  NGC 2024, a sites of massive star formation, have complex internal structures
caused by cal heating by young stars, outflows, and stellar winds. These
complex cloud structures lead to intricate emission line shapes. The goal of
this paper is to show that the complex line shapes of 12 CO lines in NGC 2024
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. We present velocity-resolved spectra of seven CO lines ranging from J=3
to J=13, and we combined these data with CO high-frequency data from the ISO
satellite. We find that the bulk of the molecular cloud associated with NGC
2024 consists of warm (75 K) and dense (9e5 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. 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 CO can be used to derive detailed physical conditions in
massive star forming regions.
