Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.mrao.cam.ac.uk/yerac/koester/koester.ps Äàòà èçìåíåíèÿ: Wed Feb 22 22:21:36 1995 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 02:39:04 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: molecular cloud

Photon dominated regions: observation and
theory
By B. K ¨o s t e r, N. S c hn e i d e r, H. S t ¨o r z e r AND J. S t u t z k i
e­mail: koester@apollo.ph1.Uni­Koeln.de
I. Physikalisches Institut der Universit¨at zu K¨oln, Z¨ulpicher Straúe 77, D­50937 K¨oln,
GERMANY
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] 158 ¯m
and 12 CO 3 ! 2 line emission of the Rosette Molecular Cloud. The FIR­ and sub­mm line
intensities are compared with theoretical predictions. We use a plane­parallel 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 mo­
lecular 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.
1. Introduction
Molecular clouds in massive star formation regions (e.g. Orion Nebula) are irradiated
by far­ultraviolet 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 condi­
tions 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 H 2 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] 158¯m, [OI] 63 ¯m and 145 ¯m lines. Deeper into the cloud, the
temperature decreases and the prominent cooling lines are the [CI] 609 ¯m 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 low­J
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.
2. Theoretical PDR models
2.1. First models and basic assumptions
The first theoretical PDR models were presented by Tielens & Hollenbach (1985) and
Sternberg & Dalgarno (1989). Both models treated a PDR as a semi­infinite 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
1

2 B. K¨oster et al.: Photon dominated regions
the evaluation of the cooling line intensities. The main parameters for these model
calculations are the hydrogen particle density (n(H) typically in the range from 10 2 to
10 7 cm \Gamma3 ) and the strength of the UV field (10 to 10 6 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 mid­J 12 CO
rotational lines or the [CII] 158 ¯m line both in galactic sources (star formations regions)
and external galaxies (e.g. Wolfire et al. (1989), Stacey et al. (1991)).
2.2. Observational hints and model improvements
[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 Boiss'e (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 12 CO and 13 CO J = 3 ! 2, J = 2 ! 1 and
J = 1 ! 0 lines cannot be explained by a single component cloud model (e.g. Castets
et al. (1990), Gierens et al. (1992)). Typically, the inter­isotopic ratios between the
same transitions are in the range of 3 to 8 and the ratios between different low­J lines
both for 12 CO and 13 CO are close to unity. In single temperature and density models
the first observational result leads to the assumption of optically thin 13 CO emission,
whereas the second one is interpreted as optically thick, close to thermalized emission.
In addition recent 13 CO 6 ! 5 observations in massive star formation regions (e.g. Graf
et al. (1993)) revealed surprisingly high intensities for this line.
The formation and destruction of 12 CO and 13 CO 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), K¨oster et
al. (1994) and Sternberg & Dalgarno (1994). In order to explain the line intensities of
13 C bearing species, it is imperative to include a realistic treatment of the 13 C chemistry
in the PDR calculations. In K¨oster et al. (1994) we presented a PDR model which
takes the clumpy structure of a molecular cloud into account by computing finite sized,
plane­parallel 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 12 C and 13 C chemistry. The calculations have been done for a wide
parameter range and succeeded to explain the observed 13 CO 6 ! 5 intensities, assuming
high density clumps (n(H) – 10 6 cm \Gamma3 ) and high UV radiation (– 10 5 times the mean
interstellar UV field). The observed low­J 12 CO and 13 CO line ratios can be explained
with PDR models assuming a density of about 10 5 cm \Gamma3 .
A PDR model with a spherical geometry which is more reasonable for a clumpy cloud
structure than the presently used plane­parallel geometry is presented by St¨orzer et al.
(1995). They point out that limb brightening and geometrical effects may lead to an
increase of the [CII]/CO intensity ratio.

B. K¨oster et al.: Photon dominated regions 3
Figure 1. This shows the correlation of the observed [CII] 158¯m and 12 CO 3 ! 2 line intens­
ities in three different regions in the RMC together with a theoretically predicted parameter
space for hydrogen particle density (n(H) = 10 3 to 10 7 cm \Gamma3 (thick vertical lines)) and UV­field
strength (ü = 10 2 to 10 6 times the mean interstellar UV­field (thin solid lines)). The models
were computed by assuming a visual extinction AV of 10. The arrow indicates in which direction
beam filling effects would shift the observed intensities.
3. [CII]/CO correlation
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 CO=H 2 self­shielding, the UV radiation decreases and CI and CO
are formed. As the PDR layer (AV Ÿ 5) 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 CO 1 ! 0 emission (e.g. Crawford
et al. (1985), Wolfire et al. (1989), Jaffe et al. (1994), see 3.2).
3.1. Rosette Molecular Cloud
The Rosette Molecular Complex (RMC) was observed in the 12 CO 3 ! 2 line with
the KOSMA (Cologne Observatory for Submillimeter Astronomy) 3m radiotelescope
and in the 158¯m [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 Hii­region 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 10 3 cm \Gamma3 Ÿ n(H) Ÿ 10 5 cm \Gamma3 and an UV

4 B. K¨oster et al.: Photon dominated regions
field range of ü Ÿ 1000. There is no overall correlation for the intensities like the one
of I CII =I CO = 4400 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 (n(H) – 10 4 cm \Gamma3 ) 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 ü) 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 (j CO ) and [CII] (j 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 12 CO 3 ! 2 line. Therefore
the cloud must have a clumpy substructure and jCO 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.
3.2. NGC2024
A similar investigation has been done by Jaffe et al. (1994). They compared the observed
[CII] and 12 CO 1 ! 0 intensities for NGC2024 with the predictions of two different PDR
models: the semi­infinite PDR model of Wolfire et al. (1989) and our finite sized PDR
model as described in K¨oster et al. (1994). They divided their observations into two
regions: 1. the cloud proper with less than 10 0 west of the radio peak and 2. the western
edge zone with 10 0 or more west of the radio continuum peak. The intensities in region
1 can be explained satisfactorily with the semi­infinite 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 semi­infinite 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 (A V Ÿ 5, 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 A V are able to provide sufficient [CII] line intensity.
On the other hand small visual extinctions and moderate UV fluxes (here models for
ü = 300 and ü = 1000) 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.

B. K¨oster et al.: Photon dominated regions 5
4. OH line emission as a tracer for high density UV irradiated clumps
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 (n cr ú 10 8 cm \Gamma3 ) 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
O+ H 2 ! OH +H; (4.1)
as far as the hydrogen particle density is above 10 6 cm \Gamma3 and the temperatures in these
layers are greater than 600 K. This leads to column densities N(OH) – 10 15 cm \Gamma2 .
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 Orion­KL (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.
5. Summary
Theoretical PDR models successfully explain a wide range of emergent line intensities
from atomic and molecular species with transitions in the sub­mm and FIR wavelength
range, originating in photon dominated regions. The comparison of the observed ratio
for the [CII] and CO (here J = 3 ! 2 and 1 ! 0) line intensities allows to estimate
the density and UV­field 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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