Table Of ContentAtomic Oxygen Abundance in Molecular Clouds:
Absorption Toward Sagittarius B2
D.C. Lis, J. Keene, T.G. Phillips, P. Schilke,
M.W. Werner & J. Zmuidzinas
2001-7
ToAppearin
The As_ophysicalJournal
Atomic Oxygen Abundance in Molecular Clouds: Absorption
Toward Sagittarius B2
D.C. Lis1, Jocelyn Keene 1,2,T.G. Phillips 1, P. Schilke 3, M.W. Werner 2, and J. Zmuidzinas 1
ABSTRACT
We have obtained high-resolution (,_35 km s-1) spectra toward the molecular cloud Sgr B2
at 63/_m, the wavelength of the ground-state fine-structure line of atomic oxygen (Of), using the
ISO-LWS instrument. Four separate velocity components are seen in the deconvolved spectrum,
in absorption against the dust continuum emission of Sgr B2. Three of these components, corre-
sponding to foreground clouds, are used to study the OI content of the cool molecular gas along
the line of sight. In principle, the atomic oxygen that produces a particular velocity component
could exist in any, or all, of three physically distinct regions: inside a dense molecular cloud, in
the UV illuminated surface layer (PDR) of a cloud, and in an atomic (HI) gas halo. For each of
the three foreground clouds, we estimate, and subtract from the observed OI column density, the
oxygen content of the HI halo gas, by scaling from a published high-resolution 21 cm spectrum.
We find that the remaining O1 column density is correlated with the observed laco column
density. From the slope of this correlation, an average [0I]/[13CO] ratio of 270 ± 120 (3a) is
derived, which corresponds to [OI]/[CO]_ 9 for a CO to 13CO abundance ratio of 30. Assuming
a 13CO abundance of 1 x 10-8 with respect to H nuclei, we derive an atomic oxygen abundance
of 2.7 x 10-4 in the dense gas phase, corresponding to a 15% oxygen depletion compared to the
diffuse ISM in our Galactic neighborhood. The presence of multiple, spectrally resolved velocity
components in the Sgr B2 absorption spectrum allows, for the first time, a direct determination
of the PDR contribution to the O! column density. The PDR regions should contain OI but not
lsCO, and would thus be expected to produce an offset in the OI - 13CO correlation. Our data
do not show such an offset, suggesting that within our beam OI is spatially coexistent with the
molecular gas, as traced by 13CO. This may be a result of the inhomogeneous nature of the
clouds.
Subject headings: Subject headings: infrared: ISM: lines and bands -- ISM: abundances
I. Introduction Klemperer 1973; Dalgarno & Black 1976) there
have been theoretical predictions indicating that
A longstanding problem for our understand- the fundamental reservoirs of these elements are
ing of the quiescent dense interstellar medium the molecular species CO, 02, and H20. It is
(ISM), often described as molecular clouds, has usually assumed that the local gas phase oxygen
been the difficulty of accounting for the gas phase abundance is about twice that of carbon. This
abundance of carbon and oxygen. Since the first comes from the observed stellar values, modified
calculations of ion-molecule schemes (Herbst &: by the depletion seen in local ISM diffuse clouds
such as those toward _ Ophiuchi and HD154368
ICalifornia Institute of Technology, Downs Laboratory
(Snow and Witt 1996; Snow et al. 1996; CardeUi
of Physics 320-47, Pasadena, CA 91125
2Jet Propulsion Laboratory 264-767, Pasadena, CA et al. 1993), i.e. the carbon abundance [C] =
91109 1.66 x 10-4 and 1.32 x 10-4, respectively, and
3Max-Planck-Institut fiir Radioastronomie, Auf dem the oxygen abundance [O] = 2.88 x 10-4, giv-
Hiigel 69, D-53121 Bonn, Germany ing an average [C]/[O] = 0.51. So nearly all car-
bonshouldbein CO,withplenty
of oxygen left et al. 1993; as well as Bergin et al. 2000 and ref-
over for 02 and H20. Millimeter-wave astronomy erences therein for a discussion of recent chemical
measurements have indeed shown the strong pres- models). Thus purely on the basis of ion-molecule
ence of CO at about 10-4 of H2 (e.g. Lada et models of ISM chemistry, there is a strong ar-
al. 1994). However, the H20 abundance is only gument for searching for O1 absorption in cool,
about 10-5-10 -7 of H2 (Jacq et al. 1988; Zmuidz- shielded clouds, as a fundamental constituent.
inas et al. 1995; Cernicharo et ai. 1997), or even However, the situation is complex. Molecules
lower, 10-_ - 10-1°, as measured by SWAS (Snell near the edges of clouds are known to be photodis-
et al. 2000a, 2000b), except for strongly shocked sociated by interstellar UV photons (e.g. Langer
gas, where it reaches 3.5 x 10-4 (Melnick et al. 1976; HoUenbach et al. 1991). These photon dom-
2000). For O2, only upper limits of < 10-s of H2
inated regions (PDRs) may show strong Ol emis-
(Liszt & Vanden Bout 1985; Fuente et al. 1993), sion (Melnick et al. 1979), CH emission (Russell
(0.33 4- 1.6) x 10-7 (Goldsmith et al. 2000) are
et al. 1980), and C! emission (Phillips & Hug-
found. This may be compared with measurements
gins 1981). In the case of CI, where relatively
of the HsO + ion abundance (Phillips et al. 1992)
high spectral and spatial resolution is available,
in the range of 10-9 -- 10-1° of H2, which lead it has proved quite hard to separate the individ-
to predictions of H20 abundances in the range ual contributions of the PDR and the quiescent
10-e - 10-7 and 02 in the range 10-5 - 10-e. dense cloud (Keene et al. 1985, 1997). One dif-
The observational status is consistent in that, af- ficulty, then, is to distinguish between the PDR
ter CO, the anticipated major oxygen molecular and quiescent dense medium contributions to the
species, H20 and 02, are deficient by at least an
atomic oxygen column density. The distribution
order of magnitude (see van Dishoeck et al. 1993
of atomic oxygen probably extends deeper into the
for a review). cloud than that of atomic carbon, because the dis-
It is known that some oxygen is tied up in sociation threshold for 02 (about 5.1 eV) is con-
H20 mantles on grains in the dense molecular siderably less than for CO (11.1 eV; Cox 2000).
clouds and this is sometimes put forward to sug- This is not usually displayed in PDR models, but
gest a [C]/[O] ratio in the gas larger than standard. in Figure la we show the atomic and molecular
Whittet (1992) estimates from 3/_m features that abundances of oxygen for a recent model (Le Bout-
N(H20)/N(H) __8.6 x 10-s on grains in the Tau- lot et al. 1993). This model shows the abundances
rus dark cloud, and Knacke & Larson (1991) find a of species in the outer PDR region, as well as
factor of 10 less H20 in the gas phase on the line- abundances in the deep, shielded molecular cloud
of-sight toward Orion BN. There is then a possi- region, where ion-molecule chemistry dominates.
bility that a quarter of the oxygen budget is lost In contrast to CH, Ol is present in both PDR
to grain mantles in high column density clouds. and shielded molecular cloud regions. OI (63 #m)
However, the clouds investigated here are on aver- emission would not be observable from deep in the
age A_ ,_ 1- 10 (see Table 1), which is somewhere cloud because this region is predicted to be too
between the values for the diffuse clouds in which cold to populate the upper level, but OI may be
the C and O abundances are directly measured, detectable in absorption if there is asuitable back-
and the very high values at which H20 mantles are ground source. In fact, Poglitsch et al. (1996) re-
seen. Thus it seems likely that the traditional as- ported an absorption feature in the OI spectrum of
sumption of [C]/[O] ,,_0.5 in the gas phase is suit- DR21, taken using the NASA KAO. This feature
able here, with the uncertainties limited to about may be due to a molecular cloud and was taken
25%, at least as is known from the effect of H20 to indicate a high relative abundance of atomic
grain mantles. oxygen in a cold cloud.
In 1994 we proposed to use the ISO-LWS in- The method we have proposed is to examine
strument to test whether the missing oxygen com- a range of objects with ISO-LWS (Fabry- Perot
ponent was present in atomic form by means of ab- mode with resolution of _-35 kms-1; Swinyard et
sorption line spectroscopy of OI at 63/_m. Indeed, al. 1996). Each of the clouds has a strong back-
atomic oxygen is predicted by chemical models to ground continuum source on the line of sight and
be present in the dense ISM (see e.g. van Dishoeck a different column density of quiescent gas, as seen
frommolecular
line data. The final intention is to in shielded clumps, and also be responsible for the
separate the PDR and quiescent cloud effects by non-appearance of the pure H_ PDR layer (see §3).
the variation of OI absorption strength with qui-
escent cloud column density. An initial report of 2. Observations
our ISO measurements was given by Keene et al.
OI (63 _m) observations of Sgr B2 were carried
(1998). The spectrum of Sgr B2 has been found
out with both grating and Fabry-Perot modes of
to show very strong Ol (63 #m) absorption due to
ISO-LWS. Figure 2 shows the Fabry-Perot scans
several separate absorption clouds, distinguishable
for positions Sgr B2(M), and 180" N and 180"
by their different velocities, which can be analyzed
S. At the central position absorption is seen for
independently to provide the variation in column
the Sgr B2 envelope at 60 kms -1, as well as
density. That forms the subject of this study. An
the foreground molecular clouds on the line of
initial report of the Sgr B2 absorption has been
sight to the Sun at -,,0 kms -1 and two nega-
made by Baluteau et al. (1997) from a low spectral
tive velocities. The observed Fabry-Perot spectra
resolution survey scan using the grating mode of
have been deconvolved with a 35 kms -1 width
ISO-LWS with resolution of _ 1,400 km s- 1, where
Lorentzian representing the LWS resolution func-
the components are not resolved. Baluteau et al.
tion (Swinyard et al. 1996) using a Maximum
also obtained very useful N-S and E-W raster
Entropy Method (MEM) deconvolution algorithm,
scans showing that, off the position of the strong
which results in a factor of 2 - 3 improvement
continuum source Sgr B2(M), OI is seen in emis-
in the spectral resolution. Improving the res-
sion. This emission at -_60 km s-1 (Fig. 2) is from
olution of astronomical images using the MEM
Sgr B2 itself, rather than from foreground clouds.
is a well established technique in astronomy (see
The molecular absorption line clouds in front
e.g. Narayan & Nityananada 1986 and references
of Sgr B2 have been studied by a number of au-
therein), provided the instrumental response func-
thors. Densities are typically found to be in the
tion is known. Although usually used for enhanc-
range 102 - 103 cm -3, with some clouds as dense
ing spatial resolution in images, the technique is
as 2 x 104 cm -3 (Greaves & Williams 1994). Tem-
perfectly suitable for enhancing spectral resolu-
peratures are found to be generally _15 - 20 K,
tion as well. It works by finding a model that,
except for the -95 to -105 kms -1 component,
when convolved with the instrumental response
which is somewhat warmer (--,35 K; Tieftrunk et
function, best reproduces the observed spectrum.
al. 1994). These temperatures and densities are
This is done by modifying the model to minimize
much too low to produce significant OI emission.
_2. Depending on the constraints of the image
These clouds are rich in molecular species, such as
or spectrum, the answer can be ambiguous. In
HCO +, HCN, HNC, CN, CCH, C3H2, CS, SiO,
these cases, maximizing the entropy ensures that
N2H +, NH3, CH3OH, SO, H2S (Tieftrnnk et al.
the model with the least information not required
1994; Greaves & Nyman 1996) with abundances
by the data is chosen. This additional constraint
similar to those observed in dark clouds. Simi-
is important in cases where little information is
larly, rich absorption spectra have also been found
present (e.g. with sparse uv-coverage in imaging),
in diffuse and translucent absorption line clouds in
but becomes less critical for well sampled data.
front of extragalactic sources (e.g. Lucas & Liszt
The achievable resolution is difficult to quantify,
1997 and references therein). Standard chemical
since it depends on the signal-to-noise ratio. Us-
models of diffuse and translucent clouds are un-
ing these techniques, we developed a simple algo-
able to reproduce such a rich chemistry, and sev-
rithm to produce a model that, when convolved
eral alternatives have been proposed, such as tur-
with the instrumental profile, fits the observed
bulent chemistry (Hogerheijde et al. 1995; Joulain
spectrum well. Since our data have high signal-
et al. 1998) or differences in the elemental deple-
to-noise ratio, and they are highly oversampled,
tions and/or the gas phase [C]/[O] ratio compared
we found that the model was unique. To estimate
to dark clouds (Turner 2000). Other possibilities the effect of the MEM deconvolution on the un-
include turbulent mixing of different cloud layers
certainty of the OI intensities, we added to the
(Chi_ze et al. 1991; Xie et al. 1995), which could
observed spectrum random noise with the same
enrich the translucent layer by molecules produced
rms as the observed spectrum and subsequently
applied the MEM algorithm to the resulting spec- will be detectable. From the model there is ,_1 Av
trum. The resulting la statistical uncertainties of such material on the line of sight through a
for the intensities of the three OI components are molecular cloud. This would correspond to a de-
estimated to be _,3-5%. tectable OI absorption with an equivalent width of
3.5 km s-1 , assuming an O/H ratio of 3.2 x 10-4.
The Sgr B2 envelope is seen in emission at both
offset positions (Fig. 2). However, no emission However, aside from the problem of not being able
is seen at the velocities of the foreground clouds. to identify the purely CI region (Keene et al. 1985),
there is another measurement which raises doubt
Molecular absorption can indeed be seen at each of
about the existence of a pure H2 region. The mod-
the velocities of the O! absorption. Figure 3 com-
pares the 13CO (1-0) absorption spectrum for the eling by Andersson and Wannier (1993) of observa-
tious of dark clouds in the lines of HI, OH, and CO
Sgr B2(M) line of sight (observed with the IRAM
leads to their interpretation that Hi halos lie much
30-meter telescope) with the LWS Ol spectrum.
closer to the molecular clouds as defined by CO
However, the HI spectrum must also be compared.
For Sgr B2(M) there exists such aspectrum (Gar- observations than predicted by standard chemi-
wood & Dickey 1989) and, as can be seen in Figure cal and photo-dissociation models as of that date.
3, HI absorption is also present for each velocity They conclude that the formation rate of H2 used
at which Ol absorption is seen. in the models is probably too large, leading to a
region of pure H2 much larger than is observed.
3. Discussion Thus, the method here, to compare OI absorp-
tions after subtracting the contribution from the
There are two major complications, which must HI halo in clouds of varying CO column density,
be dealt with before any O! absorption line can should allow the identification of any surface re-
be used to assign atomic oxygen abundance to a gion, by means of the OI intercept. If there is
molecular cloud. The first is the fact that atomic
a proportionality found between Oi and CO, this
oxygen will exist in line-of-sight HI clouds. The will provide the relative abundance of atomic oxy-
clouds, which show HI absorption at the same ve- gen in CO regions.
locity as 13C0, are very likely close to, or attached
Referring to the MEM-deconvolved OI spec-
to the molecular clouds, probably as halos (Wan-
trum of Sgr B2 (Fig. 2), we see that the spec-
nier et al. 1983; region A in Fig. lb). This problem
trum naturally separates into three velocity ranges
can be approached if good quality HI absorption
corresponding to absorption by foreground clouds
spectra are available. The second is the above- at velocities < -78 kms -1, -78 to -25 kms -1,
mentioned problem of the PDRs on the line of
and -25 to 30 km s-1, plus the absorption feature
sight. According to the models, some significant
at ,,,60 kms -1 corresponding to the envelope of
region of the PDR near the surface (,_1 Av) will
Sgr B2 itself. In this paper we are not concerned
contain H2 (region B in Fig. lb), but no CO or
with the Sgr B2 envelope feature at 60 km s-1, be-
other trace molecules whose presence defines the
cause it is likely to be contaminated by OI emis-
molecular cloud (region C). Region B, like the HI
sion, but will analyze the foreground clouds. For
region, will contain OI, and should also contribute these three velocity ranges we have O1, HI, and
to the absorption. 4 The assignment of OI to the
13CO spectra, which allow us to determine the
molecular cloud is therefore a tricky operation. Al-
atomic oxygen, atomic hydrogen, and 13CO col-
though various PDR models show differences in umn densities for the three O] velocity compo-
the depth into the cloud of the HI/H2 interface,
nents (Table 1). From the high spectral resolution
they all have an H2 region bereft of CO (region B) 13C0 and HI spectra it is plausible that the three
of roughly the same extent. OI absorption features further break into multiple
We may wonder if Ol in the PDR (region B) components (see Fig. 3).
Since we have the HI column density for each
4It is important to note that the density of these PDRs
is likely to be quite low (a few 102 - a few 103 cm-3), velocity range, using the measured [O]/[H] ratio in
so that Ol atoms are all in the ground state and the Ol diffuse clouds (3.2 x 10-4; Meyer et al. 1998) we
column density is accurately determined byan absorption can determine the OI column density associated
measurement.
with the HI region and subtract this from the ob-
served OI column density, N(OI)obs, to get the OI H in the foreground molecular clouds, compa-
column density associated with the molecular H2 rable to the value found in diffuse clouds. The
gas, N(O1)mol. I.e., whatever is left must be asso- deduced [OI]/[CO] value for the molecular gas is
ciated with the PDR H2 region (region B in Fig. --, 9 4-4 (3a) for a [CO]/[13C0] abundance ratio
lb) plus the 13C0 region (region C in Fig. lb). of 30 (Langer & Penzias 1990). Also from the
However, in any individual case it is not possible plot it appears that there is no nonzero intercept,
to separate the two contributions to N(OI)mol and (-1.1 4- 4.6) x 10is cm -2, (a la statistical uncer-
therefore not possible to claim a knowledge of the tainty, corresponding to Av = 0.08, assuming the
atomic oxygen content of molecular clouds as de- diffuse cloud OI abundance) so, within the uncer-
fined by lsCO. In principle we need to know the tainties, there is no manifestation of a PDR H2
number of PDRs on the line of sight for a given region devoid of 13CO. Incidentally, the fact that
velocity range in order to compare the total lsCO this region is not seen makes the validity of the
column density with OI for that range, or alter- assumption of similar PDRs moot.
natively by comparing mean column density per The formula given in eq. (1) corresponds to a
13C0 component with mean OI column density physical model with a number of 13CO clumps dis-
over that range. The OI and lsCO column densi- tributed along the line of sight, each one exposed
ties are given in Table 1,together with the number to the external UV field and therefore surrounded
of IsCO components in each range. by its own PDR. The other extreme is a collection
As a simplest approximation we assume that of clumps packed closely together with only exter-
the PDRs are all the same and that there is a uni- nal surfaces exposed to the UV field (one common
versal [OI]/[lSCO] coefficient, X. Then, for each PDR for all clumps; ncomp = 1). In this case,
velocity range a least squares fit gives an [OI]/[13CO] ratio of
2904-24 (la), statistically consistent with that de-
N(Ol)mol/ncomp _- N(OI)H2/PDR+ rived above, and again no nonzero intercept. Our
results are thus insensitive to the exact number of
XN(13CO)/ncomp (1) 13CO components.
Since we have three ranges, N(OI)H2/PDR and
4. Conclusion
X can be determined from the least squares line
fit. The plot ofN(OI)mol/ncomp vs. N(13CO)/ncomp
We have presenteda high signal-to-noisreatio
is shown in Figure 4. From this plot it is inferred
ISO-LWS Fabry-Perotspectrum oftheOI (63pm)
that there is indeed a reasonably constant rela-
absorptiontoward Sgr B2, which isof sufficient
tion between OI and laCO column densities and
qualitytopermit deconvolutionoftheinstrument
that [OI]/[13CO] is 2704-35 (la uncertainty for the
spectralfunction,thus revealingfourseparateve-
slope based on the statistical uncertainties in lsco
locityrangesofabsorption.This spectrum corre-
and OI column densities). After adding in quadra-
lateswellwith both the HI absorptionspectrum
ture 15% calibration uncertainties for 13CO and
and the13CO absorptionspectrum. Inordertode-
OI to the 3a statistical uncertainty for the slope,
termine theatomic oxygen abundance associated
we obtain [OI]/[laCO] = 270 ± 120 (3a; not in-
withthemolecularclouds,asdefinedby 13CO, we
cluding modeling uncertainties) for the foreground
firstsubtractthe Ol absorptioncomponents, de-
clouds on the line of sight toward Sgr B2(M). As-
duced from theHI spectrum,forthethreevelocity
suming a 13CO abundance of 2 x 10-6 relative
ranges correspondingto foregroundcloudson the
to H2 (Dickman 1978) 5 leads to an atomic oxy-
lineofsighttoward Sgr B2. We thencompute the
gen abundance of (2.7 + 1.2) x 10-4 relative to
oxygen column density remaining and correlate
thatwiththe13CO column densitypercloudcom-
5The 13CO abundace derived by Dickman (1978) corre-
sponds to local dark clouds. One might expect the laCO ponent asfoundfrom thenumber ofdistinct13C0
abundance to increase with decreasing gaiactocentric dis- velocityabsorptionfeaturesineachofthethreeOl
tance following the observed variation in the CO/13CO
abundance ratio (Langer & Penzias 1990). However, Lis &
caivalueof2x 10-6 asan average forthe clouds on the line
Goldsmith (1989) derived a 13CO abundance of 1x 10-6
ofsighttoward Sgr B2. This value isuncertain by about a
relative to H2 in the envelope of the Sgr B2 molecular cloud, factorof2.
a factor of 2 lower than the local value. We thus use the lo-
absorptionranges.The plotshows a good fitto a NASA under ISO Block Grant to the California
constant[O1]/[13CO]ratioof270 + 120 (3a)with Instituteof Technology and by NSF grant AST-
no apparent intercept.This impliesthatthereis 9980846 to the Caltech SubmiUimeter Observa-
indeeda largeatomic oxygen contentinmolecular tory. We thank J. Le Bourlot, G. Pineau des
clouds(Figlb,regionC),but thereisno evidence For_tsand E.Roueffforthepermissiontousetheir
foratomic oxygen inthe H2 regiondevoid ofCO PDR model.
predictedby PDR models. The probable reason
forthe absence of the model predictedregion B REFERENCES
isthe inhomogeneous nature ofthe cloudsurface,
Andersson, B.-G. & Wannier, P. G. 1993, ApJ,
which effectivelmyixesthevarioussurfaceregions.
402,585
The resultthen depends on how we describea
molecular cloud.Ifwe definethe cloudsasfilling Bergin,E. A.,Melnick, G. J.,Stauffer,J.R.,et
thesizescaleofthepencilbeam tothesource,the al.2000,ApJ, 539,129
OI iscorrelatedwith 13CO and itisreasonableto
Baluteau,J.-P.,Cox,P.,Cernicharo,J.etah 1997,
say thatatomic oxygen ispresentinthe CO con-
A&A, 322,L23
rainingclouds.Ifwe definea cloudas one ofthe
smallclumps presumed to be partofthe inhomo- Cardelli,J.A.,Ebbets,D.C.,& Savage,B.D. 1993,
geneous structureofthebigcloud,ithas notbeen ApJ, 413,401
proved thatatomic oxygen spatiallycoexistswith
the CO, asitmay be ina surfaceH2 region.The CaseUi,P.,Walmsley, C.M., Tafalla,M., Dote,L.,
result may be best stated that, observationally, Myers, P.C. 1999,ApJ, 523,L165
there is an apparent correlation of [OI]/[CO] _- 9
Caux, E.,Ceccarelli,C.,Casters,A.,etal.2000,
in the ISM molecular cloudson the lineofsight
A&A, 347,L1
toward Sgr B2. We finda much lower[OI]/[CO]
ratiointhe moleculargas than recentlyreported Cernicharo, J., Lira, T., Cox, P., Gonzalez-
in L1689N (_-50;Caux et al.1999). Our result Alfonso,E.,Caux, E.,Swinyard, B.M., Martin-
isalsoa littlelowerthan thatfound forthe cold Pintado,J.,Baluteau,J.P.,& Clegg,P. 1997,
clouds on the lineof sighttoward W49N (,,,15; A&A, 323,L25
Vasteletal.2000).
Chi_ze,J.-P.,Pineau desFor_ts,G.,& Herbst,E.
The observed [O1]/[13CO]column densityratio
of270:i:120 can be easilyexplainedintheframe- ApJ, 1991,373,110
work ofPDR models oflow densityclouds(Fig.6,
Cohen, R.J.1977,MNRAS, 178,547
solidline;nH= 100 cm -3,Go = 1),or highden-
sitycoudsilluminatedby a strongUV field(Fig.6, Cox, A.N.,er.2000,Allen'sAstrophysicalQuan-
dashed line;nH = 104 cm -3, GO ----I00). The tities(4thed.;New York: Springer)
PDR model predictsa lower[OI]J[13CO]ratiofor
Dalgarno,A.,& Black,J.H.1976,RPPh, 39,573
highdensitycloudsilluminatedby a weak UV field
(Fig.6,dotted line;nH ---1-04 cm-3,Go = I),ex-
Dickman, R.L. 1978,ApJS, 37,407
cept fordiffuseregions(Av < I). However, the
model does not take intoaccount CO depletion Fuente,A., Ceruicharo,J.,Garcia-Burillo,S.,&
onto dustgrainsinhigh-density,low-temperature Tejero,J.1993,A&A, 275,558
regionsthat islikelyto be important forhigh-
Garwood, R.W., & Dickey,J.M. 1989,ApJ,338,
column densityclouds,such as thosestudiedby
841
Caux et al.(1999) and Vastelet al.(2000) (see
e.g.Kramer etal.1999,Casellietal.1999).
Goldsmith,P.F.,Melnick,G. J.,& Bergin,E.A.
etal.2000,ApJ, 539,L123
This researchis based on observationswith
ISO, an ESA project with instruments funded Greaves, J.S., & Nyman, L.-A. 1996, A&A, 305,
950
by ESA Member States with the participation
of ISAS and NASA, and has been supported by
Greaves, J.S., & Williams, P.G. 1994, A&A, 290, Mehaick, G., Gull, G. E., Harwit, M. 1979, ApJ,
259 227, L29
Herbst, E., & Klemperer, W. 1973, ApJ, 185, 505 Meyer, D.M., Jura, M., Cardelli, J.A. 1998, ApJ,
493, 222
Hogerheijde, M. R., de Geus, E. J.,Spaans, M.,
van Langevelde, H. J., & van Dishoeck, E. F. Naraya, R., & Nityanananda, R. 1986, ARAA, 24,
1995, ApJ, 441, L93 127
Hollenbazah, D.J., Takahashi, T., & Tielens, Poghtsch, A., Herrmann, F., Genzel, R.; Madden,
A.G.G.M. 1991, ApJ, 377, 192 S.C., Nikola, T., Timmermann, R., Geis, N.,
Stacey, G.J. 1996, ApJ, 462, IA3
HoUenbach, D. J., & Tielens, A.G.G.M. 1999,
RevMP, 71, 173 Phillips, T. G., van Dishoeck, E.F., Keene, J.
1992, ApJ, 399, 533
Jacq, T., Henkel, C., Walmsley, C.M., Jewell,
P.R., & Baudry, A. 1988, A&A, 199, L5 Phillips, T.G., & Huggins, P.J. 1981, ApJ, 251,
533
Joulain, K., Falgarone, E., Pineau des For_ts, G.,
Flower, D.R. 1998, A&A, 340, 241 Russell, R.W., Melnick, G., Gull, G.E., & Harwit,
M. 1980, ApJ, 240, L99
Keene, J., Blake, G.A., Phillips, T.G., Huggins,
P.J., Beichman, C.A. 1985, ApJ, 299, 697 Snell, R. L., Howe, J. E., Ashby, M.L.N. et al.
2000a, ApJ, 539, L93
Keene, J., Lis, D.C.., Phillips, T.G., & Schilke, P.
1997, in Molecules in Astrophysics: Probes and Snell, R. L.; Howe, J. E.; Ashby, M.L.N. et al.
Processes, ed. E. van Dishoeck (Kluwer), 129 2000b, ApJ, 539, L101
Knacke, R.F., & Larson, H.P. 1991, ApJ, 367, 162 Snow, T.P., Black, J.H., van Dishoeck, E.F.,
Burks, G., Crutcher, R.M., Lutz, B.L., Han-
Kramer, C., Alves, J., Lada, C.J., Lada, E.A., son, M.M., & Shuping, R.Y. 1996, ApJ, 465,
Sievers, A., Ungerechts, H., & Walmsley, C.M. 245
1999, A&A, 342, 257
Swinyard, B.M., Clegg, P.E. et al. 1996, A&A,
Lada, C.J., Lada, E.A., Clemens, D.P., & Bally, 315, L43
J. 1994, ApJ, 429, 694
Tieftrunk, A., Pineau des For_ts, G., Schilke, P.,
Langer, W. 1976, ApJ, 206, 699 & Walmsley, C.M. 1994, 289, 579
Langer, W.D., & Penzias, A.A. 1990, ApJ, 357, Turner, B.E. 2000, ApJ, 542, 837
477
van Dishoeck, E.F., Blake, G.A., Draine, B.T., &
Le Bourlot, J., Pineau des For_ts, G., Roueff, E., Lunine, J. I. 1993, in Protostars and Planets
& Flower, D.R. 1993, A&A, 267, 233 III, eds. E. Leby & J. Lunine (U. Arizona Press:
Tucson), 163
Lis, D.C., & Goldsmith, P.F. 1989, ApJ, 337, 704
Vastel, C., Caux, E., Ceccareli, C., Castets, A.,
Liszt, H. S., Vanden Bout, P. A. 1985, ApJ, 291,
Gry, C., & Baluteau, J.P. 1999, A&A, 357, 994
178
Wannier, P.G., Lichten, S.M., & Morris, M. 1983,
Lucas, R., & Liszt, H. 1997, in CO: Twenty-Five ApJ, 268, 727
Years of Millimeter-Wave Spectroscopy, eds. W.
Latter et al. (Kluwer: Dordrecht), 95 Whittet, D.C.B. 1992, Dust in the Galactic Envi-
ronment (Bristol: A. Hilger), 158
Melnick, G. J., Ashby, M.L.N., Plume, R. et al.
2000, ApJ, 539, L87 Xie, T., Allen, M., & Langer, W. D. 1995, ApJ,
440, 674
ZmuidzinaJs.,,
Blake, G.A., Carlstrom, J., Keene, positionhas been correctedforthecloudemission
J., Miller, D., Schilke, P., & Ugras, N.G. 1995, averagedovera 40t'square ring.
Airborne Astronomy Symposium on the Galac-
Fig.4.--OI column densityasa functionofIsCO
tic Ecosystem: From Gas to Stars to Dust, eds.
column densityper 13CO velocitycomponent for
M. Haas et al. (ASP Conf. Set. Vol. 73), 33
the threevelocityrangesthat aredistinguishable
intheMEM-deconvolved OI spectrum. Errorbars
FIGURE CAPTIONS correspond to 1orstatisticaluncertaintiesforO!
and _3C0 column densities.A leastsquaresfitto
the data givesa slopeof270 ± 35 (la statistical
Fig. I.--(a)PDR abundances relativeto H as a
uncertainty).The interceptis(-1.1± 4.6)x 1016
functionof depth intothe cloud,afterLe Bour- cm -2 (la).This indicatesthat the OI and IsCO
lotet al.(1993). The model cloud has a vol- emission come from the same region and there
ume density n(HI + 2H2) = 104 cm -s (toward isno excess01 emissionfrom the PDR interface
the high end of the valuesderivedby Greaves & where hydrogen isalreadymolecular,but Isco is
Williams 1994,Greaves & Nyman 1996,and Lucas
photodissociated(regionB inFig.lb).
& Liszt1997),and theUV fieldintensityGo = 1
(standardinterstellarradiationfield).The total Fig.5.--Observed and model OI (63m) absorp-
oxygen and carbon abundances are3 x 10-4 and tionspectra(blackand gray curves,respectively),
1.5× 10-4,respectively.Note that 02 and H20 assuming theO! abundance of3.2x 10-4 intheHI
areminor speciesinthe model and oxygen ispri- regionand 2.6x10-4inthemoleculargas(20% de-
marilyinatomic form and inCO inthemolecular pletion)at raw LWS resolution(_35 kms-1; up-
region.The CO abundance inthe cloud interior per panel)and the deconvolved resolution(_10--
decreaseswith decreasinggas density(owing to 15 kms-1; lower panel). The part of the spec-
the decreasedCO formation rate)and increasing trum at VLSR _ 30 krns-I iscontaminated by
UV fieldintensity(owing totheincreasedCO pho- the emissionform the Sgr B2 envelopeand isnot
todissociationrate).(b)Overallgeometry ofHI, consideredhere. The discrepanciesbetween the
H2, and CO regionsin a PDR (from HoUenbach observed and model spectrum atthe deconvolved
& Tielens1999;the regiondesignationsA, B,and resolution(lowerpanel)may indicatevariationsin
C areadded here). the O! to CO abundance ratiobetween different
components. In addition,giventhe broad wings
Fig. 2.-- OI (63 _m) Fabry-Perot spectra to- of the instrumentalprofile,the Ol opticaldepth
ward Sgr B2(M) (upper panel),180" N, and 180" and column densityforthe 0 kms -_ component
S (lower-panel,upper and lower curves,respec- may be underestimated owing to the contamina-
tively).Blackand graylinescorrespondtotheraw tionby O! emissionfrom the Sgr B2 envelope.
and MEM-deconvolved spectra,respectively.The
lower-panelverticalaxeson theleftand rightcor- Fig. 6.--OI to 13C0 column densityratiofrom
respond tothe 180" N and 180"S spectra,respec- thecloudsurfaceasafunctionofvisualextinction
tively.The intensityscaleisinthe LWS Fabry- forPDR models with varying gas densitiesand
Perotunitsasgivenby the LWS analysispackage UV fieldintensitiesT.he shaded areaoutlinesthe
(ISAP). ratioderivedforthecloudsalongthelineofsight
toward Sgr B2.
Fig. 3.-- (Upper panel) Observed Oz (63 /Jm)
Fabry-Perot spectrum (_35 krns-I resolution,
grayline)and MEM-deconvolved spectrum (_10-
15 kms -I resolution,solidblack line). (Lower
panel) Hx and 13CO(1-0) absorptionspectrato-
ward Sgr B2(M) (grayand black lines,respec-
tively).The 13CO spectrum toward the central
This 2-column preprint was prepared with the AAS
macros vS.0.
8
