New observation of the OH 1667 MHz maser in the central part of ON2

CHINESE ASTRONOMY AND ASTROPHYSICS PERGAMON

Astronomy

Chinese

and Astrophysics

24 (2000)

5640

New observation of the OH 1667MHz in the central part of ON2 t *

maser

YU Zhi-yao Shanghai

Chinese

Observatory,

Academy

of Sciences,

Shanghai

200030

Abstract OH 1667MHz was observed for the first time in the central part of the OH maser source ON2 with the NanCay radio telescope of Paris-Meudon Observatory, France. The results indicate a double peak structure in the line profile. This implies that the OH 1667 MHz maser is located on a thin disk

moving Key

around

words:

the central

star.

OH 1667MHz

maser-maser

source ON2

1. INTRODUCTION

The maser source ON2 is located within a star formation rection and pointing to the well-known molecular complex parts,

Northern,

Central

See Fig. l.[‘l Many molecular

and Southern,

emissions

labelled

region, elongated in the NS diCygnus X. ON2 consists of three

respectively

have been observed

in ON2.

by ON2(N), Matthew

ON2(C), et alI

ON2(S).

carried

out

12C0 and l3 CO observations and constructed maps. Lekht et al.1’1 have monitored its Hz0 maser over a prolonged period of 14 years. Dent et a1.131observed its NH3 and HCO+ lines and constructed maps. Norris et a1.141measured the position of the OH 1667 MHz maser in ON2(C). Observation of OH 1667 MHz masers in regions of star formation can provide data on many kinematical and dynamical features of the regions. In this paper I used the Naqay radio telescope of Paris- Meudon Observatory, France, and carried out, for the first time, OH 1667MHz maser observation in ON2(C). The results t Supported

by National Natural Science Foundation

and CAS Astronomy

and Radio Astronomy

Laboratory Received 1998-07-27; * A translation 0275-1062/00/$

of Acta

revised version 1998-10-29

A&on.

- see front matter

PII: SO2751062(00)00026-6

Sin. @

Vol. 40, No. 3, pp. 230-234,

1999

2000 Elsevier Science B. V. All rights reserved.

Joint

YU Zhi-yao

revealed

a typical

surrounding

/ Chinese

double-peak

the central

Astronomy

spectrum,

24 (2000)

and Astrophysics

showing

that

the maser

56-60

57

is located

on a thin disk

star.

2. OBSERVATION The observation was carried out in November 1997 using the NanCay radio telescope of ParisMeudon Observatory. The telescope is an improved type Kraus radio telescope, possessing many special features well suited for tracking work. At frequency 1667 MHz the half-power beamwidth of the antenna is 3.5’(o) x 19’(S). The system temperature is about 45 K, and for a period of 1 hour and for a velocity resolution of 2.6 km/s, a typical sensitivity is 15 mK rms. The autocorrelation spectrometer covers the entire frequency band, divided into 4x256 channels. The position of the observed source is RA( 1950) 20h 1gm51s, Dec(l950) 37’16’ 59”. The central velocity is at 2.3 km/s

r..

8.

-.

I..

.

.

16

5s

I

Iu:

(dashed sources

clouds lines)

and Hz0 parts

IO

20

%SR (k+.)

the positions

(solid lines),

in the three

0

zut’19=

Fig. 1 Map showing molecular

-10

45’

50’

of

HI1 regions

and OH maser of the ON2 complex

Fig. 2 OH 166’7MHz circular

The spectral maser

profile of the

line in ON2(C).

polarization.

(b) Right

polarization

(a) Left circular

58

YU Zhi-yao

/ Chinese

Astronomy

3. RESULTS Fig. 2 displays the observed We see that, peak.

The measured

in Table

physical

of the OH 1667 MHz maser in ON2(C).

there exist two main peaks, a redshifted

and a blueshifted

of the two peaks in the two polarizations

Physical Parameters

of the Two Peaks in the Two Polarizations

Blueshifted

Peak

Redshifted

left-handed

VLSFl (Ws) -4.964

Flux (Jy)

are listed

right-handed

-4.121

45.80

(plr) were found to be adequately

sLr =

right plr, blue peak:

S’nb =

right plr, red peak:

Snr =

(1) the OH 1667 MHz maser in the Galaxy,

saturated

11.04

2.762

by the following

Snb =

left plr, red peak:

Flux (Jy)

5.806

1, the fluxes in the two peaks in the two polarizations

fitted

left plr, blue peak:

Peak

VLSR h/4 2.902

28.96

From the data in Fig. 2 and Table

\

1 0.4702(V&

+ ;.964)2

0.3009(VLsR - 2.;02)2 O.O2642(V&n

+ 0.03453 + 0.0958

~4.121)~

0.5701(VLsn

in ON2(C)

and the majority

expressions:

of such maser

source

(1)

+ 0.02184

- 2.762)2

is a maser

>

,

+ 0.1722

inside a region of star for-

sources

are regarded

as sources

of

amplification.

(2) Our observation for both

double-peak

of the OH 1667MHz

the left-handed

structure

(3) It is suitable saturated to about

that

proposed171, (symbols

and moreover,

both

that

to be independent

and the OH maser,

For a disk inclined coordinates

the region of the

and expanding

OH maser is located According

and rotates

as defined in Ref. [7]), the disk is assumed

are assumed

This suggests

disk structure.

disk to analyse

to assume a rotating

the HiI region.

expands

show

shows a

thin disk.

all the way until the ionized region has expanded

Elitzur 161states

shell surrounding

The results

the spectrum

one redshifted).

with the maser has a circumstellar

OH maser exists

the disk-shell

is new.

polarizations,

to use the model of thin, circumstellar

3 x 1017 cm, whereas

shock-compressed

in ON2(C)

with two main peaks (one blueshifted,

OH 1667 MHz maser,

Reid et a1.1’1 state

maser

and the right-handed

that the H II region associated

to T-r,

DISCUSSION

parameters

Polarization

that,

56-60

1.

Table 1

mation

AND

left and right polar&ions

in both polarizations

24 (2000)

and Astrophysics

of the height,

in the neutral

in the form

we

VT = Vr&,Tr, = l&f

to be thin and all the physical

the gas density

gas,

to the thin shell model

is assumed

quantities

to be proportional

saturated.

at angle

i to the line of sight,

the velocity

at the position

(2, y) on the disk is

c = vs- + VJ-

=

[(F)+ (Jy]; + [ (Jy- (Jy];.

with

YU Zhi-yao

For a saturated

/ Chinese

Astronomy

and Astrophysics

OH maser, an approximate

solution

24 (2000)

59

56-60

of the equation

of radiative

transfer

is I(L) The second

= I(0) - I(0)K.L

term on the right being dominant, I(L)

+ EL

the next approximation

is

= I(O)KL

We assume all the maser spots to have the same apparent size R. Then after a series of reduction, we find the following expression for the maser flux emitted at a point y on a disk of radius r and inclined at i to the line of sight: S=

a(VLsR

1 - VP)2 + b’

(2)

where E

a =

(PAV thermhOd i) V,,(~Y

’

r0

VP = - (1 + 9,

$cos(i)Vru(J-+, r0

b = a [

1

+?iE,]2[$+j(f-)E-“($+)‘I.

We now use the same disk model to analyse star formation

regions

model parameters: Using our analytic calculations

and the position

the peaks in Fig. 2. From our knowledge

of ON2 (Fig. 1) we selected

the following

of

thin disk

TO = 100 au, VP0 = 10 km/s, Vto = -lO,km/s, E = 0.5, p = -0.5, I = 45’. fit (1) to the observations and the theoretical result (2) and after some

we arrived

at the values of T and y shown in Table 2. Table 2

Our Calculated

Results

p (AU)

?/(AU)

Left plr

blue peak

113.5

-34.4

Left plr

red peak

115.0

+20.3

Right pk

blue peak

129.3

-33.2

Right plr

red peak

115.0

+19.6

(a) We see from Table 2 that

the blueshifted

on two different disk-shells of different values polarizations are located on the same disk-shell ‘(b) Again we see the blueshifted peaks in values of y and that the two redshifted peaks consistent with the coordinate system used in

peaks in the two polarizations

are located

of T, while the redshifted peaks in the two with the same radius. the two polarizations have close and negative have close and positive values of y;--this is our model.

60

/ Chinese Astronomy

YU Zhi-yao

(c)

It appears,

then,

there

disk-shell

of the left polarized

shell is next, redshfited absolute

exist

that

ON2(C),

disk-shell,

there are redshifted and redshift

rotating,

expanding

high resolution

observation.

The

expanding structure

disk-shell

value of y is always greater

(d) The

is generally

with OH masers

(e) Our discussion determination

reason for their separation.

star,

The

the redhsifted

maser

is furthest.

diskIn the

masers.

The

and this is

peaks. thin disk model

observation.

used in this paper

We shall be seeking before

by other

for the double-peak

to the late stage of a star’s are usually considered

can be determined

opportunity

authors15@l. evolution.

The star-forming

to have a thin shell structure.

of Zeeman

high-resolution

and

but a spherical

pair.

It dealt only with the

without

pointing

It is possible that the peaks belong to two different requires

for such

A rotating

spectrum,

of the peaks in the two polarizations,

of this point

of

blueshifted

in the right polarization,

interferometric

regard the blue and red peaks in Fig. 2a (left polzarization) are each a Zeeman

OH 1667 MHz maser

peaks of both left and right polarized

did not involve the question

of the location

but confirmation

to the central

in the left than

has been proposed

more suited

56-60

and one redshifted.

of the right polarized

shell can also account

spherical

regions associated

mapping,

model

disk of the

two blueshifted,

maser is the closest

and the blueshifted

true for both bleshifted through

in the circumstellar

3 disk-shells,

.24 (2000)

and Astrophysics

observation.

and Fig. 2b (right

out any

Zeeman pairs, If we

polarization)

pair, then the two red peaks should come from the same region,

and the

two blue peaks come from another

one region,

but their locations

in their own regions can

still be found from the disk model.

According

to our calcualtion,

if we suppose

of 900 pc for ON21’1, then the separations red peaks

are each less than

between

a distance

the two blue peaks and between

O.l”, so we can regard

the two

them each to come from one and the

same region. ACKNOWLEDGEMENT Observatory

I thank Professors

for help during the observation

E. Gerard

and J. M. Martin

of Paris-Meudon

and data treatment.

References Lekht E. E., Marques A., Mendoza Torres J. E., A&AS, 1996, 120, 415 Matthews N., Anderson M., MacdonaId G. H., A&A, 1986,155,

99

Dent W. R. F., MacdonaId G. H., Andersson M., MNRAS, 1988, 235, 1398 Norris R. P., Booth R. S., Davis R. J., MNRAS, 1980, 190, 163 Reid M. J., Moran J. M., ARA&A, Elitzur M., ARA&A,

1981, 19, 231

1992, 30, 75

YU Zbi-yao, XIANG De-lin, N. Kaifu, N. Ukita, CAA 1996, 20, 156 =AApS

1996, 16, 65

Campbell M. F., Hoffmann W. F., Thronson Jr. H. A. et al., ApJ, 1982, 261, 550