Recent theoretical and observational studies debate the similarities between the formation process of high-mass (>8 Msun) and low-mass stars. The formation of low-mass star formation is directly associated with the presence of disks and jets. According to this scenario, radio jets are expected to be common in high-mass star-forming regions. 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. We have used the Karl G. Jansky Very Large Array (VLA) to observe 18 high-mass star-forming regions in the C band (6 cm, ~1.0 arcsec resolution) and K band (1.3 cm, ~0.3 arcsec resolution). We have searched for radio jet candidates by studying the association of radio continuum sources with shock activity signposts. We have 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 non-thermal 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 non-thermal emission, likely due to strong shocks. Our sample of 18 regions is divided in 8 less evolved, infrared-dark regions and 10 more evolved, infrared-bright regions. We have 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 larger in slightly more evolved regions.
Abridged. Methods. We made use of the Karl G. Jansky Very Large Array (VLA) in the B configuration at K band, and in the A configuration at both Ku and C bands, in order to image the radio continuum emission towards 25 H$_2$O maser sites with an angular resolution and thermal rms of the order of $0.1''$ and 10 $μ$Jy beam$^{-1}$, respectively. These targets add to our pilot study of 11 maser sites presented in Moscadelli et al. (2016). The sample of H$_2$O maser sites was selected among those regions having an accurate distance measurement, obtained through maser trigonometric parallaxes, and H…
Recently, a comparison between the locations of 6.7-GHz methanol masers and dust continuum emission has renewed speculation that these masers can be associated with evolved stars. The implication of such a scenario would be profound, especially for the interpretation of large surveys for 6.7-GHz masers, individual studies where high-mass star formation has been inferred from the presence of 6.7-GHz methanol masers, and for the pumping mechanisms of these masers. We have investigated the two instances where 6.7-GHz methanol masers have been explicitly suggested to be associated with evolved sta…
The goal of the Protostellar Outflows at the EarliesT Stages (POETS) survey is to image the disk-outflow interface on scales of 10-100 au in a statistically significant sample (36) of luminous young stellar objects (YSO), targeting both the molecular and ionized components of the outflows. The outflow kinematics is studied at milliarcsecond scales through VLBI observations of the 22 GHz water masers. We employed the JVLA at 6, 13, and 22 GHz in the A- and B-Array configurations to determine the spatial structure and the spectral index of the radio continuum emission. In about half of the targe…
Ejection activities in S255IR-SMA1 and AFGL 5142 were investigated by multi-epoch VLBI observations of 22 GHz water masers, tracing bowshocks leading collimated jets. The history of ejections, revealed by the 3D maser motions and supplemented by the literature, suggests that these massive stars formed by episodic accretion, inferred via the accretion-ejection connection. This contribution centers on the role of episodic accretion in overcoming the radiation pressure problem of massive star formation - with maser VLBI and single-dish observations providing essential observational tools.
We report the results of 870-$μ$m continuum observations, using the Large APEX Bolometer Camera (LABOCA), towards 77 class-II, 6.7-GHz methanol masers identified by the Methanol Multibeam (MMB) survey to map the thermal emission from cool dust towards these objects. These data complement a study of 630 methanol masers associated with compact dense clumps identified from the ATLASGAL survey. Compact dust emission is detected towards 70 sources, which implies a dust-association rate of 99% for the full MMB catalogue. Evaluation of the derived dust and maser properties leads us to conclude that t…
The ATCA, MERLIN and VLA interferometers were used to measure the absolute positions of 35 6.7 GHz methanol masers to subarcsecond or higher accuracy. Our measurements represent essential preparatory data for Very Long Baseline Interferometry, which can provide accurate parallax and proper motion determinations of the star-forming regions harboring the masers. Our data also allow associations to be established with infrared sources at different wavelengths. Our findings support the view that the 6.7 GHz masers are associated with the earliest phases of high-mass star formation.
We investigated the ALMA science verification data of Orion KL and found a spectral signature of the vibrationally excited H2O maser line at 232.68670 GHz (nu2=1, 5,5,0-6,4,3). This line has been detected in circumstellar envelopes of late-type stars so far but not in young stellar objects including Orion KL. Thus, this is the first detection of the 232 GHz vibrationally excited H2O maser in star-forming regions. The distribution of the 232 GHz maser is concentrated at the position of the radio Source I, which is remarkably different from other molecular lines. The spectrum shows a double-peak…
Maser emission plays an important role as a tool in star formation studies. It is widely used for deriving kinematics, as well as the physical conditions of different structures, hidden in the dense environment very close to the young stars, for example associated with the onset of jets and outflows. We will summarize the recent observational and theoretical progress on this topic since the last maser symposium: the IAU Symposium 242 in Alice Springs.
Context: OH maser emission is known to be associated with high mass star forming regions. Towards some of these regions, OH masers are associated with HII regions. Towards others, believed to be in an earlier evolutionary state, OH masers are offset from HII regions. Towards these later regions, it is believed that OH masers are associated with the circumstellar disk (e.g. Edris et al. 2005; Gray et al. 2003). These disks should be hosting dense dust grains. The presence of the hot dust could be traced via the millimeter continuum emission as well as IR emission. Aims: studying the association…
We present observations of massive star-forming regions selected from the IRAS Point Source Catalog. The observations were made with the Very Large Array and the Large Millimeter Telescope to search for Class I methanol masers. We made interferometric observations of 125 massive star-forming regions in the 44 GHz methanol maser transition; 53 of the 125 fields showed emission. The data allow us to demonstrate associations, at arcsecond precision, of the Class I maser emission with outflows, HII regions and shocks traced by 4.5 $μ$m emission. We made single-dish observations toward 38 of 53 reg…
We present a survey of 56 massive star-forming regions in the 44 GHz methanol maser transition made with the Karl G. Jansky Very Large Array (VLA); 24 of the 56 fields showed maser emission. The data allow us to demonstrate associations, at arcsecond precision, of the Class I maser emission with outflows, HII regions, and shocks traced by 4.5 micron emission. We find a total of 83 maser components with linewidths ranging from 0.17 to 3.3 km s$^{-1}$ with a nearly flat distribution and a median value of 1.1 km s$^{-1}$. The relative velocities of the masers with respect to the systemic velocity…
Following an eruptive accretion event in NGC6334I-MM1, flares in the various maser species, including water masers, were triggered. We report the observed relative proper motion of the highly variable water masers associated with the massive star-forming region, NGC6334I. High velocity H$_2$O maser proper motions were detected in 5 maser clusters, CM2-W2 (bow-shock structure), MM1-W1, MM1-W3, UCHII-W1 and UCHII-W3. The overall average of the derived relative proper motion is 85 km s$^{-1}$. This mean proper motion is in agreement with the previous results from VLA multi-epoch observations. Our…
We present the results of a Nobeyama 45-m water maser and ammonia survey of all 94 northern GLIMPSE Extended Green Objects (EGOs), a sample of massive young stellar objects (MYSOs) identified based on their extended 4.5 micron emission. We observed the ammonia (1,1), (2,2), and (3,3) inversion lines, and detect emission towards 97%, 63%, and 46% of our sample, respectively (median rms ~50 mK). The water maser detection rate is 68% (median rms ~0.11 Jy). The derived water maser and clump-scale gas properties are consistent with the identification of EGOs as young MYSOs. To explore the degree of…
The massive young stellar object (MYSO) G358.93-0.03-MM1 showed an extraordinary near-infrared- to (sub-)millimetre-dark and far-infrared-loud accretion burst, which is closely associated with flares of several class II methanol maser transitions, and, later, a 22 GHz water maser flare. Water maser flares provide an invaluable insight into ejection events associated with accretion bursts. Although the short timescale of the 22 GHz water maser flare made it impossible to carry out a very long baseline interferometry observation, we could track it with the Karl G. Jansky Very Large Array (VLA). …
We present a study of star formation in the Central Molecular Zone (CMZ) of our Galaxy through the association of three star formation indicators: 6.7 GHz CH3OH masers, 22 GHz H2O masers, and enhanced 4.5 micron emission (`green') sources. We explore how star formation in the Galactic center (l < 1.3 deg, |b| < 10', where l and b are Galactic longitude and Galactic latitude) compares with that of the Galactic disk (6 deg < l < 345 deg, |b| < 2 deg). Using an automated algorithm, we search for enhanced 4.5 micron emission sources toward 6.7 GHz CH3OH masers detected in the Parkes Methanol Multi…
Water fountain stars (WFs) are evolved objects with water masers tracing high-velocity jets (up to several hundreds of km s$^{-1}$). They could represent one of the first manifestations of collimated mass-loss in evolved objects and thus, be a key to understanding the shaping mechanisms of planetary nebulae. Only 13 objects had been confirmed so far as WFs with interferometer observations. We present new observations with the Australia Telescope Compact Array and archival observations with the Very Large Array of four objects that are considered to be WF candidates, mainly based on single-dish…
We present a study of the mid-infrared environments and association with star formation tracers of 6.7 GHz methanol masers taken from the Methanol Multi-Beam (MMB) Survey. Our ultimate goal is to establish the mass of the host star and its evolutionary stage for each maser site. As a first step, the GLIMPSE survey of the Galactic Plane is utilised to investigate the environment of 776 methanol masers and we find that while the majority of the masers are associated with mid-infrared counterparts, a significant fraction (17%) are not associated with any detectable mid-infrared emission. A number…
We report on the first 1,432 sources observed using the Atacama Large Millimeter/submillimeter Array (ALMA), from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey, which aims to obtain tens of thousands of line-of-sight velocities from SiO masers in Asymptotic Giant Branch (AGB) stars in the Milky Way. A 71% detection rate of 86 GHz SiO masers is obtained from the infrared color-selected sample, and increases to 80% when considering the likely oxygen-rich stars using Midcourse Space Experiment (MSX) colors isolated in a region where [D]-[E] <= 1.38. Based on Galactic distributions,…
We have conducted VLBA phase-referencing monitoring of H2O masers around the red supergiant, S Persei, for six years. We have fitted maser motions to a simple expanding-shell model with a common annual parallax and stellar proper motion, and obtained the annual parallax as 0.413+/-0.017 mas, and the stellar proper motion as (-0.49+/-0.23 mas/yr, -1.19+/-0.20 mas/yr) in right ascension and declination, respectively. The obtained annual parallax corresponds to the trigonometric distance of 2.42+0.11-0.09 kpc. Assuming the Galactocentric distance of the Sun of 8.5 kpc, the circular rotational vel…
We present global VLBI observations of the first-excited state OH masers in the massive star-forming region Onsala 1 (ON 1). The 29 masers detected are nearly all from the 6035 MHz transition, and nearly all are identifiable as Zeeman pair components. The 6030 and 6035 MHz masers are coincident with previously published positions of ground-state masers to within a few milliarcseconds, and the magnetic fields deduced from Zeeman splitting are comparable. The 6.0 GHz masers in ON 1 are always found in close spatial association with 1665 MHz OH masers, in contrast to the situation in the massive …
The Cygnus X complex is covered by the Global View of Star Formation in the Milky Way (GLOSTAR) survey, an unbiased radio-wavelength Galactic plane survey, in 4--8 GHz continuum radiation and several spectral lines. The GLOSTAR survey observed the 6.7~GHz transition of methanol (CH$_3$OH), an exclusive tracer of high-mass young stellar objects. Using the Very Large Array in both the B and D configurations, we observed an area in Cygnus~X of $7^{\rm o}\times3^{\rm o}$ in size and simultaneously covered the methanol line and the continuum, allowing cross-registration. We detected thirteen source…
A 22 GHz water maser survey was conducted towards 178 O-rich AGB stars with the aim of identifying maser emission associated with the Sagittarius stellar stream. In this survey, maser emissions were detected in 21 targets, of which 20 were new detections. We studied the Galactic distributions of H2O and SiO maser-traced AGBs towards the Sgr orbital plane, and found an elongated structure towards the (l, b)~(340, 40) direction. In order to verify its association with the Sagittarius tidal stream, we further studied the 3D motions of these sources, but found, kinematically, these maser-traced AG…
From targeted observations of ground-state OH masers towards 702 Multibeam (MMB) survey 6.7-GHz methanol masers, between Galactic longitudes 186$^{\circ}$ through the Galactic centre to 20$^{\circ}$, made as part of the `MAGMO' project, we present the physical and polarisation properties of the 1720-MHz OH maser transition, including the identification of Zeeman pairs. We present 10 new and 23 previously catalogued 1720-MHz OH maser sources detected towards star formation regions. In addition, we also detected 16 1720-MHz OH masers associated with supernova remnants and two sites of diffuse OH…
Complex Organic Molecules in Protostars with ALMA Spectral Surveys (COMPASS) VI. Discovery of a class I methanol maser transition and its association with acetaldehyde
Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES（图卢兹大学天体物理学与行星学研究所以及法国国家科学研究中心、法国国家空间研究中心）；Physikalisch-Meteorologisches Observatorium Davos und Weltstrahlungszentrum (PMOD/WRC)（达沃斯物理气象观测站与世界辐射中心）；Department of Chemistry, Biochemistry and Pharmaceutical Sciences (DCBP), Universität Bern（伯尔尼大学化学、生物化学和药学系）；National Radio Astronomy Observatory（美国国家射电天文台）；Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Institut de Planétologie et d’Astrophysique de Grenoble, Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔行星学与天体物理学研究所，格勒诺布尔阿尔卑斯大学，法国国家科学研究中心）；Korea Astronomy and Space Science Institute（韩国天文学与空间科学研究所）；Institute of Astronomy, Department of Physics, National Tsinghua University（国立清华大学物理系天文学研究所）
We have observed a sample of 9 Extended Green Objects (EGOs) at 1.3 and 5 cm with the VLA with sub-arcsecond resolution and ~7-14 uJy/beam sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find EGO-associated continuum emission - within 1 arcsec of the extended 4.5 um emission - in every field, which is typically faint (order 10^1-10^2 uJy) and compact (unresolved at 0.3-0.5 arcsec). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal …
We conducted VLA C-configuration observations to measure positions and luminosities of Galactic Class II 6.7 GHz methanol masers and their associated ultra-compact H II regions. The spectral resolution was 3.90625 kHz and the continuum sensitivity reached 45 \uJypb. We mapped 372 methanol masers with peak flux densities of more than 2 Jy selected from the literature, 367 of them were detected. Absolute positions have nominal uncertainties of 0.3 arcsec. In this first paper on the data analysis, we present three catalogs, the first gives information on the strongest feature of 367 methanol mase…
Class I methanol masers are found to be good tracers of the interaction between outflows from massive young stellar objects with their surrounding media. Although polarization observations of Class II methanol masers have been able to provide information about magnetic fields close to the central (proto)stars, polarization observations of Class I methanol masers are rare, especially at 44 and 95 GHz. We present the results of linear polarization observations of 39 Class I methanol maser sources at 44 and 95 GHz. These two lines are observed simultaneously with one of the 21 m Korean VLBI Netwo…
We present high angular resolution observations of water masers at 1.3 cm and radio continuum emission at 1.3, 3.6 and 6 cm towards the Bok globule CB 54 using the Very Large Array. At 1.3 cm, with subarsecond angular resolution, we detect a radio continuum compact source located to the south-west of the globule and spatially coincident with a mid-infrared embedded object (MIR-b). The spectral index derived between 6 and 1.3 cm (alpha=0.3+/-0.4) is flat, consistent with optically thin free-free emission from ionized gas. We propose the shock-ionization scenario as a viable mechanism to produce…
Among the tracers of the earliest phases in the massive star formation process, methanol masers have gained increasing importance. The phenomenological distinction between Class I and II methanol masers is based on their spatial association with objects such as jets, cores, and ultracompact HII regions, but is also believed to correspond to different pumping mechanisms: radiation for Class II masers, collisions for Class I masers. In this work, we have surveyed a large sample of massive star forming regions - 296 objects divided into two groups named 'High' and 'Low' according to their [25-12]…
We have used catalogues from several Galactic plane surveys and dedicated observations to investigate the relationship between various maser species and Galactic star forming clumps, as identified by the ATLASGAL survey. The maser transitions of interest are the 6.7 & 12.2 GHz methanol masers, 22.2 GHz water masers, and the masers emitting in the four ground-state hyperfine structure transitions of hydroxyl. We find clump association rates for the water, hydroxyl and methanol masers to be 56, 39 and 82 per cent respectively, within the Galactic longitude range of 60° > $l$ > -60°. We investiga…
We investigate which structures the 6.7 GHz methanol masers trace in the environment of high-mass protostar candidates by observing a homogenous sample of methanol masers selected from Torun surveys. We also probed their origins by looking for associated H II regions and IR emission. We selected 30 methanol sources with improved position accuracies achieved using MERLIN and another 3 from the literature. We imaged 31 of these using the European VLBI Network's expanded array of telescopes with 5-cm (6-GHz) receivers. We used the VLA to search for 8.4 GHz radio continuum counterparts and inspect…
We have collected literature data on young Galactic objects such as masers with VLBI-measured trigonometric parallaxes, OB associations, HII regions and Cepheids. We have recently established that vertical disk scale height is strongly influenced by the objects of the Local arm. In the present work we used samples that do not contain objects in this arm. Based on the model of a self-gravitating isothermal disk for the density distribution, we have found the following vertical disk scale heights: h=46+/-5 pc from 69 masers with trigonometric parallaxes, h=36+/-3 pc from 59 OB associations, h=35…
MERLIN phase-referenced polarimetric observations towards the W51 complex were carried out in March 2006 in the Class II methanol maser transition at 6.668 GHz and three of the four excited OH maser hyperfine transitions at 6 GHz. Methanol maser emission is found towards both W51 Main and South. We did not detect any emission in the excited OH maser lines at 6.030 and 6.049 GHz down to a 3 sigma limit of ~20 mJy per beam. Excited OH maser emission at 6.035 GHz is only found towards W51 Main. This emission is highly circularly polarised (typically 45% and up to 87%). Seven Zeeman pairs were ide…
We present high resolution (sub-mas) VLBI maps of nuclear H2O megamasers for seven galaxies. In UGC6093, the well-aligned systemic masers and high-velocity masers originate in an edge-on, flat disk and we determine the mass of the central SMBH to be M_SMBH = 2.58*10^7Msun(+-7%). For J1346+5228, the distribution of masers is consistent with a disk, but the faint high-velocity masers are only marginally detected, and we constrain the mass of the SMBH to be in the range 1.5-2.0*10^7Msun. The origin of the masers in Mrk1210 is less clear, as the systemic and high-velocity masers are misaligned and…
