Observations of 6.7 GHz Methanol Masers with EAVN I: VLBI Images of the first Epoch of Observations
Kenta Fujisawa, Koichiro Sugiyama, Kazuhito Motogi, Kazuya Hachisuka, Yoshinori Yonekura, Satoko Sawada-Satoh, Naoko Matsumoto, Kazuo Sorai, Munetake Momose, Yu Saito, Hiroshi Takaba, Hideo Ogawa, Kimihiro Kimura, Kotaro Niinuma, Daiki Hirano, Toshihiro Omodaka, Hideyuki Kobayashi, Noriyuki Kawaguchi, Katsunori M. Shibata, Mareki Honma, Tomoya Hirota, Yasuhiro Murata, Akihiro Doi, Nanako Mochizuki, Zhiqiang Shen, Xi Chen, Bo Xia, Bin Li, Kee-Tae Kim
aa r X i v : . [ a s t r o - ph . S R ] N ov Observations of 6.7 GHz Methanol Masers with EAVN I: VLBI Images of the first Epoch of Observations
Kenta
Fujisawa , Koichiro
Sugiyama , Kazuhito
Motogi , Kazuya
Hachisuka , Yoshinori
Yonekura , Satoko
Sawada-Satoh , Naoko
Matsumoto , Kazuo
Sorai , Munetake
Momose , Yu Saito , Hiroshi
Takaba , Hideo
Ogawa , Kimihiro
Kimura , Kotaro
Niinuma , Daiki
Hirano , Toshihiro
Omodaka , Hideyuki
Kobayashi , Noriyuki
Kawaguchi , Katsunori M.
Shibata , Mareki
Honma , Tomoya
Hirota , Yasuhiro
Murata ,
12, 13
Akihiro
Doi ,
12, 13
Nanako
Mochizuki , Zhiqiang
Shen , Xi Chen , Bo Xia , Bin Li , and Kee-Tae Kim
The Research Institute for Time Studies, Yamaguchi University, 1677-1 Yoshida, Yamaguchi,Yamaguchi 753-8511, Japan Graduate school of Science and Engineering, Yamaguchi University, 1677-1 Yoshida, Yamaguchi,Yamaguchi 753-8512, Japan Shanghai Astronomical Observatory, Chinese Academy of Sciences, China Center for Astronomy, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan Mizusawa VLBI Observatory, National Astronomical Observatory of Japan (NAOJ), 2-12Hoshigaoka-cho, Mizusawa-ku, Oshu, Iwate 023-0861, Japan Mizusawa VLBI Observatory, NAOJ, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Department of Physics / Department of Cosmosciences, Hokkaido University, Kita 10, Nishi 8,Kita-ku, Sapporo, Hokkaido 060-0810, Japan College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu, Gifu 501-1193, Japan Department of Physical Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai,Osaka 599-8531, Japan Department of Physics and Astronomy, Graduate School of Science and Engineering, KagoshimaUniversity, 1-21-35 Korimoto, Kagoshima, Kagoshima 890-0065, Japan The Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1Yoshinodai, Chuou-ku, Sagamihara, Kanagawa 229-8510, Japan Department of Space and Astronautical Science, The Graduate University for Advanced Studies,3-1-1 Yoshinodai, Chuou-ku, Sagamihara, Kanagawa 229-8510, Japan Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, China Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 305-348,Republic of [email protected] Received ; accepted )
Abstract
Very long baseline interferometry (VLBI) monitoring of the 6.7 GHz methanolmaser allows us to measure the internal proper motions of the maser spots and there-fore study the gas motion around high-mass young stellar objects. To this end, wehave begun monitoring observations with the East-Asian VLBI Network. In this pa-per we present the results of the first epoch observation for 36 sources, including 35VLBI images of the methanol maser. Since two independent sources were found inthree images, respectively, images of 38 sources were obtained. In 34 sources, morethan or equal to 10 spots were detected. The observed spatial scale of the maserdistribution was from 9 to 4900 astronomical units, and the following morphologicalcategories were observed: elliptical, arched, linear, paired, and complex. The po-sition of the maser spot was determined to an accuracy of approximately 0.1 mas,sufficiently high to measure the internal proper motion from two years of monitoringobservations. The VLBI observation, however, detected only approximately 20% ofall maser emission, suggesting that the remaining 80% of the total flux was spreadinto an undetectable extended distribution. Therefore, in addition to high-resolutionobservations, it is important to observe the whole structure of the maser emission in-cluding extended low-brightness structures, to reveal the associated site of the maserand gas motion.
Key words: masers: methanol — Instrumentation: high angular resolution —Stars: formation — ISM: H II regions
1. Introduction
Although high-mass star formation has been intensively studied, it remains poorly un-derstood because of the large distance and high obscuration of the high-mass star-formingregions and short duration of critical evolutionary phases (Zinnecker & Yorke 2007, and ref-erences therein). Star forming regions are associated with maser emissions of high brightnesstemperature, and high transparency in the radio band, which are suitable for probing youngstellar objects (YSOs). Maser emissions are particularly useful for tracing circumstellar gasmotions close to the central star. The 6.7 GHz methanol maser transition, which is the bright-est among the methanol masers, is observed only from high-mass star-forming regions (e.g.,Menten 1991; Caswell et al. 1995; Minier et al. 2003; Xu et al. 2008) and considered one of thebest tracers of gas dynamics around high-mass YSOs.Some 6.7 GHz methanol masers show linearly elongated morphology with a linear ve-locity gradient (Norris et al. 1993; Phillips et al. 1998; Walsh et al. 1998; Minier et al. 2000).2hese masers can be interpreted as circumstellar disks viewed edge-on. Bartkiewicz et al.(2009) analyzed samples surveyed by very-long-baseline interferometry (VLBI) imaging usingthe European VLBI Network (EVN). They found elliptical morphology in 30% of the sam-pled methanol masers and deduced that this morphology arises from inclined rotating diskswith expansion or infall motions. In fact, rotational motions consistent with circumstellardisks have been identified as internal proper motions in a few 6.7 GHz methanol maser sources(G 16.59 − − v = 1 − µ mand SiO thermal line emissions, which are shock diagnostic. According to these authors, sucha parallel distribution suggests that methanol masers are directly associated with outflows.Supporting this inference, outward proper motions have been found in a few 6.7 GHz methanolmaser sources (Rygl et al. 2010; Sugiyama et al. 2011; Matsumoto et al. 2011; Sawada-Satohet al. 2013). Pandian et al. (2011) observed linear/arched morphology in only nine out of50 sources with the Multi-Element Radio-Linked Interferometric Network (MERLIN) and theKarl G. Jansky Very Large Array (JVLA), and they did not detect any source with a clearelliptical morphology, in contrast to the results of Bartkiewicz et al. (2009).If masers are to be used for studying high-mass star formation, the origin of the 6.7 GHzmethanol maser must be elucidated. This can be achieved with VLBI monitoring of numerousunbiased sources, from which the spatial distributions and three-dimensional velocity field (ra-dial velocity and proper motions in RA and Dec) of the maser can be statistically investigated.To date, VLBI images of 6.7 GHz methanol masers have been reported for approximately60 sources (e.g., Minier et al. 2000; Dodson et al. 2004; Sugiyama et al. 2008; Bartkiewicz et al.2009), while the masers have been detected in more than 900 high-mass star-forming regions(Pestalozzi et al. 2005, and references therein; Ellingsen 2007; Pandian et al. 2007; Xu et al.2009b; Caswell et al. 2010, 2011; Green et al. 2010, 2012). As mentioned above, the internalproper motion of the 6.7 GHz methanol maser has been measured in only a fraction of cases.Therefore, we have started a VLBI monitoring project of the 6.7 GHz methanol maser sourceswith the East-Asian VLBI Network (EAVN) to systematically investigate their internal propermotions. This study presents the initial results of this project, namely, the spatial distributionsof the 6.7 GHz methanol maser spots.Section 2 describes the criteria for target source selection and provides details of the ob-servations and data reduction methods. Section 3 presents the EAVN images, while individualsources are discussed in Section 4. Section 5 focuses on the spatial morphology and feasibility3f measuring the internal proper motion. Throughout this paper, sources are named by theirGalactic coordinates, expressed in the form xxx.xx + xx.xx following the IAU recommendationfor nomenclature, unless the source has been previously named (e.g., G 9.621+0.196).
2. Observations and Data Reduction
The target sources were selected from the methanol maser catalog of Pestalozzi et al.(2005) and the Methanol Multibeam Survey catalog (Caswell et al. 2010; Green et al. 2010)using the following criteria: 1) source declination δ > − ◦ , 2) catalogued peak flux density F p >
65 Jy, and 3) no previous VLBI observation. These criteria were satisfied by 34 sources.Two additional 6.7 GHz methanol maser sources 031.28+00.06 and 049.49 − δ < ◦ ). Observations were conducted with EAVN (Shen et al. 2004), which consists of the fol-lowing three VLBI networks: the Japanese VLBI Network (JVN; Doi et al. 2006), KoreanVLBI Network (KVN; Minh et al. 2003), and Chinese VLBI Network (CVN; Ye et al. 1991).Due to the location of the EAVN stations at latitudes below 40 ◦ N, the facility is suitable toobserving sources in the southern hemisphere. There are three main frequency bands of EAVNobservations: 6.7, 8 and 22 GHz. The two Japanese telescopes Yamaguchi and Hitachi par-ticipating in this project are described in Fujisawa et al. (2002) and Yonekura et al. (2013),respectively.
The first epoch observations of this monitoring project were conducted during six sessionsbetween 2010 and 2011. Table 2 lists the observational parameters of each session, including thedate, time, and participating telescopes. The location of the telescopes is shown in Figure 1.The projected baselines were from 6 M λ (Yamaguchi–Iriki) to 50 M λ (Mizusawa–Ishigaki)corresponding to fringe spacings of 34.4 mas and 4.1 mas, respectively, at 6.7 GHz. The typicalsize of the minor axis of the synthesized beam was 5 mas, although it varied depending on theuv-coverage.The continuum sources 3C454.3 and NRAO530 were used as the fringe finder and band-pass calibrator, respectively. Continuum sources located adjacent to target maser sources,41700 − − − − − − − and channelspacing of 0.178 km s − .The data were reduced using the Astronomical Image Processing System (AIPS; Greisen2003). Correlator digitization errors were corrected using the task ACCOR. The clock andclock-rate offsets were corrected and bandpass was calibrated using the strong continuum cal-ibrators. Next, the delay was calibrated and Doppler corrections were performed. Amplitudecalibration parameters were derived from the total-power spectra of maser lines using the tem-plate method in the task ACFIT. Template spectra at or near the VLBI observation date wereobtained from single-dish observations of each target source with the Yamaguchi 32 m tele-scope. Fringe fitting was performed using one spectral channel of the strongest maser feature,followed by self-calibration. The fringe-fitted solutions were poor for some sources at longer-baseline because the maser components of these sources, including the strongest component,were heavily resolved out. At the amplitude calibration stage using the ACFIT, we flaggedantennas for which calibration failed. Following calibration, uniformly weighted channel mapswere made every 0.178 km s − , and maser components were searched for within the imagecubes. Maser components are considered real if detected with signal-to-noise ratio (SNR) ≥ ≥ able 1. Summary of the 6.7 GHz methanol maser sources observed by VLBI
No. G-Name IRAS Coordinates (J2000) V lsr F p D Ref.R.A. Dec.( h m s ) ( ◦ ′ ′′ ) (km s − ) (Jy) (kpc)1 000.54 − − −
28 54 31.1 11.8 68 7.2 cas102 000.64 − − −
28 24 25.0 49.1 69 7.9 † cas103 002.53+00.19 17476 − −
26 39 45.3 3.1 88 4.2 cas104 006.18 − −
23 47 10.8 − − − −
23 12 34.9 16.3 91.07 3.8 gre106 008.68 − − −
21 37 10.2 43.2 102.0 4.5 gre107 008.83 − − −
21 19 25.1 − − −
20 31 44.3 5.5 70.00 5.2 † gre109 009.98 − − −
20 18 56.5 42.2 67.58 12.0 gre1010 010.32 − − −
20 05 07.8 11.5 90.05 2.39 gre1011 011.49 − − −
19 41 27.1 6.6 68.40 1.6 gre1012 011.90 − − −
18 41 28.6 42.9 64.89 4.0 gre1013 012.02 − − −
18 31 55.7 108.3 96.26 11.1 gre1014 012.68 − −
18 01 46.6 57.5 544.0 2.40 † imm1315 012.88+00.48 18089 − −
17 31 29.6 39.3 68.88 2.34 † gre1016 014.10+00.08 18128 − −
16 39 09.4 15.4 87.26 5.4 gre1017 020.23+00.06 18249 − −
11 14 54.2 71.8 77 4.4 cas0918 023.43 − − −
08 31 38.5 103 45 5.9 † cas0919 025.65+01.05 18316 − −
05 59 40.5 41.9 178 12.5 xu0920 025.71+00.04 18353 − −
06 24 15.0 92.8 364 11.8 xu0921 025.82 − − −
06 24 09.5 91.2 70 5.0 xu0922 028.83 − − −
03 45 48.5 83.5 73 4.6 cyg0923 029.86 − −
02 45 04.4 101.4 67 9.3 xu0924 030.70 − − −
02 01 05 88 87 5.9 xu0925 030.76 − − −
01 57 22.0 92 68 4.8 xu0926 030.91+00.14 18448 − −
01 44 07 104 95.2 5.6 xu0927 031.28+00.06 18456 − −
01 26 22.6 110 71 5.8 xu0928 032.03+00.06 18470 − −
00 45 47 92.8 93 7.2 xu0929 037.40+01.52 18517+0437 18 54 10.5 +04 40 49 41.1 279 2.1 xu0930 049.49 − † xu0931 232.62+00.99 07299 − −
16 58 12.4 23 162 1.68 † cas0932 351.77 − − −
36 09 17.6 1.3 231 0.4 cas1033 352.63 − − −
35 44 08.7 − − − −
34 41 45.6 − − −
33 13 55.1 − − −
29 28 16.0 − Column 1: ID number; Columns 2, 3: Galactic and IRAS names (if any), respectively; Columns 4, 5: Absolute coordinates(referenced in Column 9); Columns 6, and 7: Radial velocity and peak flux density of the brightest maser feature, respec-tively; Column 8: Source distance (referenced in section 4); Column 9: Absolute coordinate reference.Reference – cas09: Caswell (2009); cyg09: Cyganowski et al. (2009); xu09: Xu et al. (2009b); cas10: Caswell et al. (2010);gre10: Green et al. (2010); imm13: Immer et al. (2013). † : distances determined by trigonometric parallax. able 2. Summary of the first epoch of EAVN observations
Session Date Time Telescopes ∗ (y/m/d) (UT)1 2010/08/28 07:00-16:00 M, R, O, I, H, S2 2010/08/29 07:00-28:00 M, R, O, I, H, S3 2010/08/30 08:00-17:00 M, R, O, I, H4 2011/10/27 03:00-10:00 M, R, O, I, Y, H5 2011/10/28 03:00-10:00 M, R, O, I, Y, H, S6 2011/11/26 01:30-09:00 M, R, O, I, Y, U, H, S ∗ : M: Mizusawa, R: Iriki, O: Ogasawara, I: Ishigaki, Y: Yamaguchi,U: Usuda, H: Hitachi, S: Shanghai25. ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ Shanghai(S)Ishigaki(I) Iriki(R)Yamaguchi(Y) Usuda(U)Hitachi(H)Mizusawa(M)Ogasawara(O)
Longitude L a tit ud e Fig. 1.
The location of the participating telescopes. . Results The spatial distributions of the 6.7 GHz methanol maser spots were successfully obtainedfor all sources except 014.10+00.08, whose fringe was detected only in the Mizusawa − Hitachibaseline. Among the 35 VLBI images, 33 were obtained for the first time. This study hasincreased the number of reported VLBI images of 6.7 GHz methanol masers by a factor of 1.5.The VLBI images of the 35 sources in addition to their spectra are shown in Figures 2-36. In the source spectra, the solid line and hatched box represent the total- (autocorrelationof the Hitachi 32 m data) and cross-power spectra (integrated over all baselines), respectively.The spot sizes in the VLBI images indicate the peak intensity of the spots on the logarithmicscale. Radial velocities are indicated by the color index of the color bar displayed to the rightof each figure. The origin of each map is the absolute source coordinates listed in Table 3. Thespatial scale bar is displayed at the bottom corner of each figure.Besides the coordinates, Table 3 lists the following observational parameters: the radialvelocity and flux density of the peak maser spot, number of detected maser spots, spatialscale (in right ascension and declination coordinates), velocity range of the detected spectrum,morphological type, and integrated flux ratio of the correlated to the total-power spectrumof each source. As mentioned in the following subsection, we consider three sources containtwo separated star-forming regions in each image, yielding 38 imaged sources. The number ofdetected spots in each source varies from 3 to 72, with 34 sources (89%) displaying larger thanor equal to 10 spots. The spatial scale of the maser distribution is from 9 to 4900 astronomicalunits (AU).Following Bartkiewicz et al. (2009), we classified the observed morphologies into thefollowing five types: Elliptical, Arched, Linear, Paired, and Complex, as shown in Table 3.Since the classification was performed by eye, it is not strict, but provides an indication of themorphological structure. In the classification process, we only used the spatial distribution ofthe spots, the velocity distribution was not considered. The classification process is as follows:First, we recognized that almost all spots form small clusters, where we define a cluster here asa spot group including one or more spots, and the size is roughly one tenth of the total extent.When only two clusters exist in a source, the source is classified as
Paired . When three or moreclusters exist and all of them are successively and linearly distributed, the source is
Linear .When three or more clusters exist and all are distributed successively, but loosely curved ( < Arched . When four or more clusters exist and all are applicable to anellipse, the source is
Elliptical . The remaining sources are classified as
Complex .
4. Comments on individual sources − The main cluster (SE) of the 6.7 GHz methanol maser spots of this source are distributed8 able 3.
Observed parameters
No. Source Session Coordinates (J2000.0) V p F p N s Scale V range Spatial F VLBI /F RA Dec Morph.( h m s ) ( ◦ ′ ′′ ) (km s − ) (Jy) (AU ) (km s − ) (%)1 000.54 − −
28 54 28.9 8.7 0.5 3 9 × − −
28 54 31.4 13.3 63.2 55 3900 × − −
28 24 25.3 49.6 6.1 17 1300 ×
760 [48.2, 52.4] C 113 002.53+00.19 † × − −
23 47 10.8 − × − − − −
23 12 34.2 26.1 24.2 72 1400 ×
640 [15.0, 30.8] E 366 008.68 − −
21 37 10.4 43.1 21.1 25 970 ×
590 [40.7, 44.4] C 97 008.83 − −
21 19 25.4 − ×
990 [ − −
20 31 43.4 5.5 8.3 31 490 ×
120 [4.9, 6.9] L 229 009.98 − −
20 18 56.5 42.4 18.9 58 3000 × − −
20 05 07.8 11.6 21.1 22 490 ×
620 [4.1, 14.2] C 1911 011.49 − −
19 41 27.2 6.3 28.8 68 290 ×
710 [4.5, 17.1] C 4212 011.90 − −
18 41 28.8 43.1 38.3 29 1300 ×
480 [39.6, 44.2] P 4113 012.02 − −
18 31 55.9 108.3 22.8 25 520 × − † ×
650 [52.0, 60.1] C 115 012.88+00.48 5 18 11 51.39 −
17 31 30.1 39.2 18.0 67 2600 × †
517 020.23+00.06 SW 5 18 27 44.56 −
11 14 54.1 71.5 3.2 12 160 ×
280 [71.1, 73.6] P 14020.23+00.06 NE 18 27 44.95 −
11 14 47.8 60.9 1.1 14 100 ×
10 [60.2, 71.1] A 2418 023.43 − −
08 31 25.3 96.6 5.0 10 110 ×
110 [96.3, 98.4] C 7023.43 − −
08 31 39.3 103.0 8.1 28 1600 ×
550 [101.4, 107.9] P 619 025.65+01.05 ∗ ×
20 [41.5, 42.2] L 1520 025.71+00.04 1 18 38 03.15 −
06 24 15.0 95.5 34.1 13 930 × − −
06 24 09.9 91.6 8.2 53 1500 × − ∗ × − ∗ × − ∗ × − ∗ ×
20 [90.5, 92.6] P 2926 030.91+00.14 ∗ ×
320 [100.1, 103.0] L 227 031.28+00.06 ∗ × ∗ × ∗ ×
20 [41.0, 41.5] L 430 049.49 − × −
16 58 12.4 22.9 64.4 11 40 ×
120 [21.9, 23.4] P 2932 351.77 − −
36 09 17.6 1.7 4.4 18 40 ×
40 [ − − −
35 44 08.5 − ×
260 [-5.6, -2.0] L 5834 353.41 − −
34 41 45.6 − ×
460 [ − − −
33 13 55.0 − × − − − † − ×
370 [-52.4, -46.6] P 6
Column 1 – ID number (as listed in Table 1); Column 2 – Source name; Column 3 – Observational session (corresponding to thesessions listed in Table 2); Columns 4, 5 – Absolute coordinates obtained by fringe-rate mapping (for specified sources marked byasterisk or dagger, the absolute coordinates listed in Table 1 are listed here); Columns 6, 7 – Radial velocity and flux density ofthe peak maser spot located at the origin of each map, respectively; Column 8 – Number of detected maser spots; Columns 9, and10 – Spatial scale (in RA and Dec coordinates), and velocity range of the detected component, respectively; Column 11 – Spatialmorphology (E: Elliptical; A: Arched; L: Linear; P: Paired; C: Complex). Column 12 – Ratio of integrated fluxes of correlated-to total-power spectra. ∗ : sources not suitable for fringe-rate mapping because of the equatorial location. † : sources not suitable for fringe-rate mapping because of the weak flux or resolved-out.
000 AURA offset (mas) D ec o ff s e t ( m a s ) D ec o ff s e t ( m a s )
10 AU F l ux d e n s it y [ J y ] cross-powertotal-power NW SE NWE C SE
LSR Velocity [km s -1 ] Fig. 2.
The 6.7 GHz methanol maser emissions of source 000.54 − Upper-left-panel : Total-power(solid line) and cross-power (hatched box) spectra.
Lower- and upper-right-panels : Spatial distributionsof the methanol maser spots obtained from the EAVN observations. Detail are provided in Section 4. -10 0 10 20 30 40 40 50 60 70 80 F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] 500 AURA offset (mas) D ec o ff s e t ( m a s ) C Fig. 3.
Data for source 000.64 − F l ux d e n s it y [ J y ] cross-powertotal-power
500 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] E Fig. 4.
Data for source 002.53+00.19, plotted as for Figure 2. D ec o ff s e t ( m a s )
500 AU F l ux d e n s it y [ J y ] cross-powertotal-power RA offset (mas)
LSR Velocity [km s -1 ] C Fig. 5.
Data for source 006.18 −
10 0 10 20 30 40 50 60 10 15 20 25 30 35 F l ux d e n s it y [ J y ] cross-powertotal-power
500 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] E Fig. 6.
Data for source 006.79 −
500 AU D ec o ff s e t ( m a s ) RA offset (mas) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] C Fig. 7.
Data for source 008.68 −
500 AU D ec o ff s e t ( m a s ) RA offset (mas) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] E Fig. 8.
Data for source 008.83 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) cross-powertotal-power LSR Velocity [km s -1 ] F l ux d e n s it y [ J y ] L Fig. 9.
Data for source 009.61+00.19, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 10.
Data for source 009.98 − F l ux d e n s it y [ J y ] cross-powertotal-power
300 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 11.
Data for source 010.32 − F l ux d e n s it y [ J y ] cross-powertotal-power
200 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 12.
Data for source 011.49 − F l ux d e n s it y [ J y ] cross-powertotal-power
500 AU RA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] P Fig. 13.
Data for source 011.90 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] A Fig. 14.
Data for source 012.02 − RA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power
200 AU
LSR Velocity [km s -1 ] C Fig. 15.
Data for source 012.68 −
000 AURA offset (mas) D ec o ff s e t ( m a s )
200 AU RA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] C Fig. 16.
Data for source 012.88+00.48, plotted as for Figure 2. over 540 mas ×
630 mas, corresponding to 3900 AU × I self-absorption (H I SA) spectrum.Isolated spots (NW) located at 3 arcsec (2 × AU) northwest are probably associated withanother excitation source. The distribution of the maser spots of the main cluster is
Elliptical ,having roughly circular morphology, while that of NW component is
Complex . These maserspots correspond to spots labeled A − L in the ATCA image (IRAS 17470 − − This source is associated with a famous high-mass star-forming region in the Galacticcenter Sgr B2 located at a trigonometric parallax distance of 7.9 kpc (Reid et al. 2009). Thewide absorption trough in the spectrum is remarkable (Menten 1991). The methanol maserspots in this source show
Complex spatial distribution over 170 mas ×
100 mas, correspondingto 1300 AU ×
760 AU at the source (Figure 3). These maser spots are extended roughlyalong the east-west direction, although the extension is not clearly linear. The ATCA image(IRAS 17441 − + The kinematic distance of this source is 4.2 kpc. Although the north-western part is16 D ec o ff s e t ( m a s )
200 AURA offset (mas) D ec o ff s e t ( m a s )
100 AURA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power NE SW AP LSR Velocity [km s -1 ] NESW
Fig. 17.
Data for source 020.23+00.06, plotted as for Figure 2. missing, the spatial distribution is
Elliptical (Figure 4). The maser spots are distributed over180 mas ×
500 mas, corresponding to a linear area of 750 AU × − This source locates at the same line of sight toward the W 28 supernova remnant field.The 6.7 GHz methanol maser source seems to be associated with one of several dense molecularcores previously identified in this region, namely, Core 3 (Nicholas et al. 2011). The radialvelocity ( −
33 km s − ) of Core 3 approximates that of the spectral peak of the methanol maserbut differs largely from those of other molecular cores. Dame & Thaddeus (2008) suggestedthat this source locates in the galactic 3 kpc arm approximately 5 kpc from the Sun. Therefore,Core 3 (and hence the methanol maser source) is likely not connected to the gas associatedwith the W 28 field (Nicholas et al. 2011). We assumed a source distance of 5.1 kpc (Green& McClure-Griffiths 2011). The morphology is Complex (Figure 5). The maser spots are17
000 AU
200 AU D ec o ff s e t ( m a s ) RA offset (mas) F l ux d e n s it y [ J y ] cross-powertotal-power MM 1MM 2 CP D ec o ff s e t ( m a s ) RA offset (mas) . . . . . . MM 1MM 2LSR Velocity [km s -1 ] Fig. 18.
Data for source 023.43 − F l ux d e n s it y [ J y ] cross-powertotal-power
50 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] L Fig. 19.
Data for source 025.65+01.05, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 20.
Data for source 025.71+00.04, plotted as for Figure 2.
500 AU RA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] E Fig. 21.
Data for source 025.82 − F l ux d e n s it y [ J y ] cross-powertotal-power RA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] A Fig. 22.
Data for source 028.83 − F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] A Fig. 23.
Data for source 029.86 − F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] P Fig. 24.
Data for source 030.70 − F l ux d e n s it y [ J y ] cross-powertotal-power
100 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] P Fig. 25.
Data for source 030.76 − RA offset (mas)
200 AU D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] L Fig. 26.
Data for source 030.91+00.14, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 27.
Data for source 031.28+00.06, plotted as for Figure 2. D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] P Fig. 28.
Data for source 032.03+00.06, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power
50 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] L Fig. 29.
Data for source 037.40+01.52, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 30.
Data for source 049.49 − F l ux d e n s it y [ J y ] cross-powertotal-power
50 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] P Fig. 31.
Data for source 232.62+00.99, plotted as for Figure 2. F l ux d e n s it y [ J y ] cross-powertotal-power
30 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] E Fig. 32.
Data for source 351.77 − F l ux d e n s it y [ J y ] cross-powertotal-power
200 AURA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] L Fig. 33.
Data for source 352.63 − distributed over 280 mas ×
220 mas, corresponding to 1400 AU × − This source also locates near the supernova remnant W 28 similar to 006.18 − − of the ammonia line of Core 1 indicates a kinematic distance of 3.8 kpc, althoughthe uncertainty is large. The spatial distribution of the masers is clearly Elliptical , with aclockwise radial velocity gradient (Figure 6). The size of this ellipse is 400 mas from east towest, corresponding to 1400 AU. − This source locates at a distance of 4.5 kpc (Green & McClure-Griffiths 2011). Thespatial distribution of the masers is
Complex (Figure 7). The maser spots are distributed over220 mas ×
130 mas, corresponding to 970 AU ×
590 AU. These maser spots coincide well withthose labeled A-D in the ATCA image (IRAS 18032 − F l ux d e n s it y [ J y ] cross-powertotal-power
200 AU
RA offset (mas) D ec o ff s e t ( m a s ) LSR Velocity [km s -1 ] C Fig. 34.
Data for source 353.41 −
500 AU
RA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] C Fig. 35.
Data for source 354.61+00.47, plotted as for Figure 2.
000 AURA offset (mas) D ec o ff s e t ( m a s ) F l ux d e n s it y [ J y ] cross-powertotal-power LSR Velocity [km s -1 ] P Fig. 36.
Data for source 359.43 − methanol masers are located at the center of the continuum emission and the foot-point ofthe CO ( J = 2 −
1) outflow (where the CO ( J = 2 −
1) and 1.2 mm continuum images wereobtained by the SMA; Longmore et al. 2011). The CO (1 −
0) and CO (2 −
1) line profilesshow prominent infall signatures (Ren et al. 2012). The methanol maser is associated with theweaker of two submillimeter continuum cores known to exist in this region (Walsh et al. 2003). − This source locates at a distance of 5.2 kpc (Green & McClure-Griffiths 2011). Themorphology is
Elliptical but displays no simple velocity gradient (Figure 8). The maser spotsare distributed over 270 mas ×
190 mas, corresponding to 1400 AU ×
990 AU.
Trigonometric parallax measurements established the distance of this source as 5.2 kpc(Sanna et al. 2009). Various H II regions at different evolutionary phases exist in this region,labeled A − E by Garay et al. (1993). Source 009.61+00.19 is associated with D, the ultra-compact H II region. The other component E, located 10 ′′ north of D, is a candidate hyper-compact H II region (Kurtz & Franco 2002), which is well known as the strongest 6.7 GHzmethanol maser, G 9.621+0.196. Within this complex region, we imaged only 009.61+00.19,although the strongest emission in the peak total-power spectrum arises from G 9.621+0.196.The EAVN image reveals a Linear distribution of the methanol maser spots, oriented towardthe east-west direction with a linear velocity gradient (Figure 9). The scale distributed over100 mas ×
20 mas, corresponding to 490 AU ×
120 AU at the source. Only two spots appearin the ATCA image (IRAS 18032 − − This source locates at a distance of 12.0 kpc (Green & McClure-Griffiths 2011). The6.7 GHz methanol maser spots show a
Complex spatial distribution with no obvious trend inthe velocity distribution (Figure 10). The scale coverage is 250 mas ×
130 mas, correspondingto 3000 AU × − F inthe ATCA image (IRAS 18048 − lsr = 49 . − .
68 km s − , located at (∆ α, ∆ δ ) = (+130, +45) and (+180, −
60) mas from thebrightest spot at the origin of the image. The flux density of these new spots is 1.5 Jy, whichis similar to that observed in the single-dish spectrum obtained by Parkes 64 m (Green et al.2010). − The distance of this source is assumed as 2.39 kpc, the measured distance of W31 North(Mois´es et al. 2011), with which it is associated. It should be noted that this source distanceis disputed among the literatures (Green & McClure-Griffiths 2011, references therein). The6.7 GHz methanol maser spots show
Complex spatial distribution over 200 mas ×
260 mas,corresponding to 490 AU ×
620 AU at the source (Figure 11). The maser spots clus-tered in the northern part of the image correspond to those labeled E − H in the ATCAimage (IRAS 18060 − V lsr =4 . − .
95 km s − , located 220 mas south from the main cluster. The origin of the imageis not coincide with the position of the strongest component, but a relatively strong, compactcomponent at V lsr = 6 . − is selected for the origin of the image. − This source displays
Complex spatial and velocity distributions of its methanol masers(Figure 12). The overall distribution is elongated along the north-south direction over180 mas ×
440 mas, corresponding to 290 AU ×
710 AU at a kinematic distance of 1.6 kpc.These maser spots coincide with those labeled A − M in the ATCA image (IRAS 18134 − V lsr = 6 . − .
76 and 6.93 − − are detectedat (∆ α, ∆ δ ) = (+18, +409) and ( −
40, +360) mas from the brightest spot, respectively. − The observed maser spots of this source display a
Paired distribution (Figure 13), di-viding into two spatially discrete clusters separated by 340 mas, corresponding to 1300 AU ata near kinematic distance of 4.0 kpc (Green & McClure-Griffiths 2011). − This source locates at a distance of 11.1 kpc (Green & McClure-Griffiths 2011). Themaser spots show an
Arched spatial distribution over 50 mas ×
110 mas, corresponding to520 AU × − − This source, associated with W33B, is located at 2.40 kpc, determined from trigonomet-ric parallax measurements of 22 GHz water masers (Immer et al. 2013). Although the maserspots are roughly distributed along the northwest-southeast direction, we classified the morphol-ogy of this source as
Complex (Figure 15). The masers are distributed over 130 mas ×
270 mas,corresponding to 310 AU ×
650 AU. The associated submillimeter continuum emission is ex-27ended along the northeast-southwest direction, perpendicular to the methanol maser distribu-tion (Walsh et al. 2003).
This source locates at the trigonometric parallax distance of 2.34 kpc (Xu et al. 2011).The flux of its methanol maser is known to show a periodic variation with a period of 29.5 days(Goedhart et al. 2009). The 6.7 GHz methanol maser spots are distributed as a main clusterand two separated clusters, constituting
Complex morphology (Figure 16). The main cluster isseen in the ATCA image (IRAS 18089 − − E, H, and J. The clusterlocated at 1.5 arcsec in the northeast direction corresponds to spot F in the ATCA image.While spot G in the ATCA image is absent, EAVN detected new maser spots at 1.9 arcsecnortheast from the main cluster, whose V lsr = 32 . − .
88 km s − . Additional new maserspots, with V lsr = 34 . − .
99 km s − , appear around the origin. The masers are distributedover 1100 mas × × This source locates at a distance of 5.4 kpc (Green & McClure-Griffiths 2011).As mentioned in Section 3, 014.10+00.08 was detected only by a short baseline betweenMizusawa − Hitachi. Therefore, an image of this source is not made.
This source locates at a kinematic distance of 4.4 kpc (Green & McClure-Griffiths2011). Two isolated maser clusters are separated by 8.5 arcsec (equivalently, by 3 . × AU,Figure 17). The maser spots in each cluster are probably excited by different high-mass YSOs.In the southwestern maser cluster (SW), two isolated clusters are separated by 70 mas (320 AU)at the source. Therefore, the morphology of this cluster is
Paired . The velocities of maser spotsin the northeastern maser cluster (NE) vary widely (by 11 km s − ), although the spatial cov-erage is only 100 AU. The morphology of this cluster is Arched . − Trigonometric parallax measurements of 12 GHz methanol masers identified this sourceat 5.9 kpc (Brunthaler et al. 2009). The masers form two clusters separated by approximately14 arcsec, corresponding to 8 . × AU (Figure 18). Two millimeter dust continuum coreshave been reported to the north (MM1) and south (MM2) of this region (Ren et al. 2011). Eachmethanol maser cluster is associated with each mm dust core, suggesting that these separatedclusters locate in different high-mass star-forming regions. Both maser clusters appear in theATCA image (IRAS 18319 − − F and J − L in the ATCA image, respectively. Themorphology of the northern cluster MM1 is
Complex . The maser spots are distributed over18 mas ×
18 mas, corresponding to 110 AU ×
110 AU. The southern cluster MM2 is
Paired .The masers in this cluster are distributed over 280 mas ×
90 mas (1600 AU ×
550 AU).
Linear (Figure 19). The masers are distributed over7 mas × ×
20 AU. The radial velocity width is quite narrow(less than 1 km s − ). The ATCA image is elongated approximately 1 arcsec in the north-southdirection (IRAS 18316 − This source locates at a kinematic distance of 11.8 kpc (Pandian et al. 2008). Themorphology is
Complex (Figure 20). The maser spots are distributed over 80 mas ×
130 mas,corresponding to 930 AU × − − This source locates at a kinematic distance of 5.0 kpc (Green & McClure-Griffiths2011). The 6.7 GHz methanol maser spots are distributed in an
Elliptical morphology witha clockwise velocity gradient (Figure 21). The scale is 300 mas ×
300 mas, corresponding to1500 AU × − − This source locates at a kinematic distance of 4.6 kpc (Green & McClure-Griffiths2011). The spatial distribution is
Arched (Figure 22). The maser spots are distributed over370 mas ×
440 mas, corresponding to 1700 AU × − − The target source 029.86 − ∼ II regions designatedG 029.95 − − I SA and the formaldehyde absorption line spec-trum, the distances of these H II regions have been measured as 9.3 and 9.2 kpc, respectively(Anderson & Bania 2009; Downes et al. 1980). Thus, we assume a distance of 9.3 kpc for source029.86 − Arched distribution accompaniedby a clear velocity gradient (Figure 23). The scale of the distribution is 150 mas ×
340 mas,corresponding to 1400 AU × − This source locates in a star-forming region known as the W43 main complex (Motteet al. 2003). The masers are associated with the millimeter dust continuum MM2 (Motteet al. 2003), one of the most massive and luminous cores in the complex. Thus, we assumethat 030.70 − Paired (Figure 24). The maser spots are distributed over 690 mas ×
420 mas,corresponding to 4100 AU × − The kinematic distance of this source is 4.8 kpc (Anderson & Bania 2009). The 6.7 GHzmethanol maser spots form two distinct clusters isolated by 34 mas, corresponding to 160 AU;therefore, the morphology of this source is
Paired (Figure 25). The maser spots coincide withthose labeled D − F in the ATCA image (IRAS 18450 − This source locates at a kinematic distance of 5.6 kpc (Anderson & Bania 2009). Themorphology is
Linear with a clear continuous velocity gradient (Figure 26). The maser spotsare distributed over 30 mas ×
60 mas, corresponding to 150 AU ×
320 AU, and they areassociated with submillimeter continuum emission (Hill et al. 2005).
The distance of this source is 5.8 kpc (Anderson & Bania 2009). The 6.7 GHzmethanol maser spots form a
Complex distribution over 560 mas ×
470 mas, corresponding to3300 AU × − V lsr = 109 .
42 and109.87 km s − in the EVN image is absent in the EAVN image, while new maser spots appearat V lsr = 104 .
34 and 104.52 km s − , locating at (∆ α, ∆ δ ) = ( −
68, +215) from the brightestspot.
From the NIR extinction-distance relationship, the distance of this source has beenestimated as 7.2 kpc (Stead & Hoare 2010). The morphology of this source is
Paired (Figure 28).The maser spots are distributed over 290 mas ×
430 mas, corresponding to 2100 AU × µ m extended emissionobserved by Rathborne et al. (2006) and Chambers et al. (2009). The kinematic distance of this source is 2.1 kpc (Fontani et al. 2011). The morphologyis
Linear with a clear velocity gradient (Figure 29). The maser spots are distributed over30 mas ×
10 mas, corresponding to 60 AU ×
20 AU. The linear spatial distribution, the size,and the velocity gradient are similar to those of NGC 7538 IRS1 with a linear size of 90 AU(Minier et al. 1998). − This source locates in the well-known complex high-mass star-forming region W51. Fromtwo reported trigonometric parallax distances, namely 5.1 +2 . − . (Xu et al. 2009a) and 5.41 +0 . − . (Sato et al. 2010), we assign a distance of 5.41 kpc to 049.49 − − Complex morphology over 370 mas ×
310 mas, correspondingto 2000 AU × × . The southern spots detected by MERLIN were absent in the EAVN image. Thedistribution of maser spots of this source was also obtained by EVN (Phillips & van Langevelde2005; Surcis et al. 2012) and found to be consistent with that in the EAVN image. This source locates at a distance of 1.68 kpc, estimated from trigonometric parallaxmeasurements of the 12 GHz methanol maser (Reid et al. 2009). The morphology of this sourceis
Paired (Figure 31). The maser spots are distributed over 20 mas ×
70 mas, correspondingto 40 AU ×
120 AU. These spots coincide with the spots labeled B and C in the ATCA image(IRAS 07299 − − The distance of this source is 0.4 kpc with a large uncertainty (Green & McClure-Griffiths 2011). The morphology of this source is
Elliptical (although the south-western sectionis missing in the image) with a counter-clockwise velocity gradient (Figure 32). The maser spotsare distributed over 100 mas ×
100 mas (40 AU ×
40 AU) and coincide with those labeled A − Din the ATCA image (IRAS 17233 − CO bipolar outflowand OH maser outflow have been reported in this region (Leurini et al. 2008). − This source locates at a kinematic distance of 0.9 kpc, with a large uncertainty (Green& McClure-Griffiths 2011). The morphology is
Linear , but the velocity distribution is complex(Figure 33). The maser spots are distributed over 370 mas ×
290 mas, corresponding to330 AU ×
260 AU. These maser spots coincide with those labeled D and F in the ATCA image(IRAS 17278 − α , ∆ δ ) = ( − − − The kinematic distance of this source is 3.8 kpc (Caswell et al. 2011). The morphologyis
Complex (Figure 34). The maser spots are distributed over 60 mas ×
120 mas, correspond-ing to 220 AU ×
460 AU, and they coincide with those labeled A − C in the ATCA image(IRAS 17271 − CCH lines have also been detected in this region (Miettinen et al.2006).
This source locates at a kinematic distance of 3.8 kpc (Green & McClure-Griffiths 2011).31he morphology is
Complex , formed by three clusters (Figure 35). The scale of the maserdistribution is 380 mas ×
440 mas, corresponding to 1500 AU × This maser source locates at a kinematic distance of 8.2 kpc, and the apparent position isapproximately 1 arcmin east of Sgr C (Fish et al. 2003). The morphology is
Paired (Figure 36).The maser spots are distributed over 140 mas ×
50 mas, corresponding to 1200 AU ×
370 AU.
5. Discussions
The distributions of the 6.7 GHz methanol maser spots vary widely in size and structuresimilar to the results of previous studies by Bartkiewicz et al. (2009) and Pandian et al.(2011). The whole size of the spatial distribution is from 9 to 4900 AU. The 6.7 GHz methanolmaser is excited by infrared radiation when its dust temperature is 100 −
200 K (Cragg et al.2005). The spatial scale of the maser distribution can be estimated from the luminosity of theemitting source. Assuming a luminosity from 10 to 10 L ⊙ and a suitable dust temperatureof 100 K, the maser is predicted to appear at 500 − − to 10 L ⊙ because the separation of their masers exceeds 10 AU, indicating that each sourcelikely involves at least two exciting sources. In the following discussion, we assume that theselargely separated clusters associate with individual exciting sources, i.e., these are independentsources. Hence we use 38 as the number of the imaged sources, hereafter.The spatial scale of the maser distribution is plotted as a function of observed radialvelocity range in Figure 37. The spatial scale of each source is defined as the largest extent ofspot distribution in the celestial sphere, while the velocity range is the difference between thesmallest and largest velocities of the maser spots detected by VLBI (see Table 3). Open circlesdenote sources with
Linear morphology, and the filled circles represent other morphologies(
Elliptical , Arched , Paired , and
Complex ). A positive correlation is observed between the spatialscale and radial velocity range, as previously reported by Pandian et al. (2011). Note, however,that sources of
Linear morphology are the main contributors to this correlation. The spatialdistribution of all
Linearly distributed sources is below 600 AU, and their radial velocity rangeis smaller than 4 km s − , below those of other sources. If the Linearly distributed sourcesare excluded from Figure 37, the correlation between size and radial velocity range becomes32 -1 S p a ti a l s ca l e [ AU ] Velocity range [km s -1 ] Fig. 37.
Spatial scale of maser distribution as a function of velocity range. The open circles indicatesources with
Linear morphology (see text for detail). much weaker. This tendency toward small radial velocity range and spatial extent in sources of
Linear distribution has been similarly seen in the result by Pandian et al. (2011). As discussedin the following subsection, we consider that sources of such restricted size and velocity rangeare the observable parts of larger structures that are not completely revealed.
Table 4 summarizes the number of sources satisfying each morphology, as absolute val-ues and as fractions of the total number of sources. For comparison, 27 sources observed byEVN and classified into the five types are also listed (these have been previously reported inBartkiewicz et al. 2009). The number of
Elliptical sources, which is expected to trace the gasdisk around YSOs, is six (16%), small as compared with that (33%) in Bartkiewicz et al. (2009).Our samples contain relatively high numbers of
Paired (21%) and
Complex (39%) distributions,which contain no apparent systematic spatial distribution or radial velocity structures.Since structures of low intensity are not detected by VLBI, sources with
Paired or Complex morphology may in fact possess low-brightness
Elliptical , or other systematic struc-tures. The ratio of the integrated flux of CLEAN components to the total flux is listed inTable 3. To investigate the fraction of maser radiation detectable by VLBI, a histogram ofthese ratios is presented in Figure 38. The detectability is from 1% to 58%, averaging around20%. Similarly, Bartkiewicz et al. (2009) noted that, for most sources, 10 −
30% of the total fluxwas detected by EVN. Our result suggests that 80% of the maser emissions spread spatiallyinto a diffuse structure. Distribution of total-power versus correlated flux is shown in Figure 39with different symbols for each morphology. This is for testing if the detectability differs withthe morphology, but no clear tendency of distribution is seen for different morphology.Minier et al. (2002) mentioned that some of the 6.7 GHz methanol maser emissionconsists of compact core and spreading halos with sizes of a few (tens) astronomical units and afew hundred or larger astronomical units, respectively. Pandian et al. (2011) also reported that33ome maser spots possess no compact core. The spatial distribution of such dispersed maseremissions cannot be determined by VLBI observation. Therefore, as discussed for the
Linearly distributed sources, we consider that the exact spatial distributions or morphological classes ofmaser emissions are difficult to deduce from VLBI observations alone.When interpreting the proper motion of the maser spot and its associated site, anynon-observed parts must be identified. To avoid the resolved-out problem, the diffuse structureshould be observed. Assuming that the emission component spreads into a halo with a sizeof 300 AU and the distance to the source is 3 kpc, the angular size of the spread emission is0.1 arcsec. To detect this structure, an interferometric observation of spatial resolution approx-imately ≥ . × . . The observed images, whichcontain the whole emission of the 6.7 GHz masers, will assist in VLBI image interpretationand obtaining the internal proper motion of the maser spots. The ATCA results will be pub-lished elsewhere. High-resolution observations by ALMA will be important for determining thegas and dust distributions within the sources. In a future study, these distributions could becompared with the distribution of the masers. Table 4.
Spatial morphologies of EAVN and EVN samples.
Array Spatial MorphologyElliptical ∗ Arched Linear Paired ComplexEAVN 6 4 5 8 15(percentage) (16) (11) (13) (21) (39)EVN 9 3 5 1 9(percentage) (33) (11) (19) (4) (33)
The EVN data are taken from Bartkiewicz et al. (2009).Note that 27 sources, classified into the five morphologies,are included in this table. ∗ “Elliptical” is defined as “Ring” in Bartkiewicz et al.(2009). Despite being limited by the resolved-out problem, VLBI detects 1 −
58% of the maseremission, implying that the emission is concentrated in sufficiently compact spots. The numberof detected spots in each source is from 3 to 72, with an average of 29. In 34 of the sources,the spot number is larger than or equal to 10, which is sufficient for investigating the three-dimensional velocity field. The relatively long lifetime of the 6.7 GHz maser spot (Goedhartet al. 2004, Ellingsen 2007) has enabled us to observe spots at three epochs over 2 years.The relative position of the maser spot in each map was measured at an accuracy higherthan 0.1 mas, although in practice, the accuracy depends on the signal-to-noise ratio of each34 N u m b e r o f s ou r ce s Ratio of Flux [%]
Fig. 38.
Histogram of the flux ratio of the correlated to the total-power spectrum. Bins on the horizontalaxis are separated by 10%. -1 I n t e g r a t e d c r o ss - po w e r [ J y k m s - ] Integrated total-power [Jy km s -1 ] Fig. 39.
Distribution of total-power versus correlated flux. Different symbols denote each morphology:The filled circles indicate sources with
Elliptical morphology, open circles are
Linear , filled triangles are
Arched , open triangles are
Paired , cross are
Complex , respectively. component. Three repeats of this monitoring observation, planned over two years, will allowus to determine the internal proper motion to an accuracy of 1 σ = 0 .
03 mas yr − . At atypical distance of 3 kpc, this accuracy of proper motion corresponds to a tangential velocity of1.5 km s − . Since the average radial velocity width of the maser is 6.5 km s − , the internal propermotion is detectable at sufficiently high signal-to-noise ratio. In fact, from these previous JVNobservations, we measured the internal proper motions of the spots for four sources (Matsumotoet al. 2011; Sugiyama et al. 2011, 2013; Sawada-Satoh et al. 2013).35 . Conclusion To study the associated site of the 6.7 GHz methanol maser and the gas dynamicsaround high-mass YSOs, we have monitored the internal proper motion of the maser. For thesepurposes, 36 selected sources were studied by multi-epoch VLBI observations using EAVN.We present 35 VLBI images successfully obtained from the first epoch observation. Threesources contain two separated star-forming regions in each image, yielding 38 imaged sources.The distribution of the detected maser spots was from 9 to 4900 AU and displayed a range ofmorphologies. The flux detected by VLBI was 1 −
58% of the total flux, suggesting that a largefraction of the radiation is dispersed into an extended structure invisible to VLBI. To investigatethe associated site and motion of the maser, shorter baselines are required to recover thedistribution of this extended emission. Of the 38 imaged sources, we detected 10 or more spotsin 34. The accuracy of the spot position was approximately 0.1 mas. Therefore, the internalproper motions could be measured with sufficient accuracy following 2 years of monitoringobservation. From these results, we can statistically investigate the three-dimensional velocityfield around high-mass YSOs. Since most of the observed sources are located in the southernhemisphere, they can be observed with the Atacama Large Millimeter/Submillimeter Array(ALMA) in future.The authors wish to thank the JVN team for observational assistance and support.The JVN project is led by the National Astronomical Observatory of Japan (NAOJ) thatis a branch of the National Institutes of Natural Sciences (NINS), Hokkaido University,Ibaraki University, The University of Tsukuba, Gifu University, Osaka Prefecture University,Yamaguchi University, and Kagoshima University, in cooperation with Geospatial InformationAuthority of Japan (GSI), the Japan Aerospace Exploration Agency (JAXA), and the NationalInstitute of Information and Communications Technology (NICT). This work was financiallysupported in part by Grant-in-Aid for Scientific Research (KAKENHI) from the Japan Societyfor the Promotion of Science (JSPS), No. 24340034. This work is partly supported byChina Ministry of Science and Technology under State Key Development Program for BasicResearch (2012CB821800), the National Natural Science Foundation of China (grants 10625314,11121062, and 11173046), the CAS/SAFEA International Partnership Program for CreativeResearch Teams, and the Strategic Priority Research Program on Space Science, the ChineseAcademy of Sciences (Grant No. XDA04060700).
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J., et al. 2011, ApJ, 733, 25Ye, Shuhua, Wan, Tongshan, Qian, Zhihan Radio interferometry: Theory, techniques, andapplications: Proceedings of the 131st IAU Colloquium, Socorro, NM, Oct. 8-12, 1990 (A92-5637624-89). San Francisco, CA, Astronomical Society of the Pacific, 1991, p. 386-389.Yonekura, Y., Saito, Y., Saito, T., et al. 2013, ASP Conf. Ser. 476: New Trends in Radio Astronomyin the ALMA Era: The 30th Anniversary of Nobeyama Radio Observatory (eds. R. Kawabe, N.Kuno, S. Yamamoto), 415Zinnecker, H., & Yorke, H. W. 2007, ARA&A, 45, 481 ppendix. Spatial Distribution in Unified Scale VLBI images of 38 sources are shown in the figures 40-77 in the same spatial scale (5310AU in each RA and Dec) for all sources. These figures correspond to figures 2-36. A 500 AUscale bar and the spectrum are shown in the corner of each figure.
500 AU F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power C RA offset (mas) D ec o ff s e t ( m a s ) Fig. 40.
The 6.7 GHz methanol maser emissions of source 000.54 − Inset : Total-power (solid line) and cross-power (hatched box) spectra.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) E F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 41.
Data for source 000.54 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C -10 0 10 20 30 40 40 50 60 70 80 F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 42.
Data for source 000.64 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) E F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 43.
Data for source 002.53+00.19, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 44.
Data for source 006.18 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) E -10 0 10 20 30 40 50 60 10 15 20 25 30 35 F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 45.
Data for source 006.79 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 46.
Data for source 008.68 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) E F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 47.
Data for source 008.83 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) L F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 48.
Data for source 009.61+00.19, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 49.
Data for source 009.98 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 50.
Data for source 010.32 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 51.
Data for source 011.49 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 52.
Data for source 011.90 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) A F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 53.
Data for source 012.02 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 54.
Data for source 012.68 −
00 AU RA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 55.
Data for source 012.88+00.48, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 56.
Data for source 020.23+00.06 SW, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) A F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 57.
Data for source 020.23+00.06 NE, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 58.
Data for source 023.43 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 59.
Data for source 023.43 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) L F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 60.
Data for source 025.65+01.05, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 61.
Data for source 025.71+00.04, plotted as for Figure 40.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) E F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 62.
Data for source 025.82 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) A F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 63.
Data for source 028.83 −
00 AU RA offset (mas) D ec o ff s e t ( m a s ) A F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 64.
Data for source 029.86 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 65.
Data for source 030.70 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 66.
Data for source 030.76 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) L F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 67.
Data for source 030.91+00.14, plotted as for Figure 40.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 68.
Data for source 031.28+00.06, plotted as for Figure 40.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 69.
Data for source 032.03+00.06, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) L F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 70.
Data for source 037.40+01.52, plotted as for Figure 40.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 71.
Data for source 049.49 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 72.
Data for source 232.62+00.99, plotted as for Figure 40.
00 AURA offset (mas) D ec o ff s e t ( m a s ) E F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 73.
Data for source 351.77 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) L F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 74.
Data for source 352.63 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 75.
Data for source 353.41 −
00 AURA offset (mas) D ec o ff s e t ( m a s ) C F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 76.
Data for source 354.61+00.47, plotted as for Figure 40.
00 AU RA offset (mas) D ec o ff s e t ( m a s ) P F l ux d e n s it y [ J y ] LSR Velocity [km s -1 ]cross-powertotal-power Fig. 77.
Data for source 359.43 −00.10, plotted as for Figure 40.