Paul's Interstellar Medium Pages

The radiative feedback from massive stars on molecular clouds is observed directly in socalled Photon Dominated Regions (PDRs), where the stellar ultraviolet (UV) radiation heats the ambient gas, affects its thermal balance and chemistry, and partly dissociates the H₂ gas (hence the name Photo Dissociation Regions, which is also often used). The resulting atomic hydrogen (HI) can be observed directly as HI zones around HII regions. Characteristic of PDRs is a layered structure, which is ideally observed in edge-on PDRs such as the Orion bar. PDRs cool mainly by fine-structure line emission such as the [CII] 158 µm line. Emission in this line is ubiquitous in the Galaxy, where it is the strongest cooling line of neutral gas. The role of this line in extended PDRs can be seen in the L1630 region.

The Orion bar

The Orion bar is a perfect example of an edge-on PDR, offering, at its distance of approximately 450 pc, excellent spatial resolution of the stratified structure of the PDR. The bar is an edge-on ionization front at the South-East edge of the Orion Nebula (M42). The neutral gas outside the bar reveals a number of successive layers with increasing distance from the exciting stars (see image below). Closest to the ionization front, the emission of photo-excited polycyclic aromatic hydrocarbons (PAHs) peaks. Somewhat further from the ionization front, vibrationally excited H₂ is found, which radiates through fluorescence following the absorption of ultraviolet photons. Still farther from the ionization front, the emission from CO peaks.

The stratified structure in the Orion bar PDR is shown by the different locations of PAH emission at 3.3 µm (blue), H₂ vibrational line emission (green) and CO emission (red). The exciting stars are towards the North-West.

Combining the H₂ data with millimetre line mapping, the stratification of chemical and excitation conditions as a function of distance from the ionization front is revealed in detail, and the layers where H₂, CO, CS and HCO⁺ are formed can be accurately traced (see cross-cuts below). A density enhancement parallel to the bar is found at about 22" from the ionization front. This feature may result from compression due to the slow thermal expansion of the hot zone closer to the ionization front.

The stratified structure in the Orion bar PDR shown by the different normalized intensities of various tracers as a function of distance from the ionization front (IF), averaged over a strip of 45" width parallel to the ionization front, and centred on the most prominent molecular clump in the centre of the Orion bar. The abscissa indicates distance from the IF, here assumed to be located at the half-power point of the radio continuum. Note that the HCO⁺ profile comes from interferometric measurements that are not sensitive to structures larger than about 50".

The extended photon dominated region L1630

The principal cooling line of the photodissociated gas in PDRs is the 158 µm [CII] line. This line arises in the warm (a few 100K) gas where CO is photodissociated (but hydrogen remains molecular due to its strong self-shielding), and carbon subsequently ionized. With an upper level at 91K, the [CII] 158 µm line is an ideal coolant for these regions and an excellent PDR tracer. We have used the FIFI instrument on the Kuiper Airborne Observatory to make a large-scale map of the [CII] line toward the L1630/Orion B molecular cloud. The map covers an approximately 35'×45' area which includes the NGC2024 HII region, the star ζ Ori, the reflection nebula NGC2023, and the Horsehead nebula. Emission in the [CII] line is very widespread. Extended emission associated with the NGC2024 HII region and its envelope accounts for more than half of the [CII] flux. Over this approximately 1.5×2.5 pc region, the amount of gas-phase carbon in the form of C⁺ is comparable to the amount of carbon in CO. This result, together with the [CII] distribution, implies that [CII] emission arises on the surfaces of clumps throughout the cloud rather than in a single layer at the HII region boundary. Away from the HII region, most of the [CII] emission comes from the western edge of the L1630 cloud and probably results from excitation by external OB stars. The overall extent of the [CII] emission is comparable to that of millimeter molecular lines, but the distributions are different in detail (see image below).

The distribution of the [CII] 158 µm emission from the L1630/NGC2024 complex, shown in contours on top of an optical photograph of the region. Lowest contour and contour intervals are 3.6×10^-4 erg s^-1 cm^-2 sr^-1. The angular resolution is 1'. The cross indicates the position of the O9 supergiant ζ Ori.

Principal publications:

Anatomy of the photodissociation region in the Orion bar
Tielens, A.G.G.M., Meixner, M.M., Van der Werf, Paul P., Bregman, J., Tauber, J.A., Stutzki, J., & Rank, D.
Sci, 262, 86 (1993)
[ ADS entry ]

Structure and chemistry of the Orion bar photon-dominated region
Van der Werf, Paul P., Stutzki, J., Sternberg, A., & Krabbe, A.
A&A, 313, 633 (1996)
[ ADS entry ]

Ultraviolet illuminated molecular cloud boundaries: extended [CII] 158 micron emission toward L1630
Jaffe, D.T., Zhou, S., Howe, J.E., Herrmann, F., Madden S.C., Poglitsch, A., Van der Werf, Paul P., & Stacey, G.J.
ApJ, 436, 203 (1994)
[ ADS entry ]