EDP Sciences
Free access
Volume 496, Number 3, March IV 2009
Page(s) 731 - 739
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:200810958
Published online 20 January 2009
A&A 496, 731-739 (2009)
DOI: 10.1051/0004-6361:200810958

The molecular environment of the massive star forming region NGC 2024: Multi CO transition analysis

M. Emprechtinger1, M. C. Wiedner1, R. Simon1, G. Wieching1, 2, N. H. Volgenau1, F. Bielau1, U. U. Graf1, R. Güsten2, C. E. Honingh1, K. Jacobs1, D. Rabanus1, 3, J. Stutzki1, and F. Wyrowski2

1  I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
    e-mail: emprecht@ph1.uni-koeln.de
2  Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
3  European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile

Received 12 September 2008 / Accepted 7 January 2009

Context. Sites of massive star formation have complex internal structures. Local heating by young stars and kinematic processes, such as outflows and stellar winds, generate large temperature and velocity gradients. Complex cloud structures lead to intricate emission line shapes. CO lines from high mass star forming regions are rarely Gaussian and show often multiple peaks. Furthermore, the line shapes vary significantly with the quantum number $J_{\rm up}$, due to the different probed physical conditions and opacities.
Aims. The goal of this paper is to show that the complex line shapes of 12CO and 13CO in NGC 2024 showing multiple emission and absorption features, which vary with rotational quantum number J can be explained consistently with a model, whose temperature and velocity structure are based on the well-established scenario of a PDR and the “Blister model”.
Methods. We present velocity-resolved spectra of seven 12CO and 13CO lines ranging from $J_{\rm up}=3$ to $J _{\rm up}=13$. We combined these data with 12CO high-frequency data from the ISO satellite and analyzed the full set of CO lines using an escape probability code and a one-dimensional full radiative transfer code.
Results. We find that the bulk of the molecular cloud associated with NGC 2024 consists of warm (75 K) and dense ( $9\times 10^5$ cm-3) gas. An additional hot (~300 K) component, located at the interface of the HII region and the molecular cloud, is needed to explain the emission of the high-J CO lines. Deep absorption notches indicate that very cold material (~20 K) exists in front of the warm material, too.
Conclusions. A temperature and column density structure consistent with those predicted by PDR models, combined with the velocity structure of a “Blister model”, appropriately describes the observed emission line profiles of this massive star forming region. This case study of NGC 2024 shows that, with physical insights into these complex regions and careful modeling, multi-line observations of 12CO and 13CO can be used to derive detailed physical conditions in massive star forming regions.

Key words: ISM: molecules -- ISM: HII regions -- submillimeter -- stars: formation

© ESO 2009