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3.3. Line Emission From the Gas

The line intensities emerging from the gas component of the model clouds, from various elements in their different ionization stages are subjected to the `detectability' criteria of ISO spectrometers ISO-SWS and ISO-LWS. The detectability basically depends on the strength of the line vis-a-vis neighbouring continuum emitted by the dust. This is quantified by the ratio of the powers falling on the relevant spectrometer resolution elements due to the line (Pline) and its neighbouring continua on both sides (P+cont & P-cont). We define the detectability parameter, D = log (Pline / sqrt[P+cont x P-cont]), which is a measure of the line to continuum ratio. The spectral resolution, which is a function of the wavelength, ranges between 1000-20000 (de Graauw et al., 1996, Swinyard et al 1996). In the present study, only those well detectable lines have been considered for which D > -1.00, which is a rather conservative criterion.

Table 5. List of all the emission lines which have been considered
Element
(ionization stage)
lambda (µm)
Element
(ionization stage)
lambda (µm)
Element
(ionization stage)
lambda (µm)
C II
O I
N I
O III
O I
N III
O III
Ne III
Si II
157.7
145.6
121.8
88.4
63.2
57.2
51.8
36.0
34.8
S III
O IV
Ne V
Ar III
S III
Ne III
Ne V
Ar V
Mg V
33.5
25.9
24.3
21.8
18.7
15.6
14.3
13.5
13.1
Ne II
S IV
Ar III
Ar V
Ne VI
Ar II
Mg V
Ar VI
Mg IV
12.8
10.5
8.99
7.91
7.66
6.99
5.61
4.53
4.49

Line emission within the wavelength range covered by the ISO spectrometers (2.5 µm - 200 µm) is considered here. The entire optical depth range is represented by three values which includes the two extreme ones (tau100 = 0.0045 & 0.34). All the three density distribution laws (r0, r-1 and r-2) have been considered for all three types of exciting ZAMS stars. It was found from the CLOUDY runs that a superset of 27 lines describe the emission line spectrum for all the above models. These are listed in Table 5. However, this number drops to about half (for the lowest optical depth and constant density model) after the ISO detectability criterion is applied. A compact summary of the predicted emergent line luminosities and the line to continuum ratios are presented in Table 6, for all three types of the embedded stars, but only for the case of uniform density distribution and the lowest optical depth case. However, to demonstrate the role played by (a) the optical depth and (b) the radial density distribution law, on the emergent spectrum, results from selected models are shown in Figures 8, 9 & 10 for the cases of exciting stars of type O4, O7 and B0.5 respectively. At the bottom of these figures, the positions of a few prominent fine structure lines, which are important coolants for the gas, are marked.

Table 6. The line luminosity (L) and detectability (D) parameters for the case of constant density distribution (r0) and tau100 = 0.0045. The results correspond to the embedded ZAMS stars of spectral types O4, O7 and B0.5.
Lines lambda O4 O7 B0.5
µm L (erg/sec) D L (erg/sec) D L (erg/sec) D
CII 157.7 1.22(35) 1.05 5.58(34) 1.23 1.94(34) 1.30
OI 145.6 2.46(33) -0.77 4.58(32) -0.95 - -
NI 121.8 1.36(34) -0.28 4.63(33) -0.13 1.41(33) -0.04
OIII 88.4 2.74(36) 1.62 2.88(35) 1.38 1.97(33) -0.07
OI 63.2 3.25(34) -0.56 7.73(33) -0.32 1.52(33) -0.21
NIII 57.2 1.06(36) 0.87 1.45(35) 0.89 2.17(33) -0.08
OIII 51.8 6.37(36) 1.57 6.64(35) 1.51 4.27(33) 0.21
NeIII 36.0 2.34(35) 0.32 1.93(34) 0.27 - -
SiII 34.8 9.78(34) -0.06 1.83(34) 0.25 2.78(33) 0.50
SIII 33.5 1.59(36) 1.14 3.16(35) 1.48 1.98(34) 1.37
OIV 25.9 6.15(35) 0.70 1.93(33) -0.66 - -
ArIII 21.8 4.49(34) -0.44 7.79(33) 0.02 2.08(32) -0.26
SIII 18.7 1.59(36) 1.14 3.17(35) 1.72 1.96(34) 1.87
NeIII 15.6 2.55(36) 1.54 2.14(35) 1.81 3.71(32) 0.46
ArV 13.1 8.06(33) -0.87 - - - -
NeII 12.8 8.09(34) 0.11 8.05(34) 1.53 1.45(34) 2.25
SIV 10.5 3.73(36) 0.40 1.14(35) 0.38 - -
ArIII 8.99 5.97(35) -0.30 1.05(35) 0.48 2.79(33) 0.44
ArII 6.99 - - - - 1.00(34) 1.31

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