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Astron. Astrophys. 318, 608-620 (1997)

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1. Introduction

The first detection of infrared quadrupole emission of H2 in any astronomical object was the 1976 observation by Gautier et al. (1976) of what is now known as OMC-1. The H2 emission covers an area of several square arcminutes and is located in the giant Orion Molecular Cloud, at a distance of about 450 pc. It contains the much-studied Becklin-Neugebauer object and the Kleinmann-Low nebula, as well as a number of compact infrared emission sources. According to the review by Genzel & Stutzki (1989), BN-KL is a very dense, clumpy molecular core in which most of the radiation emanates from a few major luminosity sources, of which BN itself and IRc2 - located about [FORMULA] south-east of BN - are the most important.

For more than a decade the standard H2 map of OMC-1 was that obtained by Beckwith et al. (1978), using the 1-0 S(1) line. Constructed at an angular resolution of [FORMULA], this showed OMC-1 to have two extended lobes and a number of emission peaks. But, overall, the spatial distribution of H2 emission was fairly smooth. Recent, higher resolution, images of OMC-1 (Lane 1989; Allen & Burton 1993; Schild et al. 1995), however, show it to be a highly dynamic region, with a number of fingers or jets seemingly driven by the central luminous sources referred to by Genzel & Stutzki (1989). Some of the most northern extensions of the fingers described by Allen & Burton have also been identified in HST narrow band images (e.g. O'Dell 1995). They have the appearance of Herbig Haro objects and are referred to as HH 205, HH 206 and HH 207. Their optical detection indicates that they are not deeply embedded in the molecular cloud anymore.

BN is considered to be a young star of mass [FORMULA] and IRc2 a similar object, somewhat more massive at [FORMULA]. More or less strongly collimated outflows from such young stars appear to be common, producing Herbig Haro objects where they collide with dense surrounding gas. In their analysis of images from the north-west quadrant of OMC-1, Allen & Burton (1993) consider compact knots of material - or "bullets" as Mac Low (1995) terms them - to have been ejected either from BN or IRc2, traveling up to [FORMULA] cm in less than 1000 years. More recently, Stone et al. (1995) have attempted to model the observed structure in terms of Rayleigh-Taylor instabilities produced by the radially symmetric collision of an old, slow stellar wind with a more rapidly propagating young wind. The possibility of such fragmentation because of shell instabilities has already been known for some years (e.g. [FORMULA] yczka & Tenorio-Tagle 1985)

In this paper, we present images of OMC-1 taken in a number of wavelengths sensitive to ro-vibrational transitions of H2, whose upper energy levels correspond to temperatures ranging from 6500 K (1-0 S(0)) to 39 000 K (8-6 O(5)). These show up various structures in the cloud from the full jet-like distribution of intensity seen in the 1-0 S(1) image to diffuse nebulosity surrounding BN and the region known as Peak 2. We attempt to analyse the structures shown using wavelet analysis. We make use of spectroscopy to determine the temperature profile of Peak 2 and ratio images to produce temperature and column density maps of the region. Wavelet analysis is used to help determine whether temperature or density variations are responsible for the main structures observed in the images.

Another motivation for the work presented here was to see if it were possible to detect emission from H [FORMULA] in OMC-1. This fundamental molecular ion has long been known to be an important constituent of the interstellar medium (e.g. Dalgarno 1994). However despite a number of documented attempts to observe H [FORMULA] spectra (Oka 1981; Geballe & Oka 1989; Black et al. 1990), it has so far defied detection. All these attempted detections tried to observe cold (T [FORMULA] 50 K) H [FORMULA] absorption against suitable star light in the L window. Black et al (1990) gave an upper limit of the H [FORMULA] column density of [FORMULA] cm-2 towards the highly obscured sources NGC 2264 IRS and AFGL 2591. This limit is close to that predicted by theoretical models. Geballe & Oka (1989) produced an upper limit of 3 - 4 [FORMULA] cm-2 in front of BN.

Successful detections of H [FORMULA] emission in the giant planets (Drossart et al. 1989; Trafton et al. 1993; Geballe et al. 1993) and elsewhere (Miller et al. 1992) have been made in the L and K windows. In the observations reported here we attempted to observe similar H [FORMULA] emissions in a similarly active region. The H2 emissions from OMC-1 suggest that it is hot enough to produce H [FORMULA] emissions. However our observations detected no H [FORMULA]. Instead, we can only present upper limits to the H [FORMULA] column density, although these are more sensitive to the assumed temperature of the column than those obtained from attempted detections of cold H [FORMULA] via absorption.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998