map of the 6.7 ghz class ii methanol maser emission of the protoplanetary disk g23.01-0.41
TRANSCRIPT
ISSN 1063-7729, Astronomy Reports, 2011, Vol. 55, No. 5, pp. 445–455. c© Pleiades Publishing, Ltd., 2011.Original Russian Text c© S.V. Polushkin, I.E. Val’tts, 2011, published in Astronomicheskii Zhurnal, 2011, Vol. 88, No. 5, pp. 484–495.
Map of the 6.7 GHz Class II Methanol Maser Emissionof the Protoplanetary Disk G23.01–0.41
S. V. Polushkin and I. E. Val’ttsAstro Space Center, Lebedev Physical institute, Russian Academy of Sciences,
Moscow, RussiaReceived September 15, 2010; in final form, October 22, 2010
Abstract—We present images of the star-forming region G23.01–0.41 at 6.7 GHz in the Class II methanolmaser transition 51−60A
+, produced from archival observations on the European VLBI Network. Ourmap of the source and its maser spots contains 24 maser components. The data for each spot—absolutecoordinates, coordinates relative to the calibration feature, peak flux and flux integrated over the spot,size, position angle, velocity along the line of sight, and line full width at half-maximum—are collectedin tabular form. The spatial region occupied by the maser spots is approximated by a 200×130 milliarcsecellipse in position angle PA = −0.40◦, centered on the absolute coordinates α0 = 18h34m40.282s, δ0 =−09◦00′38.27′′ (J2000). If the source is a protoplanetary disk, then, for the distance estimate derived fromtrigonometric parallax, its diameter is 1800 AU, and the mass of the central protostar is 23.5 M�.
DOI: 10.1134/S1063772911040068
1. INTRODUCTION
The isolated star-forming region G23.01–0.41 inthe small constellation Scutum, which is located inthe direction of the Galactic center, is available for ob-servations in both the Northern and Southern hemi-spheres. It is located at the edge of a diffuse HIIzone [1] and does not have an optical identification.The source’s maser emission was first observed byCaswell and Haynes [1] in the main OH lines onthe 64-m Parkes radio telescope during a systematicsurvey of the Galactic plane. H2O maser emissionwas also detected 1.1′ from the OH maser by Forsterand Caswell [2], who derived the coordinates of bothmasers using VLA observations.
The methanol maser was discovered by KarlMenten [3] at 6.7 GHz in the Class II 51−60A
+
line using the 43-m Green Bank telescope, andwas later observed with the Parkes telescope in theClass I 70−61A
+ line at 44 GHz [4] and Class IItransitions at 6.7 GHz (51−60A
+) [5] and 12 GHz(20−3−1E) [6]. The source also has methanolemission at 95 GHz (Class I maser, 80−71A
+
transition [7]), 157 GHz (five thermal lines, Class IIJ0−J−1E transitions [8]) and 107 GHz (weak maserand a thermal line, Class II 31−40A
+ transition [9]).Weak, short-period variability was observed si-
multaneously in the three strongest features of thespectrum at 6.7 GHz (at 74.19, 74.75, and75.09 km/s) between January 1999 and March 2003[10].
The source is not identified with any IRAS source.Emission in the thermal lines of CS and NH3,
which trace dense gas, was observed in the sur-vey [11]. This source was later selected for a moredetailed study in the NH3 lines due to evidence forthe presence of H2O maser emission, the absence ofcontinuum emission, and the strong color excess inthe near IR [12]. It was shown in [12] that the masersource G23.01–0.41 is in a very early evolutionarystage, before the appearance of an ultracompact HIIzone around a massive star embedded in a gas–dustenvelope.
The data on this source are summarized in thecatalog of Class I methanol masers available athttp://www.asc.rssi.ru/MMI [13].
The most complete study of this source is that ofFuruya et al. [14]. Based on observations in molec-ular lines and the continuum at 3 mm (dust emis-sion), they showed that G23.01–0.41 is a compact(≤0.1 pc), massive (∼400 M�), hot (∼100 K), densemolecular core with an associated bipolar outflow.
The most accurate estimate of the distance tothe source is provided in [15], and was derived fromtrigonometric parallax based on Very Long BaselineArray observations of the CLass II methanol maserat 12.2 GHz: 4.59+0.38
−0.33 kpc. This is substantiallysmaller than the value of 10.7 kpc, which has usuallybeen assumed for this source, and is lower than thekinematic estimate obtained for the standard modelof the Milky Way.
445
446 POLUSHKIN, VAL’TTS
The Class II maser G23.01–0.41 was includedin the European VLBI Network (EVN) projectEM061E at 6.7 GHz, observed in 20071 . Theseobservational data are now available via open accessat the EVN archive at http://www.evlbi.org/. Wedescribe our processing and interpretation of thesedata here.
The aim of the present study is to investigate thefine structure of the maser source, and to compareand systematize the observational data on this star-forming region in both the continuum and in thermaland maser lines.
2. DATA, DATA REDUCTION,AND RESULTS
2.1. Observations
The observations of the maser G23.01–0.41 werecarried out over 6 h on March 17, 2007 (3:30–9:30) at the rest-frame frequency of the 51−60A
+
transition at 6.66678195 GHz, pointing at the co-ordinates RA = 18h34m40.39s, Dec = −09◦00′38.5′′(J2000). Four quasars were observed for the cali-bration: J1751+0939, J1825−0737, J2101+0341,and J2253+1608. Eight antennas located in Medic-ina (Italy), Hartebeesthoek (South Africa), Torun(Poland), Westerbork (the Netherlands), Effelsberg(Germany), Jodrell Bank (United Kingdom), Cam-bridge (United Kingdom), and Noto (Italy) partici-pated in the observations. Auto-correlation spectrafor each antenna are presented in Fig. 1. Themaximum distance between antennas (Jodrell Bankand Hartebeesthoek) was 8441 km, corresponding toan angular resolution of 0.001′′. The spectrometerwas a 1024-channel autocorrelator with a full widthof 2 MHz, i.e., 100 km/s. The frequency spectralresolution was 1.9531 KHz, i.e., 0.0878 km/s invelocity.
2.2. Data Processing
The archival data were contained in nine files inFITS format. They were distributed together withdata from an initial calibration carried out using a setof standard procedures and numerous auxiliary fileswith comments on the initial calibration and plots,collected together in a folder labeled PIPELINE.
We carried out the data processing in the Astro-nomical Image Processing System package (AIPS,NRAO, USA; see the AIPS Cookbook at thewww.nrao.edu website), the DS9-code for the visual-ization of FITS files and binary tables (see description
1 The EVN is a collaboration of European, Chinese, SouthAfrican and other astronomical institutes sponsored by na-tional scientific councils.
below), and the MATLAB package. The AIPSpackage is designed for the interactive compilationof radio-interferometry data and the calibration,construction, and analysis of astronomical imagesobtained from these data via Fourier-transform pro-cedures.
The data were read into the AIPS package usingthe task FITLD, specifying that the input FITS filesshould be merged. This produces the AIPS data fileG2301.UVDATA.
To link the calibration tables, we read in theem061e.tasav file from the PIPELINE folder. The setof pipeline calibration tables included the following:
AN1—containing the names and coordinates ofthe antennas;
CL1—initial calibration table for all the data, cre-ated during the input of data into AIPS;
SN1—solution given by the task APCAL (part ofthe amplitude calibration procedure);
CL2—a priori amplitude calibration data (CL2 =CL1 + SN1);
FG1—flags that exclude the edges of the fre-quency range and data obtained when the telescopewas on source;
SN2—solution given by the task FRING for theinitial phase calibration using the four calibrators;
CL3—complete amplitude and phase calibration(CL3 = CL2 + SN2);
BP1—bandpass calibration data for the frequencyrange obtained using the calibrators J1751+0939,J2253+1608, and J2101+0341.
We used the CL3, FG1, and BP1 tables. Thesewere copied to the data file G2301.UVDATA usingthe task TACOP.
After inspecting the spectrum using the taskPOSSM, a map of the 200 channels in the mainpart of the spectrum (channels 401 to 600) was con-structed, applying the calibration tables (using thetask IMAGR). In principle, this procedure is sufficientto determine the absolute coordinates and sizes of themaser features relative to the calibration sources. Forthe channel with the maximum intensity, we obtainedRA = 18h34m40.28932s, Dec = −09◦00′38.15128′′(J2000).
To improve the signal-to-noise ratio, which isnecessary to determine the properties of the weakestfeatures, it is more correct to reference the data to thebrightest features of the maser itself, i.e., to use self-calibration. Self-calibration applied to determine theparameters of the weak features is usually performedusing the brightest feature in the spectrum, whichis initially modeled as a point source. The brightestfeature is located in channel 534. However, analysisof the maps showed that the feature in this channelis blended, has a large angular size, so that it cannot
ASTRONOMY REPORTS Vol. 55 No. 5 2011
PROTOPLANETARY DISK G23.01–0.41 447
2.2
1.8
1.4
1.0
CM
116
12
8
4
EF
2
0
1.25
1.15
WB
3
1.45
1.05
NT
6
1.35
1.25
1.15
1.0
MC
5
1.4
1.2
1.1
JB
41.5
1.3
1.1
TR
71.3
1.2
400 450 500 550 600
1.1
HH
8
1.5
1.3
400 450 500 550 600Channel number
Fig. 1. Uncalibrated auto-correlation spectra of G23.01–0.41 obtained on the EVN on May 20, 2010 at 14:54:37 UT at6.7 GHz. The name of the antenna is listed in the upper left corner of each plot. Y axis is represented in arbitrary units.
be considered a point source and therefore cannot beused for self-calibration. We instead used the datain channel 559 for the self-calibration; this channelis sufficiently bright, and has only one feature with asmall angular size.
Using the task SPLIT, the data for the maser wereseparated from the data for the calibrators in the fileG2301.UVDATA; in this step, the amplitudes andphases were calibrated using the CL3 table, and theoutput was written in the file G2301.SPLIT. We alsowrote the data for channel 559 into a separate fileusing SPLIT.
The sequence of self-calibration procedures fol-lowed the scheme suggested at the ERIS 2009 school(http://astrowiki.physics.ox.ac.uk/ERIS2009). First,the image for channel 559 (file 559.SPLIT) was con-structed using the task IMAGR, calibrating relativeto the reference quasar. Next, the file 559.SPLITwas used together with the obtained image to self-calibrate these data using the task CALIB, whichformed a calibration table SN in the file 559.SPLITand created a new calibrated file 559.CALIB. Usingthis file, a new image of channel 559 was constructedusing IMAGR and so on, gradually decreasing thesolution intervals and increasing the number of“clean” components used for the self-calibration.
During this procedure, the quality of the imageimproves with each iteration. We carried out thephase calibration three times. In the first step,one “clean” component was taken and the intervalbetween solutions in the SN table (solution interval)was equal to four. In the second step, the firstten “clean” components were used and the intervalbetween solutions was two. In the third step, the firstten “clean” components were used, and the intervalbetween solutions was 1.3. Next, joint amplitudeand phase calibrations were carried out twice: first,using the first ten “clean” components, then thefirst 20, with the interval between solutions in thetable SN being 12 and 6, respectively. After allthe iterations, all the solutions were written into amaster SN table using the task CALIB, executedusing the initial data file and final image file. Figure 2presents images of channel 559 before and after self-calibration. The final SN table was copied to theinitial file G2301.SPLIT using the task TACOP andimages of all the maser features were constructedusing IMAGR, carrying out 500 iterations with a512 × 512 image and a pixel size of 1 milliarcsecond(mas).
For each maser feature, the maps from chan-nels corresponding to this feature were combined intogroups using the task SQUASH. The coordinates
ASTRONOMY REPORTS Vol. 55 No. 5 2011
448 POLUSHKIN, VAL’TTS
40.28040.29040.30018
h
34
m
40.310
s
40.28540.29540.305
–09
°
00
′
37.90
′′
38.35
37.95
38.00
38.05
38.10
38.15
38.20
38.25
38.30
Dec
(J2
000)
R.A. (J2000)
40.28040.29040.30018
h
34
m
40.310
s
40.28540.29540.305
–09
°
00
′
37.90
′′
38.35
37.95
38.00
38.05
38.10
38.15
38.20
38.25
38.30
Dec
(J2
000)
R.A. (J2000)
Fig. 2. Image of the reference channel before and after self-calibration for the same noise level.
and sizes of the groups, i.e., of the maser spots, weredetermined using the task SAD.
We analyzed the resulting parameters using theDS9 code developed at the Smithsonian Astro-physical Observatory (Cambridge, USA; http://hea-www.harvard.edu/RD/ds9/), which enables inspec-tion of the maps for every channel, and determinationof the coordinates and fluxes of the maser features.This code is available for Solaris, Linux, MacOSX,and Windows, and was loaded as a single executablefile and did not require additional installation. All ver-sions have a graphical interface that can be adoptedfor the user. More detailed information is given in [16].
We obtained the velocities and fluxes in individualchannels and then approximated them with Gaus-sians using the utility cftool from MATLAB.
2.3. Results
Our results are presented as spectra (Figs. 1–5)and maps (Figs. 6–8), and are summarized in Table 1.The single-dish spectrum obtained by the Parkestelescope [5] has two brightest features with simi-lar radial velocities, with fluxes of ∼400 Jy (VLSR =75.3 km/s) and ∼340 Jy (VLSR = 74.9 km/s) at theline peak. The brightest feature in the EVN spectrum
ASTRONOMY REPORTS Vol. 55 No. 5 2011
PROTOPLANETARY DISK G23.01–0.41 449
85807570Velocity, km/s
200
100
0
Flu
x, Jy
Fig. 3. Cross-correlation spectrum of the maser G23.01–0.41 on the short Jodrell Bank–Cambridge baseline.
1009080706050Velocity, km/s
150
100
50
0
Flu
x, Jy
Fig. 4. Complete cross-correlation spectrum of the maser G23.01–0.41 obtained using all eight EVN antennas. See thecaption to Fig. 7 for an explanation of the parts of the spectrum represented by squares and circles.
has a lower radial velocity. We assigned a line-of-sight velocity of 75 km/s to the strongest feature ofthe EVN spectrum (channel 534, task SETJY).
Figure 4 shows the full calibrated cross-correla-tion spectrum of the maser, with the line-of-sight ve-locities plotted along the horizontal axis. For clarity,
Fig. 5 presents the spectrum in terms of channels,since all processing in AIPS is done in channels.
The map contains 24 maser spots, each associatedwith several spectral channels containing emission atthe same coordinates and velocity. The columns inTable 1 give for each spot (1) the spot number in order
ASTRONOMY REPORTS Vol. 55 No. 5 2011
450 POLUSHKIN, VAL’TTS
400600800
150
100
50
0
Flu
x, Jy
Channel number
Fig. 5. The spectrum of G23.01–0.41 in terms of channels. See the caption to Fig. 7 for an explanation of the parts of thespectrum represented by squares and circles.
of right ascension, (2), (3) the absolute coordinatesfor J2000, (4), (5) the peak flux and integrated flux,(6), (7) the coordinates relative to the calibration spot,(8), (9) the size corrected for the beam pattern, (10)the position angle, (11), (12) the velocity with respectto the Local Standard of Rest (LSR) and the width ofthe spectral line.
Figure 6 shows a complete map of all 24 maserspots obtained with phase calibration using the ref-erence quasars and with additional self-calibrationusing channel 559 (velocity 72.81 km/s). These datawere combined by reducing the fluxes to the flux ofthe calibration component (the tasks SQASH andCOMB). A spatial separation of the spots corre-sponding to the separation of the spectral features(Fig. 4) is clearly seen.
Figure 7 shows the same map with the relativefluxes of the spots represented on a logarithmic scaleas circles and squares of various sizes proportional tothe logarithm of the flux. The velocities along the lineof sight are also indicated. The squares correspondto velocities from 70 to 77 km/s and the circles tovelocities from 79 to 81 km/s.
It is possible to approximate the arrangement ofsome of the spots (spots 2 to 15) by an ellipse usingtheir relative coordinates (columns 6 and 7 in Table 1).
We obtained the following parameters for thisellipse: a = 0.20′′, b = 0.13′′, PA = −0.40◦, x0 =0.20′′, y0 = −0.13′′ or α0 = 18h34m40.282s, δ0 =
−09◦00′38.27′′ (J2000), where a and b are semi-major and semi-minor axes of the ellipse, PA is itsposition angle, x0 and y0 are the relative coordinatesof the center of the ellipse, and α0 and δ0 are theabsolute coordinates of the center of the ellipse. Theresults of this fitting are presented in Fig. 8.
3. MODEL OF THE MASER REGION
According to the data presented in [17], methanolmaser sources have several morphological types. Asimilar classification was suggested earlier in [18, 19]:
simple—the masers emit in a narrow range ofvelocities (ΔV < 1 km/s) and have a spectrum witha single peak, with the sizes of maser spots from asingle group of channels being less than several mas;
linear—the maser spots form a line in the planeof the sky, whose angular extent can vary from 9 to54 mas, with a monotonic velocity gradient observedin some sources;
ring—a very common morphology in which themaser spots form an ellipse and emit in a fairly narrowrange of velocities (3–14 km/s), probably represent-ing a maser ring encircling a central star;
curved—the maser spots are located along a 70–220 mas, with the structure as a whole possibly dis-playing systemic velocity gradients;
complex—lacking any spatial and spectral struc-ture, with the maser-spot sizes varying from 30 ×20 (mas)2 to 350 × 200 (mas)2;
ASTRONOMY REPORTS Vol. 55 No. 5 2011
PROTOPLANETARY DISK G23.01–0.41 451
40.26540.27040.27540.28040.28540.29040.29540.30018
h
34
m
40.305
s
R.A. (J2000)
–09
°
00
′
37.9
′′
38.0
38.1
38.2
38.3
38.4
38.5
38.6
Dec
(J2
000)
Fig. 6. Full 6.7-GHz map of all 24 maser spots in G23.01–0.4 according to the EVN data with phase calibration based on thequasar reference sources and additional self-calibration using channel 559 (velocity 72.83 km/s). The fluxes of features werecombined with weights.
double—two groups of spots are separated by ∼1′′
with a velocity scatter of ≥10 km/s, with the semi-major axes of the individual groups being perpendic-ular to the line joining them.
In our opinion, G23.01–0.41 is a ring-type source.In the studies of Class II methanol masers presentedin [17], a large fraction of the sources (29%) displaythis structure. At the same time, the structure ofG23.01–0.41 is more complex: two outflows, or pos-sibly two parts of the spiral structure of the disk, areclearly visible. These outflows cannot be artefacts,since strong maser features are present in them.
Disks around young stars are considered to be acommon phenomenon [18–21]. They contain a largeamount of hot molecular gas, making the detectionof maser emission likely. If a methanol maser formsin a circumstellar (or “accretion” or “protoplane-tary” [18]) disk, and the disk is inclined to the line ofsight, its projection onto the plane of the sky will bean ellipse.
For our further processing of the data, we usedthe model with an inclined disk rotating around a
protostar or young star presented in [22]. We used thevelocities of the individual features to derive the rota-tional velocity of the disk Vrot, the rate of expansion(or compression) of the disk Vexp, and the velocity ofsystem as a whole with respect to the LSR Vsys via aminimization of the function χ2 from [22]:
χ2 =1
N − 3
N∑
k=1
1σ2
V
(Vk − Vsys
− xk
aVrot sin i − yk
aVexp tan i
)2
,
where σV is the spectral resolution, equal to0.0878 km/s, Vk is the velocity of the kth component,a and b are the values of the semi-major and semi-minor axes obtained when fitting the disk with an
ellipse, and i = arccos(
b
a
)is the angle between the
line of sight and the normal to the plane of the disk.Since the sign of i is not known, it is impossible todetermine the direction of rotation using this method.
ASTRONOMY REPORTS Vol. 55 No. 5 2011
452 POLUSHKIN, VAL’TTS
40.2740.2840.2918
h
34
m
40.30
s
R.A. (J2000)
–09 ° 00
′
37.8
′′
38.6
38.0
38.1
38.2
38.3
38.4
38.5
Dec
(J2
000)
24 (74.39)
22 (73.39) 18 (73.87)
10 (73.63)
13 (74.99)
17 (73.4)
14 (72.38)
16 (70.35)
19 (79.4)
21 (80.36)23 (80.72)
8 (79.0)
6 (82.4, 81.78)
2 (80.83)
5 (79.6)
4 (81.36)
3 (83.31)
1 (69.83)
7 (73.29)9 (74.9)
11 (73.91)
12 (75.75, 75.62)
15 (77.34, 76.8)20 (73.08, 72.83)
Fig. 7. One version of the map of the G23.01–0.41 methanol maser. The squares and circles show components with velocitiesin the ranges 70–77 km/s and 79–81 km/s. The numbers in parentheses give velocities along the line of sight (in km/s).
–0.3–0.2–0.100.1
Δα
, arcsec
0.2
0.1
0
–0.1
–0.2
–0.3
–0.4
–0.5
Δδ
, ar
csec
Fig. 8. Approximation of locations of 6.7 GHz maser spots in G23.01–0.4. See text for details.
ASTRONOMY REPORTS Vol. 55 No. 5 2011
PROTOPLANETARY DISK G23.01–0.41 453
Tabl
e1.
Par
amet
ers
ofm
etha
nolm
aser
spot
sin
G23
.01–
0.41
deri
ved
from
the
6.7
GH
zE
VN
data
No.
αδ
Pea
kflu
x,Jy
Inte
gra-
ted
flux,
JyΔ
αΔ
δE
llips
em
ajor
axis
Elli
pse
min
orax
isP
osit
ion
angl
eV
LS
R,
km/s
ΔV
,km
/s
12
34
56
78
910
1112
118
h34
m40
.273
01s
−09
◦ 00′
38.1
0693
′′1.
384
1.21
10.
3280
3′′
0.02
529′
′–
––
69.8
30.
55
218
3440
.273
48−
0900
38.3
6636
53.8
1051
.145
0.32
274
−0.
1801
6–
––
80.8
30.
40
318
3440
.273
37−
0900
38.3
1238
4.48
94.
244
0.32
101
−0.
2341
40.
0108
2′′
0.00
000′
′70
◦83
.31
0.45
418
3440
.274
18−
0900
38.3
4905
5.84
04.
991
0.31
070
−0.
2168
30.
0009
60.
0000
071
81.3
60.
43
518
3440
.274
36−
0900
38.3
6297
19.9
3018
.756
0.30
806
−0.
2307
50.
0001
10.
0000
068
79.6
0.43
618
3440
.276
76−
0900
38.4
0995
112.
283
118.
916
0.27
259
−0.
2777
30.
0019
60.
0003
870
82.4
,81.
780.
50,0
.43
718
3440
.281
68−
0900
38.0
6905
8.13
97.
844
0.19
968
0.06
317
0.00
095
0.00
000
6173
.29
0.43
818
3440
.282
02−
0900
38.4
6158
2.73
65.
829
0.19
465
−0.
3293
60.
0133
90.
0042
026
79.0
0.82
918
3440
.286
49−
0900
38.0
9921
154.
871
139.
901
0.12
831
0.03
301
––
–74
.90.
44
1018
3440
.288
05−
0900
38.1
2832
12.8
317
.732
0.10
525
0.00
390
0.00
419
0.00
379
8373
.63
0.72
1118
3440
.288
11−
0900
38.1
2818
11.3
3512
.681
0.10
442
0.00
404
0.00
000
0.00
186
073
.91
0.52
1218
3440
.288
73−
0900
38.1
7646
236.
628
511.
125
0.09
520
−0.
0442
40.
0113
00.
0046
111
75.7
5,75
.62
0.88
,0.6
2
1318
3440
.289
32−
0900
38.1
5128
237.
731
443.
813
0.08
652
−0.
0190
60.
0102
50.
0022
11
74.9
90.
84
1418
3440
.289
51−
0900
38.1
9487
1.43
41.
479
0.08
356
−0.
0626
50.
0026
70.
0000
08
72.3
80.
53
1518
3440
.289
64−
0900
38.1
7988
34.7
7249
.620
0.08
173
−0.
0476
60.
0049
70.
0000
092
77.3
4,76
.80.
66,0
.71
1618
3440
.289
74−
0900
38.3
6270
1.52
11.
576
0.08
024
−0.
2304
80.
0019
70.
0000
079
70.3
50.
46
1718
3440
.291
65−
0900
38.1
8594
8.69
88.
548
0.05
189
−0.
0537
20.
0007
00.
0000
028
73.4
0.47
1818
3440
.293
31−
0900
38.1
3380
9.53
014
.178
0.02
739
−0.
0015
80.
0085
40.
0017
761
73.8
70.
53
1918
3440
.293
32−
0900
38.4
6470
9.75
510
.441
0.02
713
−0.
3324
80.
0028
30.
0010
159
79.4
0.44
2018
3440
.295
16−
0900
38.1
3222
59.2
1873
.703
0.00
000
0.00
000
0.00
344
0.00
250
154
72.8
30.
44
2118
3440
.297
84−
0900
38.5
4661
2.18
52.
318
−0.
0398
5−
0.41
439
0.00
282
0.00
095
9780
.36
0.47
2218
3440
.298
05−
0900
38.0
8451
5.95
37.
024
−0.
0429
50.
0477
00.
0032
70.
0024
617
73.3
90.
40
2318
3440
.299
11−
0900
38.5
5415
13.8
9424
.177
−0.
0586
6−
0.42
193
0.00
697
0.00
498
1380
.72
0.47
2418
3440
.300
15−
0900
37.9
8450
64.9
0069
.463
−0.
0740
00.
1477
20.
0029
30.
0009
259
74.3
90.
43
Not
e:T
hepr
esen
ceof
ada
shin
colu
mns
(8)–
(10)
indi
cate
sth
atth
esi
zeof
the
met
hano
lspo
tis
muc
hle
ssth
ebe
am.
ASTRONOMY REPORTS Vol. 55 No. 5 2011
454 POLUSHKIN, VAL’TTS
Table 2. Parameters of the protoplanetary disk in theClass II 6.7 GHz methanol maser in G23.01–0.41
Parameter Value
Distance D 4.59 kpc
Disk radius R 900 AU
Angle between the line of sight and thedisk normal i
49◦
Velocity with respect to the LSR Vsys 77.8 km/s
Velocity of expansion (compression) Vexp −1.5 km/s
Rotational velocity Vrot 5.0 km/s
Mass of the protostar M 23.5 M�
The coordinates of the center of the ellipse xk andyk are derived from the relative coordinates Δαk andΔδk by rotating through the position angle PA andshifting by the vector (x0, y0)T:
⎛
⎝xk
yk
⎞
⎠
k �=0
=
⎛
⎝cos PA − sin PA
sin PA cos PA
⎞
⎠
×
⎛
⎝Δαk
Δδk
⎞
⎠ −
⎛
⎝x0
y0
⎞
⎠ .
We derived the values Vsys = 77.8 km/s, Vrot ∼5 km/s, Vexp = −1.5 km/s, i = 49◦, and χ2 = 360.
If we know the rotational velocity of the disk andassume that the rotation is Keplerian, we can calcu-late the mass of the central object (protostar) usingthe formula
M =RV 2
rot
G,
where R is the disk radius, Vrot the rotational velocity,and G the gravitational constant. Taking the dis-tance to G23.01–0.41 to be 4.59 kpc [15], we obtainR ≈ 900 AU and M ≈ 4.9 × 1034 g = 23.5 M�. Forclarity, the parameters of the protoplanetary disk aresummarized in Table 2.
According to [19], the mass of the central object inthe Keplerian disk model could be between 3 M� and120 M�; i.e., the derived parameters are consistentwith the model applied. Similar values for the sizesof disks and the masses of the central protostars werederived, for example, in [18–21].
A 3-mm continuum map of G23.01–0.41 waspresented in [14]. The location of the Class IImethanol maser at 6.7 GHz coincides with the maxi-mum emission at 3 mm. The observations of [14] werecarried out using the Nobeyama Radio Observatory
millimeter array, and have a resolution about threeorders of magnitude lower than the EVN observa-tions. On the scale of the Nobeyama resolution [14],our observations would look like a dot. Moreover, themass of the gas–dust cloud was estimated in [14] tobe Mdust = 380 M�, but the distance to the sourcewas taken to be 10.7 kpc. Correcting for the factthat the distance to G23.01–0.41 is 4.59 kpc [15], themass of the cloud is Mdust = 98 M�. Observationsof G23.01–0.41 in the CH3CN(6–5) thermal lineindicated that the velocity of the hot molecular cloudwith respect to the LSR is Vsys = 77.4 km/s [14],in agreement with the value Vsys = 77.8 km/s weobtained from the χ2 minimization; however, therotational velocity of the hot molecular nucleus,Vrot ∼ 0.6 km/s, differs from the value found by us,Vrot ∼ 5 km/s. This may be due to the fact that therotational velocity in [14] was derived for a regionthat is much larger than the region studied by us,and the rotation is not rigid-body. The rotationalvelocity of the protoplanetary disk can be estimatedas the difference between the velocities of two groupsof features in the spectrum. Such an estimate givesVrot � 6 km/s, in approximate agreement with thevalue obtained via the χ2 minimization.
In addition to the continuum and thermal lineemission, it would be very important to compare theClass II methanol emission generated in the disk tothe Class I methanol maser emission, which, as arule, is distributed over more extended regions (see,e.g., [23–25]). Though Class I methanol emissionis observed at both 44 GHz (70−61A
+) [4] and95 GHz [7] in G23.01–0.41, no maps of the Class Imethanol maser are available, hindering a comparisonof the spatial locations of the Class I and II maserfeatures.
4. CONCLUSION
Using 6.7 GHz EVN observations in the 51−60A+
Class II maser methanol transition, which were avail-able from the open EVN archive, we have obtaineda complete calibrated cross-correlation spectrum forG23.01–0.41 and mapped its maser emission.
The map contains 24 maser spots, each associatedwith several spectral channels in which emission atthe same coordinates and velocities is present.
For each spot, we obtained the absolute coordi-nates, peak flux, integrated flux, size, position angle,line-of-sight velocity with respect to the LSR, andspectral line width.
The arrangement of some of the maser spots canbe approximated by an ellipse. Morphologically, themaser emission is associated with a protoplanetarydisk in the shape of a ring; two outflows are present,
ASTRONOMY REPORTS Vol. 55 No. 5 2011
PROTOPLANETARY DISK G23.01–0.41 455
which may represent two parts of the spiral structurein the disk.
We found the size (diameter) of the disk to be1800 AU, and the mass of the central object to be23.5 M�.
The position of the methanol maser emitting at6.7 GHz coincides with the peak of the continuumemission at 3 mm [14], and the velocity of thehot molecular cloud obtained from observations ofthermal CH3CN(6–5) emission, Vsys = 77.4 km/s,agrees with the value Vsys = 77.8 km/s we obtainedby modeling the protoplanetary disk.
We found the rotational velocity of the protoplane-tary disk to be approximately 5 km/s.
ACKNOWLEDGMENTS
This study was partially supported by the RussianFoundation for Basic Research (project code 10-02-00147a), the Basic Research Program of the Divi-sion of Physical Sciences of the Russian Academy ofSciences “Active processes and stochastic structuresin the Universe” (project code OFN-16), the FederalProgram “Science and Education Human Resourcesof Innovation-Driven Russia” for 2009–2013 in theframework of program No. 1.1, “Scientific researchby the staff of science–education centers” and pro-gram 1.2.1, “Science studies lead by Doctors of Sci-ence” (project no. 16.740.11.0155. We thank thelecturers of the ERIS-2009 School A.M.S. Richardsand S. Bourk for consultations on data processingusing the AIPS package. We also thank the Euro-pean VLBI Network for providing open access to theEVN archival data.
REFERENCES1. J. L. Caswell and R. F. Haynes, Austral. J. Phys. 36,
417 (1983).2. J. R. Forster and J. L. Caswell, Astron. Astrophys.
213, 339 (1989).3. K. M. Menten, Astrophys. J. 380, L75 (1991).4. V. I. Slysh, S. V. Kalenskii, I. E. Val’tts, and
R. Otrupcek, Mon. Not. R. Astron. Soc. 268, 464(1994).
5. J. L. Caswell, R. A. Vaile, S. P. Ellingsen, et al., Mon.Not. R. Astron. Soc. 272, 96 (1995).
6. J. L. Caswell, R. A. Vaile, S. P. Ellingsen, andR. P. Norris, Mon. Not. R. Astron. Soc. 274, 1126(1995).
7. I. E. Val’tts, S. P. Ellingsen, V. I. Slysh, et al., Mon.Not. R. Astron. Soc. 317, 315 (2000).
8. V. I. Slysh, S. V. Kalenskii, I. E. Val’tts, et al., Astro-phys. J. Suppl. Ser. 123, 515 (1999).
9. J. L. Caswell, Yi Jiyune, R. S. Booth, andD. M. Cragg, Mon. Not. R. Astron. Soc. 313, 599(2000).
10. S. Goedhart, M. J. Gaylard, and D. J. van der Walt,Mon. Not. R. Astron. Soc. 355, 553 (2004).
11. G. Anglada, R. Estalella, J. Pastor, et al., Astrophys.J. 463, 205 (1996).
12. C. Codella, L. Testi, and R. Cesaroni, Astron. Astro-phys. 213, 339 (1997).
13. I. E. Val’tts, G. M. Larionov, and O. S. Bayandina,arXiv:1005.3715v3 [astro-ph.GA] (2010).
14. R. S. Furuya, R. Cesaroni, S. Takahashi, et al., As-trophys. J. 673, 363 (2008).
15. A. Brunthaler, M. J. Reid, K. M. Menten, et al.,Astrophys. J. 693, 424 (2009).
16. W. A. Joye and E. Mandel, in Astronomical Da-ta Analysis Software and Systems XII, Ed. byH. E. Payne, R. I. Jedrzejewski, and R. N. Hook, ASPConf. Ser. 489, 295 (2003).
17. A. Bartkiewicz, M. Szymczak, H. J. van Langevelde,et al., Astron. Astrophys. 213, 339 (1989).
18. R. P. Norris, S. E. Byleveld, P. J. Diamond, et al.,Astrophys. J. 508, 275 (1998).
19. C. J. Phillips, R. P. Norris, S. P. Ellingsen, andP. M. McCulloch, Mon. Not. R. Astron. Soc. 300,1131 (1998).
20. V. Minier, R. S. Booth, and J. E. Conway, Astron.Astrophys. 362, 1093 (2000).
21. V. Minier, J. E. Conway, and R. S. Booth, Astron.Astrophys. 369, 278 (2001).
22. L. Uscanga, Y. Gomez, A. C. Raga, et al., Mon. Not.R. Astron. Soc. 390, 1127 (2008).
23. L. Kogan and V. Slysh, Astrophys. J. 497, 800 (1998).24. V. I. Slysh, I. E. Val’tts, S. V. Kalenskii, and V. V. Gol-
ubev, Astron. Zh. 76, 892 (1999) [Astron. Rep. 43,785 (1999)].
25. S. V. Polushkin, I. E. Val’tts, and V. I. Slysh, Astron.Zh. 86, 134 (2009) [Astron. Rep. 53, 113 (2009)].
Translated by L. Yungel’son
ASTRONOMY REPORTS Vol. 55 No. 5 2011