From HifiISMWiki

Photon-dominated regions (PDRs)

Figure 1: Transition from atomic to molecular gas as a function of the cloud depth (A_V) for different species in a model of a PDR with strong UV field. Depending on the corresponding chain in the chemical network (H, C or N are illustrated here), the molecular species is formed at a different depth: H₂ at A_V = 0.5 mag., CO at A_V = 2 mag and NH₂ & NH₃ at A_V = 4 mag, corresponding to different possible definitions of a ``molecular cloud boundary´´

PDRs are transition zones in the interstellar medium between a region of ionized or atomic gas characterized by a low UV optic al depth and a region of high-density, typically molecular gas with a large UV optical depth. In PDRs the external radiation field completely determines the thermal and chemical structure of the interstellar gas. As such, PDRs are direct manifestation s of the energy balance of interstellar gas and their study allows to understand how the ISM survives the presence of the stars that it forms.

PDR observations

Herschel PDR observations can reveal the chemical structure of the transition zone. Available PDR observations have already un covered the chemical differentiation of the regions. Unfortunately, many key species have not been observable before.

Figure 2: The Horsehead Nebula is a well known PDR. The UV radiation from σ Ori induces a complex chemistry. Different molecular species form at a different depth of the cloud so that a layer structure is produced (Teyssier et al. 2004).

PDR models

Figure 3: Comparison of the abundance profile of CH and OH computed from different current PDR models for equal input parameters for a cloud with a density of 10^5.5 cm^-3 and an impinging radiation field of 10⁵ times the average Galactic radiation field. In spite of a unified set of assumptions on the geometric structure of the PDR, there exist so many open questions on the microphysics and chemistry in the cloud that the predicted abundances deviate by several orders of magnitude.

Detailed models of PDRs have been constructed over the past decades. They allow us to determine, with increasing confidence: the density and temperature structure, and the strength of the impinging radiation field. However, basic parts of the interplay between chemistry and dynamics and radiation transfer are still poorly understood. Thus different models predict completely different abundances for key species in the chemical network.

Only Herschel can measure the abundance of CH, NH, OH in their ground-states. By obtaining a comprehensive inventory of species, containing in particular the light hydrides, key nodes in the chemical network, the current models can be tested and revised.

With Herschel it will be possible to:

• Determine the dynamical structure of cloud surfaces
• Obtain an exact measure for the energy balance in PDRs
• Probe the most relevant chemical processes in PDRs