~ Pergamon

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Adv. Space Res. Vol. 30, No. 9, pp. 2059-2070, 2002 © 2002 Published by Elsevier Science Ltd on behalf of COSPAR

Printed in Great Britain 0273-1177/02 $22.00 + 0.00

PII: S0273-1177(02)00588-4

T H E L A R G E - S C A L E S T R U C T U R E OF G I A N T M O L E C U L A R C L O U D S :

S W h S O B S E R V A T I O N S O F [C I] 3P 1 A N D 13CO J - 5 > 4 E M I S S I O N

>3P 0

Ren6 Plume, 1 2 Gary Melnick 1, John Howe, 3 and Frank Bensch 1 4

1Harvard-Smithsonian Center for Astrophysics 60 Garden St., Cambridge, MA 02138, USA ~Department of Physics and Astronomy, University of Calgary, 2500 University Drive N. W.,

Calgary, Alberta, T2N-1N4, Canada 3Department of Physics and Astronomy, University of Massachusetts, Amherst, MA 01003

4 physikalisches Institut, UniversitSt KSln, Ziilpicher Strasse 77, KSln, Germany

A B S T R A C T

The Submillimeter Wave Astronomy Satellite (SWAS) is a NASA Small Explorer class mission de- signed to search for H20 and 02 emission in molecular clouds, and to investigate the large-scale structure of these clouds via mapping of the 3P1 --+ 3P0 transition of neutral atomic carbon (C I) and the J = 5 -+ 4 transition of 13CO. In this paper we will demonstrate the superb ability of SWAS to obtain maps of the physical conditions (i.e. density and temperature) in molecular clouds over scales much larger than have been previously possible. We will also show how, over large scales, the [C I] emission is consistent with that predicted by models of Photodissociation Regions (PDRs). Finally, we will briefly describe how the low-resolution [C I] observations can be used to infer clump sizes, even when the clumps are smaller than the beam. © 2002 Published by Elsevier Science Ltd on behalf of C O S P A R .

I N T R O D U C T I O N One of the goals of the Submillimeter Wave Astronomy Satellite (SWAS) is to examine the large-

scale structure of giant molecular clouds (GMC). To address this question, SWAS was designed to observe the 3P1 --+ 3P0 transition of neutral atomic carbon (C I) and the J = 5 -+ 4 transition of 13CO. Given SWAS's superb instrumental stability (Melnick et al. 2000), and the fact that it orbits above the Earth's atmosphere, the SWAS satellite has the unique ability to map these two high frequency, radio transitions over large scales (,,~ 1 ° x 1 °) in molecular clouds.

The [C I] 3/°1 --+ 3P 0 fine-structure transition (u = 492.1607 GHz) arises from a state that is only 23 K above ground. In addition, since the critical density of this transition is only ,,, 1000 cm -3 (SchrSder et al. 1991), this line is easily observable from the bulk of the low-density, low-temperature gas in molecular clouds. In fact, the large-scale distribution and morphology of [C I] emission closely resembles that of low-J transitions of 13CO (e.g. Ikeda et al. 1999; Plume et al. 2000; 1999; 1994). Apart from revealing the overall distribution and abundance of [C I] emission a number of issues remain unresolved. (1) Can [C I] be used as a tracer of gas not visible in CO? Studies of the large- scale distribution of H I have found moderately high column density H I clouds with no associated CO emission (Taylor 2000). A study of the carbon abundance in these clouds, could help determine their evolutionary state (i.e. whether or not they are transitioning from atomic to molecular clouds).

2059

2060 R. Plume et al.

(2) What is the origin of the [C I] emission? Models of photodissociation regions (PDRs) suggest that [C I] emission should arise in a thin layer at the surface of a UV illuminated molecular cloud (e.g. HollenbaCh et al. 1991; van Dishoeck and Black 1988). However, other models (e.g. Pineau des For~ts et al. 1992) suggest that [C I] emission may also arise deep in the interiors of clouds that are well-shielded from UV radiation. (3) What is the nature of the clump and interctump medium? It is well-known that molecular clouds are not monolithic slabs but contain significant sub-structure (e.g. Falgarone and Phillips 1996; Stutzki and Giisten 1990). Given this fact, is it possible to use the large-beam (3.5' × 5') observations of SWAS to examine the sub-structure (structure smaller than the SWAS beam size) of molecular clouds on large spatial scales?

The J -- 5 --~ 4 rotational transition of 13CO (v -- 550.926 GHz) arises from a state that is 79K above ground. Given the high excitation temperature and critical density of this transition (~ 3 × 105 cm-3), 13CO J -- 5 --+ 4 is a good tracer of the warm, dense gas in molecular clouds. However, given the poor atmospheric transmission at the frequencies associated with mid-to-high J transitions of 13CO, and the small beamsizes of large submillimeter telescopes, studies of distribution of warm, dense gas have been limited to small spatial scales (Howe et al. 1993; Schultz et al. 1992; Graf et al. 1990; Schmid-Burgk et al. 1989; Harris et al. 1987). Fortunately, due to the absence of an obscuring atmosphere, SWAS is able to map the distribution of 13CO J = 5 --+ 4 over large-scales in molecular clouds. Combined with low-J 13CO observations from ground-based observatories, we are able to map the physical conditions (e.g. density, temperature) of the gas over scales much larger than have previously been possible.

In the Results Section of this paper we will present highlights from the recent large-scale mapping projects of the Submillimeter Wave Astronomy Satellite, show how SWAS has been used to success- fully address some of the issues raised above. For additional information, the reader is directed to the August 20, 2000 issue of the Astrophysical Journal Letters, which is dedicated to the recent SWAS results.

OBSERVATIONS The SWAS observations presented in this paper were taken in a 2 year period between launch on

December 1998 and December 2000. The majority of the observations were obtained in a mapping mode and contain ~ 2 minutes of on-source integration time per point. However, the observations near the central (0,0) positions of each of the maps are often a few hours in duration. The beam spacing of the SWAS observations is 1.6' (half-beam spacing). For more information on the SWAS instrument, beam Sizes, data acquisition and reduction, see Melnick et al. (2000).

The 13CO J -- 1 -+ 0 observations were taken in at various times during the same 2 year period using the Five College Radio Astronomy Observatory (FCRAO) 14 meter telescope. We used the 16 element SEQUOIA array receiver coupled with a digital autocorrelator (FAAS) backend. The bandwidth of the autocorrelator is 40 MHz, with a channel spacing of 78 kHz, and a channel resolution is 94 kHz. The beam width is 47" (FWHM), and the map spacing is 44". All data are presented in units of T;~, however, to calculate densities and column densities the data are converted to main beam temperatures (Tmb) using efficiencies of Y,~b = 0.90 for SWAS (Melnick et al. 2000) and ~mb = 0.48 for the FCRAO data. Unlike ground-based telescopes operating at submillimeter frequencies, the absence of an atmosphere and the great instrumental stability of SWAS ensure constant calibration for all the SWAS data.

R E S U L T S Comparison of t h e [C I] and 13CO M a p s

Figures 1-4 present maps of the SWAS observations, as well as observations from ground-based telescopes, in four very different interstellar clouds.

Orion A (Figure 1) is a well-studied giant molecular cloud at a distance of ~-, 415 pc. The large

Large-Scale Structure of GMCs

[CI] 3PI-3P o 13CO J = 1--0 13CO J = 5--4

2061

E -2

-3

--4

-5

-6

-7

20 10 0 -10 Aa (Arcminutes)

10 20 30 10 20 30 40 50 20 40 60 80

TA'dV (K km/s) TA*dV (K km/s) TA*dV (K kin/s)

Fig. 1. Three panel figure showing observations of the Orion A molecular cloud. Offsets are given with respect to the central position defined as c~ = 5h:35m:14.5 s, 6 = --5°:22':37 " (J2000). Ovals in the bottom right-hand corner of each panel indicate the beamsize. At a distance of 415 pc, the SWAS beam has a linear size of 0.5 pc. A detailed description of each panel is as follows : left) The integrated intensity (f T~dV) of [C I] 3P1 --+ 3P 0 as observed by SWAS. middle) The 13C0 J = 1 --+ 0 integrated intensity as observed by FCRAO. right) The x3CO J = 5 --+ 4 integrated intensity as observed by SWAS.

[CI] 3P1-3P 0 13CO J = 5---4 13CO J = 1-0

20

15

10

<

'~ o

-5

-10

10 5 0 -5 10 5 0 -5 10 5 0 -5 Aa (Arcminutes)

Fig. 2. Three panel figure showing observations of M17SW. Offsets are given with respect to the central position defined as c~ = 18h:20m:22.1 s, 6 = --16°:12':37 " (J2000). At a distance of 2200 pc, the SWAS beam has a linear size of 2.8 pc. Contour levels for all maps are 0.1 to 0.9 of the peak intensity in steps of 0.1. A detailed description of each panel is as follows : left) The integrated intensity (fT~dV) of [C I] 3/91 -+ 3t9 o as observed by SWAS. Peak intensity is 57.2 K km s -1. middle) The 13CO J = 5 --~ 4 emission as observed by SWAS. Peak intensity is 56.9 K km s -1. right) The 13CO J = 1 --~ 0 emission as observed by FCRAO. Peak intensity is 38.2 K km s -1.

2062 R. Plume et al.

20

-~ 0

-20

-40

-60 , [CI,],,

140 120 100 80 60 40 20 Act (Arcminutes)

-20 -40

!

20

0

.~ ~ -20

-40

-60

50 40~"

20

140 120 100 80 60 40 20 0 -20 -40 Ao~ (Arcminutes)

Fig. 3. top) The integrated intensity (f T~dV) of [C I] aP1 -+ 3P0 as observed by SWAS in p Oph A. bottom) The 13CO J = 1 ~ 0 integrated intensity as observed by FCRAO. Offsets are given with respect to the central position defined as c~ = 16h:26m:23.4 s, ~ = -240:23':02 " (32000). At a distance of 160 pc, the SWAS beam has a linear size of 0.2 pc.

KOSMA 3m MCLD 123.6+24.9

CfA 1.2m: Polaris Flare 12CO(1-0) color: 12CO(2-1) contours: 13CO(2-1)

10

30 ] [ 20

130 125 120 123.8 123.6 123.4 123.2 Galactic Longitude ~ i 1 pc

• Positions observed by SWAS

Fig. 4. Two panel figure showing numerous observations of the Polaris Flare (MCLD123.6+24.9). The central position is defined as c~ = 2h:00m:06.6 s, 6 = 87o:42':04 " (J2000) but, in the figure, galactic coordinates are given. At a distance of 150 pc, the SWAS beam has a linear size of 0.2 pc. A detailed description of each panel is as follows : left) The integrated intensity (fT~,dV) of 12CO J = 1 --+ 0 from the CfA 1.2m "mini" telescope located in Cambridge, MA. right) Observations from the 3m KOSMA telescope in Zermatt, Switzerland. The greyscale shows the 12CO J = 2 -~ 1 integrated intensity and the contours display the I~CO J = 2 --+ 1 integrated intensity. The small grey triangles superimposed on this image present the locations of the SWAS [C I] 3P1 --~ 3Po observations.

Large -Sca le St ruc ture o f G M C s 2063

scale structure of Orion A has been the subject of many surveys : low energy rotational transitions of 12CO and 13CO (e.g. Heyer et al. 1992; Castets et al. 1990; Bally et al. 1987; Maddalena et al. 1986), CS J -- 1 --+ 0 and J = 2 --+ 1 (Tatematsu et al. 1998; 1993; Lada et al. 1991), continuum emission (350#m - Lis et al. 1998; 450ttm and 850#m - Johnstone & Bally 1999), and [C I] (Ikeda et al. 1999).

M17SW (Figure 2) is another well-studied GMC at a distance of 2200 pc. The M17SW cloud is illuminated by an extremely powerful UV field (104 Go where 1 Go is the average strength of the interstellar radiation field) originating from an OB association about 1 pc east of the cloud. Given the powerful UV field, and the fact that the ionization front is edge-on, M17SW has been the subject of many studies of cloud clumping and its effect on photochemistry (Stutzki & Gfisten 1990; Genzel et al. 1988; Stutzki et al. 1988; Keene et al. 1985).

In contrast to Orion A and M17SW, the p Ophiucus A cloud (Figure 3) is a nearby (~ 160 pc) region of low-mass star formation. Studies of the dust and molecular gas in this region have found high column densities (N(C180) ~ 2 x 1016 c m - 2 and A v "-' 5 0 - 100 mag; Wilking & Lada 1983). In addition, recent maps of the 1.3 mm dust continuum have shown several bright condensations (Motte et al. 1988).

Finally, the Polaris Flare, also known as MCLD123.5+24.9, (Figure 4) is a high-latitude, transclu- cent (Av = 0.5 - 2 mag) cloud which is also nearby (~ 150 pc). It is thought to be fairly quiescent and is not known to contain any star formation activity. 12CO maps of the region (Bensch 2001)

o 2.5

8

~ 1.5

t_~ 0.5

0

3

M,7 o c-, i 1 o o

I l l : ' ~ 5

r_~L~:H h: H I , :H ha H h : . h~,~iH ~, 0

0 10 20 30 40 50 60 70 80 0 5 10 15 20 25 30 35 40 45 50

TmbdV(13CO 14)) (Kkm/s) TmbdV(13CO 14)) (Kkm/s)

3 3

Io-sF-I L I ° o 2.5 ~ 25 p Oph A

2 ¢n 2

1.5

0 GH d , , . I , , , , I , . , ~R~qd,,,d,.,qP,~. h H,I 0

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 10 20 30 40 50 60

TmbdV( 13CO 14)) (K km/s) TmbdV( 13CO 14:)) (K kngs)

Fig. 5. Plots of [C I]/13CO J = 1 -+ 0 integrated intensity ( f T,~bdV ) ratio vs the laCO J = 1 -+ 0 integrated intensity (which is a direct measure of column density or Av). The open circles present the data from each position observed by SWAS. The solid squares represent averages of the ratio in consecutive 5 K km s -1 bins in 13CO J -- 1 -+ 0 integrated intensity, except for the Polaris Flare where the averages are performed in bins that are 1 K km s -1 wide.

2064 R. Plume et al.

Table 1. Column Densities

Source <N[C I]> <N(13CO)> am-2 cm-2

Orion A 2.0 x 10 IT 2.5 x 1016 M17SW 1.9 x 1017 1.1 x 1016 p Oph A 8.1 x 1016 1.4 x 1016 Polaris Flare 2.6 x 1016 2.1 x 1015

<N[C I ]> /<N(CO)>

0.15 0.31 0.11 0.22

show fairly bright emission with small (~ 0.2 pc) clumps that are visible in 13CO maps. These observations also indicate that the gas is farly cold (7K _< T• < l l K ) .

C l o u d S t r u c t u r e and t he Or ig in o f t h e [C I] E m i s s i o n Figures 1 - 4 show that, over large-scales, the [C I] emission is very extended, and is morphologically

similar to that of the the low-J 13CO emission. This behaviour has been noted in earlier [C I] studies of other GMCS (e.g. Plume 1999; 1994), and is not surprising given the similar excitation conditions between the two lines and the fact that even a weak UV field (Go = 1) can dissociate CO to form [C I]. An examination of the [C I]/13CO integrated intensity ratio, however, shows some interesting

f T.bdV[C I] behaviour. Figure 5 plots fT,,,~dY('3COJ=l--,O) versus f T, nbdV(13COJ = 1 -+ 0) for Orion A, M17SW,

p Oph A, and the Polaris Flare. In spite of the very different physical conditions in each of these clouds, Figure 5 shows some very remarkable results: 1) the typical [C I]/13CO integrated intensity ratio is similar in all clouds (~ 0.5 for p Oph A and the Polaris Flare, ~ 0.6 in Orion A, and ~ 0.8 in M17SW). 2) The [C I]/13CO integrated intensity ratio is constant over the bulk of each cloud, but increases asymptotically towards decreasing values of 13CO integrated intensity (or Av).

In Table 1 we present the cloud-averaged, total [C I] and 13CO column densities for each of the four clouds, as well as the average [C I]/CO column density ratio (assuming the 12CO/13CO abundance ratio is 55:1). For p Oph A and the Polaris Flare, the column densities were calculated by assuming that the emission is optically thin and that the levels were populated according to LTE (using kinetic temperatures of 20 K and 10 K respectively). For Orion A and M17SW, the column densities were calculated more exactly using a Large Velocity Gradient (LVG) code to solve the coupled equations of radiative transfer and statistical equilibrium (see Plume et al. 2000 and Howe et al. 2000 for details). Despite the vastly different temperatures, UV radiation fields, and [C I] and 13CO column densities that vary by an order of magnitude, Table 1 shows that the [C I]/CO abundance ratio only varies by a factor of 3.

Both the asymptotic behaviour seen in the data in Figure 5, and column densities shown in Table 1 can be explained by PDR models that assume the [C I] emission originates in a thin region at the surface of a UV illuminated cloud (e.g. Hollenbach et al. 1991; van Dishoeck and Black 1988). These models show that as one moves from the high column density cloud .core to the low column density edge, the [C I]/I~CO abundance ratio increases asymptotically. In addition, the column density of the [C I] emitting zone remains relatively constant at ~ 1017 cm -2, even if one changes the gas density and the UV field strength by several orders of magnitude. This theoretical carbon column density lies well within the range of that seen in Table 1. Only in the heart of the M17SW cloud does the observed [C I] column density exceed a few x 1017 cm -2. In this case it is necessary to invoke several PDRs along the line-of-sight in order to explain the observations.

E - ~

100

90

80

70

60

50

40

30

20

10

Large-Scale Structure of GMCs

I I

0

]

0

, i i I , , ~ I , i

1 2 fTmb(CI)dV/fTmb(lZCO l - 0 ) d V

, , : , :~ i~ ,~ g~ 0.05 0.1 0.15

Clump Radius (pc)

, I

3

0.2

2065

Fig. 6. Model of the distribution of spherical clump sizes as a function of both the kinetic temperature of the gas and the observed [C I]/13CO J = 1 -~ 0 integrated intensity ratio ( f T~bdV ). The model shown assumes that nH~(C I) = 10 5 cm -3. The location of the boxes indicate the clump size for a typical kinetic temperature and observed [C I]/13CO ratio in the four different clouds studied. The solid lines extending from each of the boxes indicate the possible range in clump sizes, given the possible ranges in the gas temperature and observed [C I]/13CO ratio.

Given the fact that clouds are not monolithic slabs, but are known to contain significant sub- structure in the small spatial regions that have been mapped at high-resolution (e.g. Falgarone and Phillips 1996; Stutzki and Giisten 1990), is it possible to use our large-beam carbon observations to examine the clumpy structure in GMCs over much larger scales? To answer this question we will follow the approach of Jaffe (1995), although we present models and observational results specific to the 13CO J -- 1 -+ 0 transition.

Assume that molecular clouds are made up of an ensemble of spherical clumps, each of which has the same constant density and radius (R~l). UV radiation illuminates the clumps creating a layer of [C I] at the surface of each clump whose thickness is given as AR, whereas in the interior, the carbon is primarily locked up in CO. If the [C I] and 13CO emission are both optically thin, it can be shown that:

f TmbdY[C I] u 2 - 3 ",3co Acz f~3co X[C I] A R f Tmbdy(13COJ = 1--+ O) u21 A~3co fc t X[13C0] Rd (1)

AcI and A~3co are the Einstein A values of 7.9 x 10 -s and 6.3 × 10 -s respectively. X[C I] and X[laCO] are the carbon and 13CO abundances (relative the H2) and are ~ 10 -4 and 2 x 10 -4 respectively. The frequencies of the two transitions involved are W~co = 110.201 GHz and Vcl = 492.1607 GHz. Therefore, we can reduce the above equation to:

f TmbdY[C I] A R = 9.43 f'3c° (2) f TmbdV(13COJ = 1 --+ O) fc t Rd

The remaining unknowns are the partition functions (ntot,,t/n~,~r) written as f,~co and fct , which

2066 R. Plume et al.

can be calculated as a function of kinetic temperature, and the clump radius (Rd). The thickness of the [C I] emitting layer (AR) can be estimated from PDR models which predict a relatively constant carbon column density of ~ 1017 cm -2.

NH2 (C I) 1021 324 A R - nH2(C I) -- 3.086 x 101snH2(C I) -- nH2(C I) (3)

where AR is given in pc. Therefore, we can derive the average clump radius from only three param- eters: 1) the kinetic temperature of the gas (as it affects the partition functions), 2) the H2 density in the [C I] layer (nil2 (C I)), and 3) the [C I] and 13CO integrated intensities:

1 f TmbdV(13CO) Rd(pc) 3055nH~(C I) fTmbdY[C I] (4)

Figure 6 shows the radius of a typical clump in each of the four clouds in this study. To obtain the clump radius from Eq. (4), we used gas temperatures either from the literature or from our own calculations (see the next section), average [C I]/13CO integrated intensity ratios as shown in Figure 5, and we assumed that nH2(C I ) ---- 10 5 c m -3 . Figure 6 shows that the average clump radius in Orion A is ,~ 0.1 pc, whereas in M17SW, p Oph A, and the Polaris Flare Rcl "-~ 0.08 pc. These clumps sizes are similar to those determined from direct, high-resolution studies of cloud structure (Rcl > 0.04 pc for densities > few × 104 cm -a - Falgarone & Phillips 1996; Rd > 0.05 pc for n = × 105 - l0 s cm -a - Stutzki & Gfisten 1990). The similarity between the observed clump sizes and those determined from our simple model suggests that large-beam [C I] observations can, indeed, be used to probe cloud structure over large scales in GMCs, even when the clumps are significantly smaller than the beamsize. It must also be noted that, in clumpy cloud model, the asymptotic increase of the [C I]/13CO integrated intensity ratio at the cloud edges (Figure 5) suggests that the clumps in this region are smaller than those throughout the rest of the cloud.

Clearly the results from this simplistic model are only preliminary. Nevertheless, they are enticing enough to require further, more sophisticated modelling in this direction. We are currently developing a clumpy PDR model which will correctly handle photochemistry and radiative transfer, and which can accomodate density gradients within the clumps, a distribution of clump sizes, and an interclump medium. Using this model we plan to more precisely examine the large-scale distribution of cloud sub-structure using the SWAS [C I] observations.

Physical Conditions in Molecular Clouds Large-scale maps of the [C I], and low-J 12CO and 13CO emission show the overall distribution

and morphology of the bulk of the gas in GMCs. However, with only the low energy transitions, one cannot easily or accurately determine the physical conditions in the gas (i.e. the density and tem- perature) since these transitions cannot discriminate between high or low temperature and density. Studies of higher rotational transitions help, but these studies have been limited to small spatial scales (Howe et al. 1993; Schultz et al. 1992; Graf et al. 1990; Schmid-Burgk et al. 1989; Harris et al. 1987). However, by combining the SWAS 13CO J -- 5 --+ 4 observations with ground-based observations of 13CO J -- 1 --+ 0, we are able to study the physical conditions of the gas over scales much larger than have been previously possible.

To determine the physical conditions in Orion A and M17SW we used a Large Velocity Gradient (LVG) code to find the density, column density, and kinetic temperature that best reproduced the observed 13CO J = 1 -+ 0 and J = 5 --+ 4 line stengths. For a more detailed description of the LVG modelling procedures see Plume et al. (2000) and Howe et at. (2000).

Large -Sca l e St ruc ture o f G M C s 2067

Ki,'net!c Tem~

, I , t , I

13CO Col. Den. I I I

, I , 1 , 1

80 20 40 60 80 100 15.5 16 16.5 17 Tmn (K) Log N(13CO)

Fig. 7. left) Map of the gas kinetic temperature as determined by fitting the 13CO J = 1 --+ 0 and J = 5 --+ 4 observations to LVG models. The five boxes indicate the regions where spectra were co-added in order to produce higher signal-to-noise 13C0 J = 5 --+ 4 spectra, right) Map of the 13CO column density as determined from the LVG model fits.

N(13CO) (cm -2) T k (K) n(H2) (cm-3)

20

15

10 ..=

5

¢,0 0

-5

-10

10 5 0 -5 10 5 0 -5 10 5 0 -5

Am (Arcminutes)

Fig. 8. Model results for the distribution of : (left panel) 13CO column density. Contour levels are 0.1 to 0.9 of the peak value of 6.1 x 1016 cm -2. (raiddlepanel) The kinetic temperature. Contour levels are 0.3 to 0.9 in steps of 0.2 of the peak value of 63 K. (right panel) The H2 volume density. Contour levels are 0.1 to 0.9 of the peak value of 8.3 × 105 cm -a.

2068 R. Plume et al.

Figure 7(left panel) shows a map of the gas kinetic temperature in Orion A. The average gas temperature in this region is 43 =t= 17 K. South of A5 ---- -20' , where there is no obvious 13CO J - 5 --+ 4 emission, we selected five separate regions (the five boxes in Figure 7) that follow the 13CO J -- 1 --+ 0 emission seen in Figure 1. To improve the signal-to-noise ratio we averaged all J = 5 --~ 4 spectra falling within each of these regions. We then used the co-added spectra in the five boxes to perform the LVG fitting. The results are displayed inside each of the boxes in Figure 7, clearly demonstrating that the gas in the southern part of Orion A is, on average, 20 K colder than that in the northern part of the cloud. These gas kinetic temperatures are in general agreement with previous 12CO observations, which show that the 12CO J -- 1 --+ 0 peak temperatures south of BN/KL (~ 15 K) are colder than those to the north (> 30 K) (e.g. Heyer et al. 1992). Figure 7(right panel) presents the total laCO column density in the region as determined from our LVG analysis. The 13CO column density is fairly constant throughout the region at log(N/cm -2) = 16.4 ± 0.3 and remains at this level even in the southern part of the cloud, where the temperatures are considerably lower.

Figure 8 shows the LVG modelling results for M17SW. The left panel shows the 13CO column density which closely resembles the 13CO J -- 1 -+ 0 emission seen in Figure 2. The average laCO column density is 1.2 × 1016 cm -2. Figure 8(middle panel) presents a map of the kinetic temperature of the gas. The average gas temperature in M17SW is ,,~ 35 K but there is a clear bias towards higher temperatures on the Eastern side of the cloud. This is consistent with the idea that the gas is heated by UV radiation from the OB association located about 1 pc east of the cloud. The most striking result, however, is the density distribution seen in Figure 8(right panel). The average density in M17SW is ~ 3 × 105 cm -3 but the spatial distribution of density is very irregular. In fact, there is no correlation at all between column density and density. The low column density regions are about as likely to contain high density gas as are the high column density regions and vice versa.

These results show that SWAS is superbly suited to mapping the physical conditions over large scales in molecular clouds (> 1 square degree). In the near future, we plan on extending this analysis to additional clouds, such as p Oph A (Figure 3).

This work was supported by the NASA grant NAS5-30702 (to SWAS) and NSF grant AST 97- 25951 to the Five College Radio Astronomy Observatory.

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