1. INTRODUCTION 2. OBSERVATIONS

THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 123 : 487È513, 1999 August ( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE VSOP PRELAUNCH H O MASER SURVEY. I. VLBA OBSERVATIONS 2 VICTOR MIGENES1 AND SHINJI HORIUCHI VSOP Project, National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181, Japan

VYACHESLAV I. SLYSH, IRINA E. VALÏTTS, AND VLADIMIR V. GOLUBEV Astro Space Center, P. N. Lebedev Physical Institute, Profsoyuznaya 84/32, Moscow 117810, Russia

PHILIP G. EDWARDS, EDWARD B. FOMALONT,2 AND RIKAKO OKAYASU Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara-shi, Kanagawa 229, Japan

PHILIP J. DIAMOND NRAO, P.O. Box O, Socorro, NM 87801

AND TOMOHUMI UMEMOTO, KATSUNORI M. SHIBATA, AND MAKOTO INOUE VSOP Project, National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181, Japan Received 1997 May 14 ; accepted 1999 March 26

ABSTRACT We present the Ðrst results from an H O maser survey conducted with the VLBA for determining 2 We observed 60 sources in total : four late-type stars, 29 candidate sources for space VLBI missions. star-forming regions, and 27 H II regions. These are the Ðrst interferometric observations of any kind for 50% of the sources. Approximately 30% of the sources we observed exhibit highly compact structure and very strong emission, which make them ideal targets for space VLBI missions. Subject headings : H II regions È ISM : structure È masers È surveys È techniques : interferometric 1.

INTRODUCTION

space baselines, only sources that are very strong and compact will be detected. As a result a number of continuum and maser surveys have been undertaken in order to observe a large number of sources and to determine, based on the visibilities from the longest ground baselines, their potential for space VLBI. Moellenbrock et al. (1996) observed 142 compact extragalactic sources at 22 GHz, while maser surveys have been conducted by Kemball, Diamond, & Mantovani (1988) and Hansen et al. (1992). These latter surveys have concentrated on OH maser emission from star-forming regions and have observed only a few sources. In 1995 we started a series of VLBI observations of water vapor and hydroxyl masers toward star-forming regions, late-type stars, and megamasers with the longest possible ground baselines. These observations were conducted with the Mark IIIa recording system, and the results will be published elsewhere. In this paper we will concentrate on the more recent observations of H O maser sources with 2 the VLBA, during 1996 June and October, by the VSOP Survey Working Group (SWG).

The Institute of Space and Astronautical Science (ISAS) of Japan launched an 8 m radio telescope into Earth orbit on 1997 February 12. The VLBI Space Observatory Programme (VSOP) satellite HAL CA has been used successfully with ground telescopes over the past year to produce images of active galaxies and maser sources with unprecedented resolution at frequencies of 1.6, 5.0, and 22 GHz. Observing has been done with left circular polarization (LCP) only. For details on the current observing status of HAL CA, see Hirabayashi et al. (1998). Approximately 50% of the in-orbit time has been devoted to General Observing Time (GOT) proposals that have been submitted by the astronomical community. These observations have been supported by numerous ground stations and have produced high-quality images (which will be reported elsewhere). Of the remaining time, about 25% has been used for testing, calibration, and repointing of the satellite and 25% will be devoted to a survey program, conducted by a missionÈled Survey Working Group. During the Survey Program a large number of continuum sources and OH and H O maser sources will be observed with the satellite and 2 to four ground telescopes to study their general charthree acteristics on a large statistical base. A systematic and uniform survey of a large number of sources will be invaluable in determining the characteristics of the submilliarcsecond emission of maser sources. It will also allow the study of their ““ global ÏÏ high spatial resolution properties and the statistical signiÐcance of any trends to be assessed. However, because of the ultrahigh spatial resolution and the sensitivity limitations from the

2.

OBSERVATIONS

The H O maser sources were selected from published 2 Comoretto et al. (1990), Brand et al. (1992) surveys by (Arcetri astrophysics group), Forster & Caswell (1989), and suggestions from Takahiro Iwata (private communication) with the following criteria : 1. Low-mass protostars : sources with far-infrared luminosity less than 300 L , outÑow sources or molecular cores _ with H O maser emission with a peak Ñux greater than 2 15 Jy. 2. Massive star-forming regions : sources with farinfrared luminosity greater than 10,000 L and H O 2 masers with a peak Ñux greater than 120 Jy. _ 3. Evolved stars : H O masers with a peak Ñux greater 2 than 35 Jy.

1 Current address : University of Guanajuato, Department of Astronomy, Apdo. Postal 144, Guanajuato, CP36000, GTO, Mexico. 2 Permanent address : NRAO, 520 Edgemont Road, Charlottesville, VA 22903.

487

488

06053[0622 . . . 06055]2039 . . . 06058]2138 . . . 06061]2151 . . . 06117]1315 . . . 06300]6058 . . . 06446]0029 . . . 06501]0143 . . . 07006[0654 . . . 07207[1435 . . . 07423[2001 . . . 07427[2400 . . .

02232]6138 . . . 03258]3104 . . . 04579]4703 . . . 05137]3919 . . . 05168]3634 . . . 05274]3345 . . . 05302[0537 . . . 05329[0512 . . . 05358]3543 . . . 05413[0104 . . .

00420]5530 . . .

IRAS NAME

G240.31]0.07

AFGL 5142 Ori KL OMC-2 S231 G206.01[15.48 NGC 2071 Mon R2 S252-A G188.95]00.89 AFGL 5182 S269 IRC ]60169

W3-C W3(OH) NGC 1333 IRAS 1

OTHER NAME 00 02 02 03 04 05 05 05 05 05 05 05 05 06 06 06 06 06 06 06 06 07 07 07 07

42 21 23 25 57 13 16 27 32 32 35 41 44 05 05 05 06 11 30 44 50 00 20 42 42

05.85(0.01) 39.79(0.03) 17.3c 08.30(0.05) 57.113(0.004) 46.259(0.004) 53.6c 30.014(0.001) 47.0c 59.361(0.001) 51.1958(0.0006) 18.897(0.002) 30.622(0.003) 21.588(0.003) 36.649(0.001) 53.257(0.001) 06.462(0.006) 46.916(0.005) 00.53(0.01) 39.07(0.06) 09.646(0.005) 38.892(0.004) 44.417(0.002) 16.490(0.001) 45.069(0.001)

R.A. (1950.0) ]55 ]61 ]61 ]31 ]47 ]39 ]36 ]33 [05 [05 ]35 [01 ]00 [06 ]20 ]21 ]21 ]13 ]60 ]00 ]01 [06 [14 [20 [24 56.0(0.1) 41.2(0.3) 58c 47.2(1.0) 58.84(0.05) 06.55(0.10) 21.0c 40.11(0.02) 23.2c 52.60(0.06) 13.015(0.008) 09.4(0.2) 42.6(2.0) 27.78(0.05) 45.04(0.01) 02.90(0.01) 14.69(0.05) 34.44(0.05) 48.7(0.1) 28.7(1.5) 05.74(0.15) 58.5(0.5) 42.60(0.02) 13.43(0.02) 23.99(0.01)

DECL. (1950.0) 30 53 38 05 02 19 34 45 24 11 44 04 20 22 39 39 51 50 58 29 44 53 35 01 00

TABLE 1

[45.8 [41.0 [48.8 8.1 [20.2 [21.2 [21.0 [2.1 8.0 11.3 [15.0 2.5 15.0 11.3 16.0 8.3 [10.3 18.8 [32.3 44.8 52.0 22.4 53.9 50.9 80.2

RADIAL VELOCITY (km s~1) 0.8 0.8 15 7 7 6 1.7 18 26 20 1.7 5 15 12 8 10 4 7 6 7 5 1 1 1 10

VELOCITY WIDTHa (km s~1)

SUMMARY OF RESULTS FOR H O MASERS 2

4 1 5500 71 15 17 85 16 1450 18 45 35 13 250 18 46 100 75 6 140 1.5 1.7 32 12 6

50 Mj

\0.5 \0.2 5500d 0.5 0.6 \0.3 \0.5 0.2 50d 5d 23d 4d 0.2d 10d 6d 0.2d 1.4d 0.8d \1 1.2d 1.1d 0.5d \0.1d 1.5d 0.5d

600 Mj

CORRELATED AMPLITUDE (Jy)b

6.6

2.8 5.3

6.0

3.8

0.50 0.95 2.0 3.5

2.2 0.35 2.7 12.0 6.5 1.9 0.5 0.45 1.8

4.3

DISTANCE (kpc)

H II H II H II SFR SFR SFR SFR SFR SFR H II H II IRAS H II H II H II H II SFR H II Star SFR SFR SFR SFR SFR SFR

CLASSIFICATION

489

L1204A G108.60]00.49 Cep A NGC 7538S NGC 7538 IRS 1 G111.26[00.77

G097.53]03.17 09753]0319 GL 2789 IC 1396(N) LkHa 234

R Crt NGC 6334(N) M16A W43S G34.26]0.15 S76E G35.20[0.74 G35.20[1.74 S106 FIR

OTHER NAME 10 17 18 18 18 18 18 18 20 21 21 21 21 21 21 22 22 22 22 23 23 23

58 17 15 43 50 53 55 59 25 14 30 30 38 39 41 14 19 50 54 11 11 13

06.0c 32.205(0.001) 17.33(0.01) 26.87(0.03) 46.278(0.001) 46.620(0.002) 41.049(0.006) 13.0303(0.0003) 32.585(0.001) 23.855(0.006) 37.99(0.01) 37.0c 10.622(0.001) 09.788(0.001) 57.374(0.002) 14.656(0.0003) 49.819(0.002) 38.73(0.01) 19.042(0.003) 36.0739(0.0003) 36.645(0.005) 58.905(0.002)

[18 [35 [13 [02 ]01 ]07 ]01 ]01 ]37 ]54 ]55 ]55 ]50 ]58 ]65 ]52 ]63 ]59 ]61 ]61 ]61 ]59 03 43 46 42 11 49 36 09 12 30 40 40 00 02 53 06 36 44 45 10 11 39

21.0c 58.5(0.1) 27.53(0.05) 31(4) 09.88(0.04) 19.57(0.04) 28.44(0.09) 10.75(0.06) 50.816(0.006) 57.791(0.003) 31.12(0.01) 36.0c 42.718(0.001) 31.33(0.01) 08.436(0.003) 35.309(0.004) 28.586(0.005) 52.38(0.03) 46.71(0.05) 29.880(0.002) 49.84(0.01) 05.998(0.004)

DECL. (1950.0) 3.7 [10.0 [65.0 96.2 53.7 28.4 30.9 39.5 6.1 [88.6 [71.7 [70.8 [51.8 1.3 [11.7 [36.5 [10.2 [54.3 [8.9 [56.9 [60.0 [52.8

RADIAL VELOCITY (km s~1) 18 32 10 10 23 10 3 10 23 6 4 6 23 20 14 8 27 5 27 23 15 5

VELOCITY WIDTHa (km s~1)

NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a From spectrum based on a baseline length of 700 km. b Flux measured from strongest spectral component. c Baseline length below 450 Mj. d Adopted position ; no position measurement.

23116]6111 . . . 23139]5939 . . .

18895]0108 . . . 20255]3712 . . . 21144]5430 . . . 21306]5540 . . . 21306]5539 . . . 21381]5000 . . . 21391]5802 . . . 21418]6552 . . . 22142]5206 . . . 22198]6336 . . . 22506]5944 . . .

18537]0749 . . .

18152[1346 . . . 18434[0042 . . .

10580[1803 . . .

IRAS NAME

R.A. (1950.0)

TABLE 1ÈContinued

47 2300 1050 56 120 17 18 10 5.5 670 600 3500 45 125 76 5.5 24 15 300 17 53 225

50 Mj

3.6d 110 2 \1 58 \1 1.5 \1 1 25 1.5 25 14 56 \2 \1d ... \0.5 9 \0.5 4 18

600 Mj

CORRELATED AMPLITUDE (Jy)b

0.5 1.7 2.0 7.4 3.8 2.4 2.3 2.9 4.2 11.1 8.7 8.7 6.0 0.8 1.0 4.6 0.9 5.3 0.7 2.7 2.5 4.5

DISTANCE (kpc)

Star SFR H II SFR SFR H II H II H II SFR H II H II H II SFR SFR SFR H II H II IRAS H II H II H II H II

CLASSIFICATION

TABLE 2 RELATIVE POSITION OF H O MASER COMPONENTS 2 Source Name W3(OH) . . . . . . . . . . . . . .

05137]3919 . . . . . . . . .

05274]3345 . . . . . . . . . Ori KL . . . . . . . . . . . . . . .

OMC-2 . . . . . . . . . . . . . . . S231 . . . . . . . . . . . . . . . . . . G206.01[15.48 . . . . . . NGC 2071 . . . . . . . . . . .

Mon R2 . . . . . . . . . . . . . .

G188.95]00.89 . . . . . .

06061]2151 . . . . . . . . . S269 . . . . . . . . . . . . . . . . . .

IRC ]60169 . . . . . . . . .

Radial Velocity (km s~1)

R.A. (mas)

Decl. (mas)

Flux (Jy)

[64.1 [62.0 [59.3 [56.7 [54.0 [52.3 [49.4 [49.0a [46.1 [44.3 [36.6 [35.7 [26.8 [25.5 [21.9a [4.6 [2.1a [4.6 [3.4 1.3 2.5 4.2 5.1 5.9 7.2 8.0a 8.8 11.8 14.3 16.0 17.7 18.5 19.8 20.6 36.7 11.4a 19.4 [16.4a [14.7 [1.7 2.9a 5.8 10.8 11.5 13.1 15.1a 16.6 20.5 8.8 9.7 11.4a 12.6 13.3 [1.4 6.6 8.7a [5.6 [9.8a 16.1 17.3 19.4a 20.7 [33.4 [30.8a

[1028.3(0.7) [1027.9(0.7) [994.5(1.2) [994.7(0.6) [386.3(0.6) [989.8(3.4) [31.6(3.3) [1.2(0.5) [2060.4(0.8) [2097.0(1.1) [2141.2(3.5) [2141.1(1.4) 33.8(6.1) [55.8(3.4) [0.1(0.2) 183.8(2.7) 0.0(0.1) 9048.6(17.7) 6001.3(6.8) [617.6(1.9) 2261.2(1.8) 2267.0(2.7) 5831.4(4.0) 2083.2(4.3) [1097.2(1.8) 0.6(0.3) [1101.4(0.9) 14706.5(14.2) 11016.2(35.1) [5201.0(6.2) [5201.4(3.9) [403.8(1.5) [5257.8(5.4) [449.5(1.4) [8119.2(12.7) [0.4(0.4) 8143.0(5.1) 0.2(0.4) 2.2(1.2) 95.6(0.7) [0.1(0.2) [350.0(200.0) [195.6(6.8) [161.4(6.7) [2150.9(8.1) [0.1(0.3) [2267.4(6.5) 57.6(6.5) [2.1(0.5) 2.4(1.1) [1.0(0.7) [2.6(0.9) [5.1(0.7) [3532.1(8.2) [2643.8(4.5) 0.8(0.3) [42.6(0.8) 0.1(0.4) [596.0(7.5) [3.5(3.0) [0.7(0.6) [999.0(3.1) [458.4(138.7) 6.0(2.1)

119.9(0.3) 120.1(0.3) 129.4(0.5) 127.3(0.3) [100.8(0.3) 130.9(1.5) 77.5(1.4) [0.6(0.2) 248.2(0.3) 224.4(0.5) 183.6(1.5) 183.8(0.7) 278.9(2.3) 224.0(1.7) 0.0(0.2) [266.4(6.9) 0.0(0.1) 13708.9(50.7) 5822.2(25.1) 1948.8(10.4) 344.0(8.8) 475.3(15.5) 4256.3(16.5) 586.7(21.9) [3066.2(11.4) [2.3(2.2) [3040.8(5.8) 19613.2(65.8) 19740.9(85.7) [2490.1(16.7) [2586.5(13.7) [3033.3(9.9) [2585.7(13.7) [3048.3(9.1) 14478.2(31.2) [3.2(2.1) 25816.1(41.4) [0.1(0.4) [1.4(1.1) 240.9(21.4) [0.6(5.5) 10000.0(5000.0) 340.2(214.0) 1072.7(209.0) 5505.1(256.0) [7.1(19.1) 6385.8(250.0) 545.4(205.6) [44.5(20.4) 32.0(49.4) [3.1(4.3) [28.8(37.0) [51.7(28.5) [19843.8(11.4) [16995.2(4.6) 0.9(0.4) 4.4(0.6) 0.2(0.5) [567.0(10.6) 3.8(3.2) 1.2(1.0) [595.2(1.9) [7.5(57.1) [0.5(0.6)

76.5 55.9 22.1 124.0 175.0 219.0 1200.0 5100.0 274.0 38.2 4.6 8.5 14.0 6.0 16.0 8.4 11.5 63.0 152.0 19.0 261.0 118.0 29.0 38.0 56.0 1400.0 288.0 527.0 148.0 80.0 37.0 29.0 87.0 24.0 18.0 18.1 0.9 42.0 27.5 2.0 30.0 1.3 1.7 1.9 4.4 12.2 1.8 1.5 3.6 9.0 225.0 28.0 4.1 8.2 2.8 42.5 3.8 142.0 9.4 4.2 76.0 13.0 4.8 6.0

490

TABLE 2ÈContinued

Source Name 06446]0029 . . . . . . . . .

07427[2400 . . . . . . . . .

RCRT . . . . . . . . . . . . . . . .

NGC 6334(N) . . . . . . .

NGC 6334(N)a . . . . . .

W43S . . . . . . . . . . . . . . . . .

G34.26]0.15 . . . . . . . .

G40.50]2.54 . . . . . . . .

G35.20[1.74 . . . . . . . .

G097.53]03.17 . . . . . .


R.A. (mas)

Decl. (mas)

Flux (Jy)

44.4 44.8a 46.9 47.3 52.0 70.0 78.6 80.2a [6.5 [5.2 [2.2 [0.1 0.0 0.7 3.9a 6.7 8.0 9.3 [27.7 [17.7 [9.2 [7.7a [5.7 [4.1 [32.6 [30.4 [28.8 [27.7b [26.5 [25.0 [20.7 [11.9 95.5 96.2a 97.1 98.9 103.4 34.1 38.2 42.2 46.4 50.3 51.0 53.7a 55.2 56.0 56.7 57.7 59.2 60.7 61.1 66.1 21.5 28.2a 31.2 37.8 39.5a 40.1 41.2 42.0 45.8 [81.0 [77.3a [75.5 [74.2 [71.7

[1.1(0.3) 0.5(0.2) [167.6(0.5) [169.5(0.9) [128.7(1.3) 95.7(4.1) 852.1(8.7) 0.7(0.8) 14.7(2.3) 32.7(2.9) [54.3(2.9) [77.5(1.9) [78.0(1.8) [86.6(2.1) [6.0(2.4) [23.9(4.2) 21.4(1.6) 21.6(2.5) 823.3(1.4) 504.3(4.1) 326.2(1.5) 0.6(0.6) 161.9(1.5) 532.4(1.9) [9.3(1.9) 7.3(1.2) 1.5(1.1) [0.2(0.3) 0.7(0.8) [0.3(2.1) [9.8(1.9) [9.7(2.0) 906.9(2.5) [0.6(2.5) 967.5(5.4) 252.8(3.7) 240.0(20.0) [364.0(2.7) [250.1(3.0) [1924.9(2.6) [27.9(0.9) [15.0(1.5) [837.9(5.3) [0.2(0.3) 89.0(1.1) 84.9(1.3) [432.8(1.8) [1453.6(0.7) [782.5(0.7) [684.7(0.8) [711.7(3.0) [974.0(20.0) [10000.0(500.0) 0.4(0.9) [11905.6(20.4) 84.2(1.5) 0.0(0.5) 3.8(1.7) 65.0(1.8) 59.3(0.7) 76.5(1.5) 0.7(2.0) 0.6(0.6) 780.8(8.7) 719.8(5.8) [10409.1(10.0)

[60.2(19.5) 3.2(14.3) [396.0(24.0) [193.4(32.5) [109.4(28.7) [63.0(2.6) 311.9(7.0) 0.9(0.9) 4.8(5.2) 44.3(6.9) 104.0(2.6) 103.6(6.7) 100.8(4.5) 93.5(6.2) 3.8(3.2) 100.4(14.4) 103.6(5.5) 92.6(6.3) [5150.2(2.1) [1480.0(9.6) [1895.5(2.2) [0.2(0.3) [1191.4(2.5) [1598.3(4.8) 0.2(3.2) 10.1(3.5) 1.4(1.6) 0.2(0.3) [1.8(1.2) 5.5(3.3) [28.0(3.1) [8.1(2.8) [297.9(8.6) 6.6(4.6) [95.8(7.5) [83.5(6.0) [150.0(20.0) 1608.1(31.9) 1690.3(43.3) 1490.2(28.2) [38.1(8.7) [14.4(17.2) 1743.0(29.0) 5.2(0.4) [1322.5(10.9) [1284.7(13.8) 2257.5(18.1) 3182.7(8.0) 1701.2(9.2) 1778.2(7.9) 1614.2(37.2) 3190.0(200.0) 500.0(200.0) 1.8(1.6) 1890.4(46.5) 43.8(26.5) [1.8(6.0) 10.3(31.4) 76.2(26.3) 169.4(11.1) 116.0(26.4) [55.6(1.3) [0.3(0.4) 32.8(3.1) 87.8(1.6) [8616.9(6.0)

115.0 125.0 11.3 7.5 4.0 1.0 0.5 1.8 11.5 16.2 16.4 11.8 16.7 14.1 44.5 5.3 10.7 11.5 411.0 31.0 340.0 2300.0 490.0 83.0 127.0 47.0 252.0 440.0 285.0 145.0 89.0 198.0 7.7 32.7 7.0 10.3 3.5 2.7 1.7 6.0 19.5 12.6 20.0 85.0 52.0 27.0 35.0 65.0 28.0 200.0 140.0 1.4 5.0 14.0 13.5 8.9 13.8 8.4 6.8 2.0 9.6 105.0 105.0 25.0 105.0 3800.0

491

492

MIGENES ET AL.

Vol. 123

TABLE 2ÈContinued

Source Name GL 2789 . . . . . . . . . . . . . . . .

IC 1396N . . . . . . . . . . . . . .

LkHa 234 . . . . . . . . . . . . . .

L1204A . . . . . . . . . . . . . . . . .

G108.60]00.49 . . . . . . . .

Cep A . . . . . . . . . . . . . . . . . .

NGC 7538S . . . . . . . . . . . .

NGC 7538 IRS 1 . . . . . .

G111.26[00.77 . . . . . . . .


R.A. (mas)

Decl. (mas)

Flux (Jy)

[68.8 [54.3 [51.8a [49.5 [41.6 [41.0 [39.0 [14.4 [1.7a [0.7 0.3 1.3 9.3 [11.3 [8.2 [5.1 [4.3a [2.2 [1.1 [25.6 [18.9 [12.5 [10.2a [2.4 [53.8a [51.3 [50.8 [43.7 [19.7 [11.2 [10.4 [8.9a [7.9 [6.8 [0.3 3.7 8.0 [70.9 [66.9 [63.2 [60.1 [56.9a [55.3 [53.6 [60.0a [59.4 [57.7 [53.0 [54.3 [52.8a

153.3(3.5) [1.0(1.6) [0.3(0.2) [1.9(0.8) [0.4(1.3) 0.4(1.2) 1.5(1.2) [511.0(1.1) 0.0(0.2) 3.9(1.7) 10.2(0.7) 19.4(0.8) 8010.0(26.0) [17.3(2.3) 148.2(3.8) 125.8(1.3) 0.0(0.7) [21.9(1.5) 27.0(2.8) [70.7(1.1) [119.3(1.8) [127.9(1.7) 0.1(0.5) 157.5(3.8) 0.0(0.1) 41.1(2.0) 56.4(0.9) [25.1(10.2) [11.4(1.4) 1638.9(2.3) [2.1(1.2) [0.1(0.1) 1607.8(11.8) [2.7(2.5) [160.1(1.6) 1638.5(10.6) 1622.9(13.1) 97.8(0.8) 34.6(1.4) [213.8(1.0) [116.8(0.9) 0.0(0.1) 33.2(1.5) 206.6(0.8) [1.3(1.0) 203.1(2.3) [296.4(30.0) 7170.0(100.0) 1.4(1.4) [0.7(0.5)

113.1(1.1) [0.4(0.9) 0.0(0.1) [0.2(0.3) 8.5(0.6) 9.1(0.5) 7.9(0.6) [166.7(0.5) [0.1(0.1) [0.2(0.6) 0.5(0.3) 1.5(0.3) [10640.0(20.0) 128.6(2.1) 164.4(3.9) 161.6(1.0) 0.1(0.2) 3.4(0.9) [14.5(2.9) 51.3(0.9) 141.8(1.8) 324.0(1.3) 0.2(0.3) 201.6(3.6) 0.1(0.1) [9.8(0.9) [8.8(0.2) [29.7(2.1) 500.9(2.3) [2608.1(3.6) [6.1(1.9) 0.0(0.1) [2806.3(17.9) 1.5(4.5) 695.2(1.7) [2803.4(13.9) [2792.7(20.8) 293.7(0.4) 275.7(0.9) 669.0(0.6) 346.7(0.6) 0.0(0.1) 30.0(1.5) 77.6(0.7) [0.5(0.3) 157.8(0.8) 161.2(16.7) 3229.5(7.7) [2.3(0.4) 0.3(0.1)

90.0 6.5 40.0 11.6 7.9 8.1 7.5 24.5 87.0 34.0 69.0 40.0 50.0 20.1 11.0 37.9 45.0 41.1 13.7 16.1 15.1 14.4 14.6 4.9 31.5 3.3 1.6 1.7 23.0 22.0 27.3 276.0 16.0 1.5 5.1 4.2 13.0 1.4 0.9 1.4 2.3 14.0 3.3 6.2 32.0 29.0 3.6 16.9 21.5 230.0

NOTE.ÈErrors shown in parentheses. a Reference component. Measured or adopted positions of the reference components are given in Table 1. b The reference component of NGC 6334(N)a. Its position relative to the Ðrst reference component is on the Ðrst row of the entries for NGC 6334(N).

A total of 60 sources were observed during two observing periods in 1996 June and October : 29 star-forming regions, four late-type stars, and 27 H II regions. The VLBA was used on 1996 June 5 to observe over 350 continuum sources at 5 GHz and 24 maser sources at 22 GHz over a period of 24 hr. The continuum results are described by Edwards & Fomalont (1998) and Fomalont et al. (1999). The maser observations were made with one

8 MHzÈwide channel, which was subdivided into 512 spectral channels during correlation. This provided a spectral resolution of 15.6 kHz per channel or 0.21 km s~1. Each maser source was observed for 5 minutes, providing a sensitivity of 100 mJy beam~1 and an angular resolution of 0.31 mas. In general, fringes were detected on the long baselines to VLBA-MK and/or VLBA-SC for 80% of the sources. The continuum sources 3C 273 and 3C 84 were observed at

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. NGC1333-IRAS1

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FIG. 1a FIG. 1.ÈSpectrum for 38 sources at 50 Mj (top) and the spectrum at 600 Mj (bottom)

22 GHz as well for delay calibration and bandpass correction. On 1996 June 16 a much shorter run was performed, and three additional sources were observed. These sources, W3(OH), Orion KL, and R Crt have been well studied with the VLBI technique and so were able to be used to help calibrate and check the systemÏs performance. These observations were made in the standard VSOP observing mode : two 16 MHz channels with two-bit sampling. The correlator provided 512 spectral channels, corresponding to a spectral resolution of 31.3 kHz. The scans were 6 minutes in length, but the sources were observed for over 1 hr. The

sources 3C 84 and 3C 273 were observed for delay calibration and bandpass correction. Later, on 1996 October 16, the VLBA, E†elsberg, Noto, and Metsahovi antennas were used to observe another 34 H O maser sources. The maser observations were made 2 one 16 MHzÈwide channel, which was subdivided into with 512 spectral channels during correlation. This provided a spectral resolution of 31.25 kHz per channel or 0.42 km s~1. Each maser source was observed for 6 minutes, providing a sensitivity of 100 mJy beam~1 and an angular resolution of 0.21 mas. In general, fringes were

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FIG. 1b

detected on the long baselines to VLBA-MK and/or VLBA-SC for all sources. The continuum sources 3C 120, DA 193, and 3C 418 were observed at 22 GHz as well for delay calibration. The postprocessing procedure was done in the standard VLBA manner (Diamond 1995). The amplitude calibration was performed by using the system temperature and gain curves provided by the VLBA. The fringe-Ðtting process was performed in two steps : (1) the residual delay was estimated on the continuum sources 3C 273, 3C 84, and DA 193, and (2) the residual rate was determined on a strong spectral feature for each source independently. The

bandpass correction was determined on 3C 273, 3C 84, and DA 193, and, once applied, the data were corrected for the Doppler shifts of each antenna. The short scans resulted in only a few sources having sufficiently good (u, v)-coverage to allow self-calibration and mapping, so fringe-rate mapping techniques were used for all sources. 3.

RESULTS

A total of 61 sources were observed, and the results, for the detected sources, of these observations are summarized in Table 1. The table shows the IRAS and common names,

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. MON R2

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FIG. 1c

the systematic velocity with respect to the local standard of rest, the velocity width of the H O maser emission, the 2 correlated Ñux of the strongest spectral feature on baselines of 700 and 8000 km, distance (if known), and the source classiÐcation (where SFR stands for star-forming region, STAR stands for for circumstellar sources, and H II denotes H II regions). The position columns also indicate the positional error from our observations based on the fringe-rate mapping technique. The absolute positions of the various Ðelds was tied to the observations of the various continuum calibrators used whose positions are well known to milliarcsecond accuracy. The sources W3(OH), Orion KL, and

R Crt were observed as test sources for the run since they have been well studied. This was the Ðrst interferometric observation, of any kind, for approximately 50% of the sources. Most of the sources were detected and by the fringe-rate mapping technique absolute positions accurate to within 0A. 1 were determined. Eleven of the sources were not detected on any baseline (L1448, S287A, G45.47]0.13, L998A, IC 1396W, 03245]3002, L1660, HHL 50, IC 1396E, L1204B and Cep C), and one source (IRAS 00420]5530) was detected only on the shortest baseline, PT-LA (237 km). Most of these maser sources are associ-

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FIG. 1d

ated with low-mass protostars. It is most likely that these sources were completely resolved, although extreme variability could also be the reason. Water vapor masers are well known to show highly variable emission on di†erent timescales (Simonetti et al. 1992 ; Benson, Little-Marenin, & Cadmus 1992 ; Claussen et al. 1996, and references therein). Flux variability is perhaps more evident in the circumstellar maser environment, where it seems to be associated with the rotational period, than in star-forming regions in which spectral features can vary between 10% and 50% in Ñux density on timescales of a month. Since the sources were selected because of their strong water vapor emission, it is

possible that Ñux variability in combination with high spatial resolution resulted in the nondetection. This question should be resolved with monitoring observations of these sources to determine the degree of variability. 3.1. Comments on Individual Sources Forty-four sources have been mapped with the fringerate method. The phase of all the channels was phasereferenced to a single feature, for each source, so that the position for the reference feature would be the phase center of the image. In this manner the relative positions among the channels, with respect to the phase center, is accurate to

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. 07006-0654

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FIG. 1e

within 1 mas in most cases. The spectra of the mapped sources are shown in Figure 1. The upper panel for each source shows the 50 Mj spectrum, and the lower panel shows the 600 Mj spectrum. The spectra are composed of an average over all baselines within the speciÐed range. Among all the sources nine had only one component in the spectrum, and the fringe maps consist of one spot. The fringe-rate plots indicate their size is less than 2 mas. The remaining sources have multiple components in their spectra, and their maps consist of several unresolved spots distributed over an area of up to 10 arsec2, as shown in Figure 2. The maser spot distribution of the source was

obtained from low spatial resolution maps. As HAL CAÏs orbit has a perigee height of 560 km, the baselines up to 50 Mj are comparable to baselines from the closest ground telescopes to HAL CA during the satelliteÏs perigee pass. W 3(OH).ÈThe map of W3(OH) in 1996 June is similar to the map obtained by Alcolea et al. (1992) in 1981È1982. It shows 12 maser spots located in three clusters and two separate spots. The clusters are aligned in the east-west direction and have mean radial velocity [49 km s~1, [58 km s~1, and [41 km s~1. The brightest spot at [49 km s~1 is completely unresolved on the all baselines up to 300 Mj.

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FIG. 1f

05274]3345 (AFGL 5142).ÈThe position of this source is in agreement with the VLA position obtained by Tofani et al. (1995) and coincides with the position of the farinfrared source (Jenness, Scott, & Padman 1995). See the spectrum in Figure 1b. OMC-2.ÈThe strongest feature at the radial velocity 11.4 km s~1 (see Fig. 1b) coincides with the 1.3 mm continuum and far-infrared source FIR 4 (Chini et al. 1997), while the weaker feature, at 19.4 km s~1, coincides with FIR 3 (Chini et al. 1997). At the latter position Tofani et al. (1995) found H O maser components at radial velocities 2 and 12.0 km s~1, which is di†erent from the 6.1 km s~1 radial velocity of the weaker feature in the present survey.

Orion KL .ÈWe mention Orion KL brieÑy because though it did not appear strong and compact enough for HAL CA observations in our survey results (see the spot distribution in Fig. 2b), there was a strong maser burst that started in 1998 January. The Ñux has increased by a factor of 1000 times stronger than usual. The maser peak Ñux exceeded 2 MJy, which was the Ñux measured when the VLBA and HAL CA observed Orion in 1998 March. In spite of HAL CAÏs serious loss of sensitivity, the spectrum of the Ñare was detected on all VLBA-HAL CA baselines. These have been the Ðrst fringes with HAL CA at 22 GHz. No fringes were detected with baselines longer than 700 Mj, which suggests the source is heavily resolved. For

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. G40.50+2.54 (S76E)

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FIG. 1g

more details of the observations and results, see Kobayashi et al. (1998). S231.ÈOnly two spectral features separated by 1.7 km s~1 were present in our observations (see Fig. 1b) ; both come from the same position within 2.5 mas, which coincides with the position of the strongest spectral feature at the radial velocity [16.9 km s~1 in the VLA survey by Tofani et al. (1995) (source S233 in their list). G206.01[15.48.ÈThis is a low-mass YSO IRAS 05413[0104 in the dark cloud L1630. The H O maser consists of two components separated by about2 0A. 2 (see Fig. 1b). The position of the H O maser coincides with far2

infrared (Jenness et al. 1995) and submillimeter (Chini 1997) sources identiÐed with Herbig-Haro object HH 212. Our VLBA imaging took place 1 month after a series of four more sensitive VLBA maps made by Claussen et al. (1998). Our absolute fringe-rate position in Table 1, which refers to the red feature, is in agreement with the VLA position (right ascension di†erence is 0A. 66, or 6 p). We see only two features, while Claussen et al. (1998) found 17È40 features, probably because of a longer integration. On the synthesized map the blue component has split into two components separated by 2.58 mas. The position angle of the line connecting red and blue components on our map is

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FIG. 1h

30¡ ^ 0¡.2, in reasonable agreement with the position angle 23¡ ^ 2¡ found by Claussen et al. (1998), and the separation between them is 194.10 and 195.47 mas for the two blue subcomponents, respectively, consistent with the mean separation of 200 mas in the Claussen et al. (1998) data. NGC 2071.ÈSee spectrum in Figure 1c. There are two groups of spots on the map with overlapping radial velocities, separated by 5A. 6 (Fig. 2a). Their positions roughly correspond to the position of two 10 km infrared sources IRS 1 and IRS 3 (Persson et al. 1981). The absolute position of the southern group corresponding to IRS 1 is consistent with

the VLA position obtained by Tofani et al. (1995) and Torrelles et al. (1998). In the northern group corresponding to IRS 3 we found only two spots, while Torrelles et al. (1998) in a more sensitive VLA map obtained 2 months later the present 13 spots delineating a proposed protoplanetary disk of radius 20 AU with a velocity gradient 0.35 km s~1 AU~1. The two spots in our map are separated by 33 AU and have velocity di†erence 3.8 km s~1, which is consistent with them belonging to the protoplanetary disk. Mon R2.ÈThere is one strong feature and several weak features in the spectrum ; see Figures 1c and 2a. The absol-

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. IC1396(N) Flux Density (Jy)

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FIG. 1i

ute fringe-rate position of the strong feature coincides with the VLA position obtained by Tofani et al. (1995) and with the far-infrared source IRS 3 (Jennes et al. 1995). The weaker features are separated from the strong one by less than 5 mas and may coincide with it within the errors. G188.95]0.89 (AFGL 5180).ÈThe strongest feature (Figs. 1c and 2a) at the radial velocity 8.7 km s~1 coincides with one of the components on the VLA map by Tofani et al. (1995), while the two weaker features are approximately coincident with the southern group of masers on the VLA map. There is a 1.3 mm continuum emission at this position (Chini et al. 1986).

IRAS 06061]2151 (AFGL 5182).ÈThe two close maser spots separated by 43 mas have a 1.3 mm continuum emission associated with them (Chini et al. 1986). See the spectrum in Figure 1d. NGC 6334(N).ÈThis is also known as NGC 6334A (Moran & Rodr• guez 1980) ; see Figure 1f. The spectrum of the 1978 observation of Moran and Rodr• guez is dominated by the feature at about [9 km s~1 with a Ñux density 800 Jy. In 1996 the strongest feature was at [7.7 km s~1 and had a Ñux density 2300 Jy. The group of features between [30 and [20 km s~1 was practically absent in the 1978 spectrum. The fringe-rate map consists of six spots

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5 0

0

-5

-10


-30

S158 Flux Density (Jy)

Flux Density (Jy)

Cep A 50 40 30 20

45 40 35 30 25 20 15 10

10

5 0

0 0

-10

-20

-30

50

Flux Density (Jy)

Flux Density (Jy)

10

40 30 20

-50

-55

-50

-55

-60

-65

-70


-70

45 40 35 30 25 20 15 10

10

5 0

0 10

0


-30

FIG. 1j

with a maximum separation between spots of about 5200 mas (Fig. 2c), corresponding to a linear separation of 9000 AU. The southernmost component at V \ [27.7 km s~1 is the center of a separate compact group of eight spots with a separation of only 42 mas, or 73 AU. We called this group NGC 6334(N)a, and the map is shown in Figure 2c ; it is listed on Table 2. The position and overall distribution of maser spots is consistent with the VLA map produced by Forster (1992) (source 351.42]0.64). M16A.ÈThe position of this maser coincides with the far-infrared source I (McBreen, Fazio, & Ja†e 1982). The H O maser is unique in several respects : 2

1. It has a radial velocity [65 km s~1, which is very di†erent from the radial velocity of the foreground molecular cloud 24.9 km s~1 2. It has an unusually broad line of several km s~1 with a smooth proÐle, suggesting that it is not a blend of several narrow components. 3. The source is heavily resolved by the VLBA even at the shortest baseline of about 20 Mj (see Fig. 1f ), suggesting an angular size of about 10 mas ; a good candidate for mapping with the VLA. W 43S.ÈThe spectrum is shown in Figure 1f. Five maser spots corresponding to Ðve spectral features were mapped

No. 2, 1999

VSOP PRELAUNCH SURVEY. I. 11126-0077

10

Flux Density (Jy)

Flux Density (Jy)

S158B 8 6

250 200 150

4

100

2

50 0

0 -40

-50

-60

-70

-80

10

Flux Density (Jy)

Flux Density (Jy)

503

8 6

-45

-50

-45

-50

-55

-60

-65


-65

250 200 150

4

100

2

50 0

0 -40

-50


-80

FIG. 1k

in this source (Fig. 2c). The absolute fringe-rate position of the reference feature at 96.2 km s~1 coincides with the 1.3 mm continuum source G29.96 FIR I (Mooney et al. 1995). Hofner & Churchwell (1992) with the VLA found two H O maser spots at the positions consistent with our map if 2 di†erence in angular resolution is taken into account. the G34.26]0.15.ÈThis source has a rich spectrum (Fig. 1g), with positions determined for 15 spectral features. The maximum velocity separation between the features is 32 km s~1, and the maximum angular separation is 4900 mas, or 18,600 AU (Fig. 2c). The spectrum has little resemblance to the 1977 spectrum obtained by Downes et al. (1979), although the overall velocity range is the same. Nevertheless our map is very similar to the map of Downes et al. (1979), which was also constructed using fringe rates. All the spectral features that are present in both spectra originate from the same positions within the errors. The spectral features at radial velocities 53.8, 56.0, 56.7, 57.7, 58.2, and 60.4 km s~1 are at the same positions as their respective features on the Downes at al. (1979) map (if the maps are adjusted to the same reference feature). Some features of our map are missing from the Downes et al. (1979) map. These are weak features at 34.1, 38.2, and 42.2 km s~1. On the other hand several high velocity features near 80 km s~1, which were present in the 1977 spectrum, were completely absent in our spectrum. There is a 1.3 mm continuum source at the position of the maser (Chini et al. 1986). S76E (G40.50]2.54).ÈThere are only three components in the spectrum (Fig. 1g), which are separated by 12,000 mas, or 29,000 AU. This is one of the largest linear separations in our sample (Fig. 2d). The position of the H O maser coincides with the far-infrared source (Jennes et 2al. 1995). G35.20[0.74.ÈThere is one spectral feature with a rather large line width for a maser : 0.6 km s~1 (Fig. 1g),

and its position agrees with the VLA position reported by Forster & Caswell (1989) and with the position of 1.3 mm continuum source G35.20 FIR (Mooney et al. 1995). G35.20[1.74 (W 48).ÈThis source exhibits 1665 and 1667 MHz OH maser emission (Slysh et al. 1996) originating from the same region as the water vapor maser emission. The 1665 MHz masers are approximately 1.4 mas in size and strong enough to be a reasonable candidate for space VLBI observations. See the spectrum in Figure 1g. The H O masers are spread within 100 mas (Fig. 2d). Its 2 coincides with the H O maser component HC 1 position 2 with the infrared source (Ho†ner & Churchwell 1996) and MIR 1 (Persi et al. 1997) and the 1.3 mm continuum source (Chini et al. 1986). S106 FIR.ÈAt the position of this single-component maser, there is a 450 km far-infrared sourceÈan intermediate-mass class 0 protostar (Richer et al. 1993) and 1.3 mm continuum source (Chini et al. 1986). See the spectrum in Figure 1h. G097.53]3.17 (IRAS 21306]5540).ÈThe spectrum of this maser is dominated by a very strong feature with a Ñux density of 3800 Jy and at a radial velocity of [71.7 km s~1 (see Fig. 1h), and several small features on either side. The Ðve features (Fig. 2d) are spread over an area 780 by 170 mas, with the largest linear separation being 7000 AU. The strongest feature at the radial velocity [71.7 km s~1 is very far away from the rest of the features and is o†set by 13A. 5. The strongest component is heavily resolved with the angular size about 5 mas. GL 2789 (V 645 Cyg).ÈThere are two groups of spectral features : at [40 km s~1 and at [52 km s~1 (Fig. 1i). On the map (Fig. 2d) the two groups are concentrated in two clusters separated by 9 mas, or 54 AU, with one group blueshifted and the other redshifted with respect to the systematic velocity. The 1979 observation of Lada et al. (1981) revealed only the group at [50 km s~1 and one feature at

300

05137+3919

NGC 2071

200

100

0 40

20

0

-20

-40

-60

MON R2

18895+0089

Relative Declination (mas)

40

20

0

-20

-40

-60

60

40

20

0

S269

06446+0029

200

0 0

-200

-200

-400 -400 -600

0

-500

-1000

0

-50

-100

Relative Right Ascension (mas) FIG. 2a FIG. 2.ÈSpot distribution maps for sources with more than one component in the spectrum

504

-150

07427-2400

10860+0049 0

300

-10

200

100 -20

0 -30 -100 800

600

400

200

0

60


Cep A

40

20

0

-20

-40

S158B

500 0

0

-2500

-500 2000

1000

0

200

Ori KL

0

-200

R Crt

100

15000

50

0 0

15000

0

50

Relative Right Ascension (mas) FIG. 2b

505

0

-50

-100

NGC6334(N)


NGC6334(N)a

W43S

G34.26+0.15

Relative Right Ascension (mas) FIG. 2c

506

G40.50+2.54 (S76E)


G35.20-1.74 (W48)

09753+0319

GL2789

Relative Right Ascension (mas) FIG. 2d

507

IC1396(N)


LKHalpha234

L1204A

S158

Relative Right Ascension (mas) FIG. 2e

VSOP PRELAUNCH SURVEY. I. NGC6334N

10

3.0 2.5 2.0 1.5

0

20

0.0

0.0

0 G35.20-0.74

15

5

5

0

0

500 400 300

4

200

2

100

0

0

0 09753+0319

120

80

Janskys

100

5

50 4 3 2

60

1

10

0

0 L1204A

40

15 10

30 20

5

0

100 200 300 400 500 600 Mega-Wavelengths

30

22142+5206

20

40

40

20

40

60

GL2789 60

LKH234

140

80

5

Janskys

IC1396N

15 10

Janskys

600

6

100

15 10

Janskys

8

20

10

Kilo Janskys

Janskys

10

20

25

Janskys

Janskys

20

700

12

G35.20-1.74

30

21144+5430

14

60

0.2

25 Janskys

Janskys

160 140 120 100 80 60 40 20 0

80

0.5

S106FIR

Janskys

0.6

S76E

16

Janskys

0.8

40

G34.26

20 0

100

1.0

0.4

1.0

5

W43S

Janskys

15

M16A Kilo Janskys

3.5

Kilo Janskys

Janskys

IRC+60169

509

10

0

0

S158

11126-0077

0


250 Janskys

Janskys

60 50 40 30

200 150 100

20 50

10 0

0


0

0


FIG. 3.ÈVisibility plots of the strongest maser feature for the sources not resolved with the VLBAÏs full resolution

an intermediate velocity [44.5 km s~1, which was shifted from the Ðrst group by 160 mas, i.e., much farther than [40 km s~1 group on our map. The [44.5 km s~1 feature was not present in our spectrum. Our fringe-rate absolute

position is consistent with the VLA position (Tofani et al. 1995). IC 1396N.ÈSee the spectrum in Figure 1i. Four spectral features are located on a straight east-west line 20 mas long,

510

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Vol. 123

FIG. 4.ÈL eft : Structure function for G34.26]0.15. Right : Line width vs. size relationship.

and another feature is about 500 mas away (Fig. 2e). The group of four spectral features was mapped with the full resolution of the VLBA, and the results will be reported elsewhere (Slysh et al. 1999). The disk delineated by the low-velocity features seem to indicate the presence of a protoplanetary disk similar to the one found in NGC 2071 by Torelles et al. (1998). There is one more distant feature at 8000 mas with the radial velocity 9.3 km s~1, which is not shown on the map. The fringe-rate absolute position is consistent with the VLA position (Tofani et al. 1995) and coincides with the position of the 1.25 mm continuum source (Wilking et al. 1993). L kHa 234 and L 1204-A.ÈLkHa234 is a Herbig Ae/Be star within the NGC 7129 cold molecular cloud probably driving the large-scale mass loss in the region, while L1204-A is a young stellar object with no obvious stellar association or H II region that is probably associated with

FIG. 5.ÈLine width vs. size relationship

an earlier stage of star formation (see Figs. 1i and 1j, respectively). Both objects are characterized by strong H O 2 maser emission and outÑows (Fig. 2e). Their fringe-rate absolute position are consistent with the VLA position (Tofani et al. 1995), and there are far-infrared sources at the positions (Jennes et al. 1995). Cep A.ÈOur map (Fig. 2b) is consistent with previous maps of H O maser spot distribution made by Lada et al. 2 Rowland, & Blair (1984), and Torrelles et al. (1981), Cohen, (1998) showing two groups associated with continuum sources HW 2 and HW 3. The absolute position of our reference feature at the radial velocity [8.9 km s~1 coincides with the VLA position of the same feature measured by Torrelles et al. (1998) to within 0A. 04. No one-to-one correspondence of maser spots between our map and previous maps could be found, although the VLA map by Torrelles et al. (1998) had been made only 15 months earlier. Tentative identiÐcation of maser spots from our map with those on the map made 13 yr earlier (Cohen et al. 1984) gives an upper limit to the proper motion of about 25 mas yr~1, which is much larger than the proper motion of OH maser spots measured by Migenes, Cohen, & Brebner (1992). Most probably we compared position of di†erent components with too small a lifetime for the proper-motion estimates. The maser spots are so short-lived that measurements much more closely spaced should be made with sufficient spatial accuracy. There is a 450 km continuum emission at the position of the H O masers (Moriarty-Schieven, Snell, & Hughes 1991). See2 the spectrum in Figure 1j. NGC 7538 IRS 1 and NGC 7538S.ÈThese are two H O 2 masers separated by [email protected]. NGC 7538 IRS 1 consists of three closely spaced spots and one spot that is 7A away. The fringe-rate absolute position of the strongest feature at the radial velocity [60 km s~1 is 1A. 25 di†erent from the position given by Forster et al. (1978). The VLBI map of NGC 7538 IRS 1 obtained by Genzel et al. (1978) 19 yr ago contains only two spots separated by 0A. 28. On our map there are three spots, with separation from the reference feature of 0A. 257 and 0A. 337 for spectral features at the radial velocities of [59.4 km s~1 and [57.7 km s~1, respectively. If two of

No. 2, 1999

VSOP PRELAUNCH SURVEY. I.

the three features are the same that were present in the map of Genzel et al. (1978), then the proper motion relative to the 60 km s~1 feature would be 46 km s~1 or 181 km s~1 (at the distance 2.5 kpc), depending on which of the two features is identiÐed with the [57.2 km s~1 feature in the Genzel et al. (1978) map. Those values of the transverse velocity seem to be too big compared to the radial dispersion, and the derived proper motion is not real. More probably the spectral features did not survive during the long time interval of 19 yr. See the spectra in Figures 1j and 1k (S158 and S158B). The fringe-rate absolute position of the strongest feature in NGC 7538S at the radial velocity [56.9 km s~1 is o†set from the position given by Forster et al. (1978) by 1A. 1, which is probably within the errors of their position measurements. Both our map (Fig. 2b) and the map produced by Genzel et al. (1978) 19.75 yr ago contain seven spots. It is possible to identify four spots common to both maps. These are at the radial velocities [60.1 km s~1 ([60.0 km s~1), [56.3 km s~1 ([56.9 km s~1), [54.8 km s~1 ([55.3 km s~1), and [53.5 km s~1 ([53.6 km s~1) (the Ðrst value refers to the spot from Genzel et al. 1978, and the value in parentheses refers to the present map). The relative proper motion between the four spots corresponds to the transverse velocity 50È70 km s~1, which is again much larger than the dispersion of radial velocity. G111.26[0.77.ÈThe absolute fringe-rate position of the strongest component (Fig. 1k) at the radial velocity [52.8 km s~1 is less than 74 mas from the position of the similar component measured by Tofani et al. (1995). Three other components from the VLA map are not present in our map, probably because of our poorer sensitivity. The main component has a weaker spot 3.75 mas apart. This is the smallest angular separation measured by the fringe-rate method in this sample. The measured distance between the two features was veriÐed by the full resolution VLBA imaging. 4.

DISCUSSION

4.1. Compactness of H O Masers 2 With present technology the biggest limitation for space VLBI satellites is the deployment of a large antenna with sufficient surface accuracy. There are other considerations such as cooling systems, feed loss, the total bit rate of the transmitter, surface reÑectivity, etc., which must comply with payload mass and power restrictions as well. In any case the limitations generally a†ect the sensitivity of the system. The expected sensitivity on a HAL CAÈsingleVLBA antenna baseline, at 22 GHz, is approximately 12 Jy (7 p). This value is estimated for a channel width of 15.6 kHz and assuming a coherence time of 150 s, and efficiency factor of 0.88 for 2 bit sampling.3 Though the sensitivity depends on the spectral resolution required, sources with Ñux density greater than 12 Jy are the best candidates for VSOP observations. From Table 1 we can list 13 sources that meet this criteria : NGC 6334(N), M16A, G34.26]0.15, IRAS 21144]5430, G097.53]3.17, GL 2789, IC 1396N, LkHa 234, Cep A, and G111.96[0.77, plus the well-known sources W3(OH), Orion KL and R Crt. The most striking feature from the plots in Figure 1 is how the complex and multifeatured spectra for the sources become very simple with high spatial resolution. Essen3 See the VSOP ProposerÏs Guide and Announcement of Opportunity for further information, http ://www.vsop.isas.ac.jp.

511

tially, the strongest feature in the spectrum appears to be the most compact. Most of the sources do show more complex spectra on shorter baselines, but many of the spectral features are mostly resolved on 2000 km (250 Mj) baselines. Considering the spectral resolution of the observations, most sources exhibit a single-peak spectrum with the VLBAÏs maximum spatial resolution. There are strong variations in the appearance and number of spectral features from baseline to baseline for most of the sources. The distribution of the maser components seems to be very compact. For most sources the largest angular separation between components is 2AÈ3A. The notable exceptions are NGC 6334(N), G40.50]2.54, G097.53]3.17, IC 1396N, S158, and G34.26]0.15, the components of which are separated by up to 12A. There is no correlation between the Ñux density of the strongest spectral feature and distance to the source. Table 2 shows the list of detectable components and their position, relative to the observed positions indicated on Table 1. The radial velocity of the components is also listed. The Ñux values listed are based on measurements from the shorter baselines. Figure 3 displays the source visibility for the strongest spectral component for some of the sources. The visibilities were averaged over 10 s (with little editing of the data beforehand). A number of sources reveal interesting and complex structure for the strongest spectral component. There is a clear trend in the visibility plots for the sources to be completely resolved on the longest baselines. Some of the more northern sources exhibit slightly better (u, v) coverage and so can be mapped with normal techniques : a discussion of these results will be presented elsewhere. 4.2. Maser Spot Distribution There seems to be no regular pattern in the maps of H O masers. One exception is the aligned chain of spots in 2IC 1396N, which was mentioned above. Neither is there any clear correlation in the so-called velocity structure function, i.e., between the separation of the components and their velocity di†erence. A power-law dependence would be expected if the position of maser spots were related to the turbulence of the parent molecular cloud. For the Kolmogorov turbulence a power-law index of 1/3 is predicted. Figure 4 presents a plot of the velocity di†erence versus angular separation in G34.26]0.15, which has the largest number of components. The straight line is a least-squares approximation by a power-law function with the power-law index 0.10 ^ 0.08. It is evident that the Ðt is poor with a correlation coefficient r of 0.13, and the scatter of points is large. Any power-law index from 0, meaning no dependence of velocity di†erence on separation, to the 1/3 of Kolmogorov turbulence is compatible with the data within the scatter. For the whole sample of H O masers one can construct a 2 so-called LarsonÏs relation line widthÈsize diagram, the (1981). Plume et al. (1997), based on their CS line data, found a weak correlation between the line width and size for molecular cloud cores associated with H O masers in the 2 \ [1.3 to range of core sizes from log R(pc) log R(pc) \ 0.2 : log *V (km s~1) \ b ] q log R(pc)

(1)

with the correlation coefficient r \ 0.26, b \ 0.92 ^ 0.02, and q \ 0.35 ^ 0.03. Using the present sample of VLBA data, the velocity rangeÈsize relation can be extended to

512

MIGENES ET AL.

much smaller sizes, to log R(pc) \ [4.3 (10 AU). The data are shown in Figure 5. The correlation coefficient is somewhat higher, r \ 0.54, and the approximation parameters are b \ 1.37 ^ 0.13 and q \ 0.19 ^ 0.06. With the allowance for a large scatter the H O data are consistent with the 2 CS data of Plume et al. (1997) in the region around 0.1 pc, where the two sets of data meet. One may conclude that the velocity rangeÈsize dependence, if any, is quite Ñat in the size interval from 5 ] 10~5 to 1.6 pc. Since the velocity range is almost independent of size, the virial mass should be linearly proportional to the radius. The range of virial masses in the H O sample is from 0.03 M for 2 _ G111.26[0.77 to 2.1 ] 104 M for G34.26]0.15. Some of the sources are common to _ this sample and to the CS sample of Plume et al. (1997) ; however, the size and mass estimates di†er greatly. For example, for W43S, Plume et al. (1997) estimate a core radius of 0.9 pc and a mass of 1.8 ] 103 M , while the H O maser region in W43S has a _ 2 radius of 0.037 pc and virial mass 90 M . For _ mass G34.26]0.15 the CS diameter is 0.7 pc, and the virial 4.5 ] 103 M , while the H O maser core has a size of _ 2 104 M . The density of the 0.09 pc and virial mass 2.1 ] _ maser cores is 3 orders of magnitude higher than the density in the CS core. The maser cores probably represent the densest regions of the molecular cloud cores, with the maser spots representing still denser condensations embedded in the H O maser cores. 2 4.3. Association with Far-Infrared Sources Jenness et al. (1995) found compact submillimeter continuum emission sources in 91% of 44 water maser sources. Our high-precision absolute coordinate measurements gave improved positions for many masers, which conÐrmed or added new identiÐcations with the far-infrared, submillimeter, or 1.3 mm continuum emission sources as explained in the comments on individual sources. Thus the association of water masers with far-infrared/submillimeter/ millimeter continuum sources seems to be Ðrmly established. These sources are believed to be class 0 protostellar objects, consisting of a dense protostellar core and an extended envelope in the process of infall on the core. The far-infrared emission comes from the cold dust of the envelope, with a temperature of about 20 K (Andre, WardThompson, & Barsony 1993). It is evident that the water masers cannot be produced in the envelope since they require a much hotter and denser environment for their excitation. The core that includes a protostar and an accretion disk (Lin & Pringle 1990) seems to be a more plausible site for the water maser production. We suggest that it is the accretion disk that is the origin of water masers, associated with class 0 protostellar objects. One of the possible disk models of a water maser was proposed for the globule IC 1396N (Slysh et al. 1999), which is also associated with millimeter and submillimeter continuum emission. 4.4. Proper Motion For several sources in our sample, earlier epoch images with comparable angular resolution are available. These are W3(OH), Ori KL, NGC 7538 IRS and S, and Cep A. The time di†erence between the epoch of the present observations (1996) and previous observations is 15È20 yr. This is a very large time span, which allows accurate measurement of the proper motion even with the moderate precision of position measurements by the fringe-rate method. In

Vol. 123

Ori KL we identiÐed six maser spots that could be considered as common to maps of di†erent epochs, and the derived relative proper motion is in the range from 16 to 54 km s~1, which is consistent with previous measurements (Genzel et al. 1981). For W3(OH), the proper motion is much higher, from 50 to 160 km s~1, in contradiction with the measured expansion velocity of 20 km s~1 (Alcolea et al. 1992). For NGC 7538 and Cep A, similarly high propermotion velocities were derived. We attribute this apparent high-velocity motion to the ““ Christmas tree ÏÏ e†ect, when masers fade in one place and Ñare up in another independently. Another model of the high-velocity proper motion is based on the assumption that the high-velocity motion of the maser spots is real, but only those maser spots that move in the transverse direction are observable. This is possible for a maser emitting in the direction perpendicular to the velocity vector such as the shock front model. In the shock model a thin layer of excited molecules is formed parallel to the shock front, and the optical depth is the highest in the plane of the layer, or perpendicular to the shock motion vector. Based on our measurements, we cannot distinguish between the two modelsÈ““ Christmas tree ÏÏ and shock front. In view of the highly variable nature of the water masers, the ““ Christmas tree ÏÏ model seems to be more adequate. 5.

CONCLUSIONS

In the summer of 1997, during the initial system checks and tests, it became obvious that there were serious problems with the 22 GHz system on board. Though the problem is not completely understood, the sensitivity of the 22 GHz system has become considerably worse so as to eliminate the possibility of any observations at 22 GHz. Orion KL may remain the only source strong enough for HAL CAÏs present condition at 22 GHz. Though HAL CAÏs observations will be now limited to 1.6 and 5 GHz, we should mention that RADIOAST RON (the Russian space VLBI mission) does have 22 GHz capabilities, and the results of this study will help determine target sources for that mission, which projects launch around 2005. These are the Ðrst results from our ongoing maser surveys at 22 and 1.6 GHz. The sources that have been determined to be good candidates for space VLBI will be monitored frequently, with both VLBI and single-dish observations, to study their variability and the e†ects this may have on future observations. Some interesting conclusions can be made from the results of the present survey. As the main goal of the survey was a search for compact maser components, one can conclude that there is a large range of angular sizes of maser spots from 10 to less than 0.1 mas. The large angular sizes may be caused by a particularly strong interstellar scattering in certain directions in the Galactic plane. Another important result of the present survey is a strong conÐrmation of the association of water masers with class 0 protostellar objects consisting of a protostar with the accretion disk and extended, cool infalling envelope. The masers can be associated with the accretion disks. We are grateful to the VLBA NRAO sta† for the excellent performance of the observations. The work of V. I. S., I. E. V., and V. V. G. was partly supported by grants 95-0205826 and 95-02-16916 from the Russian Foundation for Basic Research.

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REFERENCES Alcolea, J., Menten, K. M., Moran, J. M., & Reid, M. J. 1992, in AstroHirabayashi, H., et al. 1998, Science, 281, 1825 physical Masers, ed. A. W. Clegg & G. E. Nedoluha (Berlin : Springer), Hofner, P., & Churchwell, E. 1992, in Astrophysical Masers, ed. A. W. 225 Clegg & G. E. Nedoluha (Berlin : Springer), 167 Andre, P., Ward-Thompson, D., & Barsony, M. 1993, ApJ, 406, 122 ÈÈÈ. 1996, A&AS, 120, 283 Benson, P. J., Little-Marenin, I. R., & Cadmus, R. R. 1992, in Astrophysical Jenness, T., Scott, P. F., & Padman, R. 1995, MNRAS, 276, 1024 Masers, ed. A. W. Clegg & G. E. Nedoluha (Berlin : Springer), 271 Kemball, A. J., Diamond, P. J., & Mantovani, F. 1988, MNRAS, 234, 713 Brand, J., et al. 1992, A&AS, 103, 541 Kobayashi, H., Shimoikura, T., Horiuchi, S., Murata, Y., Omodaka, T., & Chini, R. 1997, A&A, 325, 542 Hirabayashi, H. 1998, in Proc. of Japanese Astronomical Society annual Chini, R., Kreysa, E., Mezger, P. G., & Gemuend, H.-P. 1986, A&A, 154, meeting, Yamagata Univ., Q28a (in Japanese) L8 Lada, C. J., Blitz, L., Reid, M. J., & Moran, J. M. 1981, ApJ, 243, 769 Chini, R., Reipurth, B., Ward-Thompson, D., Bally, J., Nyman, L.-A., Larson, R. B. 1981, MNRAS, 194, 809 Sievers, A., & Billawala, Y. 1997, ApJ, 474, L135 Lin, D. N. S., & Pringle, J. E. 1990, ApJ, 358, 515 Claussen, M. J., Marvel, K. B., Wootten, A., & Wilking, B. A. 1998, ApJ, McBreen, B., Fazio, G. G., & Ja†e, D. T. 1982, ApJ, 254, 126 507, L79 Migenes, V., Cohen, R. J., & Brebner, G. C. 1992, MNRAS, 254, 501 Claussen, M. J., Wilking, B. A., Benson, P. J., Wooten, A., Myers, P. C., & Moellenbrock, G. A., et al. 1996, AJ, 111, 2174 Terebey, S. 1996, ApJS, 106, 111 Mooney, T., Sievers, A., Mezger, P. G., Solomon, P. M., Kreysa, E., Cohen, R. J., Rowland, P. R., & Blair, M. M. 1984, MNRAS, 210, 425 Haslam, C. G. T., & Lemke, R. 1995, A&A, 299, 869 Comoretto, G., et al. 1990, A&AS, 84, 179 Moran, J. M., & Rodr• guez, L. F. 1980, ApJ, 236, L159 Diamond, P. J. 1995, in ASP Conf. Ser. 82, Very Long Baseline InterferomMoriarty-Schieven, G. H., Snell, R. L., & Hughes, V. A. 1991, ApJ, 374, 169 etry and the VLBA, ed. J. A. Zensus, P. J. Diamond, & P. J. Napier (San Persi, P., Felli, M., Lagage, P. O., Roth, M., & Testi, L. 1997, A&A, 327, Francisco : ASP), 227 299 Downes, D., Genzel, R., Moran, J. M., Johnston, K. J., Matveyenko, L. I., Persson, S. E., Geballe, T. R., Simon, T., Lonsdale, C. J., & Baas, F. 1981, Kogan, L. R., Kostenko, V. I., & Ronnang, B. 1979, A&A, 79, 233 ApJ, 251, L85 Edwards, P. G., & Fomalont, E. B. 1998, in ASP Conf. Ser. 144, Radio Plume, R., Ja†e, D. T., Evans, N. J., II, Martin-Pintado, J., & GomezEmission from Galactic and Extragalactic Compact Radio Sources, ed. Gonzales, J. 1997, ApJ, 476, 730 J. A. Zensus, G. B. Taylor, & J. M. Wrobel (San Francisco : ASP), 127 Richer, J. S., Padman, R., Ward-Thompson, D., Hills, R. E., & Harris, A. I. Fomalont, E. B., Edwards, P. G., Hirabayashi, H., Frey, S., Paragi, Z., 1993, MNRAS, 262, 839 Gurvits, L. I., & Scott, W. K. 1999, AJ, submitted Simonetti, J. H., Diamond, P. J., Upho†, J. A., Boboltz, D., & Dennison, B. Forster, J. R. 1992, in Astrophysical Masers, ed. A. W. Clegg & G. E. 1992, in Astrophysical Masers, ed. A. W. Clegg & G. E. Nedoluha Nedoluha (Berlin : Springer), 108 (Berlin : Springer), 311 Forster, J. R. & Caswell, J. L. 1989, A&A, 213, 339 Slysh, V. I., et al. 1996, MNRAS, 283, L9 Forster, J. R., Welch, W. J., Wright, M. C. H., & Baudry, A. 1978, ApJ, 221, Slysh, V. I., ValÏtts, I. E., Migenes, V., Fomalont, E., Hirabayashi, H., 137 Inoue, M., & Umemoto, T. 1999, ApJ, submitted Genzel, R., et al. 1978, A&A, 66, 13 Tofani, G., Felli, M., Taylor, G. B., & Hunter, T. R. 1995, A&AS, 112, 299 Genzel, R., Reid, M. J., Moran, J. M., & Downes, D. 1981, ApJ, 244, 884 Torrelles, J. M., Gomez, J. M., Rodriguez, L. F., Curiel, S., Anglada, G., & Hansen, J., Booth, R. S., Dennison, B., & Diamond, P. J. 1992, in AstroHo, P. T. P. 1998, ApJ, 505, 756 physical Masers, ed. A. W. Clegg & G. E. Nedoluha (Berlin : Springer), Wilking, B. A., Mundy, L. G., McMullin, J., Hezel, T., & Keene, J. 1993, AJ, 255 106, 250