Ten new H2O masers discovered in star-forming regions

@ Pergamon Press Ltd Printed in Great Britain 02751062/92$10.00+.00

Chin. A&on. Astrophys.(l992)16/3,307-314 A translation of Acta Astron. Sin. (1992) 33/l, 87-94

Ten new Hz0 masers discovered in star-forming regions t XIANG De-lin’l*

B. E. Tune?

1. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008 2. National Radio Astronomy Observatoy, P.O. Box 2, Green Bank, WV 24944-0002, U.S.A.t 3. National radio Astronomy Observatory, Charlottesville, VA 22903-2475, U.S.A.t

Abstract It is commonly believed that recently formed low-and intermediatemass stars have bipolar outflows, are strong IR emitters, and may have Hz0 masers which may be related to HH objects. Based on the statistics of the 47 known cases from our investigations, we have discovered 15 new Hz0 masers out of a sample of 97 IRAS objects having the properties of early star formation with the 43 m radio telescope in Green Bank, NRAO, U.S.A. Ten new Hz0 masers, i.e. IC 1805-W, L1641-N, S287-A, L483, L3791RS3, L3791RS2, 20188+3928, IC1396-N, L1204-A and L1204-B, with certain positions and flux densities are published in this paper. The mean differences in positions between these new Hz0 masers and the central IRAS point sources of outflows are IAal = 23”, IASI = 15”. Key words:

star formation-H:aO

maser-molecular

outflow-IRAS

object

1. INTRODUCTION It is now generally believed that during the earliest stages of their evolution, most stars undergo a phase of very energetic mass ejection, frequently characterized by t Supported by National Academy of Sciences Received 1991 November

Natural

Science

Foundation

and Astronomy

Branch

of Chinese

11

$ The National Radio Astronomy Observatory is operated Under agreement with the National Science Foundation, U.S.A.

by Associated

Universities,

Inc.

308

XIANG

De-lin & B.E. Turner

are found to be close to the dense cores ~7~1 These outflows appear to be driven by strong stellar winds, whose other notable manifestations include rapidly moving Herbig-Haro (HH) objects, high-velocity HzOmasers, shock-excited molecular hydrogen emission regions, and optically visible jets, While optical observations have been building the case for some type of stellar wind or outflow phase of evolution for recently formed low-and intermediate-mass stars, radio and infrared observations have been providing evidence that mass outflow may be associated with the formation of massive star&l. In p~tic~~, the discovery of high-velocity HzOmaser sources around luminous embedded objects led to the suggestion that the very dense (i.e.107- lo* cmm3), compact (e.g. lo-1OOAU) maser knots were accelerated to high velocities (50-100 km/s) in an intense stellar HzOmaser sources with HH obwind131. Indeed, th e association of low-luminosity jects has led to the su~estion that masers and HH objects may be very similar objects observed at different sta.ges of evolution in an outflow from around a young star14*51. And approximately 70% of all luminous HzOmasers have high-velocity (i.e. [‘l, which suggests that an outflow phase may be common IDI > 10 km/s) components to high-mass as well as low-mass stars during their early evolutionl’l. Thus it is of interest to establish various kinds of relations~ps jects and their properties, outflows, and HrOmasers, to improve about the early stages of star formation.

between IR.AS obour understanding

Tentative relationships between luminosities of HzOmasers and IR objects have earlier been discussed[7f81 but these studies involved only small numbers of objects (a dozen or so). In order to improve our understanding of the early evolutionary stages of star formation, a greatly increased sample is needed to establish better relationships between IRAS objects, molecular outflows and related HzOmasers. Based on a study of the literature [112~Q~101 it was found that 47 of a total of 144 known outflows are associated with known HzOmasers. All but 7 of the 144 known outflows are also associated with known IRAS sources. This study further showed that the positions of associated HzOmasers are very close to the positions of the central IRAS point sources that drive the outflows. The mean difference in positions is [Aal = 14” IAs = 9” for the 47 objects. Up to 1991, 157 molecular outflows have been discoveredl” I , of which 52 are associated with HzOmasers and 46 out of these 52 with central IR point sources. The apparent association of those two kinds of objects has prompted us to establish the relationships with higher significance in the star forming regions by extending the sample number. For this it is necessary to find a way to discover more new associated objects. Accordingly, we (including collaborators N. Kaifu, N. Ukita and 0. Kameya) proposed in 1990 to conduct these studies in two ways: (1) to search for new dense cores and outflows in star forming regions near known HzOmasers by observing 30 sources centered at the positions of IRAS point sources which were found within 30” of our candidate HzOmasers. These candidates were selected beforehand from 366 known galactic interstellar HzOmasers[10~121. The Nobeyama 45m telescope was used to search the J=l-0 lines of 12C0, 13C0 and Cl80 with a highest resolution of 20”.

Hz0 Masers

309

Nobeyama 45m telescope was used to search the J=l-0 lines of “CO, 13C0 and Cm0 with a highest resolution of 20". (2)To search for new Hz0 masers from the 97 outflows in the set of 144 known outflows that were not yet observed in the HsOline, centered at the central IRAS point sources or visible stars that drive the outflows.

2. OBSERVATION

AND

IDENTIFICATION

The Green Bank 43m telescope was used in January 1991for the observations of Hz0 masers. Its sensitivity is N 83y/K at 22.235 GHz. Front end dual channel masers provide Tsys of 60 K. The spectrometer was the 1024 channel MkIV autocorrelator used in the 2 ~10~12 channels configurations. Searching and confirming observations used band widths of 20 MHz (f134 kms-‘) and 10 MHz providing velocity resolutions of 39 kHz (0.52 kms-l) (f67 kms-’ ) respectively, respectively. For the Hz0 maser line, the rest frequency used is 22.23508 GHz, the beamwidth (HPBW) = 84”. Based on the statistical characteristics of 47 known Hz0 masers associated with molecular outflows the position uncert~nty is expected to be small (5 16”). We used g-point square maps with 30” between points, 2” and 4m integrations for searching and confirming observations respectively. Maps were iterated until positions accurate to I 15” were obtained. Accurate telescope pointing was established (5 10”) by observing nearby continuum sources such as W3 (OH), 0528+134, MonR2, 1730+130,2005+403 and BL Lac et al., which were always within one hour of the HsOsources observed. If we exclude the strongest 5% of Hz0 maser from the 47 outflow objects which exhibit them, a typical flux for the remaining 95% of Hz0 maser is 10 Jy thus it is necessary to achieve better limits than 10 Jy. Assuming 8 JyfK at 1.3 cm at the 43 m telescope, a Tsys of 60 K, a spectral resolution of 0.52 km/s (39kHz), due-chant operation, and Mk IV autocorrelator, a 2 minutes integration in beam switching mode gives a limit of 2.2 Jy(5a). Atmospheric extinction corrections have been made using the antenna efficiency as a function of hour angle established by Maddalena (1990). The maximum efficiency is VA M 0.25, and always exceeded 0.17 for the range of declination and hour angle used. 10 out of 15 newly discovered Hz0 masers are listed in Table 1 and their spectra are shown in Figure 1. Table 1 gives parameters of 10 sources whose positions and flux densities have been fairly well determined. The upper table lists the custom source name, observing date, position, flux, LSR velocity, and line width. The lower table gives the name, position and distance of the central IR.AS point source or visible star which drives the known CO outflow fl*‘l. In Figure 1 the abscissa is LSR velocity (km/s) and the ordinate is flux density S(Jy) at 22.235 GHz. All sources in the list have been identified. No previously known Hz0 masers lie nearby. We have also checked that HsOlines shown in Figure 1 do not arise from stronger, more distant HzOsources seen in the antenna sidelobes. All linewidths are in the typical

XIANG De-lin & B.E. Turner

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257

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Hz0 Masers

Spectral line profiles of the Newly Discovered Hz0 Masers

Fig. 1 Table 1

Parameters of the Newly Discovered Hz0 H?O Maser ob,. date

source

T

R. A. (1950)

Masers New Detections

Dec. (1950)

(0,

of

VW Pm/a)

I

‘2 *>

1

1c1805-w

910124

02

25

12.9

20

12

10.7

2

L1641-N

910122

05

33

54.7

-06

23

5a

12.1

8.6

0.52

3

s2a7-A

910125

06

57

54.5

-04

32

19

5.5

31.2

0.52

4

L4a3

910125

la

14

50.4

-04

40

15

10.1

5.9

0.52

5

L379IRS3

910125

18

26

32.6

-15

17

17

9.6

6

L379IRS2

910125

la

27

43.1

-15

16

43

61

-33.3

0.78

-50.7

192

1.04

20.4

0.52

7

20188 + 3928

910125

20

18

50.4

39

28

20

9.5

8

IC1396-N

910125

21

39

07.9

58

02

31

33.9

-3.6

0.76

9

Ll204-A

910125

22

19

45.6

63

36

04

96.0

-22.0

3.3)

10 L1204-B

910125

22

19

52.6

63

21

14

6.8

-7.0

0.71

Kooro

CO Outflow

4.1

0.52

Neerb7

R.A.(1950) m, 1) 0,

Source 1

Iclaos-w

02

25

14.5

20

10

2.3

2

L1641-N

05

33

52.7

-06

61

24

02

0.5

3

S287-A

06

57

54.5

-04

32

22

1.4

4

L483

18

14

50.6

-04

40

49

0.25 0.2

5

L379IRS3

18

26

32.9

-15

17

51

6

L3791RS2

18

27

43.4

-15

16

45

2.0

7

2018a +392a

20

18

50.7

39

28

la

4.0

8

IC1396-N

21

39

10.3

58

02

29

0.75

36

33

0.9

22

12

0.9

L1204-A

22

19

50.7

63

10 L1204-B

22

19

55.7

63

9

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LSR velocity

end lioeridth

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the strongest

feature

rscpectively.

XIANG De-lin & B.E. Turner

314

range of Ha0 maser. They are neither thermal emission nor that of transitions from other molecules in the frequency range observed.

3. PRELIMINARY

RESULTS

AND CONCLUSIONS

The results of the first observing run are as follows: (1) In 1990 four or five possible new molecular outflows have been found at the Nobeyama 45m telescope in JapanI141. (2) In 1990-1991 we found 15 new Ha0 masers out of 97 candidate sources. We searched with the Green Bank 431x1telescope in U.S.A. Five of the fifteen newly discovered Hz0 masers aud the 4 or 5 possible new molecular outflows need to be further observed to determine their accurate positions and flux densities. We are also now able to extend the sample number to over 70 for further analyses and studies as described above. We report here the ten new Hz0 masers discovered in galactic star-forming regions whose positions and flux densities have been determined. Other new HQO masers, outflow source and the analyses of the relationships between molecular outflows and their properties, Hz0 masers and IRAS objects will be pub~shed elsewhere.

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