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Photon dominated regions: observation and
theory
B. Köster,
N. Schneider,
H. Störzer and
J. Stutzki
e-mail:
koester@apollo.ph1.Uni-Koeln.de
I. Physikalisches Institut der Universität zu Köln,
Zülpicher Straße 77, D-50937 Köln, GERMANY
Abstract:
We give a brief introduction to the physics and chemistry of photon dominated
regions (PDRs) and the state of art of theoretical PDR modelling. We present
observations of the [CII] 
and 
line emission of the Rosette Molecular Cloud. The FIR- and sub-mm line
intensities are compared with theoretical predictions. We use a plane-parallel
finite sized PDR model with a wide range of densities and UV field strengths in
order to model the observed regions. PDR models predict that OH is efficiently
produced and excited in high density molecular cloud regions which are
illuminated by a strong UV field. Therefore OH is a tracer for high density
clumps inside UV penetrated molecular clouds. The ISO mission will provide the
opportunity to observe the OH rotational lines.
Molecular clouds in massive star formation regions (e.g. Orion Nebula) are
irradiated by far-ultraviolet radiation (FUV, 6-13.6 eV) emitted by young,
nearby, OB type stars. The theory states that these FUV photons determine the
chemical and physical conditions of the cloud, creating a so called
photon dominated region (PDR, in earlier papers also named
photodissociation region) on the surface of individual molecular clumps.
Several UV field and density dependent processes determine the gas heating. The
two most important mechanisms are photoelectric heating and UV pumping of 
so that in the outermost PDR layers, the temperature can increase up to
several 100 K (or even more than 1000 K in case of a strong UV field). The gas
cools via FIR fine structure lines, e.g. the [CII] 
, [OI] 
and 
lines. Deeper into the cloud, the temperature
decreases and the prominent cooling lines are the [CI] 
fine
structure line and the (sub)millimeter rotational transitions of CO. In the
cold interior of the cloud the temperature is about 10 K, resulting from the
balance of cosmic ray heating and low-J CO rotational line cooling.
Numerous observations of galactic and extragalactic sources in the lines given
above supported the scenario of the PDR structure of molecular clouds. In this
paper we briefly discuss the current state of PDR modelling and present some
observational results which motivated us to develop a new PDR model for a
clumpy cloud structure. Some new applications of these models are presented in
the following sections.
The first theoretical PDR models were presented by Tielens & Hollenbach (1985) and
Sternberg & Dalgarno (1989). Both models treated a PDR as a semi-infinite plane-parallel
slab in which the UV radiation illuminates the PDR from one side and the
cooling lines can only emit toward the same direction. Under these model
assumptions the temperature structure and the chemical composition of a PDR is
calculated, including the evaluation of the cooling line intensities. The main
parameters for these model calculations are the hydrogen particle density
(
typically in the range from 
to 
)
and the strength of the UV field (10 to 
times the mean UV interstellar
radiation field, Draine (1978)). The variation of these parameters may lead
to a drastic change of the chemical and physical structure of the PDR and
results in a typical set of observable line intensities. The computed
intensities for a wide parameter range were presented by several authors
(e.g. Tielens & Hollenbach (1985),van Dishoeck & Black (1988),Sternberg & Dalgarno (1989),Burton et al. (1990)). These PDR models
succeed to explain the observed line integrated intensities of e.g. the low-
and mid-J 
rotational lines or the [CII] 
line
both in galactic sources (star formations regions) and external galaxies
(e.g. Wolfire et al. (1989),Stacey et al. (1991)).
[CII] observations in molecular clouds revealed, that the [CII] emission
extends too far into the molecular cloud than could be explained by assuming a
homogeneous cloud. The [CII] emission distribution can be well modelled within
a clumpy cloud scenario (Stutzki et al. (1988),Howe et al. (1991),Stacey et al. (1993),Meixner & Tielens (1993)). In a
clumpy cloud, provided that there is a high clump to interclump density
contrast, the UV field can penetrate deep into the cloud and forms PDRs on the
surface of the clumps. Detailed UV radiative transfer calculations by
Boissé (1990) confirmed these results. It is thus necessary to consider a
clumpy molecular cloud structure. A first step in this direction is to compute
PDR models with a finite extent.
The observed intensity ratios between the 
and 
, 
and 
lines
cannot be explained by a single component cloud model (e.g.
Castets et al. (1990),Gierens et al. (1992)). Typically, the inter-isotopic ratios between the
same transitions are in the range of 3 to 8 and the ratios between different
low-J lines both for 
and 
are close to unity. In
single temperature and density models the first observational result leads to
the assumption of optically thin 
emission, whereas the second
one is interpreted as optically thick, close to thermalized emission. In
addition recent 
observations in massive star
formation regions (e.g. Graf et al. (1993)) revealed surprisingly high intensities
for this line.
The formation and destruction of 
and 
in the
CII/CI/CO transition zone of a PDR depends on several chemical processes.
Photodissociation, fractionation reactions and neutral reactions build a
sensitive chemical network in the formation and destruction of CO and its
isotopomers. For details refer to van Dishoeck & Black (1988), Köster et al. (1994) and
Sternberg & Dalgarno (1994). In order to explain the line intensities of 
bearing species, it is imperative to include a realistic treatment of the
chemistry in the PDR calculations. In Köster et al. (1994) we
presented a PDR model which takes the clumpy structure of a molecular cloud
into account by computing finite sized, plane-parallel PDRs. In these models
the more accurate treatment for the calculation of the depth dependent UV
radiation from Roberge et al. (1991) was used. Furthermore, we included a proper
and 
chemistry. The calculations have been done
for a wide parameter range and succeeded to explain the observed 
intensities, assuming high density clumps (
) and high UV radiation (
times the mean interstellar
UV field). The observed low-
and 
line ratios
can be explained with PDR models assuming a density of about 
.
A PDR model with a spherical geometry which is more reasonable for a clumpy
cloud structure than the presently used plane-parallel geometry is presented by
Störzer et al. (1995). They point out that limb brightening and geometrical effects
may lead to an increase of the [CII]/CO intensity ratio.
The [CII] fine structure line and the CO rotational lines have different
emission regions in PDRs. [CII] radiation emerges from the surface of a PDR as
far as the UV radiation is strong enough to ionize carbon. Deeper into the
cloud, due to scattering and absorption of dust grains and 
self-shielding, the UV radiation decreases and CI and CO are formed. As the PDR
layer (
) is geometrically rather thin at the relatively high
densities required to explain the observed line intensities, the chemical
structure of individual clumps has never been resolved observationally up to
now. Several authors have investigated the correlation between [CII] and 
emission (e.g. Crawford et al. (1985),Wolfire et al. (1989),Jaffe et al. (1994), see
3.2).
Figure 1: This shows the correlation of the observed [CII] 
and
line intensities in three different regions in
the RMC together with a theoretically predicted parameter space for hydrogen
particle density (
to 
(thick vertical
lines)) and UV-field strength (
to 
times the mean
interstellar UV-field (thin solid lines)). The models were computed by assuming
a visual extinction 
of 10. The arrow indicates in which direction beam
filling effects would shift the observed intensities.
The Rosette Molecular Complex (RMC) was observed in the 
line with the KOSMA (Cologne Observatory for
Submillimeter Astronomy) 3m radiotelescope and in the 
[CII]
line with the Kuiper Airborne Observatory Schneider (1995). In
Figure 1 we plot the emergent intensities of the two lines in a
parameter space of hydrogen density and UV-flux used in our PDR models. We
distinguish three different regions in the RMC: 1. the interface between the
HII-region and the molecular cloud (marked with open triangles), 2.
positions in the vicinity of the embedded IR source IR06314+0427 (marked with
filled circles) and 3. the molecular cloud core (marked with filled triangles).
The measurements cover a density range of 
and an UV field range of 
. There is
no overall correlation for the intensities like the one of 
obtained by Crawford et al. (1985) in bright galactic
sources and galaxies. In contrast, the intensity ratio strongly depends on the
emitting region. The strongest CO and [CII] emission is found near the IR
source, which is embedded in a clump of dense (
) molecular gas. Schneider (1995) conclude that the central OB
association of the Rosette Nebula NGC2244 is not the dominant UV source in this
region, as its UV intensity at a distance of about 30pc to the IR source is too
low (about a few 
) to account for the observed [CII] intensity. The IR
source itself could be a late O type star or a small cluster of OB type stars
with lower luminosities which provide the necessary UV flux.
The cloud core and the HII interface regions yield lower CO intensities
and strongly varying [CII] intensities. Due to the higher UV flux in the HII interface region, the [CII] line is quite strong. In the remote part of
the molecular cloud, far away from the HII region, the UV flux is lower
and the [CII] emission decreases. On the other hand the CO emission at the HII interface region is low, as in these strong UV flux regions carbon is
photoionized and/or CO is photodissociated.
In the model calculations the beam filling factors for CO (
) and
[CII] (
) are set to 1 which is certainly not true for all observed
positions. The calculated mean density is about a factor of 10 lower than the
critical density for the 
line. Therefore the
cloud must have a clumpy substructure and 
might be lower than 1.
Provided that CII has about the same beam filling factor as CO, the plotted
values are shifted in the direction indicated by the arrow in Figure 1.
Nevertheless, without knowing the correct beam filling values, the PDR models
are able to fix a lower limit for the densities and the UV fluxes.
A similar investigation has been done by Jaffe et al. (1994). They compared the
observed [CII] and 
intensities for NGC2024
with the predictions of two different PDR models: the semi-infinite PDR model
of Wolfire et al. (1989) and our finite sized PDR model as described in
Köster et al. (1994). They divided their observations into two regions: 1. the
cloud proper with less than 
west of the radio peak and 2. the western
edge zone with 
or more west of the radio continuum peak. The intensities
in region 1 can be explained satisfactorily with the semi-infinite PDR models.
This indicates optically thick line emission both for [CII] and CO. However in
region 2 the CO line intensity drops whereas the [CII] intensity is still
high. The semi-infinite models fail to explain such ratios. The authors pointed
out that the most reasonable scenario is one in which the mean column
densities of the clumps decreases to the east. Thus they chose the finite PDR
models for low visual extinctions (
, the visual extinction is a
direct measure for the geometrical extent of a cloud) and thus succeed to
reproduce the emission from this region with these models. The [CII] line
intensities are not sensitive to the geometrical extent of a cloud, as CII
always exists in the outer layers of clumps/PDRs. Thus even clouds with a very
small 
are able to provide sufficient [CII] line intensity. On the other
hand small visual extinctions and moderate UV fluxes (here models for 
and 
) lead to low CO column densities and optical depths for
the CO lines, as most of the carbon is still in the form of CII and thus the CO
intensities are quite low.
To prove the existence of high density condensations in molecular clouds which
are illuminated by strong UV fields, it is necessary to measure a line of a
species which is exclusively formed in these high density clumps. Such a
species is the OH molecule. Because of its high critical density (
) OH is not excited in the low density interclump
medium. Theoretical model calculations show that OH is produced very
efficiently in the outer layers of high density/strong UV field regions
(Sternberg & Dalgarno (1994)). OH is produced by the endothermic reaction
as far as the hydrogen particle density is above 
and the
temperatures in these layers are greater than 600 K. This leads to column
densities 
. Thus several OH
lines should be quite strong and provide an excellent tracer for high density
clumps. OH rotational lines have been observed so far only in shock heated
regions like Orion-KL (Melnick et al. (1987)) but not in any photon dominated
region. The frequencies lie in the range which is not observable with ground
based telescopes. However with the KAO or at least with the higher sensitivity
of ISO (Infrared Space Observatory) the lines should be detectable. Direct
observation of the OH rotational lines would thus give strong evidence for the
current PDR models.
Theoretical PDR models successfully explain a wide range of emergent line
intensities from atomic and molecular species with transitions in the sub-mm
and FIR wavelength range, originating in photon dominated regions. The
comparison of the observed ratio for the [CII] and CO (here 
and 
) line intensities allows to estimate the density and
UV-field conditions in the observed regions, i.e. to give at least a lower
limit for these values. For clumps with low total column densities, e.g. at
the edges of molecular clouds in more diffuse regions, models with a finite
geometry are essential to explain the observations.
OH is efficiently produced and excited in high density clumps which are
irradiated by strong UV fields, but not in the low density interclump medium.
OH is thus an excellent tracer for high density PDRs and should be detectable
with ISO.
This research was supported by the Deutsche Forschungsgemeinschaft through
Grant SFB 301.
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