The diversity of methanol maser morphologies from VLBI observations Tables 1-3 and 6, Figures 3 and 6 are only available in electronic form via http://www.aanda.org

The diversity of methanol maser morphologies from VLBI observations thanks: Tables 1-3 and 6, Figures 3 and 6 are only available in electronic form via http://www.aanda.org

A. Bartkiewicz Toruń Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland
[annan;msz]@astro.uni.torun.pl
   M. Szymczak Toruń Centre for Astronomy, Nicolaus Copernicus University, Gagarina 11, 87-100 Toruń, Poland
[annan;msz]@astro.uni.torun.pl
   H.J. van Langevelde Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
langevelde@jive.nl Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
   A.M.S. Richards Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, M13 9PL, UK
a.m.s.richards@manchester.ac.uk
      Y.M. Pihlström Department of Physics and Astronomy, MSC07 4220, University of New Mexico, Albuquerque, NM 87131, USA
ylva@unm.edu National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
Received; accepted
Key Words.:
stars: formation – ISM: molecules – masers – instrumentation: high angular resolution
Abstract

Context:The 6.7 GHz methanol maser marks an early stage of high–mass star formation, but the origin of this maser is currently a matter of debate. In particular it is unclear whether the maser emission arises in discs, outflows or behind shocks running into rotating molecular clouds.

Aims:We investigate which structures the 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.

Methods: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 inspected Spitzer GLIMPSE data at 3.6–8 m from the archive.

Results:High angular resolution images allowed us to analyze the morphology and kinematics of the methanol masers in great detail and verify their association with radio continuum and mid-infrared emission. A new class of  ”ring-like” methanol masers in star–forming regions appeared to be suprisingly common, 29% of the sample.

Conclusions:The new morphology strongly suggests that methanol masers originate in the disc or torus around a proto- or a young massive star. However, the maser kinematics indicate the strong influence of outflow or infall. This suggests that they form at the interface between the disc/torus and a flow. This is also strongly supported by Spitzer results because the majority of the masers coincide with 4.5 m emission to within less than 1″. Only four masers are associated with the central parts of UC H II regions. This implies that 6.7 GHz methanol maser emission occurs before H II region observable at cm wavelengths is formed.

1 Introduction

Methanol masers are commonly assumed to be associated with the environments of high-mass protostars, which provide the conditions required for methanol first to form on grains and then to be sublimated off, and finally to excite the maser transitions (Dartois et al. dartois99 (); Cragg et al. cragg02 ()). Methanol maser emission at 6668.519 MHz is one of the strongest and most widespread (Menten menten91 ()) of the the first observable manifestations of a newly formed high-mass star. Emission towards the archetypical source W3(OH) is characterized with a brightness temperature of up to  K from a spot of intrinsic size of 00014 (Menten et al. menten92 ()). The gas environment of even distant (5 kpc) high-mass stars can thus be probed on scales as small as 5 AU when observed with milliarcsecond (mas) resolution using Very Long Baseline Interferometry (VLBI).

All major methanol targeted surveys of high angular resolution taken to date are summarized in Table 1. This indicates the diversity of the sample selections and observing parameters, which might have affected the data interpretation.

The first observations of the 6.7 GHz maser line at arcsec resolution, using the ATCA, concentrated on the brightest sources (Norris et al. norris93 ()), followed by more extensive surveys (Phillips et al. phillips98 (); Walsh et al. walsh98 ()). The relative positions of individual maser spots were determined with 005 accuracy and the distribution of bright (0.5 Jy beam) maser spots was resolved for the majority of targets. Various morphological structures, such as simple, linear, curved, complex and double, were found. The linear sizes varied between 190 and 5600 AU. Norris et al. (norris93 ()) found that in 10 out of the 15 sources imaged the masers are located along lines or arcs of which five sources show a clear velocity gradient along the line. They proposed that the linear structures with velocity gradients are produced by the masers residing in rotating discs seen edge-on. Phillips et al. (phillips98 ()) increased the sample of masers studied with the ATCA to 45 objects, finding that 17 of them show morphologies and monotonic velocity gradients consistent with the circumstellar disc hypothesis. Assuming a Keplerian disc, the enclosed masses range from 1 to 75 M. In a sample of 97 sources Walsh et al. (walsh98 ()) found 36 masers with some linear structure but this was clearly-defined for only 9 sources. Therefore, the Keplerian disc hypothesis accounts for only a small proportion of their sources; most maser sites do not exhibit a systematic velocity gradient. They suggested that the masers form rather behind shock fronts.

The methanol masers are often not associated with detectable continuum emission at centimeter wavelengths. Twenty-five of sources in the sample of Phillips et al. (phillips98 ()) are associated with an ultra-compact H II (UC H II) region wherein the methanol masers are slightly offset from the peak continuum emission. They argued that the methanol sources without an H II region are possibly associated with less massive stars than those with coincident radio continuum emission. Walsh et al. (walsh98 ()) also found that most of their maser sources are not associated with radio continuum brighter than 1 mJy, implying that the phase of methanol maser occurs before an observable UC H II region is formed. This suggestion was confirmed for another sample of high-mass protostellar candidates (Beuther et al. beuther02 ()).

The detailed spatial structure of the methanol maser emission in these sources should provide further clues to their origin. To date, only a few observations at mas resolution have been published. Minier et al. (minier00 ()) observed 14 bright sources with the EVN. In 10 targets they found elongated structures with linear velocity gradients, which can be interpreted in terms of a circumstellar edge-on disc model. However, the estimates of central mass with this model for all but one source seemed to be far lower than expected for a high-mass star. Minier et al. (minier00 ()) suggested that this could be because the detectable masers delineate only part of the full diameters of the discs. They also proposed other models, such as accelerating outflows and shock fronts. Dodson et al. (dodson04 ()) used the LBA to image five maser sites with linear morphologies at arcsecond resolutions. Their milliarcsecond resolution data were interpreted using a model of an externally generated planar shock propagating through a rotating dense molecular clump or star-forming core.

Van der Walt et al. (vanderwalt07 ()) argued that the model of Dodson et al. is inconsistent with the observed kinematic properties of the masers. They concluded that the observed rest frame distribution of maser velocities can be reproduced well with a simple Keplerian-like disc model. Source NGC 7538 IRS 1 is understood to be a good example of an edge-on Keplerian disc (Minier et al. minier00 (); Pestalozzi et al. pestalozzi04 ()). However, high angular resolution mid-infrared (MIR) data were used to demonstrate that the outflow scenario is also plausible since the maser is not oriented perpendicular to the outflow as expected (De Buizer & Minier debuizer05 ()). The kinematic and spatial distribution of the 12 GHz methanol masers in W3(OH) were successfully fitted by a model of a conical bipolar outflow (Moscadelli et al. moscadelli02 ()).

The initial methanol imaging surveys were mostly of relatively low (arcsec) resolution, whilst very long baseline interferometer (VLBI) studies probably missed fainter emission, such as from the edges or the far side of putative discs. For the first time, we have studied a large sample, detecting 31 sources at mas resolution and 10 mJy sensitivity, with sufficient astrometric precision to complete robust identifications. This enables us to test the competing hypotheses for the origins of methanol 6.7 GHz maser emission, namely circumstellar discs, outflows, or propagating shock fronts. In this paper, we present EVN111The European VLBI Network observations of the methanol line and VLA observations of continuum emission for a homogeneous sample of the methanol masers discovered in the Torun untargeted survey (Szymczak et al. szymczak00 (), szymczak02 ()). The preliminary results of methanol observations were partly published in Bartkiewicz et al. (bartkiewicz04 (), bartkiewicz06 (), bartkiewicz09 ()). As part of this survey the discovery of a ring structure in G23.65700.127 was reported by Bartkiewicz et al. (bartkiewicz05 ()). Our observations have enabled us to detect a wide diversity of methanol maser geometries and demonstrate for the first time that in a large fraction of sources the distribution of the spots is ring-like.

\onltab

1

 Survey Telescope Spectral resolution Angular resolution 1 Number of targets Median peak flux density
(km s) (mas) (mJy beam) (Jy)
Norris et al. 1993 ATCA 0.3 1500 500 15 475
Phillips et al. 1998 ATCA 0.35 1500 50 33 41
Walsh et al. 1998 ATCA 0.18 1500 300 97 23
Minier et al. 2000 EVN 0.04 few 14 272
Dodson et al. 2004 LBA 0.2 5.3 2 5 148
This paper EVN 0.09; 0.18 312 31 3.6
the value not given
Table 1: Summary of previous high angular resolution studies of 6.7 GHz methanol masers.

2 Observations and data reduction

2.1 Sample selection

The sources were selected from two previous samples obtained using the Torun 32 m antenna: the blind survey of the 6.7 GHz methanol maser line (Szymczak et al. szymczak02 ()) and the methanol survey of IRAS-selected objects (Szymczak et al.  szymczak00 ()). The untargeted flux-limited (31.6 Jy) complete survey of the Galactic plane region and enabled the detection of 100 sources of which 26 were new. The same field includes 22 sources discovered in the earlier survey of IRAS-selected objects. These 48 objects were chosen as a sample for detailed studies. We note that the mean single-dish methanol maser flux density of these 48 sources is 16 Jy, a factor of 2 lower than that of the rest sources in the original samples. This may have introduced a selection effect for masers that are more distant, have intrinsically weaker maser emission, or are less aligned with the line of sight.

Depending on the maser flux densities the source coordinates obtained using the 32 m dish are accurate to within 2570″. We undertook astrometric measurements using the first two MERLIN222The Multi-Element Radio Linked Interferometer Network antennas to be equipped with 6.7-GHz receivers (Mark II and Cambridge). These single baseline observations detected 30 of the 48 objects, providing positions with sub-arcsecond accuracy (see Sect. 2.2). We included three additional objects in the same region that had not been detected in the Torun surveys, for which accurate positions are reported in the literature. These are: G22.35700.066 (Walsh et al. walsh98 ()), G25.41100.105, and G32.99200.034 (Beuther et al. beuther02 ()). The total sample selected for VLBI observations comprised 33 sources in the Galactic within the region defined by and (Table 2).

\onltab

2

Source Observing Phase-calibrator Separation
Gll.lllbb.bbb run ()
G21.40700.254 3a J18250737 3.1
G22.33500.155 3a J18250737 2.5
G22.35700.066 4a J18250737 2.3
G23.20700.377 2 J18250737 2.6
G23.38900.185 2 J18250737 2.1
G23.65700.127 2 J18250737 2.4
3a J18250737 2.4
4a J18250737 2.4
G23.70700.198 2 J18250737 2.5
3a J18250737 2.5
G23.96600.109 3a J18250737 2.5
G24.14800.009 3a J18250737 2.4
G24.54100.312 2 J18250737 2.4
G24.63400.324 4a J18250737 2.9
G25.41100.105 4a J18250737 3.1
G26.59800.024 4a J18250737 4.1
G27.22100.136 2 J18250737 4.5
G28.81700.365 4b J18340301 2.2
G30.31800.070 4b J18340301 3.2
G30.40000.296 4b J18340301 3.5
G31.04700.356 4b J18340301 3.5
G31.15600.045 4b J18340301 3.8
G31.58100.077 4b J18340301 4.1
G32.99200.034 4c J19070127 4.2
G33.64100.228 1 J19070127 3.5
G33.98000.019 4c J19070127 3.5
G34.75100.093 4c J19070127 3.0
G35.79300.175 1 J19070127 2.7
G36.11500.552 1 J19070127 3.4
4c J19070127 3.0
G36.70500.096 4c J19070127 3.1
G37.03000.039 3b J18560610 2.6
G37.47900.105 3b J18560610 2.4
G37.59800.425 3b J18560610 1.9
G38.03800.300 3b J18560610 2.2
G38.20300.067 3b J18560610 2.0
G39.10000.491 3b J18560610 1.2
source from Walsh et al. (walsh98 ())
sources from Beuther et al. (beuther02 ())
Table 2: Sample of methanol masers observed with the EVN. The names are the Galactic coordinates of the brightest spot of each target obtained in post-processing EVN data. The dates of each observing run are listed in Table 4. The phase-calibrator names and angular separations from the targets are given.

2.2 MERLIN astrometry

The MERLIN observations at a rest frame frequency of 6668.519 MHz were carried out during observing runs between 2002 May and June and 2003 March and May. The typical on-source observing time was about 1 hr for each target, and frequent observations of nearby phase reference sources and other calibrators were completed. Standard single-baseline data reduction procedures were applied (Diamond et al. diamond03 ()) using AIPS (the Astronomical Image Processing System). We searched for emission from each target in its vector-averaged spectrum by shifting the phase center from 200″  to 200″  in right ascension and from 500″  to 500″  in declination (at 1″  intervals) to locate the position giving the maximum intensity for the main maser feature. We simultaneously inspected the phase, which should be close to 0°  at this position in the spectral channels containing the main feature. Finally, we produced a large (40″40″) dirty map of the main feature for the channel of the highest spectral signal-to-noise ratio, centered on the estimated position. The brightest spot was then assumed to be as the maser position. A typical beam was 20020 mas at a position angle of 20°. Because of the very poor uvcoverage, we were unable to derive the maser structures.

Methanol maser emission was detected towards 30 of the 48 sources observed, giving absolute positions of sufficient accuracy for follow-up EVN observations. The MERLIN single-baseline astrometric accuracy was between 03 and 1″  in most cases, depending on the source brightness. However, the absolute position uncertainty of sources at Dec35 increased to 5–10″. The mean differences between the coordinates obtained using a single dish and using the MERLIN single baseline were 30″6″  and 20″4″  in right ascension and declination, respectively. No emission was detected towards the remaining 18 sources above a sensitvity limit of 0.3 Jy (Table 3). The possible causes of non-detection are: variability, large errors in the single dish positions, interference (for a few targets), or extended emission, resolved out by the interferometer.

\onltab

3

Source RA, Dec V
Gll.llbb.bb (J2000) (km s)
G21.5700.03 18 30 36.5, 10 06 43 117
G22.0500.22 18 30 35.7, 09 34 26 54
G24.9300.08 18 36 29.2, 07 05 05 53
G26.6500.02 18 39 51.8, 05 34 52 107
G27.2100.26 18 40 03.8, 04 58 09 9
G27.7800.07 18 41 47.5, 04 33 11 112
G28.0200.44 18 44 02.1, 04 34 14 17
G28.4000.07 18 42 54.5, 04 00 04 69
G28.5300.12 18 42 57.7, 03 51 59 25
G28.6900.41 18 42 13.6, 03 35 07 94
G28.8500.50 18 42 12.8, 03 24 26 83
G29.3100.15 18 45 23.1, 03 17 23 48
G33.7400.15 18 53 26.9, 00 39 01 54
G33.8600.01 18 53 05.2, 00 49 36 67
G34.1000.01 18 53 31.9, 01 02 26 56
G37.5300.11 19 00 14.4, 04 02 35 50
G38.1200.24 19 01 47.6, 04 30 32 70
G38.2600.08 19 01 28.7, 04 42 02 16
Table 3: Targets not detected with the MERLIN single baseline. The source names, coordinates, and peak flux velocities are taken from Szymczak et al. (szymczak02 ()).

2.3 EVN observations

The EVN observations of 33 targets in the 6.7 GHz methanol maser line were carried out in seven observing runs between 2003 and 2007 (projects EN001, EN003, EB031, EB034). The observing parameters are summarized in Table 4 including the date, duration of each run, working antennas, cycle time between the maser and phase-calibrator, spectral resolution, typical synthesized beam size, and 1 noise level in a spectral channel.

Observing Date Duration Telescopes Cycle time Spectral channel Synthesized Rms noise
run separation beam(;PA) per channel
(hr) (min) (km s) (masmas;°) (mJy beam)
1 2003 Jun 08 12 CmJbEfOn 6.004.00 0.09 616; 1 10
2 2004 Nov 11 12 CmDaEfMcNtOnTrWb 3.751.75 0.09 616; 7 4
3a 2006 Feb 22 10 CmJbEfHhMcNtOnWb 3.751.75 0.09 515;31 10
3b 2006 Feb 23 10 CmJbEfHhMcOnWb 3.751.75 0.09 614;34 7
4a 2007 Jun 13 10 CmJbEfHhMcNtOnTrWb 3.251.75 0.18 612;20 4
4b 2007 Jun 14 10 CmJbEfHhMcNtOnTrWb 3.251.75 0.18 611;25 4
4c 2007 Jun 15 10 CmJbEfMcNtOnTrWb 3.251.75 0.09 613;35 6
Cm–Cambridge, Da–Darnhall, Jb–Jodrell Bank, Ef–Effelsberg, Hh–Hartebeesthoek, Mc–Medicina, Nt–Noto, On–Onsala, Tr–Toruń,
Wb–Westerbork
Table 4: Details of EVN observations.

Each observing run included scans of 3C345, which was adopted as a bandpass, delay and rate calibrator. Five or six sources were, typically, observed in each run, selected to be within a few degrees of each other in projection on the sky and of similar maser emission velocities. A phase-referencing scheme was applied in which a nearby, sufficiently bright phase-calibrator for each session was selected from the VLBA list. Details are given in Table 2. We used a spectral bandwidth of 2 MHz yielding a velocity coverage of 100 km s. In all sessions, the Mk V recording system was used with the exception of the first epoch when data were recorded on tapes (Mk IV system). The data were correlated with the Mk IV Data Processor operated by JIVE with 1024 spectral channels. In 4a and 4b runs only, when all nine antennas were operating, data were correlated with 512 spectral channels in two passes i.e., separately for LHC and RHC polarization, because of the correlator limitations. Left- and right-hand circular polarization data were averaged to increase the signal-to-noise ratio.

2.3.1 Calibration and imaging

The data calibration and reduction were carried out in AIPS, employing standard procedures for spectral line observations. First, the amplitude was calibrated using measured antenna gain curves and system temperatures. In the second step, the parallactic angle corrections were added. The Effelsberg antenna was used as a reference when calibrating the data from all sessions. The instrumental delays for each antenna were determined using 3C345. Rates and delays were calibrated using phase-calibrator and 3C345 observations. The phase-calibrator was mapped and a few iterations of self-calibration were completed, gradually shortening the time interval from 120 min to 1 min. Flux densities of 240, 202, 80, and 237 mJy were obtained for phase-calibrators J18250737, J18340301, J18560610, and J19070127, respectively. The maser data were corrected for the effects of the Earth’s rotation and its motion within the Solar System and towards the LSR. After applying all corrections from the calibration sources, we compiled preliminary maps of the channel containing the brightest and most compact peak. We then used the clean components of that map, if possible, as the starting model for further rounds of self-calibration. In two cases, high quality images could only be obtained after the first round of self-calibration using a default model at the pointing position.

We searched for emission using large (2″2″) maps over the entire band. We then created naturally-weighted 0.5″0.5″  cleaned images to use in analyzing maser properties. The beam sizes for each data set are listed in Table 4. The pixel separation was 1 mas. The rms noise levels in line-free channels were typically between 4 and 10 mJy beam depending on the run. The positions of all maser spots (above 5) in each individual channel map were determined by fitting two-dimensional Gaussian components. The formal fitting errors were, typically, 0.01–0.15 mas in right ascension and 0.02–0.5 mas in declination, depending on the source strength and structure.

The astrometric accuracy for the 29 sources with phase-referenced maps is limited by four factors. Firstly, the phase-reference source positions have an accuracy of 1.5 mas. Secondly, the antenna positions have an accuracy of 1 cm, corresponding to an uncertainty of 1 mas in RA and 2–3 mas in Dec. Thirdly, the separations between the targets and phase reference sources were 45, which translate into a potential phase solution transfer error equivalent to 2 mas in RA and 4–5 mas in Dec. Fourthly, the position uncertainty due to noise is given by (beamsize)/(signal-to-noise ratio), which is 1 mas for all our reference features. This infers a total astrometric uncertainty of 3 mas in RA and 6 mas in Dec. For the remaining four sources, we were unable to improve on the original MERLIN positions.

2.4 VLA continuum observations

In order to investigate the presence, position, and distribution of radio continuum emission associated with the 6.7 GHz methanol maser emission, we used the VLA at 8.4 GHz in A configuration (the project AB1250). Data were taken on 2007 August 18 for 12 hrs in a standard VLA continuum mode towards 30 sources in the sample. We did not observe the three sources that had not been included in the Torun surveys. We used 3C286 as a flux calibrator and two phase-calibrators, 185170355 and 183231035, from a standard VLA list. To increase signal-to-noise ratio, we employed the fast switching mode with a cycle time between the phase-calibrator and the target of 50 s250 s. This sequence lasted for 20 min for each target.

The data reduction was carried out following to the standard recipies from AIPS Cookbook Appendix A (NRAO 2007). The amplitude and phases of 3C286 were corrected using the default source model and 3C286 was then used to find the phase-calibrator flux densities. The antenna gains were calibrated using the phase-calibrator data. Some bad points were flagged and finally the images were created with natural weighting. The 1 noise level in the maps was typically 50 Jy beam and the beam was 035025.

3 Results

3.1 Maser emission

We successfully mapped a total of 31 out of 33 methanol masers observed with the EVN. We were unable to image G31.15600.045 and G37.47900.105 because of the weakness (30 mJy) of the emission and to strong spike artefacts in the channels at the maser velocity, respectively. We were unable to improve on the MERLIN astrometry for G33.64100.228 and G35.79300.175 due to a problem with the EVN phase-referencing that appeared during the first observing run. We created fringe rate maps of the brightest channels of the targets but still failed to determine the absolute position of these two sources. The target sources were near zero declination (from 05 to 25). Furthermore, because of the use of only four EVN telescopes, the uvplane coverage was poor for NS baselines. It is probable that these factors together with a too long phase-referencing cycle time precluded a proper phase calibration. The position of the third source, G36.11500.552, observed in the first run, was easily ascertained during the 4c run when eight antennas were working and the cycle time between the maser and phase-calibrator was shorter.

The results are summarized in Table 5. The names of the maser sources correspond to the Galactic coordinates of the brightest spot of each target. The absolute coordinates, the LSR velocity (V), and the intensity (S) are given for the brightest spot of each target. We also indicate the velocity range of emission V. The area containing all maser emission from each source was parameterised by measuring the extent of the maser emission along the line given by a least squares fit to the maser spot distribution (major axis and position angle) and in the perpendicular direction (minor axis). The morphological class based on the relative positions of methanol maser spots and the angular separation of the brightest spot of each source from the nearest 4.5 m source (see Sect. 4.2) are also given in Table 5.

Figure 1: Spectra and maps of 6.7 GHz methanol maser emission of sources detected with the EVN. The names are the Galactic coordinates of the brightest spot as listed in Table 5. The thin bars under the spectra show the velocity ranges of spots displayed. The coordinates are relative to the brightest spots (Table 5). The sizes of circles are proportional to the logarithm of the intensities of maser spots. The colors of circles relate to the LSR velocities as indicated in the spectra, respectively. For the sources with ring-like morphologies, the best-fit ellipse and its center are marked by a dotted curve and a cross, respectively. The crosses coincide (within the uncertainties) with Spitzer IRAC MIR emission (Sect. 4.2).
Figure 1: continued.
Figure 1: continued.
Figure 1: continued.
Figure 1: continued.
Figure 1: continued.
Figure 1: continued.
Figure 1: continued.

In Fig. 1, we present the spectra and distribution of the methanol maser emission for the 31 imaged targets. The spectra were extracted from the map datacubes using the AIPS task ISPEC. They represent the total amount of emission seen in the maps. In order to display the detailed structures of masers, we show all the spots detected in each of the individual channel maps. If spots appear at the same positions within half of the beamwidth in at least three or two consecutive channels, for observations with a spectral resolution of 0.09 km s and 0.18 km s, respectively, we refer to them as a cluster. The relevant parameters of all maser clusters for each source are listed in Table The diversity of methanol maser morphologies from VLBI observations thanks: Tables 1-3 and 6, Figures 3 and 6 are only available in electronic form via http://www.aanda.org: the position (RA, Dec) relative to the brightest spot (given in Table 5), the peak intensity (S), and the LSR velocity (V) of the brightest spot within a cluster. The velocity full-width at half-maximum (FWHM) and the fitted peak amplitude (S) are given if the spectrum of the cluster has a Gaussian profile.

Source Position (J2000) V V S Area Class
Gll.lllbb.bbb RA(h m s) Dec(° ′  ″) (km s) (km s) (Jy beam) (masmas) PA(°) (″)
G21.40700.254 18 31 06.33794 10 21 37.4108 89.0 3.00 2.76 13839 87 C 0.23
G22.33500.155 18 32 29.40704 09 29 29.6840 35.6 3.10 1.71 4911 16 L 0.67
G22.35700.066 18 31 44.12055 09 22 12.3129 79.7 9.20 10.54 330174 5 C 0.51
G23.20700.377 18 34 55.21212 08 49 14.8926 77.1 13.20 9.30 313255 69 R 0.56
G23.38900.185 18 33 14.32477 08 23 57.4723 75.4 6.00 21.55 205134 59 R 0.16
G23.65700.127 18 34 51.56482 08 18 21.3045 82.6 10.80 3.62 351345 82 R 0.50
G23.70700.198 18 35 12.36600 08 17 39.3577 79.2 23.30 6.06 130110 83 A 0.74
G23.96600.109 18 35 22.21469 08 01 22.4698 70.9 4.20 5.47 354 45 L 0.19
G24.14800.009 18 35 20.94266 07 48 55.6745 17.8 1.40 3.60 283 11 L 0.16
G24.54100.312 18 34 55.72152 07 19 06.6504 105.7 6.80 7.75 13753 78 A 0.45
G24.63400.324 18 37 22.71271 07 31 42.1439 35.4 13.40 3.03 7321 60 R 1.01
G25.41100.105 18 37 16.92106 06 38 30.5017 97.3 5.20 3.43 225162 79 R 0.64
G26.59800.024 18 39 55.92567 05 38 44.6424 24.2 3.30 3.04 361152 76 R 0.39
G27.22100.136 18 40 30.54608 05 01 05.3947 118.8 16.10 12.54 10479 6 C 0.89
G28.81700.365 18 42 37.34797 03 29 40.9216 90.7 5.20 3.14 11528 45 A/R 4.71
G30.31800.070 18 46 25.02621 02 17 40.7539 36.1 1.90 0.52 506 50 L 0.87
G30.40000.296 18 47 52.29976 02 23 16.0539 98.5 6.70 2.77 19997 47 C/R 2.28
G31.04700.356 18 46 43.85506 01 30 54.1551 80.7 6.30 1.99 6827 72 R 1.73
G31.15600.045 18 48 02.347 01 33 35.095 6.62
G31.58100.077 18 48 41.94108 01 10 02.5281 95.6 4.80 2.72 217105 79 A/R 4.23
G32.99200.034 18 51 25.58288 00 04 08.3330 91.8 5.20 6.21 11568 80 C 1.48
G33.64100.228 18 53 32.563 00 31 39.180 58.8 5.30 28.3 16761 66 A 1.22
G33.98000.019 18 53 25.01833 00 55 25.9760 58.9 6.90 3.78 8943 82 R 0.95
G34.75100.093 18 55 05.22296 01 34 36.2612 52.7 3.10 1.95 4911 56 R 0.47
G35.79300.175 18 57 16.894 02 27 57.910 60.7 2.80 9.70 102 65 L 1.12
G36.11500.552 18 55 16.79345 03 05 05.4140 73.0 14.80 11.74 1201297 79 P 2.42
G36.70500.096 18 57 59.12288 03 24 06.1124 53.1 10.60 7.58 6418 16 C 0.32
G37.03000.039 18 59 03.64233 03 37 45.0861 78.6 0.70 0.69 21 15 S 0.61
G37.47900.105 19 00 07.145 03 59 53.350 1.73
G37.59800.425 18 58 26.79772 04 20 45.4570 85.8 4.50 3.91 9428 87 C 1.24
G38.03800.300 19 01 50.46947 04 24 18.9559 55.7 4.20 2.17 3123 10 C 0.28
G38.20300.067 19 01 18.73235 04 39 34.2938 79.6 6.00 0.83 18258 44 C 1.74
G39.10000.491 19 00 58.04036 05 42 43.9214 15.3 3.30 2.07 18337 52 C 0.82
coordinates derived from the single MERLIN baseline data
class of morphology as described in Sect. 3.3: S – simple, L – linear, R – ring, C – complex, A - arched, P – pair.
Table 5: Results of EVN observations

3.2 Radio continuum emission

We detected 8.4 GHz continuum emission in eight of the fields centered on methanol masers. Table 7 lists the continuum source names (derived from the Galactic coordinates of the 8.4-GHz peak fluxes), the peak and the integrated intensities, and the angular size of the radio continuum emission at the 3 level. We also provide the name of the nearest maser from the sample and the angular separation between the continuum peak and the brightest spot of the nearest methanol maser.

The contour maps of all detections are shown in Figs. 2 and 3. The majority of sources are single peaked and their angular sizes range from 06 to 38. Both G24.14800.009 and G36.11500.552 have integrated flux densities that are equivalent to their peak flux densities within the noise, suggesting that these sources are unresolved. The values given for these sources in Table 7 correspond to the angular size upper limits. G31.58200.075 is one of the weakest sources (S=0.43 mJy beam) but has an exceptionally complex structure. It is extended (4″3″) and contains multiple emission peaks. The typical upper limit (3) for the fluxes of the remaining 22, non-detected sources is 0.15 mJy beam.

The 6.7-GHz methanol maser emission is found to be within 02 of the 8.4-GHz continuum position peaks of G24.14800.009, G28.81700.365, and G36.11500.552. The maser spots of source G26.59800.024 are 08 from the NE edge of the radio continuum source. Therefore, these four sources are closely associated with the methanol masers (Fig. 2). The continuum object G31.16000.045 is located 119 from the nominal position of the maser source G31.15600.045, but this maser has a position uncertainty of 10″  because we were unable to image it with the EVN (Sect. 3.1), so it may also be associated with the radio continuum. On the other hand, the continuum source G31.58200.075 has a separation of 9″  from the maser G31.58100.077, but the latter has a position accuracy of a few mas implying that the source and maser are unlikely to be associated.

We conclude that only 4 (possibly 5) of 30 masers are associated with radio continuum at 8.4 GHz. This is consistent with previous findings (Phillips et al. phillips98 (); Walsh et al. walsh98 (); Beuther et al. beuther02 ()) that the 6.7 GHz methanol masers are rarely associated with centimeter wavelength continuum emission. However, 24-GHz ATCA observations detected continuum emission associated with methanol masers toward which no continuum at 8.4 GHz had been previously detected (Longmore et al. longmore07 ()). This opens the possibility of methanol masers being associated with hyper-compact H II regions (HC H II), which are ptically thick at frequencies 10 GHz.

Radio continuum S S Size Nearest maser Separation
source Major axis Minor axis PA
(mJy beam) (mJy) (″) (″) (°) (″)
G21.38500.254 13.00 65 3.8 1.8 50 G21.40700.254 76.8
G24.14800.009 1.05 1 0.6 0.4 35 G24.14800.009 0.11
G26.59800.024 4.30 42 3.8 2.5 35 G26.59800.024 0.80
G28.81700.365 0.81 0.8 0.6 0.5 20 G28.81700.365 0.08
G30.33000.090 8.70 13 1.3 0.8 25 G30.31800.070 85.6
G31.16000.045 2.40 22 2.0 1.5 70 G31.15600.045 11.9
G31.58200.075 0.43 15 4.0 3.0 0 G31.58100.077 9.00
G36.11500.552 0.25 0.2 0.7 0.3 45 G36.11500.552 0.20
angular size upper limits (see Sect. 3.2)
Table 7: Results of VLA observations at 8.4 GHz
Figure 2: The 8.4 GHz continuum emission towards four methanol maser sites taken using VLA on 2007 August 18. The names of radio continuum sources (RC) relate to the Galactic coordinates of their peak fluxes. The peak and integrated fluxes as well as the levels of rms (1) are given under each map and the beamsizes are presented at the left bottom corners. Contours trace the levels of 3 (1, 2, 4, 8, 16, 32, 64, …). Crosses represent the 6.7 GHz methanol maser spots registered using EVN.
\onlfig

3

Figure 3: The 8.4 GHz continuum emission detected using VLA on 2007 August 18. The names of radio continuum sources (RC) are the Galactic coordinates of peak fluxes. The peak and integrated fluxes as well as the levels of rms (1) are given under each map and the beamsizes are presented at the left down corners. Contours trace the levels of 3(1, 2, 4, 8, 16, 32, 64, …).

3.3 General properties of the 6.7 GHz methanol masers

In 31 sources, we detected a total of 1934 maser spots that form 333 clusters. The spectral profiles of 265 (80%) clusters are well fitted with a Gaussian. The mean FWHM is 0.410.01 km s and the median value is 0.37 km s. This is consistent with results from single dish spectra at  0.05 km s resolution (Menten menten91 (); Caswell et al. caswell95 ()). Nineteen sources have complex spectra, that is indicative of spectral blending, so that the line width of individual features cannot be properly determined solely from the spectrum.

We compared the basic parameters of the spectra and distributions of all masers from the sample. However, we did not find any relationships between the line parameters such as FWHM, brightness temperature, velocity range of the maser emission, and the size and geometry of the maser region.

The sources show a wide diversity of structures. The following types of morphology can be identified (Table 5):

Simple – the emission appears in a narrow velocity range (V=0.7 km s) as a single peaked spectrum. The maser spots form one cluster of size smaller than a few mas. G37.03000.039 is the only source with these properties. Its spectrum is obviously blended.

Linear – the maser spots form a line in the plane of the sky. The angular extent of these maser structures ranges from 9 to 54 mas. In some sources (G30.31800.070, G35.79300.175) a monotonic velocity gradient is clearly seen. There are five linear sources in the sample.

Ring – this morphology appears to be ubiquitous in our sample. The distributions of no less than nine sources display a ring structure. Using the GNU Octave script developed by Fitzgibbon et al. (fitzgibbon99 ()), we fitted an ellipse to the spatial positions of the maser spots for each source. The results are summarized in Table 8. The semi-major (a) and semi-minor (b) axes range from 27 to 192 mas and from 15 to 128 mas, respectively. The average size of the semi-major axis and the standard dispersion in the mean is 8920 mas. The eccentricity (e) of the best-fit model ellipses ranges from 0.38 to 0.94. The average eccentricity and the standard dispersion of the mean is 0.790.06. The emission spans a modest velocity range of (3.113.4 km s). All nine sources possess MIR counterparts that coincide with the ellipse center to within less than 25 (Table 5). In these objects it is very likely that ring-like maser emission surrounds a central embedded star (see Sect. 4.2). Three other sources (G28.81700.365, G30.40000.296, G31.58100.077) have a ring-like morphology, although the separation between the MIR candidate counterpart and the ellipse center is greater than 25 (Table 5). This is probably caused by the larger uncertainties in the maser positions, since all three sources are at declinations near 0°. These are assigned a tentative classification of the ring-like class in Table 5.

Arched – maser spots are distributed along an arc of between 70 and 220 mas in length. The entire structure may show a systematic velocity gradient. Three (or possibly five) sources exhibit this morphology.

Complex – 9 (possibly 10) sources, i.e. about one third of the sample, do not show any regularities in their spatial and velocity distributions. These sources vary greatly in size from 3123 mas to 330174 mas.

Pair – this class was defined by Phillips et al. (phillips98 ()), comprising two maser groups separated by 1 arcsec with 10 km s difference in velocity. The major axes of individual clusters are perpendicular to the line joining them. In our sample, we found only one source with such a morphology.

The most striking aspect of this study is that we find that the commonest morphology of sources with a systematic maser structure is a ring-like distribution of emission, seen in 29% of objects. These rings probably surround young massive objects (see Sect. 4). A similar proportion (29-32%) of sources possess a complex morphology. Linear sources with a monotonic velocity gradient are relatively rare in the sample (16%).

3.4 Individual sources

This section presents comments on individual sources, including additional observational data relevant to the main aims of this paper. If not stated otherwise, we present the linear sizes of masers derived using the near (and far) kinematic distances given in Szymczak et al. (szymczak05 ()).

G22.35700.066. The ATCA observations detected three maser spots (Walsh et al. walsh98 ()), while the EVN revealed 31 spots in 10 clusters. The strongest emission detected with both interferometers, at 80.0 km s, coincides to within 17. We detected new emission at close to 88.5 km s, 020 - 027 north of the brightest spot, but a 77.0 km s spot reported by Walsh et al. (walsh98 ()) was not redetected.

G23.65700.127. The parallax of this source was measured (Bartkiewicz et al. bartkiewicz08 ()), showing that the circular distribution of masers has a linear radius of 405 AU, which differs significantly from the sizes inferred from the kinematic distances. The source was observed at three epochs (Tables 2, 4) and the ring-like morphology clearly persisted over time spans of 1.25 and 2.5 yrs. A detailed description of the brightness variability in the source will be presented in a forthcoming paper.

G23.70700.198. Walsh et al. (walsh98 ()) detected 7 masers in a velocity range of 74.981.4 km s, randomly scattered over a 015 area. The first epoch of EVN maps of this source (run 2) detected 23 clusters (140 spots) of which 19 form a 71 mas (corresponding to 360/750 AU for near/far kinematic distance, respectively) long arc in the NS direction, which has a velocity span of 8 km s. A clear velocity gradient is seen in the overall arched distribution. The remaining clusters (all blue-shifted) are located 100 mas to the west (two clusters) and 20 mas to the east (two clusters), relative to the brightest spot. All four clusters are weak and were not detected at a later epoch (run 3a), but this data had poorer sensitivity. The brightest methanol maser component (Table 5) coincides in position (within 82 mas) and velocity (within 0.1 km s) with weak (60 mJy beam) HCO maser emission at 4.8 GHz (Araya et al. araya06 ()). We note that this is well within the absolute positional accuracy of the HCO maser. Both masers lie at the edges of two probable H II regions (Araya et al. araya06 (), their Fig. 2) with a peak intensity of 6.1 mJy beam at 5 GHz (VLA C-configuration). Our VLA A-configuration data at 8.4 GHz do not show any emission above the 0.15 mJy beam sensitivity limit, nor was this source detected at 8.6 GHz with a 2 mJy beam limit (Walsh et al. (walsh98 ()). Therefore, the H II regions are intrinsically weak at frequencies 5 GHz or they are possibly resolved at subarcsec angular resolution.

G25.41100.105. Beuther et al. (beuther02 ()) observed this source with the ATCA and found only two components at velocities of 94 and 97 km s at positions that coincide to within 02 with the brightest spots in the EVN maps. We detected 30 spots, probably because of our 50 times higher sensitivity, although variability may also be involved. The distance of 9.5 kpc (Sridharan et al. sridharan02 ()) implies that the linear radius of the ring distribution is 980 AU.

G26.59800.024. The maser is located 085 from a cometary H II region (Fig. 2) with a spectral index of 0.23 between 1.4 and 5 GHz (Becker et al. becker94 ()). This corresponds to linear distances of 1530(11400) AU. The flux density of 4.4 mJy at 8.4 GHz, compared with 55 mJy at 5 GHz, implies that the turnover frequency is near 5 GHz. The methanol maser probably forms behind a shock front induced by the H II region. The strongest maser component, at 24.2 km s, coincides in velocity with a 24.3 km s absorption feature (59.8 mJy) in the HCO line at 4.8 GHz (Sewilo et al. sewilo04 ()).

G36.11500.552. The brightest component of the NW maser structure is 02 away from the weak point continuum source at PA = 120° (Fig. 2). The shape of this complex suggests the existence of an outflow. However, the kinematics are not consistent with an outflow model (Sect. 4.2.3), nor do CO (2-1) line maps at 29″ resolution detect any molecular outflow (Zhang et al. zhang05 ()).

Source Centre Semi–axes PA e
RA,Dec a b
(mas,mas) (mas) (mas) (°)
G23.20700.377 62, 71 126 45 60 0.93
G23.38900.185 34, 75 95 56 45 0.81
G23.65700.127 69, 93 133 128 10 0.38
G24.63400.324 31, 17 45 15 45 0.94
G25.41100.105 61, 46 103 70 90 0.73
G26.59800.024 161, 73 192 111 84 0.81
G31.04700.356 30, 15 37 18 47 0.87
G33.98000.019 11, 18 42 20 80 0.88
G34.75100.093 9, 16 27 16 83 0.80
coordinates relative to the brightest spots as listed in Table 5.
the position angle of semi-major axis (north to east).
Table 8: Parameters of ellipses fitted to the maser spot distributions.

4 Discussion

4.1 Kinematic models of the origins of methanol masers

The diverse morphologies of methanol masers indicate that there is no straighforward explanation of the origin of this emission, as discussed previously (Norris et al. norris93 (); Phillips et al. phillips98 (); Minier et al. minier00 ()). The main hypotheses for the origin of methanol masers assume that they originate in a circumstellar disc or torus, in outflows or a shock colliding with a rotating molecular cloud. The results that we obtained by applying the existing models to our data are summarized below.

4.1.1 Rotating and expanding ring

Ring-like masers, which are prevalent in the present sample, may represent an inclined disc or torus around a massive protostar or young star. There is a tendency for flatter structures to have a larger velocity width, in all sources apart from G23.65700.127, (see Table 5), which suggests that we observe the effects of inclination and all motion is intrinsic to the plane of the ring. To test this hypothesis, we applied the model of a rotating and expanding thin disc (Uscanga et al. uscanga08 ()) to the nine ring-like masers. First, the coordinates of the spots (x, y) were transformed to a reference system (x, y) in which the origin was the center of the ellipse and the x’–axis was directed along the major axis of the projected ellipse (see Fig. 1 in Uscanga et al. uscanga08 ()). We then attempted to reproduce the kinematics using the LSR velocities (V) of the maser spots to determine rotation (V), expansion (V), and systemic (V) velocities of each source. The solutions were based on the minimisation of the function expressed in Eq. (8) by Uscanga et al. (uscanga08 ())

where was the spectral resolution of the observations corresponding to the uncertainty in the LSR velocity. The inclination angle is the angle between the line-of-sight and the normal to the ring plane, which is defined to be i. The semi–major and semi–minor axes (a,b) were taken from Table 8. We note, that we cannot determine the sign of the inclination angle from the data available and the direction of the rotation is therefore ambiguous, nor distinguish between outflow and contraction. Additional information (e.g., spectroscopic and interferometric observations of molecular clouds at mas resolution) are necessary to solve these questions. For the purpose of the model, we assumed that the brightest and the most complex half of the ellipse is closer to the observer. The results of fitting are summarized in Table 9 and an example of the fit of the model to the data, for G33.98000.019, is presented in Fig. 4.

We note that in general the expansion/infall velocity is higher than the rotation component in the majority of sources, as can be clearly seen in the maser spot distributions (Fig. 1). If rotation velocity was instead higher than that of expansion or infall, the extreme values of velocities could be produced where the major axis and ellipse intersect (at a position angle of 0°  from the major axis). In four of nine sources, the opposite result is found that higher blue- or red-shifted velocities appear where the minor axes intersect the ellipses (see plots of G23.20700.377, G23.38900.185, G24.63400.324, and G25.41100.105). The average position angle of the most extreme velocity with respect to the major axis in all nine rings, and the standard dispersion in the mean, is 52°11°. This suggests that the masers originate in the zone where the radial motions exist and expansion/infall plays a role. This could occur at the interface between the disc/torus and outflow. A similar result was reported for the archetypical object Cep A (Torstensson et al. torstensson09 ()), where the major axis of the elliptical distribution of 6.7 GHz methanol masers is perpendicular to the bipolar outflow. Their LSR velocity distributions show similar characteristics in that expansion or contraction dominates over the rotation.

Source V V V i
(km s) (km s) (km s) (°)
G23.20700.377 1.17 3.96 79.46 69 168
G23.38900.185 1.26 1.71 74.91 54 101
G23.65700.127 7.29 2.61 81.90 16 641
G24.63400.324 8.64 2.25 38.96 71 544
G25.41100.105 0.09 1.17 95.84 47 206
G26.59800.024 0.81 0.81 24.58 55 30
G31.04700.356 410 2.43 80.68 61 211
G33.98000.019 0.45 2.97 61.86 62 123
G34.75100.093 1.17 2.88 51.17 53 22
Table 9: Parameters derived by fitting kinematics of the rotating and expanding disc model. The signs and of the rotation and expansion velocities refer to the clockwise or anti-clockwise rotation and outflow or inflow for positive i. Both rotation and flow are reversed in a case of negative i. For each source, both signs together with the sign of i could be reversed since our model does not give the directions unambiguously.
Figure 4: Velocity of the maser spots in G33.98000.019 versus azimuth angle measured from the major axis (north to east). The open circles represent the data and are proportional to the logarithm of the flux densities. The sinusoidal line represents the best-fit kinematical model of a rotating and expanding disc (with infintesimal thickness) using the parameters listed in Table 9.

4.1.2 Linear maser as edge-on disc?

A thin disc seen edge-on would appear to have a linear morphology. Norris et al. (norris98 ()) argued that maser radiation propagates most strongly in the plane of the disc due to the greater column depth, to explain why so many methanol masers with linear morphology appeared in the data then available.

We do not confirm this selection effect and note that the increased sensitivity detects more complex structures of masing regions. Only 16% of our sample are linear masers (compared to 29% ring-like masers) and they are also not the brightest, although it is possible that if there is strong expansion or infall, this would produce a steeper velocity gradient in edge-on discs and possibly make the masers fainter.

We calculated the mass that each linear maser structure would contain if it originated in a disc in Keplerian rotation, using a method similar to that of Minier et al. (minier00 ()). Assuming that the masing area corresponds to the diameter of the disc, the average central mass of these five linear structures is 0.12 M or 0.44 M for the near or far kinematic distances, respectively. These subsolar values are very unlikely for massive protostars. We agree with Minier et al. (minier00 ()) that the underlying assumption is wrong and we do not detect the full diameter of the disc. However, if we assume that the true extent of the linear masers is similar to the average size of the major axis of the nine ellipses (188 mas), this implies a mean central mass of 76 M or 190 M for the near or far kinematic distance, respectively. These values seem unrealistically high. Another solution is that the masers are not bound by Keplerian rotation and we argue that the most likely explanation is that the linear morphology results from a different scenario. Linear structures with ordered velocity gradients may be produced readily by geometric effects in outflows. It seems significant that Szymczak et al. (szymczak07 ()) detected molecular line emission from HCO, CO, and CS towards these five sources using the IRAM 30 m telescope, supporting the outflow scenario. In addition, De Buizer et al. (buizer09 ()) imaged SiO outflows towards five methanol masers with linear morphologies. They found that the spatial orientations of the outflows were inconsistent with the methanol masers tracing discs. Linear masers produced by outflows seemed to provide a much more plausible scenario. Finally, we also note that the linear masers have a smaller extent than most other maser structures (Table 5). In particular, the entire G35.79300.175 structure is only 10 mas long corresponding to the typical size of an individual maser cluster in other sources. We conclude that most of the linear masers are more likely to be associated with outflows than with edge-on discs, although it is possible that more sensitive observations might indicate that some are part of ring-like or other structures. G25.41100.105 (see Sect. 3.4) provides such an example, since Beuther et al. (beuther02 ()) found only two maser spots, whilst in the present study we detected 30 spots, forming a ring-like morphology.

4.1.3 Propagating shock front

Dodson et al. (dodson04 ()) proposed another model for linear methanol masers. A low speed planar shock propagating through the rotating molecular cloud would produce a linear spatial distribution of maser spots if the shock was propagating close to perpendicular to the line of sight. The linearity would be distrupted where the shock interacted with density perturbations in the star-forming regions. The main diagnostic for this scenario is the perpendicular orientation of velocity gradients within individual clusters with respect to the main large-scale velocity gradient. We analyzed the internal gradients of clusters and found this behaviour in three out of five masers with linear morphologies (G22.33500.155, G23.96600.109, and G30.31800.070). In addition, the arched source, G33.64100.228, shows similar characteristics in four (out of six) clusters, which have internal velocity gradients perpendicular to the longest axis of the overall structure. All these masers have young massive objects in close proximity less then 122 away (Sect. 4.2), which could be responsible for the external shocks. Proper motions studies are needed to verify this scenario.

4.1.4 Bipolar outflow

The bipolar outflow model for HO masers associated with a high–mass young stellar object was proposed by Moscadelli et al. (moscadelli00 ()) and confirmed in IRAS201264104 by proper motion studies (Moscadelli et al. moscadelli05 ()). We also tested this model for all sources in this study. The assumptions of the model are as follows: masers originate in the surface of a conical bipolar jet due to the interaction between the ionised jet and the surrounding neutral medium, and the velocity of a maser spot, V, is directed radially outward from the central star at a constant value. The center of the coordinate system is at the vertex of the cone, the z–axis is along the line of sight, and the x–axis coincides with the projection of the outflow on the plane of the sky (see Fig. 4 in Moscadelli et al. (moscadelli00 ())). Taking the central velocity of the maser emission (V) to be the systemic LSR velocity, we minimized the function as expressed in Eq. (3) of Moscadelli et al. (moscadelli00 ())

where n is the number of spots, V is the velocity of the spot #j calculated using Eqs. (1) and (2) from Moscadelli et al. (moscadelli00 ()), and is the observed LSR velocity of the spot.

Source RA,Dec V PA
(mas, mas) (km s) () () ()
G23.70700.198 33, 45 52.7 57 115 37 1.2
G24.54100.312 52, 15 15.5 112 52 48 2.4
coordinates relative to the brightest spots as listed in Table 5.
Table 10: Parameters derived from fitting the biconical outflow model of Moscadelli et al. (moscadelli00 ()).

We obtained reasonable fits for only two of the arched sources, G23.70700.198 and G24.54100.312. The following best-fit model parameters are listed in Table 10: the position of the vertex, the opening angle of the cone (2), the inclination angle between the outflow axis and the zaxis (), and the direction of the xaxis, PA, which is the position angle of the outflow on the plane of the sky. A comparison between the modelled and observed data and a sketch of the orientation of the outflow was presented previously in Fig. 1 in Bartkiewicz et al. (bartkiewicz06 ()). It is significant that the vertices of the cones calculated for both sources coincide with infrared sources within the position uncertainties (Sect. 4.2). In addition, molecular line emission at similar LSR velocities was reported towards both sources (Szymczak et al. szymczak07 ()). As we mentioned previously, Araya et al. (araya06 ()) imaged HCO maser emission at 4.8 GHz towards G24.54100.312. All of these findings ensure that the outflow scenario is plausible for these two objects.

We tested the outflow model intensively on the four maser sources associated with H II regions. However, we did not achieve a reasonable fit that would place the vertex of the cone at the center of the radio continuum object, whilst reproducing the maser spot kinematics, for any of these sources.

4.2 Association with MIR emission

Early work by Szymczak et al. (szymczak02 ()) showed that 80% of methanol masers have infrared counterparts in IRAS and/or MSX catalogues. Since maser data with position accuracy as good or better than these IR catalogues (30″5″) have become available, the fraction of secure identifications has diminished (Pandian et al. pandian07 ()). We can now attempt identifications with sub-arcsec resolution data.

We used Spitzer IRAC data to test the association between methanol masers and MIR emission. Images at 3.6, 4.5, 5.8, and 8.0 m from the GLIMPSE survey, all at 06 resolution (Fazio et al. fazio04 ()), were retrieved from the Spitzer archive333http://irsa.ipac.caltech.edu/applications/Cutouts/ and compared with the radio data using AIPS.

Figure 5: Histogram of number of the methanol masers as a function of angular separation from associated MIR sources at 4.5 m. Bin size is 05. The sources are divided into two groups with position measurements of high accuracy (Dec35) and lower accuracy (Dec35 plus those measured with MERLIN alone).

The angular separation between the brightest maser spot of each source in our sample and the nearest MIR source at 4.5 m () is given in Table 5 and a histogram of the results is shown in Fig. 5. The average separation for the entire sample is 118019 and the median value is 089, whereas, for the subsample of 19 objects at Dec35 with coordinates measured using the EVN, the corresponding values are 061009 and 051, respectively. The remaining 14 targets with Dec35, or with positions derived from the MERLIN data alone, show a larger with the mean and median values of 196035 and 161, respectively. Since the images of these sources had poor -plane coverage and hence less accurate astrometry, we suggest that their higher estimates of have a higher uncertainty and that most associations are also likely to be genuine.

We conclude that the majority of maser sources coincide, within 1″  with MIR emission, i.e., the maser emission from each source falls within one Spitzer pixel (nominal size 12, see Fazio et al. fazio04 ()) in the IRAC 4.5 m image. This strongly reinforces the finding by Cyganowski et al. (cyganowski08 ()), who reported coincidences to within 5″  for 46 out of 64 methanol sources with positions measured using the ATCA. Extended emission in the IRAC 4.5 m band has been postulated to be a tracer of shocked molecular gas in protostellar outflows (e.g., Davies et al. davis07 (); Cyganowski et al. cyganowski08 () and references therein). This IRAC band contains H and CO lines that may be excited by shocks. Using shock models, Smith & Rosen (smithrosen05 ()) predicted that H emission in outflows is 5–14 times stronger in the 4.5 m band than in the 3.6 m band. Cyganowski et al. (cyganowski08 ()) identified more than 300 objects with extended 4.5 m emission, which may relate to outflows in massive stars. Four maser sources from our sample, G23.96600.109, G35.79300.175, G37.47900.105, and G39.10000.491, were included in their catalogue.

In order to search for extended emission, we created images of the 4.5 m3.6 m excess by subtracting the 3.6 m image of each of our sources from its 4.5 m counterpart, and compared these with the 4.5 m image. All the sources have extended 4.5 m emission. Figure 6 shows the 4.5 m3.6 m excess superimposed on the 4.5 m image for selected sources. Source G21.40700.254 illustrates how the maser emission located precisely inside the brightest pixel of the 4.5 m3.6 m image outside the 4.5 m peak. There is also evidence that maser clusters in at least three sources are associated with 4.5 m emission excess. The image at the position of G24.14800.009 shows that the maser coincides exactly with maxima of both the 4.5 m3.6 m excess and of the 4.5 m emission. Weak, extended 4.5 m emission is also seen at the edges of two neighbouring sources and in diffuse lanes. The maser G37.59800.425 coincides with a maximum in a very asymmetric distribution of 4.5 m3.6 m emission excess that is displaced by 15 from a peak of 4.5 m emission, implying that the methanol maser and the excess in the 4.5 m IRAC band are very strongly related. Similar evidence is provided by G38.20300.067, where the maser is offset by more than 8″  from a bright MIR object. The maser is 17 away from a weak bump in a large lane of diffuse 4.5 m3.6 m emission excess. Inspection of IRAC images for other bands suggests that the maser is probably associated with a faint MIR object.

All the counterparts in the sample have MIR properties typical of embedded young massive objects (e.g., Kumar et al. 2007) associated with the methanol masers (Ellingsen 2006). We defer a detailed discussion of the MIR spectral indices of individual objects, because several bright sources (e.g., G23.38900.185, G23.65700.127, and G24.63400.324) are saturated in the IRAC images, while for others only upper limits to the 3.6 m flux can be derived (e.g., G23.96600.109, G37.03000.039, and G38.20300.067). We note only, that the MIR objects associated with methanol masers that have a ring-like morphology have much stronger emission at 8 m, and all the objects that are saturated in the IRAC images have regular maser structures. In contrast, no object saturated in the IRAC images has a complex maser morphology. This suggests that the MIR counterparts of masers with less regular structures are deeply embedded massive stars that are younger than the counterparts of those with more regular, ring-like maser structures. One can speculate that in those younger objects the methanol maser originates in a limited number of confined regions, whilst more regular structures emerge during later evolutionary stages.

In summary, we have found strong evidence of a close coincidence of 6.7 GHz methanol masers with 4.5 m emission excess. This provides a firm argument that methanol emission originates in those inner parts of an outflow or disc/torus where the molecular gas is shocked.

This hypothesis is strongly supported by the kinematic model that can be successfully applied to the ring masers in our sample, which involved a rotating and expanding disc wherein expansion dominates the velocity field. Moreover, a fraction of masers seem to fit the model of a bipolar jet or that of a shock front colliding with the surrounding molecular material.

\onlfig

6

Figure 6: Spitzer IRAC 4.5 m (grey) and 4.5 m3.6 m (contour) images centered on selected methanol sources (at coordinates taken from Table 5). The grey scale in MJy sr is indicated in the horizontal bar and contour levels are 1 MJy sr(1, 2, 4, 8, 16, 32, 64, 128). The methanol maser G21.40700.254 coincides exactly with the brightest pixel in the 4.5 m3.6 m image, at the map origin, which is offset from the 4.5 m peak. There are at least two other nearby MIR sources with 4.5 m emission excesses. Note the absence of further sources with extended 4.5 m3.6 m emission excess in this 30″30″  field. The maser source G24.14800.009 coincides with the peaks of emission both at 4.5 m and for the 4.5 m3.6 m excess. The maser G37.59800.425 is offset from the brightest 4.5 m emission. Note the highly asymmetric morphology of the extended 4.5 m3.6 m excess emission and the coincidence of the maser emission with a maximum of this excess. In the case of G38.20300.067, the maser emission does not coincide with either of the two brightest sources but is close (17) to a weak bump of 4.5 m3.6 m excess located in a very extended arc (25″) of diffuse 4.5 m emission. Further inspection of IRAC images at 5.8 m suggests that this bump could be a weak MIR object.

5 Conclusions

We have completed a 6.7 GHz methanol line imaging survey of 33 maser sources in the Galactic plane. High quality EVN images were obtained for 31 targets showing their mas-scale structures, from which we derived the absolute positions of 29 sources with a few mas accuracy. In most cases, the masers exhibit complex structures. The observed morphologies can be divided into five groups: simple, linear, ring-like, complex, arched, and pair. It is surprising that about 29% of the sources exhibit ring-like distributions of maser spots that were not apparent in previous VLBI surveys, which were less sensitive and concentrated on brighter masers. We find that many of the other ordered structures, notably linear and arched, can be interpreted as originating from the interaction between collimated or biconical outflows and the surrounding medium.

A simultaneous survey at high angular resolution of continuum emission at 8.4 GHz towards the maser targets revealed that only 16% of the methanol masers appear to be physically related to H II regions. In three cases, the maser coincides with a peak in a compact and relatively weak continuum object. These are believed to be young UC H II regions. In general, these results seem to imply that 6.7 GHz methanol masers appear before the ionised region is detectable at cm wavelengths. The hypothesis that these masers are possible associated with HC H II regions needs to be verified.

We used the Spitzer GLIMPSE survey to demonstrate that the majority of methanol masers are closely associated with MIR emission. Analyzis of the MIR counterparts suggests that masers with a regular, ring-like morphology are associated with more evolved proto- or young stars, relative to the counterparts of masers with more complex, irregular structures. Moreover, both the kinematics of the masers with a ring-like distribution and the characteristics of associated MIR emission are consistent with a scenario in which the methanol emission is produced by shocked material associated with a disk or torus, possibly from an interaction with the outflow.

Acknowledgements.
Our special acknowledgements to Dr. Peter Thomasson at Jodrell Bank Observatory and to Dr. Bob Campbell at JIVE for detailed support in many stages of this project. We also thank Dr. Riccardo Cesaroni, Dr. Luca Moscadelli, Dr. Lucero Uscanga, Dr. Krzysztof Goździewski and Kalle Torstensson for useful discussions. This work was benefited from the Polish MNiI grant 1P03D02729 and from research funding from the EC 6th Framework Programme.
The European VLBI Network (EVN) is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. MERLIN is a National Facility operated by the University of Manchester at Jodrell Bank Observatory on behalf of STFC. The Very Large Array (VLA) of the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
G21.40700.254
89.040 0.0000 0.0000 2762 0.32 2672
89.303 0.0192 9.7250 1426 0.35 1513
90.094 8.9947 26.3260 307 0.18 308
91.148 136.3023 5.0420 891 0.26 888
91.412 13.1560 23.6830 117
91.587 27.3535 1.5320 422 0.26 420
G22.33500.155
35.626 0.0000 0.0000 1714 0.32 1642
38.174 10.9782 47.4060 125 0.53 125
G22.35700.066
79.708 0.0000 0.0000 10544 0.37 11350
80.235 1.0074 6.0100 10067 0.47 10190
81.114 2.8964 18.3490 1422 0.48 1497
81.465 155.0868 46.3230 492
83.574 139.2789 18.6790 309
84.101 141.5278 66.5690 156 0.58 158
84.804 132.0327 75.7690 176
88.143 55.7575 208.5390 602 0.27 636
88.494 68.1772 277.2010 250
88.670 56.6464 207.5440 476
G23.20700.377
72.553 101.3966 64.2970 141 0.31 140
73.959 32.5775 17.5310 194 0.28 190
75.190 0.5751 9.9430 226 0.23 223
75.014 25.7012 1.9800 421 0.83 374
75.453 269.6437 26.1920 893 0.34 946
75.629 57.3581 41.8210 647 0.34 683
76.069 30.8269 11.6140 229 0.42 228
76.596 11.6581 3.1990 4748 0.43 4761
77.123 0.0000 0.0000 9292 0.34 9468
77.475 29.4751 4.2880 3387 0.46 3463
77.739 11.0845 4.9910 3096 0.47 2852
78.793 53.0729 33.4270 541 0.44 563
78.881 217.1237 30.9130 286 0.28 293
78.969 47.4388 30.9230 577
79.233 50.4581 32.3460 1319 0.38 1412
79.496 135.9039 191.481 0 277 0.21 294
79.672 113.8403 117.084 0 213 0.41 221
79.936 57.8547 71.4830 315 0.29 303
80.727 163.9145 147.143 0 7092 0.36 6989
81.869 136.2582 139.260 0 8967 0.37 8838
81.869 141.0800 140.772 0 5874 0.60 5918
82.045 157.2072 145.562 0 1104 0.25 1119
82.836 151.9599 147.620 0 1190 0.43 1012
84.858 47.3987 112.077 0 791 0.56 679
G23.38900.185
72.641 78.2268 32.6440 6398 0.46 6279
73.784 52.4158 32.4260 9326 0.47 8831
74.399 46.8761 38.8880 2978 0.41 2715
74.575 43.2789 31.9400 4700 0.28 4730
74.750 43.4540 63.9750 311
74.750 41.0708 14.0150 267
74.838 31.5347 70.8960 19723 0.71 21320
74.926 41.8691 49.9560 1272 0.17 1232
75.453 0.0000 0.0000 21554 0.26 21587
75.541 57.2121 153.9020 397
Table 6: Maser clusters towards 31 sources. If a spectrum of a maser cluster does not show a Gaussian profile we enter the sign in the Cols. of FWHM and S.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
75.805 64.5014 143.6490 3708 0.32 3768
76.069 78.5948 144.9390 6571 0.37 6686
76.332 93.2091 154.8360 4516 0.62 4683
76.596 70.0292 145.2360 1072 0.55 1086
76.948 20.9791 75.4450 721 0.31 727
77.035 69.1893 147.1100 4860 0.28 4795
77.475 73.8030 150.8570 234 0.19 234
77.563 31.8834 157.9720 485 0.24 470
G23.65700.127
77.563 5.0421 229.8720 816 0.42 754
77.563 22.2581 232.7000 333 0.25 355
78.266 71.1275 228.1640 1059 0.52 932
78.881 54.5928 171.6190 336 0.50 305
79.584 105.3428 117.0720 162 0.75 170
80.024 124.7437 212.4980 1595 0.23 1607
80.112 39.3568 213.5540 977 0.32 938
80.639 21.7712 13.2960 850 0.49 691
80.727 14.3113 66.4860 375 0.20 391
80.990 156.9775 5.2880 124 0.25 135
81.166 2.9967 47.4670 362 0.30 370
81.166 26.3472 70.2780 68 0.55 77
81.518 7.2432 3.7020 588 0.42 564
81.781 46.3136 156.5250 126 0.39 117
81.869 10.2711 10.8770 806 0.30 820
81.869 13.2471 13.0970 90 0.19 92
82.133 1.5481 43.7250 132 0.31 124
82.309 44.5072 156.8130 393 0.28 391
82.573 0.0000 0.0000 3623 0.48 3401
82.836 41.5298 162.2880 168 0.29 174
83.100 210.9484 11.3450 182 0.24 196
83.276 47.8646 35.5030 1018 0.33 1036
83.539 69.2069 83.7510 80 0.27 81
83.979 0.7674 18.7780 143 0.62 125
83.979 52.8072 32.5950 557 0.58 531
84.243 133.7606 4.1400 358 0.27 373
84.858 45.6887 140.0160 548 0.44 552
86.264 17.8008 15.6640 797 0.37 766
86.791 19.7200 18.7010 746 0.37 702
87.231 16.7410 26.9680 96 0.24 96
87.670 20.3523 29.5260 280 0.26 272
G23.70700.198
58.578 20.4283 18.1540 377
71.410 101.2879 0.5490 200 0.19 207
71.762 105.4053 5.6330 128 0.19 141
72.993 10.2000 57.1700 667 0.23 707
73.344 1.6921 33.7300 92 0.40 95
73.608 0.3488 27.4820 100 0.22 102
74.223 3.2461 38.1980 672 0.38 683
74.575 0.3933 23.6850 1845 0.26 1756
75.102 0.0089 29.0830 404 0.47 416
75.190 16.6285 9.9600 306 0.52 340
75.629 0.3577 26.8210 249 0.50 260
76.157 0.9722 19.8400 3319 0.46 3297
76.596 0.9589 17.6580 2054 0.83 2033
77.299 0.8772 16.2390 3473 0.29 3625
78.266 14.6856 14.3980 219 0.31 219
78.530 13.7935 14.4180 358 0.32 366
Table 6: continued.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
77.651 0.4334 3.7530 2234 0.70 2239
78.090 0.5982 1.0340 2144 0.95 2099
79.145 0.0000 0.0000 6059 0.79 5368
79.496 17.3246 13.9080 117 0.61 109
80.112 14.3234 14.3790 202 0.36 200
80.903 4.2421 8.0300 142 0.48 129
81.078 3.9200 8.6280 159 0.56 153
G23.96600.109
67.428 21.9498 23.0050 387 0.30 369
68.219 23.3000 22.9600 431 0.41 404
70.942 0.0000 0.0000 5487 0.41 5390
G24.14800.009
17.441 4.3548 21.7190 1048 0.20 1048
17.529 0.1980 0.0970 2459 0.51 2493
17.792 0.0000 0.0000 3648 0.40 3702
18.407 0.6149 5.9860 240 0.33 235
G24.54100.312
103.754 35.7935 59.5820 233 0.23 233
105.688 0.0000 0.0000 7753 0.34 7453
106.215 38.6463 37.3600 316 0.29 278
107.973 10.8681 7.8090 238 0.33 239
106.479 5.5553 2.9680 2788 0.49 2554
107.270 0.0253 1.0010 2700 0.47 2677
107.533 1.8840 2.4950 2005 0.38 2052
108.324 94.8606 14.3270 106 0.32 108
108.588 26.0490 13.4890 638 0.37 534
109.467 39.7491 16.3690 339 0.57 339
109.643 39.9396 16.9440 407 0.38 413
110.082 41.4218 18.0810 728 0.39 742
G24.63400.324
34.725 18.6698 25.4370 457
35.428 0.0000 0.0000 3027 0.29 3544
43.862 62.2684 36.4860 2370 0.56 2322
45.795 41.9147 45.7150 257 0.41 257
46.322 43.8591 45.1230 312 0.38 313
47.904 42.4775 45.4230 136 0.54 136
G25.41100.105
93.765 34.1096 111.2790 142
94.117 109.0956 19.4400 82
94.820 115.8608 101.1940 847
94.644 16.7363 86.3980 2374 0.51 2345
94.820 3.5683 20.6650 1025 0.50 1037
96.928 3.5146 123.8050 323
96.928 3.2029 5.4840 2251 0.42 2241
97.104 46.6480 16.7050 2468 0.28 2504
97.280 0.0000 0.0000 3433 0.56 3690
98.861 176.2362 50.0100 139
G26.59800.024
22.952 2.1488 0.1130 182 0.45 183
24.182 0.0000 0.0000 3043 0.34 3239
24.709 84.5157 169.5700 1368 0.44 1276
25.061 335.4955 131.1280 714 0.51 671
25.588 78.8583 159.7380 76
25.939 338.8875 124.8040 97 0.94 99
Table 6: continued.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
G27.22100.136
105.424 11.6570 77.8590 253 0.42 237
106.391 11.2849 77.8360 94 0.59 91
107.182 33.2614 55.4880 245 0.42 232
109.731 33.8577 32.9640 1239 0.26 1193
110.258 34.2955 34.2190 1182 0.43 1179
111.225 33.0149 35.1860 467 0.30 466
112.280 19.4363 27.0680 563 0.35 538
112.719 18.6697 27.1140 906 0.29 917
114.828 20.1700 27.8700 166 0.25 169
115.268 18.4171 23.1670 150 0.39 152
115.444 19.0119 24.4340 158
115.707 24.1941 47.1650 236 0.31 225
116.586 24.7798 7.8580 437 0.23 457
117.114 53.1908 8.6200 6519 0.33 6591
117.729 8.9344 12.1430 448 0.23 469
117.817 54.3145 7.6670 5642 0.55 5621
118.168 9.3558 11.4900 1991 0.33 1990
118.432 55.3082 7.0300 4831 0.35 4755
118.783 0.0000 0.0000 12535 0.39 12021
118.959 8.6878 13.1750 765 0.67 801
119.223 70.7368 6.8450 209
119.311 9.2317 5.7490 232 0.33 224
119.311 34.3642 4.9240 135 0.51 111
119.750 9.4066 11.3470 101 0.38 103
120.365 15.4465 18.4780 536 0.23 536
120.190 20.1998 41.8670 1339 0.40 1367
121.069 17.4294 44.0510 218 0.29 218
G28.81700.365
87.735 73.2105 56.0600 325 0.51 331
88.087 63.9892 72.9580 183
90.723 0.0000 0.0000 3137 0.36 3211
91.074 2.8641 17.4230 1440 0.62 1440
91.250 2.7787 3.5790 896 0.68 933
91.601 4.0115 20.0900 2511 0.31 2755
92.129 3.1337 10.3800 743 0.47 761
92.656 2.7143 26.7830 2116 0.38 2103
G30.31800.070
35.192 23.8385 22.6390 300 0.53 303
36.070 0.0000 0.0000 514 0.68 528
36.949 5.6770 5.0420 144 0.70 144
G30.40000.296
98.104 4.4898 11.8250 2139 0.31 2151
98.455 0.0000 0.0000 2765 0.32 3129
100.388 31.0281 45.2180 1581 0.29 1603
101.618 27.7091 85.7960 331 0.72 326
103.727 19.1733 86.2030 1914 0.41 1923
103.727 67.4799 134.7790 497 0.28 503
104.430 66.4200 135.2860 260 0.43 267
Table 6: continued.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
G31.04700.356
81.058 59.6159 28.5990 277
78.070 0.9674 7.7850 281 0.82 267
79.125 1.2973 3.2920 1169 0.73 1180
80.179 0.4664 1.5320 544 0.55 555
80.882 0.0000 0.0000 1989 0.55 1960
81.058 0.3989 1.5250 1563 0.55 1568
82.991 47.8577 6.9740 617 0.28 651
84.045 25.8591 9.6530 101
G31.58100.077
95.643 0.0000 0.0000 2722 0.35 2891
97.752 60.9387 57.6080 349
98.104 37.3458 7.0480 809 0.31 814
98.455 32.2616 3.7570 376 0.39 386
98.631 149.7072 94.0260 493
98.806 61.9691 79.9160 292 0.41 321
98.806 149.8767 93.2150 2045 0.68 2039
99.509 158.1329 69.4150 358
99.685 151.6794 56.5090 1099 0.39 1223
G32.99200.034
89.724 13.3545 44.5810 321
90.339 51.5775 49.8690 396
90.602 14.7555 34.4810 1967 0.40 1953
90.954 26.7960 1.1600 4650 0.32 4838
91.393 4.7370 6.0210 493 0.50 505
91.481 28.3665 0.9760 1856 0.50 1867
91.656 16.1355 37.9820 465
91.832 0.0000 0.0000 6212 0.35 6786
92.184 34.7340 14.9630 394 0.28 394
92.447 35.5635 14.7320 163
92.711 24.9990 19.6750 199
94.204 51.0915 1.6040 600 0.26 574
94.731 49.8900 4.5590 445 0.22 446
G33.64100.228
58.840 0.0000 0.0000 28300 0.31 29569
59.540 9.5996 12.3000 4105 0.29 4259
59.800 14.3994 20.1000 12260 0.22 12665
60.330 69.5970 15.9000 20402 0.30 21448
60.420 61.9473 25.9000 2280 0.50 2688
60.510 60.8973 27.0000 1621
60.860 13.3494 28.6000 1672 0.30 1716
60.940 65.3971 14.6000 10447 0.35 10558
61.120 76.3467 2.3000 356 0.35 372
61.300 56.2475 17.0000 207 0.26 213
61.300 128.3944 55.9000 176
61.820 82.1964 7.6000 365 0.40 358
62.260 122.5447 63.5000 893 0.56 812
62.180 111.2951 50.0000 109
62.440 42.5981 18.2000 5440 0.31 5758
62.700 42.5981 17.8000 20690 0.35 20872
62.970 42.5981 16.6000 9325 0.48 9620
63.140 30.4487 19.7000 1090 0.30 1124
G33.98000.019
58.886 0.0000 0.0000 3782 0.33 4057
59.501 5.1225 0.1130 434 0.30 454
60.292 50.1990 23.9650 315 0.45 290
Table 6: continued.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
60.555 29.6685 1.7760 254 0.29 258
61.258 49.1925 16.8830 784 0.37 797
61.609 30.1755 5.5850 515 0.60 469
62.312 30.5595 3.2980 276 0.40 248
64.157 16.2345 27.9140 497 0.50 471
65.387 32.4450 40.9110 236 0.25 237
G34.75100.093
50.627 28.8631 27.8740 224 0.59 215
51.769 10.8418 3.5670 718 0.46 702
52.736 0.0000 0.0000 1954 0.68 1817
53.175 3.5980 0.9420 1763 0.44 1845
G35.79300.175
60.060 2.4000 0.8600 889 0.27 952
60.680 0.0000 0.0000 9702 0.62 9616
61.300 1.9500 0.8100 6338 0.48 6327
61.820 3.0000 1.2100 5960 0.75 5927
62.350 4.3500 2.2100 1225 0.60 1347
G36.11500.552
70.395 1072.0323 421.7720 10779 0.38 9795
70.834 1075.7487 431.4520 254 0.33 261
71.186 5.1454 0.8900 1066 0.40 1113
71.449 1068.0478 420.0420 617 0.35 637
71.801 1062.5353 420.6660 2921 0.28 2869
71.977 24.9857 12.5080 1386 0.29 1398
72.416 1064.1711 421.8460 2973 0.63 2681
72.504 1168.7980 300.8850 506
72.855 1063.7741 421.0850 11194 0.35 11100
73.031 0.0000 0.0000 11744 0.49 11978
73.119 39.3480 27.3450 1431 0.58 1613
73.821 1163.3500 155.9080 1134 0.19 1149
74.173 48.5034 43.3810 2845 0.39 3600
74.436 43.6441 45.4110 6641 0.54 5863
75.139 9.8909 12.9190 371 0.35 357
75.579 46.4093 46.6400 316 0.36 302
75.754 7.3175 14.7340 431 0.28 431
76.106 0.1992 16.2260 3900 0.27 4007
81.289 1122.9459 420.7460 1619 0.78 1373
81.553 1100.4663 436.9820 447 0.77 386
81.816 1103.1221 438.3080 513 0.27 515
81.904 1116.3250 407.0410 1010 0.31 1103
82.168 1113.5523 420.1690 5062 0.32 5183
83.925 1116.4418 417.6580 1652 0.66 1816
G36.70500.096
52.560 1.3392 1.1530 348 0.33 353
53.087 0.0000 0.0000 7576 0.31 7608
53.790 0.9167 0.1560 2413 0.48 2470
54.405 1.8934 1.2010 4629 0.65 4404
54.844 2.0102 9.9610 1309 0.43 1343
55.108 1.5579 3.2250 420
62.137 14.1016 52.0590 1974 0.45 1969
63.015 0.3940 45.6080 153 0.47 154
G37.03000.039
78.585 0.0000 0.0000 691 0.47 656
78.761 0.4374 0.2050 224
Table 6: continued.
V RA Dec S FWHM S
(km s) (mas) (mas) (mJy beam) (km s) (mJy beam)
G37.59800.425
82.802 91.6302 11.1510 251
85.789 0.0000 0.0000 3911 0.43 3826
85.438 0.3322 0.8860 891 0.33 902
86.931 26.0379 12.9580 2407 0.32 2482
87.195 25.4768 13.1770 465
87.283 39.3823 6.436 2543 0.45 2661
88.600 63.5139 4.0650 395
88.688 66.2298 7.5300 202
G38.03800.300
55.656 0.0000 0.0000 2168 0.34 1996
57.062 6.2921 15.5470 507
57.413 9.3731 16.4000 1046 0.38 980
58.204 13.9385 1.8140 1434 0.25 1444
59.522 20.8441 15.0610 215
59.522 19.9328 12.8000 224
59.609 19.0545 3.8880 281
G38.20300.067
79.640 0.0000 0.0000 828 0.28 707
80.518 54.8141 20.5190 169
80.606 11.2600 13.4970 157 0.38 160
83.856 101.8398 154.2200 173
83.944 35.3736 29.9400 152
83.944 35.2315 44.9650 195
84.208 33.8743 31.6240 198
G39.10000.491
14.630 5.7037 38.5700 222
14.630 7.8346 11.4310 453 0.28 446
15.772 13.3126 22.0650 314
15.860 115.0769 103.8770 623
15.860 116.6086 75.8880 545
15.860 13.3112 22.0040 769
15.245 0.0000 0.0000 2068 0.37 1803
15.684 5.1254 5.8070 1140 0.62 1158
15.948 4.9192 7.1320 1042 0.23 1108
17.617 6.6630 8.1800 696 0.32 654
Table 6: continued.
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