Table 4: Comparison of model predictions for diffuse and dense clouds with the observed abundances (compared to $2N(\rm {H_{2}})+N(\rm {H})$ ) of CH and CH₂ (X(CH) and X(CH₂)). The diffuse cloud model is for the line of sight to $\zeta$ Per (van Dishoeck & Black 1986): vDB86. The quoted density and temperature are for the centre of the cloud in the model. The dense cloud models are steady state values corresponding to the gas phase "new standard model'' of Lee et al. (1996): L96 and the gas-grain model of Ruffle & Herbst (2001): RH01 including photochemistry (P1/A at 10⁸ yr). The PDR model is for a density of 10⁵ cm^-3 and radiation field $\sim$ 650-900 times the average interstellar value (Jansen et al. 1995): J95. The observational results derived from our ISO measurements are shown as well as previous results from Lyu et al. (2001) and Hollis et al. (1995).
Density Temperature X(CH) X(CH₂) $[{\rm CH}/{\rm CH}_2]$

(cm^-3) (K) (10^-8) (10^-8)

Model Diffuse (vDB86) 500 25 1.6 2.4 0.67

predictions: Dense (L96) 10³ 10 0.25 1.6 0.018

Dense (L96) 10⁵ 10 0.0007 0.039 0.16

Dense (RH01) $2\times10^{4}$ 10 0.1 1.2 0.08

PDR (J95) 10⁵ 1.4 0.7 2.0

Results from Sgr B2 Line of Sight 0.6-3 0.2-1.1 $2.7\pm0.5$

this work: Sgr B2 0.05-0.2 0.02-0.07 $2.7\pm0.5$

W49 N 37 km s^-1 >1.4

W49 N 61 km s^-1 >0.8

Sgr A $^{\ast }$ >2.6

Previous $\zeta$ Oph 1.8^b 4.3^a 0.4^b

observational HD154368 1.9^d 0.79^c 2.4^d

results: Orion-KL $\sim$ 10⁵ 100-200 0.06^e 0.4^e 0.1^e

W51 M $\sim$ 10⁴ 100-200 0.007^e 0.07^e 0.1^e

^a Combining $N({\rm CH}_2)$ from Lyu et al. (2001) with a hydrogen column density as quoted in van Dishoeck & Black (1986).
^b Based on $N({\rm CH})=2.5 \times10^{13}$ cm^-2 from van Dishoeck & Black (1989).
^c Combining $N({\rm CH}_2)$ from Lyu et al. (2001) with a hydrogen column density from Snow et al. (1996).
^d Based on $N({\rm CH})=8 \times10^{13}$ cm^-2 from van Dishoeck & Black (1989).
^e Taking values as quoted by Hollis et al. (1995) - but note that X(CH) could be significantly higher - see text.

**Table 4:** Comparison of model predictions for diffuse and dense clouds with the observed abundances (compared to $2N(\rm {H_{2}})+N(\rm {H})$ ) of CH and CH₂ (X(CH) and X(CH₂)). The diffuse cloud model is for the line of sight to $\zeta$ Per (van Dishoeck & Black 1986): vDB86. The quoted density and temperature are for the centre of the cloud in the model. The dense cloud models are steady state values corresponding to the gas phase "new standard model'' of Lee et al. (1996): L96 and the gas-grain model of Ruffle & Herbst (2001): RH01 including photochemistry (P1/A at 10⁸ yr). The PDR model is for a density of 10⁵ cm^-3 and radiation field $\sim$ 650-900 times the average interstellar value (Jansen et al. 1995): J95. The observational results derived from our ISO measurements are shown as well as previous results from Lyu et al. (2001) and Hollis et al. (1995).
		Density	Temperature	X(CH)	X(CH₂)	$[{\rm CH}/{\rm CH}_2]$
		(cm^-3)	(K)	(10^-8)	(10^-8)
Model	Diffuse (vDB86)	500	25	1.6	2.4	0.67
predictions:	Dense (L96)	10³	10	0.25	1.6	0.018
	Dense (L96)	10⁵	10	0.0007	0.039	0.16
	Dense (RH01)	$2\times10^{4}$	10	0.1	1.2	0.08
	PDR (J95)	10⁵		1.4	0.7	2.0
Results from	Sgr B2 Line of Sight			0.6-3	0.2-1.1	$2.7\pm0.5$
this work:	Sgr B2			0.05-0.2	0.02-0.07	$2.7\pm0.5$
	W49 N 37 km s^-1					>1.4
	W49 N 61 km s^-1					>0.8
	Sgr A $^{\ast }$					>2.6
Previous	$\zeta$ Oph			1.8^b	4.3^a	0.4^b
observational	HD154368			1.9^d	0.79^c	2.4^d
results:	Orion-KL	$\sim$ 10⁵	100-200	0.06^e	0.4^e	0.1^e
	W51 M	$\sim$ 10⁴	100-200	0.007^e	0.07^e	0.1^e

Source LaTeX | All tables | In the text

	1 Introduction
	2 CH₂ rotational spectrum
	3 Observations and data reduction
	4 Sgr B2 results
	5 Abundances
	6 CH₂ in other sources
	7 Discussion
	8 Summary
	Appendix A: Observations used
	References