ELSEVIER Chinese Astronomy and Astrophysics 29 (2005) 9-19

CHINESE ASTRONOMY AND ASTROPHYSICS

A N e w Observation of ClSO (J - - l -0 ) Line Emiss ion in W31 t*

Z H A N G Y a h - p i n g 1 H U A N G Y u - m e i I S U N J in 1,2 L U D e n g - r o n g a

1Department of Astronomy, Beijing Normal University, Beijing 100875 2CAS-Peking University Joint Be@n9 Astrophysical Center, Beijing 100871

a Qinghai Station of Purple Mountain Observatory, Chinese Academy of Sciences, Delingha 817000

A b s t r a c t Using the 13.7-m millimeter wave telescope of Purple Mountain Ob- servatory, we for the first t ime observed in c l S o ( J = l - 0 ) , the northwest region (size 16' x 25') of the molecular cloud W31. We constructed contour maps in dif- ferent velocity ranges (grid separation 1'). Three compact molecular clumps are identified and their physical parameters are derived from the observed tempera- ture and line width; they are found to be rather young and stable. We discuss their distribution with respect to the associated HII regions, in regard to star formation.

K e y words : stars: formation--ISM: molecules --ISM: individual objects: W 3 1 - - radio lines: ISM

1. I N T R O D U C T I O N

Recent studies have indicated that massive stars form only in giant molecular clouds under certain conditions, and that they are eventually generated from the bigger compact clumps. Destruction of the environment by their strong violet emission and clustering in their for- mation complicate the study of the massive stars. At present, we still don' t know much about the birth and very early stage of massive stars [1]. Since the massive stars are formed in the dense cores of giant molecular clouds, they should be surrounded in thick molecular envelopes. Even for an already formed massive star, it is most likely to be deeply buried

t Supported by National Natural Science Foundation, Joint Laboratory for Radio Astronomy, CAS and Science Foundation for Undergraduates, Beijing Normal University

Received 2003-10-21; revised version 2004-03-17 * A translation of Acta Astron. Sin. Vol. 45, No. 3, pp. 242-252, 2004

0275-1062/05/S-see front matter © 2005 Elsevier B. V. All rights reserved. DOI: 10.1016/j.chinastron.2005.01.002

10 ZHANG Yah-ping et al. / Chinese Astronomy and Astrophysics 29 (2005) 9-19

in its parent cloud. Moreover, the observed young massive stars are generally quite dis- tant (D >1 kpc). All these factors cause difficulties in the observation and study of massive star formation.

W31 is an HII region and molecular cloud complex located in the Galactic plane. It is centered on the UC HII Region G10.2 - 0.3 (a =18h06m23s.6, 5 = --20°19/53~, 1950). The region we observed is the northwest part of W31, about 23 ~ away from the center. This molecular cloud contains multiple compact HII regions, including G10.47+0.03, G10.46+0.03 and G10.30-0.15, and in recent years a number of water, OH and methanol masers have been found in association with these regions [2,3]. This means that W31 is a relatively active star forming region of massive stars. Wood et al.[2] made VLA high resolution radio continuum observations on the HII regions, and Churchwell et al. [3], also using VLA, made water maser and NH3 spectral observations of the same. These observations reveal that the hot and dense molecular cloud cores are often associated with some bright and small- scale ionized gases [4]. Sun Jin et al. [5] made 13CO(I-0) mapping observations on a rather large region (28 ~ × 45 ~) of W31. For studying further the internal structure of this cloud and the physical and dynamical properties in the early stage of massive star formation, and considering that when the density of hydrogen molecules in the molecular clump is large enough (n(H2) > 104 cm-3), the ]3CO line will become optically thick, we have made, for the first time, C]sO(1-0) mapping observations on a dense region of this molecular cloud. Because W31 is located in the Galactic plane and near the Galactic center, and because the molecular clouds within it are closely packed, we may expect overlapping of clouds along the line of sight. In this paper, the molecular clouds with different line-of-sight velocities are analyzed separately, and in the mapping region three C180 (1-0) molecular clumps are observed for the first time. We have calculated their physical parameters and discussed their relationship with star formation.

2. O B S E R V A T I O N S

In January, 2002, we used the 13.7m millimeter wave telescope at Qinghai Station of Purple Mountain Observatory and made Clso(J--1-0)(109.7822GHz) mapping observations of a part of the molecular cloud W31. The main-beam width of the telescope's antenna is 55", the main-beam efficiency Tim b is 66.6%, and the pointing accuracy is better than 10". The front-end of the receiver is a 3 mm-band cooled SIS mixer, the receiver noise temperature is ~60K (double-sideband), and the system noise temperature during the observations is 138 K (double-sideband). The back-end of the spectral receiver is a 1024-channel acousto- optic spectrometer (AOS) with a total bandwidth of 168.6MHz (at ~ l l0GHz, equivalent to a velocity coverage ~450 km/s) and a spectral resolution of 250 KHz (equivalent to a velocity resolution ~0.68km/s at ~l l0GHz). The black body equivalent to the ambient temperature is adopted for the temperature calibration. The antenna temperature obtained directly from observations is T~ (corrected for the atmospheric attenuation, radome loss, antenna's backward scattering and radiation loss), and the spectral radiation temperature is T~ = T~/~,~b. For the spectral resolution 0.68 km/s, the rms noise level of the system is 0.1 K (for the effective integration time 120s). The position switching mode is used in the observations, and the beam separation is lq The area mapped is 16 ~ × 25q

ZHANG Yah-ping et al. / Chinese Astronomy and Astrophysics 29(2005) 9-19 11

3. R E S U L T A N D A N A L Y S I S

Fig.1 shows a contour map of the velocity-integrated intensity ( f T~dV) of the ClSO(1-0) line emission in the observed region. The range of velocity integration is -40 k m / s ~ 100 km/s.

Table 1 Posi t ion and ClsO(1-0) line width of HI I regions

No l b RA DEC AV IRAS Association Type 1950 1950 (km/s)

1 10.48 +0.03 18 05 39.97 -19 51 44.3 4.733 HII 2 10.473 +0.027 18 05 40.36 -19 52 19.1 4 .57 18056-1952 UC HII 3 10.462 -{-0.034 18 05 38.78 -19 52 43.5 3.66 18056-1952 UC HII 4 10.46 -I-0.02 18 05 39.2 -19 53 12 4.43 HIt 5 10.44 +0.01 18 05 41.3 -19 54 26 3 . 0 9 18056-1954 HII 6 10.322 -0.155 18 06 03.22 -20 05 03.6 5.28 UC HII 7 10.320 -0.156 18 06 02.11 -20 05 41.3 4.04 18060-2005 HII 8 10.301 -0.147 18 05 57.67 -20 06 27.5 4.49 UC HII 9 10.290 -0.218 18 05 52.22 -20 06 27.6 4.14 UC HII 10 10.241 -0.077 18 05 34.73 -20 07 30.7 1.59 UC HII

Table 1 presents the positions of the HII regions in the mapped region, taken from the high resolution and high sensitivity VLA observations at 5 GHz, 1.4GHz and other frequencies [6-9], together with the observed half-maximum ClSO line widths at those posi- tions. The last two columns give the associated source and type of the HII region.

Fig.2 is a typical multi-peaked spectrum in the northwest region of W31 (c~ --- 18h05m59 s, 5 ---- --20°01/00 ", 1950), and it can be interpreted in terms of two or more molecular clouds with different velocities along the line of sight. For studying better the structure of this re- gion, we will make separate maps for the velocity intervals (7.8 - 15.9) km/s, (41 - 50) km/s and (61 - 70) kin/s, and label the separate clouds, C1, C2 and C3. The molecular cloud C1 still needs some further observations, so for the moment we shall discuss only the other two clouds. Their integrated intensity maps are shown in Fig.4 (for C2) and Fig.3 (for C3). The area mapped is 8 ~ x 81 for C2 and 7 ~ x 14 ~ for C3.

3.1 M o l e c u l a r C l o u d C3

Fig.3 is the contour map of integrated intensity for the molecular cloud C3 in W31, in which the bold solid line is the isoline for the integrated intensity equal to 0.6 times the peak integrated intensity. The molecular cloud C3 contains three compact clumps; from top to bottom, they are labelled C3A, C3B and C3C.

There are many maser sources in association with the molecular cloud C3. Their positions are marked in Fig.3 with different symbols: filled squares for water masers, empty diamonds for methanol masers, and filled triangles for OH masers. Their numerical data are listed in Table 2.

All the three kinds of masers being good tracers of massive star formation, their presence here implies that the clump C3A is a very active massive star forming region. C3A contains also many HII and compact HII regionsl such as G10.47+0.03, G10.46+0.03 and so on, and three of the compact HII regions are associated with masers. As shown by the VLA high resolution NH3 spectral observations by Cesaroni et al. [1°], G10.47(No.2) contains three compact NH3 molecular clumps, and G10.46 (No.3) contains 2 compact NH3 molecular clumps. This implies that multiple massive stars are forming here.

Commonly, the CIsO line can be used for tracing dense cores of molecular clouds, and the dense cores and clumps generally indicate the positions of star formation. In the

12 Z H A N G Yan-ping et al. / Chinese A s t r o n o m y and As trophys ics 29 (2005) 9 -19

_ ~ g * ~ '_

_I ,,.*,%

- 20 ~ 00'

/

I Q \

"N

l

RA(195O) Fig. 1 Contour map of the integrated intensity of c lSo(1-0) line emission in the northwest region of W31. The contour levels s tar t at 0.1 K.km/s, and then increment in steps of 0.8K.km/s. Bold "+" t s denote the UC HII regions, thin "+"Is the HII regions, and the numbers correspond to the serial numbers in Table 1.

ZHANG Yan-ping et al. / Chinese Astronomy and Astrophysics 29(2005) 9-19 13

0.2

~ 0 . 1

0

- O . 1 ,

r

0 50 100 V LSR / (km/s)

Fig. 2 A typical multi-peaked spectrum in the northwest region of W31

molecular cloud C3, the HII regions are not positioned at the center of the ClSO compact clump, rather, they are distributed around the center of the clump C3A. This means that the stellar wind generated by the newly formed stellar object has destroyed the structure of its parent cloud and produced a new density distribution.

Most of the HII regions and maser sources in association with the molecular cloud C3 are distributed near the clump C3A, and none of them appears near the clump C3C. This implies that star formation is quite active in the clump C3A, while C3B and C3C are relatively stable molecular clumps.

T a b l e 2 M a s e r s o u r c e s i n a s s o c i a t i o n w i t h t h e m o l e c u l a r c l o u d C 3

maser RA DEC V peak source name IRAS association number reference 1950 1950 (km/s)

H20 18 05 40.40 - 1 9 52 21.30 60.2-70.3 G10.47+0.03 18056-1952 4 [11, 12] OH 18 05 39.97 - 1 9 51 47.3 66 G10.48+0.03 18056-1952 1 [13] OH 18 05 40.0 - 1 9 52 24 75.0 G10.47+0.03 18056-1952 1 [13] OH 18 05 38.7 -19 52 34 57 G10.46+0.03 18056-1952 1 [14] OH 18 05 47.0 - 1 9 55 01 71.9-75.5 G10.45-0.02 18056-1954 2 [14, 15]

methanol 18 05 39.93 - 1 9 51 42 61--62 Gt0.48+0.03 18056-1952 2 [16] methanol 18 05 4 0 . 3 4 : 1 9 52 21 59-76 G10.47+0.03 18056-1952 9 [16] methanol 18 05 38.00 -19 52 34 61.7 G10.46+0.0 18056-1952 1 [17] methanol 18 05 46.99 - 1 9 55 09.6 68-79 G10.45-0.02 18056-1954 5 [16]

3.2 Molecu la r Cloud C2 Fig.4 is the contour map of integrated intensity of the molecular cloud C2 in W31, in

which the bold solid line is again the isoline at 0.6 times the peak value. The densest part of the molecular cloud C2 (~ =18h05m44 s, 5 = --20°06~00 ", 1950) is situated between two ultra compact HII regions. It might have been formed by the pressing together of winds from the newly formed massive stars on either side. It is relatively stable at present, but it may well be the site of the next generation of star formation.

3.3 Calcu la t ion of t he Phys ica l P a r a m e t e r s of the Molecu la r C lumps To study further the properties of the molecular clumps, we have calculated the various

physical parameters for the clumps in the clouds C2 and C3, including the spectral optical thickness, column density, volume density, size and mass of the clump. The calculations are

14 ZHANG Yan-pin 9 et al. / Chinese Astronomy and Astrophysics 29 (2005) 9-19

.19°50 /

~" .19°55 t k~

.20°00 '

18ho5m50 s 18ho5m34 s

R.A (1950)

Fig. 3 Contour map of the integrated intensity of the C 1sO emission in the molecular cloud C3 of W31. The range of velocity integration is 61--70km/s, the contour levels start at 0.1 K.km/s, and increment in steps is 0.3 K.km/s.

ZHANG Yan-ping et al. / Chinese Astronomy and Astrophysics 29(2005) 9-19 15

0

,,., ,.,.., ,.,4 ~

~ , ,~ ( 1 9 5 o )

Fig. 4 Contour map of the integrated intensity of the ClSO emission in the molecular cloud C2 of W31. The range of velocity integration is 41--50 km/s. The contour levels s tar t at 0.1 K.km/s, and increment in steps is 0.5 K.km/s.

based on LTE (local thermodynamic equilibrium), and the distance and excitation temper- ature data are taken from the observations made by Churchwell et al.[3]: Tex=14K for C2, and Te==20K[3] for C3. The optical thickness is obtained with the following formula[18]:

T~(ClSO) } ~(C180) = - lu 1 - 5.27 [J(Tox) - 0.16~] ' (1)

in which J(Tex) = 1/[exp(5.27/T~x) - 1]. According to Ref.[19], we can derive the formula for the column density of ClsO molecules to be:

N(c lSo) = 2.13 x 1014 fTR(ClSO)dv 1 : ~ ~ ) ( c m - b . (2)

And from N(H2)/N(ClSO) = 7.1 × !0 612°], we can then obtain the //2 column density N(H2) of the cloud.

For a molecular cloud in LTE, its mass MLTE (expressed in solar masses) can be cal-

16 ZHANG Yan-ping et al. / Chinese Astronomy and Astrophysics 29 (2005) 9-19

culated by the next formula[5]:

MLTE = # m H / N(H2)ds(M®), S

(3)

in which s is the projected area of the cloud on the celestial sphere, #=2.8 is the average molecular weight, and mH is the mass of hydrogen atom. The calculated port ion of the clump is the par t encircled by the bold solid line, as shown in Figs.3 and 4.

The virial mass can be obtained by the following formula[2°]:

Mvir ---- 189 x Rhm(pc)AV2(km/s)(M®), (4)

in which Rhm is the harmonic mean radius of the clump, and AV is the average half-width of ClSO line.

The calculated results for the clumps in the molecular clouds C2 and C3 are listed in Table 3. V0 is the central velocity of the clump.

Table 3 Phys ica l parameters of the molecular c lumps in the n o r t h w e s t region of W31 der ived f rom our c l S o observat ions

clump name C3A C3B C3C C2 RA(1950) 18 05 40 18 05 42 18 05 46 18 05 44

DEC(1950) -19 53 00 -19 56 00 -19 59 30 -20 05 00 V0 (kin/s) 66.06 67.76 66.08 44.49

T~(K) 20 20 20 14 AV (kin/s) 4.571 7.880 8.565 6.988

T~(K) 0.632 0.473 0.548 0.513 -r 0.059 0.044 0.051 0.076

N(ClSO)(1015cm -2) 4.3 5.5 7.0 3.9 N(H2)(1022cm -2) 3.1 3.9 5.0 2.8

MLTE (104M®) 1.807 1.140 1.035 1.051 Mvir (104 M®) 1.195 2.493 2.898 2.297

D (kpc) 5.8 5.8 5.8 6.0 Rum (pc) 3.027 2.286 1.966 2.489

n(H2)(103cm -3) 1.639 2.796 4.098. 1.824

4. D I S C U S S I O N

(1) Compared with the 13CO(1-0) mapping observations towards W31 reported in Ref.[5], because the Clso(1-0) emission is optically thinner than the 13CO(1-0) emission, some finer internal structures are revealed in the present work (see Fig. 1).

(2) From the 13CO observations on the Rosette molecular cloud, Williams et al. [21] obtained that the average density n(H2) of the clumps in tha t cloud is about 103 cm -3. As seen from Table 3, the densities of the 4 clumps obtained by us are all on the same order of magnitude as the densities of the clumps in the Rosette molecular c loud- -a molecular cloud of middle evolutionary age.

(3) As shown in Table 3, for the molecular clump C3A, we have Mvir < IF/LIE. The main error in the calculation of MLTE comes from the C l s o line brightness tempera ture or the observed antenna temperature . According to the da ta provided by the Qinghai station the relative accuracy of the observed C l s o line antenna t empera tu re is about 17%,

ZHANG Fan-ping et al. / Chinese Astronomy and Astrophysics 29(2005) 9-19 17

so the calculated MLTE will be (1.807 ± 0.307) M®, and we still have the conclusion that MLTE > Mvir. It means that this clump is unstable, and the associated HII regions and maser sources indicate that a stellar object has formed or is forming in this clump. On the other hand, for the clumps C3B and C3C we have Mvlr > MLTE, it implies that these clumps are in the state of virial equilibrium, i.e., they can avoid the fate of collapse by relying on the support of the thermal pressure. And the observations demonstrate also that there is only one maser source near the clump C3B, and that there is no infrared source or HII region in association with C3C.

(4) From Table 3 we can find that the values of hydrogen density n(H2) for the three clumps in the molecular cloud C3 are in the order C3A<C3B<C3C; however, according to the virial criterion, C3A is the most unstable with stars forming, while C3C is contrarily the most stable. Fig.5 shows the spectrum of the clump C3C (c~ = 18h05m54 s, 5 = --19°59'30 ", 1950), in which two peaks appear at velocities 64 km/s and 68 km/s. Fig.6 shows the contour maps of C3 in the different velocity channels (the area mapped is the same as in Fig.3). In the optically thin case, the column density of the molecular gas is proportional to the spectral integrated intensity, so from these maps we can know the space distribution of column density for the ClSO gases with different velocities. The column density at the position of the clump C3C is high in the 60 - -64km/s map, is low in the 64 - -68km/s map, and is again high in the 68--72 km/s map. This can be taken to mean that the clump C3C is composed of two overlapping clumps in the direction of the line of sight, each has a rather small column density, but they are combined to give a large one.

As seen from Fig.6, for both clumps C3A and C3B the maximum column density appears at the same velocity range 64--70km/s . This implies that the two clumps are closely related and have a common bulk motion. On the other hand, for the clump C3C, besides the overall motion of C3, it shows also an additional peculiar motion.

0.6

0.4

0.2 -¢

o

-0.2

C3C- ~

- L. a

50 60 70 80 V LSR / (kin/s)

Fig. 5 A typical double-peaked spectrum of the clump C3C

(5) As shown in Fig.3, the clump C3A is associated with many HII regions and maser sources. According to the theory of triggered star formation [a,22], a star or a group of stars can trigger the formation of other stars, and can even lead to the formation of several generations of stars, i.e., the so called self-propagating mode of star formation, so C3A is most likely the site of a first-generation star formation. As regards the clumps C3A and C3B, their MLTE approaches their Mvir, so they are in a quasi-stable state of equilibrium,

18 ZHANG Yah-ping et al. / Chinese Astronomy and Astrophysics 29 (2005) 9-19

Fig. 6

60-62km/s 62-64kmls 64-66km/s

RA (1950)

66-68km/s 68-70kmls 70-72kmls

RA (19so) Channel integrated intensity maps of the molecular cloud C3 in W31. The contour

levels start at 0 .1K.km/s and the increment is 0.1K.km/s.

but they may leave such a state for one of star formation by even a small trigger action. The clump C3B is very likely the site of a second-generation star formation, and the clump C3C, one of third-generation star formation. As for the clump C3A, a massive star has already formed at its center, witness the multiple compact clumps in both GI0.47+0.03 and G10.46+0.03, and even some OB-type zero age main sequence (ZAMS) stars 122]. It is only

because they are deeply imbedded in the parent cloud, so they are not directly detected in the optical waveband. In the course of the star formation, a large amount of matter has been ejected, and the stellar wind may have driven off the surrounding molecular gas, lowering the average density of the clump C3A. On the other hand, extrusion of winds from the massive stellar objects in the clump C3A may have increased the density of the clump C3B, making it a probable site for a new generation of stars. As for the clump C3C, it consists of two overlapped clumps along the line of sight, and so far no trace of star formation has been found there.

(6) As shown in Fig.3 and Table 2, the molecular cloud C3 is associated with many maser sources, and we know that water, methanol and OH masers are generally associated

ZHANG Yan-ping et al. / Chinese Astronomy and Astrophysics 29(2005) 9-19 19

with star forming regions. The methanol masers and OH masers in C3 are always found to be associated together, suggesting that the two kinds of masers have similar excitation mechanisms. Water masers are observed only in G10.47+0.03, and since water masers are generally believed to be associated with youngest stellar objects [1,1°], G10.47+0.03 ought to be the youngest stellar object in the three UC HII regions.

Maser sources with no associated HII region are only found near the clump C3B. Now, as we believe, massive stars form by accreting surrounding gas, at the progenitor stage of this UC HII region, it should have undergone a period of rapid accretion from an accretion disk accompanied by massive bipolar outflows, and the OH and methanol masers near C3B are very probably related with such accretion and outflows. Besides, Ref.[23] has indicated that masers with no associated HII region are very probably related with young massive stellar objects that are deeply imbedded in the parent molecular cloud. For further interpretation we require high resolution multi-band observations on this molecular cloud.

(7) As regards the molecular cloud C2, Fig.4 shows that the known HII regions are not associated with the cloud, and that the masses of the clumps are not enough to form stars, so the clumps are stable.

A C K N O W L E D G E M E N T S We thank all the colleagues of Qinghai Station, Purple Mountain Observatory, for the help during our observations and Prof. Jiang Bi-wei for helpful discussion.

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