Large Array (VLA) to observe 18 high-mass star-forming regions in theCband (6 cm, ≈1′′.0 resolution) andKband (1.3 cm, ≈0′′.3 resolution). We searched for radio-jet candidates by studying the association of radio continuum sources with shock activity signs (e.g., molecular outflows, extended green objects, and maser emission). Our VLA observations also targeted the 22 GHz H2O and 6.7 GHz CH3OH maser lines.Results.We have identified 146 radio continuum sources, 40 of which are located within the field of view of both images (CandKband maps). We derived the spectral index, which is consistent with thermal emission (between − 0.1 and + 2.0) for 73% of these sources. Based on the association with shock-activity signs, we identified 28 radio-jet candidates. Out of these, we identified 7 as the most probable radio jets. The radio luminosity of the radio-jet candidates is correlated with the bolometric luminosity and the outflow momentum rate. About 7–36% of the radio-jet candidates are associated with nonthermal emission. The radio-jet candidates associated with 6.7 GHz CH3OH maser emission are preferentially thermal winds and jets, while a considerable fraction of radio-jet candidates associated with H2O masers show nonthermal emission that is likely due to strong shocks.Conclusions.About 60% of the radio continuum sources detected within the field of view of our VLA images are potential radio jets. The remaining sources could be compact H IIregions in their early stages of development, or radio jets for which we currently lack further evidence of shock activity. Our sample of 18 regions is divided into 8 less evolved infrared-dark regions and 10 more evolved infrared-bright regions. We found that ≈71% of the identified radio-jet candidates are located in the more evolved regions. Similarly, 25% of the less evolved regions harbor one of the most probable radio jets, while up to 50% of the more evolved regions contain one of these radio-jet candidates. This suggests that the detection of radio jets in high-mass star-forming regions is more likely in slightly more evolved regions.

High-sensitivity observations of molecular lines with the Arecibo Telescope

ABSTRACT We report on one of the highest sensitivity surveys for molecular lines in the frequency range 6.0–7.4 GHz conducted to date. The observations were done with the 305- m Arecibo Telescope toward a sample of 12 intermediate-/high-mass star-forming regions. We searched for a large number of transitions of different molecules, including CH3OH and OH. The low rms noise of our data ($\sim \!5\,$ mJy for most sources and transitions) allowed detection of spectral features that have not been seen in previous lower sensitivity observations of the sources, such as detection of excited OH and 6.7 GHz CH3OH absorption. A review of 6.7 GHz CH3OH detections indicates an association between absorption and radio continuum sources in high-mass star-forming regions, although selection biases in targeted projects and low sensitivity of blind surveys imply incompleteness. Absorption of excited OH transitions was also detected toward three sources. In particular, we confirm a broad 6.035 GHz OH absorption feature in G34.26+0.15 characterized by an asymmetric blueshifted wing indicative of expansion, perhaps a large-scale outflow in this H ii region.

6.7 GHz CH3OH Absorption toward the N3 Galactic Center Point Source

Abstract We present evidence of 6.7 GHz methanol (CH3OH) and 4.8 GHz formaldehyde (H2CO) absorption toward the Galactic Center (GC) point source “N3.” Both absorption features are unresolved and spatially aligned with N3. The 6.7 GHz CH3OH contains a single-velocity component (centered at ∼10 km s−1), while the 4.8 GHz H2CO shows two-velocity components (centered at ∼−3 and +8 km s−1). We find that the velocities of these absorption components are similar to that of emission lines from other molecules (e.g., SiO and HC3N) detected toward this compact source (−13 to +25 km s−1; “N3 cloud”). The detection of these absorption features is a firm indication that some of the molecular gas in the N3 molecular cloud is on the near side of the continuum source. Analysis of the CH3OH absorption kinematics shows a relatively large velocity dispersion (3.8 km s−1) for the size scale of this feature (&lt;0.″1, &lt;0.01 pc at the GC), when compared with other similarly sized GC clouds in the Larson line width–size relationship. Furthermore, this line width is closer to velocity dispersion measurements for size scales of 1.3 pc, which is roughly the width of the N3 cloud (25″; 1.0 pc). We argue that this relatively broad line width, over a small cross-sectional area, is due to turbulence through the depth of the cloud, where the cloud has a presumed line-of-sight thickness of ∼1 pc.

36 GHz methanol lines from nearby galaxies: maser or quasi-thermal emission?

Methanol (CH3OH) is one of the most abundant interstellar molecules, offering a vast number of transitions to be studied, including many maser lines. However, while the strongest Galactic CH3OH lines, the so-called class II masers, show no indications for the presence of superluminous counterparts in external galaxies, the less luminous Galactic class I sources appear to be different. Here we report class I 36 GHz (λ ≈ 0.8 cm) CH3OH 4−1 → 30 E line emission from the nearby galaxies Maffei 2 (D ≈ 6 Mpc) and IC 342 (D ≈ 3.5 Mpc), measured with the 100 m telescope at Effelsberg at three different epochs within a time span of about five weeks. The 36 GHz methanol line of Maffei 2 is the second most luminous among the sources detected with certainty outside the Local Group of galaxies. This is not matched by the moderate infrared luminosity of Maffei 2. Higher-resolution data are required to check whether this is related to its prominent bar and associated shocks. Upper limits for M 82, NGC 4388, NGC 5728 and Arp 220 are also presented. The previously reported detection of 36 GHz maser emission in Arp 220 is not confirmed. Nondetections are reported from the related class I 44 GHz (λ ≈ 0.7 cm) methanol transition towards Maffei 2 and IC 342, indicating that this line is not stronger than its 36 GHz counterpart. In contrast to the previously detected 36 GHz CH3OH emission in NGC 253 and NGC 4945, our 36 GHz profiles towards Maffei 2 and IC 342 are similar to those of previously detected nonmasing lines from other molecular species. However, by analogy to our Galactic center region, it may well be possible that the 36 GHz methanol lines in Maffei 2 and IC 342 are composed of a large number of faint and narrow maser features that remain spatially unresolved. In view of this, a search for a weak broad 36 GHz line component would also be desirable in NGC 253 and NGC 4945.

Long-term with short-intervals monitor of 6.7 GHz CH3OH masers using Hitachi 32-m radio telescope to statistically research the periodic flux variability around high-mass protostars

We initiated a long-term with short-intervals monitoring project toward 442 CH3OH masers at 6.7 GHz (Dec > -30 deg) using Hitachi 32-m radio telescope of Japan in Dec 2012. Our observations have been carried out daily, monitoring a spectrum of each source with intervals of 9-10 days. In Sep 2015, the number of the targets sources and intervals were redesigned into 143 and 4-5 days to detect periodic variations with periods shorter than 30 days. We have so far obtained new detections of periodic flux variations in 31 CH3OH sources with periods of 22-409 days. These periodic flux variability with timescale between a few months and a few years must be a unique tool to study high-mass protostars themselves and their circumstellar structures on a tiny spatial scale of 0.1-1 au (∼1.5 x 107-8 km). We will also present the future prospect via monitoring of OH and H2O masers in these periodic sources using 40-m Thai National Radio Telescope that is under construction by NARIT.

VLA Overview of the Bursting H2O Maser Source G25.65+1.05

Abstract 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.