[¹²Cii]  and [¹³C ii]  158 μm emission from NGC 2024: Large column densities of ionized carbon

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
e-mail: graf@ph1.uni-koeln.de
² NASA Ames Research Center, Moffett Field, CA, USA
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
⁵ Deutsches Zentrum für Luft- und Raumfahrt, Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
⁶ Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany

Received: 31 January 2012
Accepted: 1 March 2012

Abstract

Context. We analyze the NGC 2024 Hii region and molecular cloud interface using [¹²Cii] and [¹³Cii] observations.

Aims. We attempt to gain insight into the physical structure of the interface layer between the molecular cloud and the Hii region.

Methods. Observations of [¹²Cii] and [¹³Cii] emission at 158 μm with high spatial and spectral resolution allow us to study the detailed structure of the ionization front and estimate the column densities and temperatures of the ionized carbon layer in the photon-dominated region.

Results. The [¹²Cii] emission closely follows the distribution of the 8 μm continuum. Across most of the source, the spectral lines have two velocity peaks similar to lines of rare CO isotopes. The [¹³Cii] emission is detected near the edge-on ionization front. It has only a single velocity component, which implies that the [¹²Cii] line shape is caused by self-absorption. An anomalous hyperfine line-intensity ratio observed in [¹³Cii] cannot yet be explained.

Conclusions. Our analysis of the two isotopes results in a total column density of N(H)  ≈  1.6  ×  10²³ cm^-2 in the gas emitting the [Cii] line. A large fraction of this gas has to be at a temperature of several hundred K. The self-absorption is caused by a cooler (T ≤ 100 K) foreground component containing a column density of N(H)  ≈  10²² cm^-2.