characteristics of massive star-forming molecular cores: the spectral observations of 12co, 13co and...

ELSEVIER Chinese Astronomy and Astrophysics 33 (2009) 32–47

CHINESEASTRONOMYAND ASTROPHYSICS

Characteristics of Massive Star-formingMolecular Cores: The Spectral Observationsof 12CO, 13CO and C18O and the Statistical

Comparison† �

ZHANG Li-yun � GAO YuPurple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008

Abstract With the 13.7m millimeter wave telescope of Purple Mountain Ob-servatory at Qinghai Station, the simultaneous mapping observations at the12CO(J=1-0), 13CO(J=1-0) and C18O (J=1-0) lines were performed towardsthe 24 Galactic high-mass star-forming cores, which are associated with watermasers and have available Spitzer’s infrared data. The average mapping rangewas 8′ × 8′. The C18O line emission was detected in all the cores, in which 11cores were observed to the half maximum of their C18O integrated intensities andthe rather extended (5′ − 8′) C18O maps were obtained, while the others werefailed to make such a large scale mapping because of the low SNR or the intrin-sically extended morphology of the cores. On the 11 completely mapped densecores, we analyzed their characteristics and made the statistics and comparisonson the integrated intensity ratios between 12CO and 13CO (R12/13), 13CO andC18O(R13/18), as well as 12CO and C18O(R12/18). We concluded that as atracer of dense gas, C18O is absolutely optically thin and can be used to detectthe detailed structures of the cores, and that in general the 3 ratios increasegradually from the core center to the periphery. We found that the integratedintensity ratio R12/13 ranges from 2 to 6; R13/18 fluctuates between 4 and 20, butin central regions it is concentrated in the range 6–12 with a small fluctuation;and R12/18 occupies a wider range 13-90, but it is concentrated between 13 and50 in the denser regions of the cores.

Key words: stars: formation—ISM: spectral lines—stars: emission lines—ISM:

† Supported by National Outstanding Youth FoundationReceived 2007–06–06; revised version 2007–07–13

� A translation of Acta Astron. Sin. Vol. 49, No. 2, pp. 144–158, 2008� [email protected]

/ /$0275-1062/08/$-see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.chinastron.2009.01.012

Chinese Astronomy and Astrophysics 33 (2009) 32–47

ZHANG Li-yun et al. / Chinese Astronomy and Astrophysics 33 (2009) 32–47 33

molecules

1. INTRODUCTION

Star formation is a basic topic in astrophysics, and the massive star-forming region is one ofthe indicators of the star formation in galaxies. To understand the star-formation processis very important for the knowledge about the galaxy formation and evolution. In recent 30years, the rapidly developing technology of the infrared, millimeter-wave and submillimeter-wave telescopes provides a powerful observational measure for the research of star formation,and makes it achieve a series of progresses. Among them, the theoretical research of starformation for isolated low-mass stars has been fruitful[1], but for massive stars, the starformation theory has not been perfect, the systematic observations of massive star-formingregions and the in-depth analysis on their internal properties are urgently needed. Theexisting studies indicate that massive stars form only in the dense cores of some giantmolecular clouds under certain conditions. Because of the surrounding thick molecular gasand dust envelope, the extinction at the optical and even near-infrared wavebands is verylarge, it brings some difficulties to the study about the star-formation process in the densecores of giant molecular clouds. However, at the moderately-far infrared and even longerwavelengths the gas properties can be studied through the dust, for example, the propertiesof molecular gas can be detected by the millimeter-wave molecular lines. Therefore, themillimeter-wave molecular probes (such as CO, HCN, CS, etc.) and the moderately-farinfrared continuum emissions (such as Spitzer’s 8, 24, 70, 160μm luminosities, etc.) havebeen widely used for the observational study of giant molecular cloud cores.

Besides, in recent years, the studies of star-formation regularity are mainly focussed onobtaining the relationship of star-formation rate with the interstellar medium (neutral gas)from the entirety of galaxies[2−4]. In order to perfect further the star-formation regularity,it is necessary to observe the Galactic cloud cores and study the star-formation regularity insmall-scale local regions[5]. With a very high critical density (>104 cm−3), the HCN and CSmolecules have been used as the probes of dense gas for a long time. But, HCN and CS aregenerally the optically-thick molecules. Being different from the HCN and CS molecules,the 13CO molecule is optically thin in most cases, and the C18O molecule is a completelyoptically-thin molecule, it gives us an opportunity to detect the regions closer to the corecenters.

Based on the above analysis, as the first step, we select sources from the sample of giantmolecular cloud cores observed by Shirley et al.[6] with the CS(J=5-4) line, and based on thedata of 12CO and its isotopes obtained from the 13.7m telescope at the Qinghai Station, thispaper is restricted to make a statistical comparison on the properties of the cores, to obtainthe ranges of the integrated intensity ratios between 12CO and its isotopes, as well as theoverall statistical values of these ratios. At present, the mapping observations of large-scaleC18O survey are very few. By comparing systematically the maps, especially by the mappingobservations of the optically-thin C18O, we intend to investigate the internal characteristicsof the cores to provide some reference for the future C18O observations with the telescopesof higher sensitivities. As for the second step, we expect to combine the obtained ratioswith the data of HCN, CS, HNC, HCO+, etc., as well as Spitzer’s moderately-far infrared

34 ZHANG Li-yun et al. / Chinese Astronomy and Astrophysics 33 (2009) 32–47

data, and statistically compare the different gas probes (such as 13CO, C18O, HCN, CS,etc.) and the different ratios (such as HCN/13CO, HCN/C18O, etc.) in relation with thestar-formation rate (for examples, Spitzer’s 8, 24, 70, 160μm luminosities, etc.), so as toidentify the best molecular probe for the high-density gas and star-formation rate in differentGalactic massive star-forming regions, and to determine better the relationship between thehigh-density molecular gas and the star-formation rate in a small scale.

2. SAMPLE AND OBSERVATION

In the Galaxy the 24 giant molecular cloud cores associated with water masers and availableof the Spitzer’s infrared data are selected as our observational sample. It is generally believedthat the water maser is associated with the UCH II region and massive star-forming region,and is the prominent sign of massive star formation. From the water maser sources given byCesaroni et al.[7] and the sample of the CS (J=5-4) observations made by Shirley et al.[7],our sample sources are selected according to following conditions: (1) with a distance lessthan 5 kpc, in favor of the star-formation study in the scale as small as pc; (2) with Spitzer’sinfrared imaging data released in 2006 as the probe of star-formation rate; (3) in northernsky with a declination higher than -15◦. Finally, 24 giant molecular cloud cores are selectedand listed in Table 1, in which D is the distance of the core, taken from Reference [1], vlsr

is the spectral line velocity.In Nov. 2005, with the 13.7m millimeter wave telescope of Purple Mountain Obser-

vatory at the Qinghai Station, we made the simultaneous spectral mapping observationsat the 12CO (J=1-0), 13CO (J=1-0) and C18O(J=1-0) lines towards the selected 24 giantmolecular cloud cores in the Galaxy. The main-beam width of the telescope’s antenna at110GHz is 61′′ × 67′′. For our extended sources, the moon-disk efficiency 71.5%±1.4% istaken as the main-beam efficiency η. The receiver frontend is the 3 mm super-conducting SISreceiver. In observations, the double-sideband system temperature varies between 150K and300K, depending on the weather condition. The receiver back-end consists of the 3 1024-channel acousto-optic spectrometers (AOSs) to enable the simultaneous observation at the 3spectral lines: 12CO (J=1-0) (115.271GHz), 13CO(J=1-0) (110.210GHz) and C18O(J=1-0) (109.782GHz). The total bandwidth of the AOS is 43MHz for the 13CO(J=1-0) andC18O (J=1-0) lines, and 145MHz for the 12CO (J=1-0) line, corresponding to the velocityresolutions of 0.21 km/s and 0.54 km/s, respectively. In observations, the position-switchingmode is adopted with the effective integration time of 60s and the beam separation of 60′′.But, for improving the signal to noise ratio, we have increased the integration time by re-peatedly observing every source for 2 or 3 times to make the noise temperature reduce to0.1K. In Table 1, for the C18O observations, the previous 11 sources are all observed to theplaces of their half-maximum integrated intensities, and at the edge weaker than the half-maximum integrated intensity we have observed only once. The next 7 sources are failedto observe to the places of half-maximum integrated intensities, because that they are veryextended or that the data quality is not ideal. And the signals of the remaining 6 sourcesare very weak.

The data processing is made by using the Gildas software package of IRAM.


Table 1 The list of the selected molecular cores

Core Name R.A. DEC. D vlsr

(J2000) (J2000) (kpc) (km s)CepA 22 56 18 +62 01 46 0.73 −10.7S87 19 46 20 +24 35 34 1.9 +21.3S88B 19 46 48 +25 12 56 2.1 +20.9S106 20 27 26 +37 22 52 4.1 −0.99S231 05 39 13 +35 45 54 2.3 −48.2S235 05 40 53 +35 41 49 1.6 −18.1S252A 06 08 35 +20 39 03 1.5 −16.6W3(OH) 02 27 05 +61 52 26 2.4 −31.6W75N 20 38 37 +42 37 38 3.0 +9.8DR21S 20 39 01 +42 19 30 3.0 −1.2W75(OH) 20 39 01 +42 22 50 3.0 −3.0DR21S 20 39 01 +42 19 30 3.0 −1.2G123.07−6.31 00 36 48 +63 29 02 2.2 −18.5S255 06 12 54 +17 59 22 1.3 +7.2W44 18 53 19 +01 14 57 3.7 +109.8S76E 18 56 10 +07 53 14 2.1 +58.9BFS11−B 21 43 07 +66 07 04 2.0 −11.4NGC7538 23 13 45 +61 26 51 2.8 −56.5G59.78+0.06 19 43 12 +23 43 54 2.2 +44.3G19.61−023 18 27 39 −11 56 27 4.0 +5.9G20.08−0.13 18 28 10 −11 28 48 3.4 +42.7G24.49−0.04 18 35 05 −07 31 27 3.5 +41.9G35.58−0.03 18 56 23 +02 20 28 3.5 +32.6G35.20−0.74 18 59 13 +01 40 41 3.3 +54.1OH43.80−0.13 19 11 54 +09 35 55 2.7 +30.3

3. DATA PROCESSING AND RESULTS

3.1 The Basic Information and Morphology of the CoresTaking Cep A, S87 and S106 as examples, Fig.1 gives the typical 3 spectra of 12CO,

13CO and C18O obtained simultaneously. In the left 3 diagrams, the short-dashed line, long-dashed line and solid line correspond respectively to the 12CO, 13CO and C18O emissionlines. All the 3 spectra of Cep A are expressed by the actually-observed antenna temperature;for S87, the 3 spectra from top to bottom are expressed by the 0.1, 0.2 and 2 times of theactual antenna temperature, respectively; and for S106, the C18O spectrum is expressed bythe 5 times of the actual antenna temperature. In the right 3 diagrams, the upper spectralline is the weighted average of multiple observations at the central peak position of theC18O line emission, the lower spectral line is the weighted average of the multiple C18Ospectra observed once at a different place on the edge and multiplied by 2. The spectrain the left diagrams are all taken from the peak positions of C18O integrated intensities.As is shown, the intensity of the 12CO line is greater than those of the 13CO and C18Olines, the intensity of the 13CO line is greater than that of the C18O line; the line width of12CO is the greatest, 13CO is the secondary, C18O is the narrowest; and the 3 spectra arebasically coincident in positions. For comparisons, Table 2 presents numerically the spectralinformation at the peak positions of C18O integrated intensities, in which T ∗

mb is the observedbrightness temperature corrected for the main-beam efficiency, FWHM is the full width athalf-maximum. Among all the spectra, only the 3 spectra of the source S235 do not vary


completely according to the above-mentioned rules, because of the very weak C18O emissionat this place and the rather large line width, which makes the integrated intensity of thecore become large. The diagrams on the right side show the contrast between the spectraat central peak positions and the spectra at edges. In each diagram, the upper spectralline is obtained by the weighted average of the multiple observations at the central peakposition, and for a rather high signal to noise ratio, the lower one is obtained by the weightedaverage on the multiple single observations at different places on the edge, then multipliedby two. From this figure, we can see that the signals at the edge in 5′ apart from the centerare very weak, about 4-5 folds weaker than those at the center. Fig.2 demonstrates thesuperimposed intensity contour maps of the 3 spectral lines for the 11 cores which have beencompletely mapped at the C18O line. As is shown by this figure, in general, the 12CO coreis consecutively larger than the 13CO core and C18O core.

Fig. 1 Spectral lines of 12CO and its isotopes

3.2 The Basic Physical Parameters of the CoresIn order to study the properties of the giant molecular cloud cores, we have calculated


Table 2 The spectral information at the peak positions of C18O integrated intensities

Core Name offset T ∗mb

(k) FWHM (km s)(arcmin) 12CO 13CO C18O 12CO 13CO C18O

Cep A (+0 + 0) 16.6 10.5 2.1 8.2 4.5 2.7S87 (+0 + 1) 34.0 9.8 0.95 4.3 3.5 3.4S88B (+0,−1) 27.6 7.9 0.67 4.9 3.2 3.3S231 (−1,−1) 16.1 5.8 0.76 5.2 4.0 3.0S235 (+1,−1) 17.5 7.7 0.77 5.0 2.5 5.5S252A (+1,−2) 23.5 13.7 2.3 5.9 3.9 3.0W3(OH) (+0, +0) 23.2 9.1 1.0 6.7 4.8 4.5W75N (+2,−1) 14.4 9.9 1.4 6.2 3.1 3.3DR21S (+0,−3) 27.3 15.0 2.3 7.3 3.3 3.1W75(OH) (+0, +2) 25.2 14.8 2.5 6.5 3.4 3.2S106 (+1, +1) 20.4 12.7 2.4 4.3 2.8 2.2

the physical parameters for every core. In calculations, we assume that the cores are inlocal thermodynamic equilibrium (LTE). As in molecular cores the 12CO line is generallyoptically thick, the excitation temperature of the 12CO line emission can be derived fromits peak brightness temperature[8],

Tex =5.53

ln [1 + 5.53/ (T ∗mb (12CO) + 0.84)]

, (1)

here, the radiative temperature of the cosmic microwave background is taken to be 2.73K.We assume also that the 13CO and C18O lines have the same excitation temperature as the12CO line, then anywhere in the core, the optical depths of the emission lines are:

τ(13CO) = − ln[1− T ∗

mb(13)5.29/[exp(5.29/Tex)− 1]− 0.89

], (2)

τ(C18O) = − ln[1− T ∗

mb(13)5.27/[exp(5.27/Tex)− 1]− 0.89

], (3)

and the column densities of the 13CO and C18O molecules can be derived from the followingformulae[9,10],

N(13CO)cm−2

= 4.71× 1013 Tex + 0.88exp(−5.29/Tex)

τ̄

1− exp(−τ̄ )

∫T ∗

mbdv

[Kkm s−1], (4)

N(C18O)cm−2

= 4.75× 1013 Tex + 0.88exp(−5.27/Tex)

τ̄

1− exp(−τ̄)

∫T ∗

mbdv

[K km s−1]. (5)

The characteristic size R of the core means the radius of the circle whose area equals thearea (A1/2) enclosed by the half-maximum intensity contour of the dense core. It can beobtained by the deconvolution on the beam width:

R = D

(A1/2

π− θ2

MB

4

) 12

, (6)

in which D is the distance of the core, A1/2 is the angular area within the border of half-maximum integrated intensity, θMB is the beam width.


Fig. 2 The superimposed intensity contour maps of 12CO and its isotopes. The grey map,solid line and dashed line show the integrated intensity contours of 12CO, 13CO and C18O,respectively. The contour level starts from the 40% of the peak intensity, and increases bythe step of 10%. The thick line represents the half peak intensity, the plus sign marks theposition of reference, and the dots are the sampling points in observations. From top tobottom and from left to right, the sources are Cep A, S252A, S88B, S231, S87, S235, W75,W3(OH), W75(OH) and S106, respectively.

Taking the element abundance ratios N(H2)/N(13CO) and N(H2)/N(C18O) to berespectively 5×105 and 6×106[11,12], and combining with the above characteristic size R ofthe core, the derived mean volume density of H2 molecules is

n(H2) = N(H2)/2R . (7)

Based on the LTE assumption, the mass of the core is calculated as:

MLTE = μmH2N(H2)× (πR2) , (8)

in which mH2 is the mass of hydrogen molecule, μ is the mean molecular weight consideringthe contributions of He and other heavy elements to the total mass, here it is taken to be1.36[13].

For every core, we derive piecewise the integrated intensity ratios of the 13CO and C18Olines (R13/18) and of the 12CO and C18O lines (R12/18), as well as the excitation temperatureTex and the optical depths of the 13CO and C18O lines. The calculated characteristic sizes,total volume densities and masses of the cores are listed in Table 3. The two cores DR21Sand W75(OH) given here are actually the two cores DR21 and W75(OH) in Cygnus, thereal DR21S is located in the even more south of DR21, outside our observed range[14].Besides, as the separation between DR21 and W75(OH) is only 3′, but the resolution andsampling interval are all ∼1′, the edges of the two cores can not be separated, so the givencharacteristic size is the sum of those of the two cores, as is shown by Fig.2, however, lateron we still call them as DR21S/W75(OH). Among the listed 11 cores, only 7 sources areobserved to the half-maximum border of 13CO integrated intensities; for the other 13COcores, we can give only the lower-limit mass, instead of the accurate mass, and they aremarked with the symbol ∗ in the list. The parameters of the sources Cep A and S231 are


given by taking the contour line of the 60% of the maximum 13CO integrated intensityas the border, the parameters of the source S252A are given by taking the contour lineof the 70% of the maximum 13CO integrated intensity as the border, and for the sourceW75(OH)/DR21S, it is very difficult to find a suitable contour line, so the parameters arenot given. Comparing the mean gas densities of the 13CO and C18O cores, the mean gasdensity of the core regions traced by the C18O line is obviously greater than that of thecorresponding regions traced by the 13CO line, namely the C18O line can trace the evendenser gas. Although the mass of the C18O core is less than the 13CO counterpart for 2-3times at most, but the size of the C18O core is less than the 13CO counterpart for aboutone fold, therefore the C18O core regions are more compact.

Table 3 The physical parameters of the cores

Core Name R (pc) nH2 (104 cm−3) M (M�)13CO C18O 13CO C18O 13CO C18O

Cep A* 0.65 0.35 1.24 2.93 1466 517S87 0.88 0.64 1.09 1.49 4716 1645S106 1.60 0.93 0.75 1.81 10219 4209S231* 1.69 0.82 0.23 0.48 4717 2206S235 1.48 1.03 0.80 1.03 1299 1179S252A* 1.25 0.65 0.94 1.25 7766 7362S88B 0.91 1.12 0.39 1.17 6681 3909W3(OH) 1.48 1.03 0.51 0.76 6981 3466W75N 1.89 1.47 0.55 0.53 15788 7081W75(OH)/DS21S* · · · 1.93 · · · 0.81 · · · 24627

3.3 The Integrated Intensity Ratios of the 3 Kinds of Spectral Lines of 12COand Its Isotope

For every core, Table 4 gives the ratios of the 12CO and 13CO integrated intensities(R12/13’s), averaged on the different radial ranges from the center. With the interval of 25%of 13CO maximum integrated intensity, we divide radially every core region into 4 pieces: themaximum place, the places from the maximum to the 75% maximum, the places from 75%to 50% and the places of 50% less. As the 12CO and 13CO integrated intensities in observedcore regions are all much larger than 5σ, we just make the averages on the 12CO and 13COintegrated intensities at different points in the core, then obtain the ratio between the two.For the cores Cep A, S252A, S231 and W75(OH)/DR21S, the half-maximum contour linesof 13CO integrated intensities have not been plotted completely, so only rough estimationsare made, and they are marked by the symbol ∗ in the table. The ratios in the last columnof the table are calculated by selecting 1-2 points in the outmost observed region, and thelast row of the table gives the averaged results of every column. The errors of correspondingratios do not include the systematic errors, but comprise the noises of the observed spectrathemselves, namely, they are derived from the following error transfer formula:

σ(R12/13) =

√[σ(I12)

I13

]2

+[σ(I13)×R12/13

I13

]2

, (9)

and this error is generally less than the systematic error. Similarly, the errors of the othertwo ratios R13/18 and R12/18 can be derived, and the details are not described here. FromTable 4 we can find that the ratio R12/13 decreases gradually from the periphery to the


center, but the variation is not very large, the reduction from the periphery to the corecenter is only about 50%.

Table 4 List of the ratios of the 12CO and 13CO integrated intensities

Core Name R12/13

max 75% 50% < 50% outmostCEPA* 3.2 ± 0.04 2.8 ± 0.4 3.1 ± 0.1 3.5 ± 0.4 4.0 ± 0.1S87 2.9 ± 0.03 3.1 ± 0.06 2.9 ± 0.04 3.4 ± 0.2 3.9 ± 0.5S88B 4.2 ± 0.06 4.2 ± 0.3 4.1 ± 0.08 4.6 ± 0.3 5.4 ± 2.1S106 2.8 ± 0.03 2.9 ± 0.2 3.1 ± 0.02 3.5 ± 0.2 3.5 ± 0.05S231* 3.0 ± 0.05 3.3 ± 0.1 3.8 ± 0.1 4.7 ± 0.2 4.2 ± 0.1S235 2.9 ± 0.07 2.9 ± 0.07 3.5 ± 0.07 5.6 ± 0.2 5.9 ± 0.09S252A* 2.4 ± 0.02 2.4 ± 0.09 2.7 ± 0.03 3.1 ± 0.1 3.9 ± 0.06W3(OH) 3.4 ± 0.04 3.6 ± 0.8 4.1 ± 0.6 4.6 ± 0.2 4.5 ± 0.08W75N 2.5 ± 0.09 2.6 ± 1.2 3.1 ± 0.6 3.9 ± 0.6 4.0 ± 0.2W75(OH)/DR21S* 2.7 ± 0.06 3.3 ± 0.1 3.5 ± 0.08 3.1 ± 0.3 3.1 ± 0.06average 3.0 ± 0.02 3.1 ± 0.2 3.4 ± 0.09 4.0 ± 0.1 4.2 ± 0.2

Table 5 gives the radial distributions of the 13CO/C18O ratio (R13/18) and the 12CO/C18Oratio (R12/18), calculated by dividing every core into pieces and making average on each piece,for every core, from top to bottom. This table gives the variations of the ratios from thecentral region to the periphery, as well as the excitation temperatures and optical depthsof corresponding pieces. For every core, the region is divided into pieces with one samplinginterval, outwards from the peak position of the C18O integrated intensity. As the geomet-rical shapes of the cores are different, the divided regions are also different, generally, thecore is divided into 3-7 small regions. In each small region, the weighted average is madefor the 3 spectral lines to improve the signal to noise ratio, before deriving the ratios.

Table 5 List of the ratios of the 13CO and C18O integrated intensities and of the12CO and C18O integrated intensities

Core Name Tex τ Ratio(K) 13CO C18O R13/18 R12/18

Cep A 15.5 1.45 0.153 5.5 ± 0.1 14.7 ± 0.315.8 1.25 0.104 7.8 ± 0.1 19.1 ± 0.314.5 0.88 0.079 6.7 ± 0.1 19.0 ± 0.312.8 0.71 0.074 5.7 ± 0.1 19.4 ± 0.5

S87 34.4 0.40 0.031 8.3 ± 0.4 31.6 ± 1.428.8 0.39 0.026 11.8± 0.3 41.5 ± 1.121.8 0.41 0.028 12.9± 1.1 47.0 ± 4.123.1 0.33 0.018 15.3± 1.0 66.8 ± 4.3

S88B 26.7 0.42 0.031 4.7 ± 0.3 21.3 ± 1.223.5 0.43 0.030 7.9 ± 0.3 31.1 ± 1.414.9 0.64 0.044 19.7± 0.9 51.7 ± 2.416.8 0.51 0.030 15.5± 1.0 47.5 ± 3.014.4 0.27 0.012 17.7± 2.5 86.0 ± 12.2

S106 21.1 0.80 0.090 8.5 ± 0.06 26.0 ± 0.222.0 0.77 0.096 7.7 ± 0.09 25.3 ± 0.318.9 0.54 0.051 11.6± 0.2 34.5 ± 0.613.0 1.11 0.088 12.7± 0.4 29.6 ± 0.912.0 0.54 0.041 17.1± 1.4 45.2 ± 3.810.0 0.75 0.054 8.7 ± 0.4 18.6 ± 0.912.7 0.67 0.039 13.5± 0.7 42.3 ± 2.3

S231 16.7 0.58 0.061 7.2 ± 0.2 22.8 ± 0.614.9 0.73 0.040 12.6± 0.5 39.8 ± 1.7


Table 5 (continued)

Core Name Tex τ Ratio(K) 13CO C18O R13/18 R12/18

15.4 0.72 0.030 16.5± 0.9 49.3 ± 2.813.0 0.61 0.033 15.7± 0.7 45.8 ± 1.8

S235 19.3 0.64 0.056 8.6 ± 0.6 37.4 ± 2.416.9 0.49 0.044 15.7± 2.8 83.4 ± 15.015.5 0.39 0.033 7.7 ± 1.0 48.5 ± 6.2

S252A 24.5 0.91 0.105 7.7 ± 0.1 20.1 ± 0.322.2 0.91 0.109 7.8 ± 0.1 19.9 ± 0.325.9 0.81 0.086 9.1 ± 0.1 26.5 ± 0.320.4 0.78 0.065 10.8± 0.2 29.7 ± 0.518.4 0.68 0.051 11.9± 0.3 35.3 ± 0.917.3 0.47 0.032 19.1± 1.5 65.2 ± 5.1

W3(OH) 18.9 0.63 0.052 10.2± 0.2 31.3 ± 0.717.7 0.48 0.046 10.3± 0.3 34.2 ± 1.012.8 0.53 0.062 6.2 ± 0.2 20.0 ± 0.715.6 0.42 0.030 16.5± 1.2 68.5 ± 4.9

W75N 15.4 1.21 0.126 5.1 ± 0.2 15.2 ± 0.415.5 0.71 0.104 5.7 ± 0.2 18.2 ± 0.515.6 0.61 0.059 7.0 ± 0.3 22.6 ± 0.813.5 0.98 0.077 12.0± 0.5 32.0 ± 1.411.1 1.06 0.044 14.0± 1.0 44.2 ± 2.9

W75(OH)/DS21S 21.1 1.26 0.144 6.0 ± 0.2 13.4 ± 0.523.1 1.14 0.116 8.2 ± 0.6 25.9 ± 1.820.5 0.65 0.091 9.2 ± 0.6 28.6 ± 1.721.5 0.85 0.074 10.9± 0.7 37.6 ± 2.523.9 0.67 0.070 8.5 ± 0.5 29.4 ± 1.817.1 1.21 0.103 18.1± 2.9 32.3 ± 5.2

4. ANALYSIS AND DISCUSSION

4.1 Analysis and Comparisons of the Basic Characteristics of Giant MolecularCloud Cores

Although 12CO and its isotopic molecules 13CO and C18O require the same criticaldensity for excitation, but compared with the 12CO and 13CO molecules, the abundance ofC18O molecules is very low, and compared with the 13CO line, the C18O line is opticallythinner, so the C18O line can be used to probe the even deeper regions of the cores. Asshown by Fig.2, the C18O cores are always embedded in the 13CO cores, the finer structuresin the central core regions can be resolved, and they usually have even smaller sizes. FromTable 3, we can find that compared with the C18O core, the 13CO core has a larger size,smaller molecular number density and a slightly greater mass, except that for only one cloudcore the molecular number densities given by the two lines are very close to each other. Bycalculations, the averaged radius ratio of the 13CO and C18O cores is 1.51, the ratio ofmolecular number densities is 1/1.55, and the averaged mass ratio is 1.20. For a part of thecores, the calculated 13CO line emission is not completely optically thin (see Table 5), forexamples, at the core centers of Cep A, W75N and W75(OH)/DR21S, the optical depth isgreater than 1, but the mass given by us is obtained under the optically-thin assumption,thus the masses of these 4 cores in the table are on the low side.

In this paper, the obtained physical parameters of the 13CO cores are rather consistent


with the previously published results[15] in the order of magnitude, and the obtained physicalparameters of the C18O cores are also similar with the previous results[16].

4.2 The Analysis and Discussion on 12CO/13CO Integrated Intensity RatiosThe 12CO/13CO integrated intensity ratio (R12/13) indicates the optical depth of the

13CO line emission. Under the LTE state, if we assume that the 12CO line is optically thick,but the 13CO line is optically thin, and that both of them come from the equally densegases, then R12/13 is reversely correlated with the 13CO optical depth τ(13CO):

R12/13 ∝ 1/τ(13CO) . (10)

For every core, the value of R12/13 increases gradually from the center to the edge, it indicatesthat in the central region the gas is denser and τ(13CO) is larger, and that going more andmore outward, the gas becomes more and more diffuse, therefore τ(13CO) is less and less.The left panel of Fig.3 gives the histogram of the R12/13 distribution, it demonstrates thatthe 12CO/13CO integrated intensity ratios (R12/13) are mainly concentrated between 2.5and 4.5, with a mean value of about 3.45. In the scale of galaxies, for extreme star-burstgalaxies, R12/13 is in the range 20-50, and the range for normal galaxies is 5-15[17,18]. Ourstatistical result on the giant molecular cloud cores in the Galaxy is even less than the valueof normal galaxies. Limited by the observed regions, in the reachable peripheral regions ofour observations, R12/13 (approaching to 6) has been close to the value of normal galaxies.If the observed regions could be further extended outwards, it is very possible to obtain thestatistical result similar with that of normal galaxies. But, why our statistical result is muchless than the value of star-burst galaxies, we still can not explain. Among the 11 cores, thereare 5 cores whose ratios are not increasing outward, it is mainly because that the region of13CO half-maximum integrated intensities is relatively coincident with that of 12CO. At thecenter, τ(13CO) is large, the intensity of 12CO emission is also large, but in the same regionthe 12CO emission varies more rapidly than the 13CO emission, so that R12/13 exhibits onlya relatively small fluctuation or variation, as the cases of W75(OH)/DR21S, S88B, S87 andCep A.

4.3 The Analysis and Discussion on the 13CO/C18O and 12CO/C18O IntegratedIntensity Ratios

Generally speaking, the 13CO/C18O and 12CO/C18O integrated intensity ratios (R13/18

and R12/18) also increase gradually from the core center to the periphery, particularly promi-nent is the variation of R12/18. The middle and right panels of Fig.3 display the histogramsof the ratios R13/18 and R12/18, the former varies in the range 4-20, and is mainly concen-trated between 6 and 12, the latter ranges between 13 and 90, and is mainly concentratedin the range 13-50. Because of the different distributions of C18O line intensities in differentcores, the variation of the ratio may have some fluctuation. As a comparison with the pre-vious works done by other authors, Reference [19] gives the typical value of R12/18 being inthe range 25-110, basically consistent with our result, and the value of R13/18 being in therange 3-8, with a smaller fluctuation than our result, but consistent well with our result formost ratios (6-12). In the following, we will analyze and discuss every core one by one.

S88B: From Table 5, we can find that the optical depths of the 13CO and C18O linesdecrease gradually from the central region to the edge, and that the fluctuations of the ratiosR12/18 and R13/18 are caused by the fluctuation of the excitation temperature Tex. And from


Fig. 3 Histograms of the 12CO/13CO, 13CO/ C18O and 12CO/C18O integrated intensity ratios

Fig.2, we can find as well that the centers of the distributed regions of the 3 isotopes arenot coincident, it is also a factor leading to the fluctuations of the ratios.

S106: In this core, the regions of the half-maximum integrated intensities of the 3 COlines are rather extended in the DEC direction, and there exist obviously the 13CO and C18Odouble-nuclei. In the RA direction, these regions are narrow, it leads to the fluctuations ofthe ratios in these regions, and compared with the DEC direction, both the 12CO and 13COline intensities decrease rapidly, it causes the ratios become small.

S231: Relative to the region of the C18O half-maximum integrated intensities, theregions of the 12CO and 13CO emissions are very extended, this is the reason that the tworatios are rather small at the edge.

S235: That the C18O molecule can trace the denser gas region has obtained the bestverification in this core, and in the 13CO emission region, two compact C18O nuclei canbe clearly observed. Relatively, the 12CO and 13CO emission regions are more extended, itleads to the similar situation of S231, namely, the rather small ratios at the edge.

W3(OH): In this core, the fluctuations of the ratios are caused by the apparent reductionof the excitation temperature Tex, the influences on the optically thick 12CO and 13COemissions are much greater than the influence on the C18O emission, and at the fluctuationposition the optical depth of the C18O emission is reversely increased, it makes both ratiosdecrease, and therefore causes the fluctuations of the ratios. In addition, in contrast to the12CO and 13CO emission regions, multiple nucleus regions exist in the C18O emission region,this is naturally a factor to cause fluctuations.

W75N: The separated C18O double-nucleus is also detected in this core region. Wehave divided the central region into 3 pieces and made comparisons, the calculated ratiosR12/18 and R13/18 do not differ much in the 3 small regions, but still have fluctuations. Thisis because that in the distributed regions of the 3 isotopes, even though one of the nucleusregions has a good coincidence, but the others are not coincident, and further outwards, theratios tend to increase.


S87: Although the ratios given in the table increase from the center to the periphery,but in fact this core has two velocity components. When we make contour maps, the twocomponents are processed as a single one, it makes the obtained ratio variations representonly a rough tendency.

The 3 cores CepA, S252A and W75(OH) obey basically the rule that the ratios increasegradually from the center to the periphery. This is because mainly that the C18O regionsof these 3 cores are all surrounded by the 12CO and 13CO regions, and that the intensityvariations of the 3 emission lines are rather consistent. From the table we can see as well thatthe ratios of the different cores vary in different ranges, it may be caused by the differentevolution stages of the different cores, leading to somewhat different integrated intensityratios of the 3 kinds of isotopes.

4.4 Comparison with the Incompletely-mapped Sample CoresBecause of the very weak signal of the C18O emission, and because that a part of our

sample cores are much extended in dimensions, in addition to the restriction of the observa-tion time and weather condition, 13 cores are not mapped completely. In observations, wefind that there are 9 cores whose 12CO spectral lines exhibit the complex multi-peak shape,so they are temporarily ruled out from our statistics. The statistical results of the ratios ofthe remaining 4 incompletely-mapped cores are listed in Table 6. For every core, the first

Table 6 Integrated intensity ratios in the cores incompletely-mapped at the C18O

line

Core Name Tex τ Ratio(K) 13CO C18O R13/18 R12/18 R12/13 position

BFS11-B 19.0 0.99 0.078 9.1 ± 0.5 27.7 ± 1.4 2.6 ± 0.04 max15.1 1.21 0.090 14.9± 1.1 40.0 ± 2.9 2.3 ± 0.2 75%15.1 0.99 0.089 8.6 ± 0.4 24.2 ± 1.1 2.6 ± 0.3 50%15.1 1.18 0.092 10.0± 0.4 24.7 ± 1.0 3.6 ± 0.6 < 50%13.7 0.66 0.061 10.9± 1.0 41.0 ± 3.6 · · · · · ·14.6 1.10 0.100 10.6± 0.8 25.3 ± 2.0 · · · · · ·13.9 1.03 0.078 11.2± 1.1 26.8 ± 2.7 · · · · · ·12.9 0.86 0.070 9.1 ± 1.0 23.2 ± 2.5 · · · · · ·11.4 0.84 0.069 14.8± 1.8 48.7 ± 5.9 · · · · · ·

G123.07-6.31 12.7 1.49 0.150 6.2 ± 0.3 19.3 ± 1.0 3.6 ± 0.09 max11.9 1.25 0.105 8.3 ± 0.4 23.3 ± 1.1 3.3 ± 0.2 75%10.8 0.96 0.092 8.9 ± 0.4 25.8 ± 1.2 3.5 ± 0.1 50%10.0 0.76 0.050 12.9± 1.9 56.8 ± 8.4 4.6 ± 0.2 < 50%

G59.78+0.06 16.9 0.94 0.099 8.0 ± 0.2 23.1 ± 0.5 3.7 ± 0.05 max14.9 0.67 0.096 6.2 ± 0.2 22.6 ± 0.7 3.5 ± 0.1 75%18.6 0.68 0.071 10.3± 0.4 30.1 ± 1.3 3.6 ± 0.3 50%· · · · · · · · · · · · · · · 4.5 ± 0.7 < 50%

S255 28.1 0.65 0.064 9.6 ± 0.2 34.1 ± 0.8 2.9 ± 0.04 max23.1 0.48 0.047 9.7 ± 0.2 36.8 ± 0.8 3.7 ± 0.1 75%16.9 0.39 0.033 10.8± 0.3 48.5 ± 1.4 3.8 ± 0.2 50%19.7 0.43 0.041 9.6 ± 1.0 42.0 ± 4.4 4.5 ± 0.4 < 50%

5 columns are the statistical values derived by dividing the core into pieces in the manner ofTable 5, and the last 2 columns are the statistical values of R12/13 obtained by dividing thecore by way of Table 4. The errors are also derived according to the formula in Sec.3.3. Asthe observed regions are very small, the integrated intensity ratios of the 3 kinds of spectrallines are concentrated in rather small ranges, namely, R12/13, R13/18 and R12/18 vary in theranges 2.3-4.6, 6-15 and 19-50, respectively. These statistical values are rather close to those


obtained in the central regions of the completely-mapped cores.

5. CONCLUSIONS

(1) For giant molecular cloud cores, the 12CO line is optically thick and cannot be used tostudy very well the internal physical properties of the molecular cloud cores. The 13CO lineis much optically thinner than the 12CO line and has a small scale, it can be used to explorebetter the internal structures of the cores. And in the C18O map, which has an even smallerscale than the 13CO map, the finer structures of the core can be seen. Our observationalresults agree exactly with this conclusion.

(2) For the 11 completely-mapped cores in our observational sample, the optical depthat the C18O line is much less than 1, and this is a typical optically-thin emission. Most ofthe optical depths at the 13CO line are less than 1, to be also optically thin, except for CepA, W75(OH) and DR21S, whose optical depths at the core center are greater than 1, beingoptically thick.

(3) The statistical result indicates that the gas density in the 13CO cores is obviously lessthan that of the C18O cores, the averaged number density ratio is 1/1.55. It demonstratesas well that the C18O line is the probe of higher column densities, able to trace better thedenser regions of the cores.

(4) In the cloud cores, the integrated intensity ratios between 12CO and its isotopesincrease gradually from the center to the periphery. The 12CO/13CO integrated intensityratios are in the range 2-6, and the variations from the center to the periphery are not verylarge. The 13CO/C18O ratios are in the range 4-20, the ratios near the core centers arerelatively small, in the range 6-12, equivalent to those obtained from normal galaxies. The12CO/C18O ratios are in the range 13-90, and around the core centers, they are about 13to 50.

(5) We have made the statistical analysis on several incompletely-mapped cores, theobtained statistical results are very consistent with those of the 11 completely-mapped cores.

ACKNOWLEDGEMENT We thank the colleagues at the Qinghai Station of PurpleMountain Observatory for the support and assistance during our observations.

References

1 Shu F. H., Lizano S., Adams F. C., IAUS, 1987, 417

2 Solomon P. M., Downes D., Radford S. J. E., et al., ApJ, 1997, 478, 144

3 Gao Y., Solomon P. M., ApJS, 2004, 152, 63

4 Gao Y., Solomon P. M., ApJ, 2004, 606, 271

5 Wu J., Evans N. J., et al., ApJ, 2005, 635, 173

6 Shirley Y. L., Evans N. J. II, ApJS, 2003, 149, 375

7 Cesaroni R., Palagi F., et al., A&AS, 1988, 76, 445

8 Blitz L., Thaddeus P., ApJ, 1980, 241, 676

9 Sun Jin, Li Shou-Zhong, Foundation of Molecular Astrophysics (Vol.2), Beijing: Publishing House of

Beijing Normal University, 2003, 340

10 Zhang Y. P., Sun J., et al., AcASn, 2002, 43, 152


11 Dickman R. L., ApJS, 1978, 37, 407

12 Frerking M. A., Langer W. D., Wilson R. W., ApJ, 1982, 262, 590

13 Hildebrand R. H., QJRAS, 1983, 24, 267

14 Dickel J. R., ApJS, 1978, 233, 840

15 Geng T., Yang J., Thesis for Bachelor degree, CAS, 2005

16 Yonekura Y., Asayama S., et al., ApJ, 2005, 634, 476

17 Paglione T. A. D., Wall W. F., et al., ApJS, 2001, 135, 183

18 Taniguchi Y., Ohyama Y., Sanders D. B., ApJ, 1999, 522, 214

19 Meier D. S., Turner J. L., ApJ, 2001, 551, 687

20 Tachihara K., Mizuno A., Fukui Y., ApJ, 2000, 528, 817

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