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Astron. Astrophys. 333, 280-286 (1998)

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1. Introduction

The work reported here concerns H2 emission in the reflection nebula NGC 2023 in the v=1-0 S(1) and S(2) lines at 2.121 and 2.033µm respectively. The present data represent an extension of previous work involving 1" resolution observations of a central portion of NGC 2023 in the S(1) line alone (Field et al. 1994 (F94)). The nebula, which forms part of the molecular cloud L1630, has structure extending over several arcminutes, and is associated with the B1.5V star HD 37903, of effective temperature 22000K, and distance 450 to 500pc (Racine 1968; Lee 1968; de Boer 1983). HD 37903 is the major source of excitation of the gas and dust in NGC 2023 (Sellgren et al. 1992). The excitation zone has been extensively studied. IR continuum observations are described for example in Sellgren et al. 1992 and Roche et al. 1994. References to numerous molecular detections, including H2, may be found in F94 and in Fuente et al. 1995. Data show that a shell of neutral material is located around HD 37903, at a distance of 0.1 to 0.2 pc from the star. The first observations of H2 fluorescence in NGC 2023 (and the first in any reflection nebula) were reported in Gatley et al. 1987, who mapped the v=1-0 S(1) line at a spatial resolution of 19 arcsec. Further observations of NGC 2023 are reported in Hasegawa et al. 1987, Burton et al. 1989, Tanaka et al. 1989 and F94. Burton et al.1990a found an upper limit of 17 kms-1 for the width of the H2 S(1) v = 1-0 line in the brightest zone in NGC 2023 and concluded, using other evidence from PDR models, that H2 emission results from fluorescence, rather than a shock, in that region. There has been an accompanying development of models of these so-called photon-dominated regions (PDRs) e.g. Black and van Dishoeck 1987; Sternberg and Dalgarno 1989; Burton et al. 1990b; Abgrall et al. 1992. These data and models have provided unambiguous evidence that the excitation mechanism for the major part of H2 vibrational fluorescence in NGC 2023 is through initial excitation in the Lyman and Werner bands of H2 followed by emission into highly excited vibrational states of the ground electronic state of H2 and a subsequent rovibrational cascade in the red and the IR.

Steady-state PDR models show that densities in bright emitting regions of NGC 2023 lie in the range 104 to 105 cm-3 with incident radiation fields between a thousand to a few thousand times the average interstellar VUV field (Pankonin and Walmsley 1978; Jansen et al. 1994; F94; Fuente et al. 1995; Rouan et al. 1997). H2 emission originates in a thin skin on the inner surface of the gas exposed to the stellar VUV and observed emission therefore presents a view of the surface structure of the cloud. The penetration depth of the VUV varies between several tenths of a parsec, for number densities of 103 cm-3, to 10-4 pc for 106 cm-3.

The H2 S(1) image in F94 showed a network of narrow filamentary structures and hotspots. Similar narrow bright filamentary emission has been observed in NGC 7023 (Lemaire et al. 1996 (L96)). L96 showed that there exists structure on a scale of 0.004 pc in the molecular cloud associated with NGC 7023. Using adaptive optics, Rouan et al. 1997 found for NGC 2023 that structure on the scale of a few times 10-4 pc exists in the molecular cloud at a distance of 0.03 pc (in projection) from HD 37903. In the present work we return to the milliparsec scale and describe further structure in the molecular cloud associated with NGC 2023. Emission is found to extend over a much larger area than recorded in F94. The extended emission which we record here may be found at low resolution in the original H2 image of NGC 2023 in Gatley et al. 1987.

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© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998
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