INTRODUCTION

The reflection nebula NGCá 2023 is one of the brightest sources of fluorescent molecular hydrogen in the sky, making it a laboratory for the study of H fluorescence which occurs over a wide range of physical conditions. It is powered by the B1.5V star HDá 37903, the most luminous member of a cluster of young stellar objects illuminating the front surface of the Lynds 1630 molecular cloud in Orion B (Depoy et al. 1990). NGCá 2023 forms a cavity in the surface of the cloud, some 450 pc from us, producing both a bright visual reflection nebula and a UV-excited photodissociation region. The latter is evident through, for instance, extensive C 158m line emission (Howe et al. 1991), extended red emission features (Witt & Malin 1989) and IR emission features from PAHs (Joblin et al. 1995).

Fluorescent molecular hydrogen was first discovered through observations of NGCá 2023 (Gatley et al. 1987; Hasegawa et al. 1987), on the basis of a 1-0/2-1á S(1) line ratio of . Narrow line profiles (FWHM < 16km , Burton et al. 1990a), and numerous high-excitation emission lines emitted in the far-red (from levels as high as v=7 & 8, Burton et al. 1992) are also evidence for the fluorescent excitation process in this source, as opposed to shocks. Emission line images (Burton et al. 1989; Field et al. 1994) show narrow filamentary structure on top of a diffuse emission nebula, suggestive of emission from limb-brightened undulations in the surface of the molecular cloud. However, Steiman-Cameron et al. (1997) argue that elevated intensities are due, in large, to enhancements in local density rather than limb brightening.

The application of photodissociation region (PDR) models (e.g., Tielens & Hollenbach 1985) to interpreting the emission in sub-mm CO rotational, far-IR fine structure and near-IR H lines from NGCá 2023 (Jaffe et al. 1990; Burton, Hollenbach & Tielens 1990b; Steiman-Cameron et al. 1997) suggests that a far-UV radiation field times the average interstellar value is incident on a clumpy molecular cloud, with the bulk of the gas at densities of , but with a fraction () about two orders of magnitude denser. However, we note that Wyrowski et al. (1997), on the basis of C91 radio data, suggest that the beam filling factor for high density gas is much greater than this, albeit with the dense clumps only at 100-300K as opposed to the 750K determined by Steiman-Cameron et al.

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Figure: Spectra from 1-2.5m observed at the southern location, (-11'', -78''), in NGCá 2023. Several series of vibrational-rotational lines of H are indicated. The wavelength of the lowest excitation line in each branch shown is described by the following illustrative coding:
e.g., 2-0 S(0) +, 1.190m denotes the 2-0 S-branch with + signs, starting from 2-0 S(0) at 1.190m.
Top panel (J-band): 2-0 S(0) +, 1.190m; 2-0 Q(1) , 1.238m; 2-0 O(2) vertical line, 1.293m; 3-1 S(0) triangle, 1.262m; 3-1 Q(1) filled triangle, 1.314m; 3-1 O(2) 3-sided star, 1.373m; 4-2 S(0) square, 1.343m; 4-2 Q(1) filled square, 1.398m; 5-3 S(0) pentagon, 1.433m; 6-4 S(3) hexagon, 1.446m.
Middle panel (H-band): 1-0 S(6) open circle, 1.788m; 3-1 O(5) 3-sided star, 1.522m; 4-2 O(3) , 1.510m; 5-3 Q(3) filled pentagon, 1.506m; 5-3 O(2) 5-sided star, 1.561m; 6-4 S(0) hexagon, 1.537m; 6-4 Q(1) filled hexagon, 1.602m; 6-4 O(2) 6-sided star, 1.675m; 7-5 S(0) 7-gon, 1.658m; 7-5 Q(1) filled 7-gon, 1.729m; 8-6 S(3) 8-star, 1.702m; 10-7 S(3) 10-star, 1.549m.
Bottom panel (K-band): 1-0 S(0) circle, 2.223m; 1-0 Q(1) filled circle, 2.407m; 2-1 S(0) +, 2.356m; 3-2 S(1) 3-gon, 2.386m; 4-3 S(2) 4-star, 2.435m.

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Figure: Spectra from 1-2.5m observed at the northern location, (+33'', +105''), in NGCá 2023. Symbols are as described for Figure 1.

In view of its brightness, the H emission from NGCá 2023 can be subjected to detailed spectral examination, with emission lines from highly-excited vibrational-rotational levels in the ground electronic state measured. Such observations permit stringent tests of fluorescent excitation models (e.g., Black & van Dishoeck 1987; Sternberg & Dalgarno 1989; Burton, Hollenbach & Tielens 1990b; Draine & Bertoldi 1996) for H. Deviations from a pure fluorescent cascade might result from collisional depopulation of excited levels, from formation pumping of newly created molecules, from variations in the injection levels for the cascade and/or from changes in ortho-to-para ratio. Thus they may be used to test our physical models for these processes. This paper reports observations of the 1-2.5m H emission from two locations in NGCá 2023 which can be used for this purpose. Parts of these data have previously been discussed in the PhD Dissertation of John Howe (1992) and in a review on molecular hydrogen emission (Burton, 1992).

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Table: H Line Identifications and Flux Densities from Spectra

Line		Flux Density
	(m)	(-11'', -78'')	(+33'', +105'')
		erg m
7-4 S(1)	1.063070	3.1 (0.7)	...
2-0 S(7)	1.064143		...
2-0 S(4)	1.099817	2.3 (0.6)	<1.0
2-0 S(3)	1.117492	4.0 (0.6)	2.6 (0.5)
8-5 S(3)	1.129215
3-1 S(7)	1.130405	2.6 (0.5)	2.0 (0.6)
2-0 S(2)	1.138238	3.2 (0.5)	1.7 (0.3)
3-1 S(5)	1.151873	4.2 (0.4)	1.2 (0.5)
8-5 S(1)	1.152436
2-0 S(1)	1.162223	2.9 (0.4)	1.8 (0.3)
3-1 S(4)	1.167158	2.1 (0.4)	1.2 (0.3)
3-1 S(3)	1.185697	3.1 (0.5)	1.9 (0.5)
2-0 S(0)	1.189585	1.8 (0.6)	0.4 (0.4)
8-5 Q(1)	1.201711
4-2 S(7)	1.204710
8-5 Q(2)	1.206846
3-1 S(2)	1.207589	2.8 (0.4)	4.0 (0.6)
4-2 S(6)	1.213907
8-5 Q(3)	1.214628	1.5 (0.4)	2.1 (0.5)
4-2 S(5)	1.226300	2.6 (0.4)	1.1 (0.3)
3-1 S(1)	1.232986	3.5 (0.4)	1.8 (0.2)
2-0 Q(1)	1.238343	2.8 (0.5)	1.9 (0.3)
2-0 Q(2)	1.241937	2.4 (0.5)	1.7 (0.3)
4-2 S(4)	1.242159
2-0 Q(3)	1.247324	2.2 (0.4)	0.9 (0.2)
4-2 S(3)	1.261546	5.0 (0.5)	3.2 (0.4)
3-1 S(0)	1.262065
9-6 S(1)	1.262129
2-0 Q(5)	1.263593
4-2 S(2)	1.284625	2.4 (0.4)	1.1 (0.2)
5-3 S(7)	1.289366	1.7 (0.4)	0.6 (0.2)
2-0 O(2)	1.293229	2.2 (0.4)	1.1 (0.3)
5-3 S(5)	1.310673
4-2 S(1)	1.311568	2.4 (0.5)	0.7 (0.4)
3-1 Q(1)	1.314102	4.0 (0.5)	2.5 (0.3)
9-6 Q(1)	1.315821
3-1 Q(2)	1.318072	3.3 (0.4)	1.7 (0.2)
3-1 Q(3)	1.324025	3.7 (0.4)	1.3 (0.3)
5-3 S(4)	1.326966
2-0 O(3)	1.335422	4.0 (0.4)	1.7 (0.2)
4-2 Q(7)	1.459198
4-2 O(2)	1.461130	3.0 (0.4)	1.5 (0.4)
5-3 Q(1)	1.492938	2.3 (0.3)	1.2 (0.3)
5-3 Q(2)	1.497988	1.5 (0.4)	0.7 (0.3)
6-4 S(1)	1.501553	2.3 (0.3)	1.3 (0.3)
5-3 Q(3)	1.505600	2.5 (0.4)	0.6 (0.3)
4-2 O(3)	1.509865	2.8 (0.2)	2.5 (0.4)
5-3 Q(4)	1.515792	1.0 (0.4)	0.3 (0.3)
3-1 O(5)	1.522026	1.9 (0.2)	2.2 (0.3)
7-5 S(5)	1.523598

á© Copyright Astronomical Society of Australia 1997