Observations of Solid Carbon Dioxide in Molecular Clouds with the Infrared Space Observatory

22 Pages·1999·1.33 MB·English

THEASTROPHYSICALJOURNAL,522:357¨377,1999September1
(1999.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
INFRARED SPACE OBSERVATORY WITH THE OBSERVATIONS OF SOLID CARBON DIOXIDE IN
MOLECULAR CLOUDS1
P. A. GERAKINES,2,3 D. C. B. WHITTET,2 P. EHRENFREUND,4 A. C. A. BOOGERT,5 A. G. G. M. TIELENS,5,6 W. A. SCHUTTE,4
J. E. CHIAR,6 E. F. VAN DISHOECK,4 T. PRUSTI,7 F. P. HELMICH,4,8 AND TH. DE GRAAUW5,9
Received1998September11;accepted1999April14
ABSTRACT
Spectra of interstellar CO ice absorption features at a resolving power of j/*jB1500¨2000 are pre-
2
sented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS)
of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO
2
near 4.27 km in all sources; the bending mode near 15.2 km is also detected in 12 of them. The selected
sources include massive protostars (Elias 29 [in o Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC
7538 IRS 1, NGC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center
(Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias
16); they thus probe a diverse range of environments. Column densities of interstellar CO ice relative to
2
H O ice fall in the range 10%¨23%: this ratio displays remarkably little variation for such a physically
2
diverse sample. Comparison of the observed pro—les with laboratory data for CO -bearing ice mixtures
2
indicates that CO generally exists in at least two phases, one polar (H O dominant) and one nonpolar
2 2
(CO dominant). The observed CO pro—les may also be reproduced when the nonpolar components are
2 2
replaced with thermally annealed ices. Formation and evolutionary scenarios for CO and implications
2
for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing,
rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO -
2
bearing ices.
Subject headings: dust, extinction ¨ infrared: ISM: lines and bands ¨ ISM: molecules ¨
stars: pre¨main-sequence
1. INTRODUCTION 1992; Allamandola et al. 1992), (cid:147)(cid:147)XCNˇˇ (a molecule or ion
containing the CN group; Lacy et al. 1984; Tegler et al.
The evolution of carbonaceous materials in the inter-
1995; Schutte & Greenberg 1997), and formaldehyde
stellarmedium(ISM)isakeyprobleminastrophysics,with
(H CO; tentative detection; Schutte et al. 1996a). Simple
rami—cationsasfar-reachingastheoriginsoflifeonEarth. 2
hydrocarbons may also be present (Brooke et al. 1996b;
To date, nearly 100 carbon-bearing species are known to
Chiar,Adamson,&Whittet1996).
exist in the gas phase of the ISM, most abundantly in the
Before the launch of the Infrared Space Observatory
formofcarbonmonoxide,CO(seereviewsbySnyder1997;
(ISO), high-resolution spectroscopy in the mid-IR was
van Dishoeck & Blake 1998). Of these species, a handful
limitedtomoleculeswithtransitionsinspectralregionsfree
have been detected in the solid state as constituents of icy
of signi—cant atmospheric absorption. A full inventory of
grain mantles through infrared (IR) spectroscopy. Carbon-
interstellaricesaccessibletoIRobservationisnowpossible
aceous components of icy grain mantles detected to date
with ISO, and one of the early highlights in reaching this
includeCO(Lacyetal.1984),methane(CH ;Boogertetal.
4 goalhasbeenCO (e.g.,deGraauwetal.1996b;Gu(cid:142)rtler et
1996, 1998; Whittet et al. 1996; Lutz et al. 1996; 2
al.1996;dˇHendecourtetal.1996;Lutzetal.1996;Whittet
dˇHendecourt et al. 1996), methanol (CH OH; Grim et al.
3 et al. 1998). Here we present an analysis of high-resolution
1991; Schutte, Tielens, & Sandford 1991; Skinner et al.
solidCO observations(j/*jB1500¨2000)towardalarge
2
varietyofsources.
Dust grains inside quiescent, dense molecular clouds are
1BasedonobservationswiththeInfraredSpaceObservatory,anESA
shieldedfromtheinterstellarradiation—eldandmayreach
projectwithinstrumentsfundedbyESAMemberStates(especiallythePI
countries:France,Germany,theNetherlands,andtheUnitedKingdom) temperatures as low as 8¨10 K in the absence of any inter-
andwiththeparticipationofISASandNASA. nalheatingsources(see,e.g.,Pratapetal.1997).Particlesat
2DepartmentofPhysics,AppliedPhysics,andAstronomy,Rensselaer these temperatures readily accrete species from the gas
PolytechnicInstitute,Troy,NY12180.
phase because of high sticking efficiencies and lack of effi-
3Present address: Code 691, NASA/Goddard Space Flight Center,
cient desorption mechanisms (see, e.g., Tielens & Allaman-
Greenbelt,MD20771.
4LeidenObservatory,P.O.Box9513,2300RALeiden,Netherlands. dola 1987). Where the fraction of hydrogen in atomic form
5KapteynAstronomicalInstitute,P.O.Box800,9700AVGroningen, is substantial (D0.01%¨1%), as in the outer regions of
Netherlands. dense molecular clouds (e.g., Savage et al. 1977), models
6NASA/Ames Research Center, Mail Stop 245-3, Mo(cid:134)ett Field, CA
suchasthosebyTielens&Hagen(1982)anddˇHendecourt,
94035.
7InfraredSpaceObservatoryScienceOperationsCenter,Astrophysics Allamandola, & Greenberg (1985) predict that accreting
Division, European Space Agency, Villafranca del Castillo, P.O. Box atoms will form fully hydrogenated molecules via surface
50727,28080Madrid,Spain. reactionswithH(toproducemostlyH O,CH , andNH ).
8Stichting Ruimte Onderzoek Nederland, Sorbonnelaan 2, 3584 CA 2 4 3
In denser regions, molecules that are abundant in the gas
Utrecht,Netherlands.
phase (e.g., CO, N , and O ) will accrete directly onto the
9Stichting Ruimte Onderzoek Nederland, P.O. Box 800, 9700 AV 2 2
Groningen,Netherlands. dust.
357
358 GERAKINES ET AL. Vol. 522
Carbon dioxide has several possible formation mecha- (Sandford & Allamandola 1990; Ehrenfreund et al. 1997).
nismsintheISM.WhileformationratesforCO arelowin Temperaturehistoryisalsopreservedinthestructureofthe
2
the gas phase (Herbst & Leung 1989; Millar et al. 1991), 15.2kmCO bendingmode,whosepro—lecontainsacom-
2
laboratory experiments show that CO is formed with plicated substructure when annealed (Ehrenfreund et al.
2
extremeefficiencyinirradiationexperimentsinvolvingsolid 1999).
H O and CO (see, e.g., dˇHendecourt et al. 1985; Sandford In this paper, we present an analysis of high-resolution
2
& Allamandola 1990; Moore, Khanna, & Donn 1991). spectroscopy(j/*jB1500¨2000)ofthe12CO moleculein
2
Thus, large amounts of CO are predicted near luminous the solid state as observed by the Short-Wavelength
2
protostars with strong ultraviolet (UV) —elds. Computa- Spectrometer(SWS)instrumentonboardISO.Anextensive
tional models that include grain surface chemistry (Tielens analysisofthefeaturesof13CO ispresentedbyBoogertet
2
& Hagen 1982; dˇHendecourt et al. 1985; Hasegawa & al. (1999). Section 2 contains a description of the methods
Herbst1993;Shalabiea&Greenberg1994)alsopredictthat used to derive the spectra. In (cid:176) 3, the observed features are
CO should be present in interstellar icy grain mantles, presented and are analyzed through the use of laboratory
2
where the abundance of CO produced is strongly depen- spectra, and the derived CO abundances are compared
2 2
dent on whether the grain-surface oxidation of CO is pos- withthoseofotherknowncomponentsoficygrainmantles.
sible.ThelaboratoryresultsofGrim&dˇHendecourt(1986) Finally, the implications for grain mantle chemistry in the
indicatethatthisreactionmayhaveanactivationenergyof ISMarediscussedin(cid:176)4.
10¨30 K, but such a barrier may be easily overcome in the
2. DATA REDUCTION
ISM given the long timescales for di(cid:134)usion-limited grain
surfacereactions(accretionrateD1speciesperday;Tielens ISO was launched on 1995 November 17, and the SWS
& Whittet 1997). Moreover, the observation of abundant has obtained the —rst interstellar spectra to span the entire
CO inthequiescentISMtowardElias16stronglysuggests mid-IR wavelength regime (j\2.38¨45.2 km). The SWS
2
that processing of mantle ices due to embedded sources of instrumentconsistsoftwogratingspectrometersthatusea
radiation is not required to produce interstellar CO 100 line mm~1 grating in the —rst 4 orders from 2.4 to 13
2
(Whittetetal.1998). km,anda30linemm~1gratinginthe—rst2ordersfrom11
Infrared spectra of CO -bearing ices have been well to45km(deGraauwetal.1996a).
2
studied in the laboratory in an astrophysical context by Two SWS Astronomical Observing Templates (AOTs)
Sandford & Allamandola (1990), Gerakines et al. (1995), wereusedtoobtaindatainthespectralregions containing
and Ehrenfreund et al. (1997). CO has four observable the fundamental CO absorptions (j\4¨4.5 km, and
2 2
absorptions in the mid-IR (two combination modes, one 12¨16km).AOTSWS01observesthefullwavelengthrange
stretching mode, and one bending mode), the strongest of from 2.38 to 45.2 km with one of four di(cid:134)erent speeds that
which is due to its asymmetric stretching fundamental determine the resulting spectral resolution and sensitivity.
vibration (l ) at 4.27 km (2342 cm~1): its strength is Scantimesareapproximately 1, 1, 1,and2hrwithrespec-
3 4 2
A\7.6]10~17 cm molecule~1, which may be compared tive resolving powers of 1, 1, 1, and 1 of the full SWS
8 8 4 2
with the values for the H O and CO stretching modes, resolution (which ranges from 1000 to 2000, depending on
2
A\2.0]10~16 and 1.1]10~17 cm molecule~1, respec- the wavelength region; see de Graauw et al. 1996a). AOT
tively (Gerakines et al. 1995). This transition is strong SWS06 is designed for smaller wavelength regions with
enoughtobeobservableevenwhentheCatomisreplaced longerintegrationtimesatfullresolution.Integrationtimes
byitsisotope13C(terrestrially,89timeslessabundantthan in SWS06 depend on the required sensitivity. A detailed
12C). The asymmetric stretching mode of 13CO is located descriptionoftheSWSinstrument,itscalibration,anddata
2
at4.38km(2283cm~1).Thesymmetricstretchingmode(l ) reduction techniques can be found in de Graauw et al.
1
of CO at 7.2 km (1385 cm~1) is normally unobservable (1996a),Schaeidtetal.(1996),andValentijnetal.(1996).
2
since it produces no change in the moleculeˇs dipole Details of our observations are listed in Table 1. Data
moment.Thebendingmode(l )absorbsatawavelengthof presented in Figures 1, 2, 3, and 4 were obtained in AOTs
2
15.2 km (660 cm~1) and is 7 times weaker than the asym- SWS01 and SWS06 in instrument bands 2A (j\4.08¨5.3
metric stretching mode (A\1.1]10~17 cm molecule~1; km)and3A(j\12.0¨16.5km).TheSWSobservingmodes
Gerakines et al. 1995). The l ]l and 2l ]l com- and integration times for each observation are listed in
1 3 2 3
bination modes, located at 2.697 and 2.778 km (3708 and Table 1. The full spectral resolution in SWS bands 2A and
3600cm~1),are,respectively,54and170timesweakerthan 3A are j/*jB1800 and 2000, and therefore our obser-
thestretchingmode. vationscarriedoutwithAOTSWS01speeds3and4corre-
The sensitivity of CO infrared features to ice composi- spond to j/*jD500 and 1000, respectively. Wavelength
2
tion and temperature makes them ideal probes of inter- scanning in an observation is achieved by the rotation of a
stellar cloud parameters. CO is less volatile than CO and (cid:209)at mirror located near each grating. Every region of the
2
may exist in solid form up to temperatures of 50¨90 K, spectrum is scanned at least twice, in pairs of (cid:147)(cid:147)upˇˇ and
depending on the ice in which it resides. It could therefore (cid:147)(cid:147)downˇˇ scans whose names refer to the scan direction in
provide information on carbon chemistry in a wider range wavelengthspace.
of environments than CO, which will sublime almost com- The data reduction techniques developed at SRON in
pletely at temperatures above 30 K (although a small Groningen,theSIDTatVILSPA,Spain,andMPEinGar-
amount may exist at higher temperatures when trapped in ching, Germany, were used to derive the optical depth
someicemixtures).Thepro—lesofCO featuresinpolarice spectra.Darkcurrentsforeachdetectoroverthetotalscan
2
mixtures(dominatedbyH O orCH OH) aredramatically time were removed by the interpolation of measurements
2 3
di(cid:134)erent from those of CO in nonpolar environments taken at the beginning and end of each scan. Data were
2
(dominatedbymoleculessuchasCO,O ,orCO itself)and converted to astronomical (cid:209)ux density units (Jy) from
2 2
thereby trace the chemical composition of interstellar ices instrumentsignallevel(kV)bydivisionoftherelativespec-
No. 1, 1999 SOLID CO IN MOLECULAR CLOUDS 359
2
TABLE 1
OBSERVINGPARAMETERS
POSITION(J2000) *t [s]c
int
SOURCE R.A. decl. DATE REVOLUTIONa AOTb 4.27km 15.2km
W3IRS5 .............. 022540.8 620552 1997Jan17 427 1.3 117 333
1997Jan17 427 6 250 417
GL490................. 032738.7 584701 1997Aug17 640 1.3 117 192
1998Mar27 863 6 292 384
Elias16................. 043938.9 261127 1997Oct1 686 6 3883 725
GL4176................ 134302.1 [620852 1996Mar13 117 6 240 275
1996Sep18 306 6 304 389
Elias29................. 162709.3 [243721 1996Aug9 267 1.3 117 192
1996Sep3 292 6 496 ...
SgrA*.................. 174540.0 [290028 1996Feb19 94 1.4 250 404
GCS3I................ 174614.9 [284934 1996Oct8 327 6 507 ...
GCS4.................. 174615.6 [284947 1996Sep9 297 1.3 117 192
W33A.................. 181439.4 [175201 1996Oct10 329 1.4 235 384
1997Feb25 467 6 981 389
GL2136................ 182226.3 [133008 1996Mar16 120 6 477 344
1996Sep23 311 6 240 ...
GL2591................ 202924.7 401119 1995Dec15 28d 6 833 417
1996Nov7 357 1.3 117 192
S140 .................... 221918.1 631847 1996Jun24 220 1.4 234 384
1996Aug6 263 6 501 341
NGC7538IRS1...... 231345.3 612810 1996Aug25 283 6 283 368
1996Dec6 385 1.3 117 192
NGC7538IRS9...... 231401.6 612720 1996Feb23 98 6 2003 384
NOTE.¨Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and
arcseconds.
aISOrevolution(orbit)numberduringwhichthedataweretaken.
b(cid:147)(cid:147)AOT1.xˇˇindicatesanAOTSWS01observationatspeedx.
cTimededicatedtotheregionscontainingtheIRfeaturesofCO (j\4.1¨4.4,14.0¨16.5km).
2
dDatatakeninthepreveri—cationphaseoftheISOmission.
tral response function (RSRF) for each SWS band. The 2.1. ContinuumDetermination
wavelengthdependenceoftheRSRFishighlyoscillatoryin
SWSbands3Athrough3D(12¨30km;seedeGraauwetal. Continuum —ts in the 4 km region were obtained by
1996a), and if the observed data are shifted in wavelength —ttingalow-orderpolynomial(1¨3)todatapointsfrom4.0
withrespecttothecalibrationdata,abeatingor(cid:147)(cid:147)fringingˇˇ to4.1and4.4to4.5km.Sincethebendingmodeat15.2km
pattern results when the RSRF is divided out. Fringing islocatedontheshort-wavelengthedgeofthe18kmsilicate
e(cid:134)ects were minimized using an interactive routine that absorption,a—twas—rstmadetothecontinuumfrom14to
applied small wavelength shifts to the response curves 25kmusingalow-orderpolynomialwithpointsfrom14to
before the division. In most cases, however, some small 14.5andabout18to25km(dependingonthepositionand
amount of fringing did remain in the spectra. If reliable extent of the silicate feature). The silicate feature was sub-
analyses were still possible, this residual fringing was not tracted by —tting its shape with a Gaussian curve. As an
removed since attempts to do so (such as clipping high- example, continuum —ts to the AOT SWS01 data of S140
frequencycomponentsfromtheFouriertransform)slightly areshowninFigure5.
alter the shape of existing absorptions. However, the 15.2 Toward Elias 29, GL 490, GL 2136, NGC 7538 IRS 1,
kmspectraofNGC7538IRS1,GL4176,andGL2591did Sgr A*, S140, W3 IRS 5, and W33 A, another feature of
require this procedure to remove enough of the fringing to unknown origin centered near 16.4 km became apparent
properlyanalyzethem.Oneartifactappearsinallspectraat after the subtraction. This absorption was also —t with a
14.5km(aquasiabsorptionfeaturewithanopticaldepthof Gaussian curve and subtracted out to produce the —nal
about 0.05; it is prominent in the spectra of GCS 4, GL optical depth spectra. It is likely that this feature is merely
2591, GL 490, and Sgr A*; see Figs. 1, 2, 3, and 4). Its theresultofthejumpfromSWSband3Ato3C.
central wavelength corresponds to a discontinuity present The 15.2 km features in laboratory spectra typically
intheRSRF,andthereforeitsremovalisquitedifficult(see contain a long-wavelength wing extending to about 16.3
Fig.1). km.ItisnotclearwhetherthisoccursintheSWSspectra:it
Scans in each SWS band free of obvious glitch e(cid:134)ects or may be hidden by the silicate bending mode in this region.
anomalous response jumps were combined and —t with a Makingapropercontinuum—tisthereforenotstraightfor-
straight line. This line was then used to (cid:209)at-—eld each scan ward.Inordertoestimateerrorbarsontheareasofthe15.2
individually. Up and down scans were independently (cid:209)at- kmfeatures,wehavealsomadelocalcontinuum—tstodata
—eldedtocorrectforanomaloustrendsindarkcurrent,and points in the ranges 13¨14 km and 16.5¨18 km using low-
—nal spectra were obtained by an average (weighted by orderpolynomials.Sucha—tisalsoshowninFigure5.The
variance)ofeachup/downpair. areas of the resulting 15.2 km bending modes in Figure 5
360 GERAKINES ET AL. Vol. 522
0.60
0.60
. 0.50
p
s 0.58
e 0.40
R
0.30
t. 0.56
e
D RSRF 0.20
0.54 0.10
0
0.0
1
0.1
2
0.2
h 3
t
p W3 IRS5 0.3
e 4
D
0.4

l
a
c -0.04
pti 0.0 -0.02
O
0.00
0.2
0.02
0.04
0.4
GL 490 0.06
0.08
0.6
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 1.¨Toppanels:Relativespectralresponsefunction(RSRF)forISOSWSdetectorsintheobservedspectralregions.Otherpanelscontain(leftpanels)
theobservedstretchingmodesand(rightpanels)bendingmodesofsolidCO toward(middlepanels)W3IRS5and(bottompanels)GL490andthebest
2
polar]nonpolarlab—tstoallfeaturesineachcase¨includingthe4.38km13CO stretchingfeature(notshown;seeBoogertetal.1999).Dottedlinesshow
2
polarcomponentsoflab—ts;dashedlinesshownonpolarcomponents;solidlinesshowsumofpolarandnonpolarcomponents.TheCO stretchingmodeof
2
W3IRS5issaturated((cid:209)uxatthepeakwavelengthistoolowtobemeasured)andpointswithsignal-to-noiseratios(S/N)\1havebeenomitted.The15km
spectrumofW3IRS5containsanemissionfeatureat15.55kmduetothe—ne-structurelineof[NeIII].Apparentabsorptionsnear14.5kmaredueto
imperfectratioingofahighlyfringedfeaturepresentintheRSRF(seetoprightpanel).
are di(cid:134)erent by 17%. Error bars in column density ((cid:176) 3.4) extensive study of the corresponding interstellar 13CO
2
re(cid:209)ect this uncertainty in continuum shape in lines of sight spectramaybefoundinBoogertetal.(1999),whopresenta
wherethe4.27kmstretchingmodeissaturated. separate, complementary analysis to that which follows in
Finally,thesourceofradiationinthelineofsighttoward (cid:176)(cid:176)3.1¨3.5.
Elias 16 is a background K 1 III star, and its spectrum
contains photospheric absorption features at 4.295, 4.352, 3.1. ObservationalCharacteristics
and 4.393 km (Whittet et al. 1998). Cancellation of these The 4.27 km feature is clearly saturated toward NGC
lines was carried out by ratioing the (cid:209)ux spectrum of Elias 7538IRS9,W3IRS5,andW33A(seeFigs.1,2,3,and4).
16bythatofHR8657,aK2IIIstarwithlowextinction. SourcesElias29,GL2136,GL2591,andGL4176exhibita
3. ANALYSIS OF OBSERVED ABSORPTION FEATURES shallow shoulder (qB0.1) on the long-wavelength side of
the feature, and it is suspected that this is the result of an
Observed spectra for all sources are shown in Figures 1, unidenti—ed, broad underlying component. On the short-
2, 3, and 4 with the polar:nonpolar —ts ((cid:176) 3.3.1) and in wavelengthsideofthefeature,mostsourcesdisplayawing
Figures 6, 7, 8, and 9 with the polar:annealed —ts ((cid:176) 3.3.2). likely due to a large nonpolar component as seen in labor-
Tables 2 and 3 list their characteristics. In some cases, the atory ices containing pure CO (Sandford & Allamandola
2
bending mode was unobservable because of low (cid:209)ux levels 1990;Ehrenfreundetal.1997).ThespectraofGL2136,GL
(e.g.,Elias16)orwastooweaktodistinguishfromfringing 2591 and GL 4176 also contain shallow absorption shoul-
or noise in the spectrum (e.g., GCS 4 and GL 4176). An dersontheshort-andlong-wavelengthsidesofthestretch-
No. 1, 1999 SOLID CO IN MOLECULAR CLOUDS 361
2
1.0
0.0
0.8
0.5
0.6
1.0
0.4
1.5 Elias 16
0.2
2.0
-0.020.0
0.0
0.00
0.1
0.2 0.02
h
0.3
pt GL 4176 0.04
e
D 0.4
0.06

l -0.2
a
c 0
ti -0.1
p
O
1 0.0
0.1
2
0.2
Elias 29
0.3
3
-0.02

0.0 0.00
0.02
0.2
0.04
0.4
0.06
Sgr A*
0.6
0.08
0.10
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 2.¨InterstellarsolidCO spectraandbestpolar]nonpolarlab—tspresentedasinFig.1,butforElias16,GL4176,Elias29,andSgrA*.Gas-phase
2
CO absorptionat4.27kmhasbeenestimatedforGL4176usingthe14.97kmCO gasfeature.SpuriousstructureexistsinthetroughsoftheElias16and
2 2
Elias29stretchingmodesbecauseoflow(cid:209)uxlevels.ObservationsofElias16near15kmwereprecludedbyitslowcontinuum(cid:209)ux.The15kmspectrumof
SgrA*containsanemissionfeatureat15.55kmduetothe—ne-structurelineof[NeIII].
ing mode. These absorptions are likely due to the wavelengths). Spurious structure also exists in the troughs
unresolvedP-andR-branchesofgas-phaseCO ,consistent of the 4.27 km features of Elias 29 and NGC 7538 IRS 1,
2
with the strong gas-phase l absorptions seen at 14.97 km whichmaybeclosetosaturation.
2
toward these sources. Estimated underlying gas com- The observedCO 15.2 km bending modes (Figs. 1, 2, 3,
2
ponents are plotted in Figures 1, 2, 3, and 4 and have been and 4) have at least two peaks toward most sources: apart
subtracted from the spectra before performing the labor- from those with poor signal-to-noise ratio, the only excep-
atory —ts ((cid:176) 3.3). The apparent (cid:147)(cid:147)emissionˇˇ in the spectrum tionisSgrA*.TowardGL2136,GL2591,GL4176,NGC
of Elias 16 at 4.22 and 4.27 km is not real but the result of 7538IRS1,NGC7538IRS9,SgrA*,W3IRS5,andW33
di(cid:134)erences in the up and down scans of this observation A,absorptionduetotheQbranchofthegas-phaseCO l
2 2
(because of the low (cid:209)ux level of this object at these bandcanbeseenat14.97kmwithawidthofabout0.02km.
362 GERAKINES ET AL. Vol. 522
1.0
0.0
0.1 0.8
0.2 0.6
0.3
0.4
0.4 GCS 3 I
0.2
0.5
-0.040.0

-0.02
0.0
0.00
0.02
0.1
0.04
h
t 0.06
p GCS 4
0.2
e
0.08
D
0.10

l
a
c 0
0.0
i
t
p
O 2
0.2
4
0.4
W33 A
6
0.6
0.0
0.0
0.5
0.1
1.0
1.5
0.2
2.0 GL 2136
0.3
2.5
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 3.¨InterstellarCO spectraandbestpolar]nonpolarlab—tspresentedasinFig.1,butforGCS3I,GCS4,W33A,andGL2136.No15kmdata
existforGCS3I.Thespec2trumofGCS4containsemissionat15.55km(beyondthe(cid:209)uxscaleoftheplot)duetothe—ne-structurelineof[NeIII].The
stretchingmodeofW33Aissaturated(peak(cid:209)uxistoolowtobemeasured),andpointswithS/N\1havebeenomitted.Gas-phaseCO absorptionsat4.27
2
kmhavebeenestimatedforGL2136usingthe14.97kmCO gasfeature.
2
Thegas-phaseCO featurestowardGL2591andGL4176 stronger but has only P- and R-branch structure (van Dis-
2
may have been broadened by attempts to remove fringing hoeck et al. 1996). The spectra of W3 IRS 5, Sgr A*, and
((cid:176)2),buttheirwidthscouldalsobeduetohighCO excita- GCS4alsocontainthe—ne-structurelineofdoublyionized
tiontemperaturestowardthesesources.Thegas-ph2aseCO neon[NeIII]at15.55km,indicatingthepresenceofionized
datawillbefurtheranalyzedanddiscussedbyBoonmane2t gas(anHIIregion)intheselinesofsight.
al. (1999). At this resolution, all Q-branch ro-vibrational Ingeneral,thepro—lesofalltheobservedinterstellar15.2
lines merge into one apparent absorption feature. This km features can be broken down into three components: a
unresolvedbandiseasiertoobservethanthecorresponding broad underlying feature (observed *l\16.3¨27.4 cm~1
gas-phasel bandat4.27km,whichisintrinsically10times vs.B25cm~1inthelab),apairofsharppeaksat15.15and
3
No. 1, 1999 SOLID CO IN MOLECULAR CLOUDS 363
2
-0.02
0.0
0.00
0.1
0.2 0.02
0.3 0.04
0.4
0.06
GL 2591
0.5
0.08
-0.05
0.0
0.00
0.2
0.05
0.4
0.10
0.6
0.8 0.15
h
pt 1.0 S 140 0.20
e
1.2
D 0.25

l -0.05
a
c 0.0
i 0.00
t
p
O 0.5
0.05
1.0
0.10
1.5 0.15
NGC 7538
IRS1
0.20
2.0
0 0.0
1
0.2
2
0.4
3
0.6
4 NGC 7538
IRS9
5 0.8
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 4.¨InterstellarCO spectraandbestpolar]nonpolarlab—tspresentedasinFig.1,butforGL2591,S140,NGC7538IRS1,andNGC7538IRS9.
2
Gas-phaseCO absorptionsat4.27kmhavebeenestimatedforGL2591usingthe14.97kmCO gasfeature.StructureinthetroughoftheNGC7538IRS1
2 2
stretchingmodeisduetolow(cid:209)uxlevels.ThestretchingmodeofNGC7538IRS9issaturated(peak(cid:209)uxistoolowtobemeasured),andpointswithS/N\1
havebeenomitted.
15.27 km (observed *l\1.7¨6.6 cm~1 vs. B2¨4 cm~1 in at approximately 15.4 km. In most cases this absorption is
thelab),andashouldernear15.4km(laboratorymeasure- broad and shallow, but it appears sharply peaked toward
mentsfromEhrenfreundetal.1997).Therelativecontribu- GL2136.
tions of these components vary from source to source. For WeattributethebroadunderlyingcomponenttoCO in
2
example, the height of the peaks near 15.15 and 15.27 km a polar environment (presumably an H O- and/or
2
arehighlyvariableandmeasureupto40%ofthetotalpeak CH OH-rich ice). The two sharp peaks at 15.15 and 15.27
3
optical depth (in S140). Toward GL 2136, GL 4176, NGC km have been shown to form in annealedCO ices and in
2
7538 IRS 1, S140, W3 IRS 5, and W33 A (and weakly certain nonpolar ice mixtures upon warm-up (Sandford &
towardNGC7538IRS9),athirdpeakorbroadshoulderis Allamandola 1990; Ehrenfreund et al. 1997). In the latter,
apparentonthelong-wavelengthsideofthe15.2kmfeature ice segregation is taking place; this forms aCO ice that is
2
364 GERAKINES ET AL.
120
S140 AOT01 SP4
800
100
]
y
J
[

y 80
t
si 700
n
e
60
D

x
u 600
Fl 40
20
500
4.0 4.2 4.4 4.6 4.8 5.0 13 14 15 16 17 18 19
0.0
0.00
h 0.2
pt 0.05
e 0.4
D
al 0.6 0.10
c
ti
p 0.8 0.15
O
1.0
0.20
4.0 4.2 4.4 4.6 4.8 5.0 13 14 15 16 17 18 19
Wavelength [m m] Wavelength [m m]
FIG. 5.¨Demonstrationofcontinuum—tsusingtheAOTSWS01speed4dataofS140.Leftpanels:CO stretchingmoderegion.Rightpanels:CO
2 2
bendingmoderegion.Inthe15.2kmregion,twoextremecasesaregivenbythesolidanddottedlines.Thesolidlinesrepresentathird-orderpolynomial—tto
continuumbetween13¨14and18¨19kmcombinedwithGaussian—tstothestructureofthe16¨18kmfeature.Thedottedlineisa—fth-orderpolynomial—t
topointsadjacenttotheCO feature.
2
embeddedinthematrixoftheothercomponent.Thebroad 3.2. LaboratoryData
shoulder does not seem to correlate with the other com-
ponents, and recent lab results (Ehrenfreund et al. 1999) Laboratory experiments by Sandford & Allamandola
indicate that this absorption may be a result of (1990) and Ehrenfreund et al. (1997) have shown that at 10
CO :CH OHicecomplexes(cf.(cid:176)3.3). K the bending mode consists of one broad absorption for
2 3
TABLE 2
PROPERTIESOFOBSERVED4.27kmFEATURES
l
0 *l
SOURCE (km) (cm~1) q (cm~1) S/Na COMMENTSb
0
W3IRS5 .............. ... ... [3 D20 28 Saturated
GL490................. 4.262 2346.2 0.46^0.05 21.3 27
Elias16................. 4.265 2344.6 1.8‘0.5 19.6 63 Long-jwing
~0.2
GL4176................ 4.264 2345.4 0.40^0.02 17.8 97 Long-jwing
Elias29................. 4.267 2343.5 2.77^0.2 17.1 49
SgrA*.................. 4.269 2342.3 0.70^0.01 16.7 100 Short-jwing,4.38kmHem
GCS3I................ 4.267 2343.3 0.48^0.02 15.4 67
GCS4.................. 4.270 2342.0 0.22^0.02 12.8 40
W33A.................. ... ... [5 D20 58 Saturated
GL2136................ 4.269 2342.7 2.40^0.08 18.5 144
GL2591................ 4.266 2344.3 0.55^0.01 24.0 215 Broad,short-&long-jwings
S140 .................... 4.262 2346.5 1.25^0.04 23.9 50
NGC7538IRS1...... 4.265 2344.7 2.03^0.11 16.4 40
NGC7538IRS9...... ... ... [5 D20 15 Saturated
aMediansignal-to-noiseratioacrosstherangeofthefeature.
b(cid:147)(cid:147)SaturatedˇˇindicatesanopticaldepthtoogreattobemeasuredbyISOSWS.
0
0.0
1
0.1
2
0.2
h 3
t
p W3 IRS5 0.3
e 4
D
0.4

l
a
c -0.04
pti 0.0 -0.02
O
0.00
0.2
0.02
0.04
0.4
GL 490 0.06
0.08
0.6
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 6.¨InterstellarsolidCO spectratowardW3IRS5andGL490,andthebestlab—tswithpolar]annealedmethanol-richicestoallfeatures¨
2
includingthe4.38km13CO stretchingfeature(notshown)¨ineachcase.Dottedlinesshowpolarcomponents;dashedlinesshowannealedcomponents;
2
solidlinesshowsumofpolarandannealedcomponents.
TABLE 3
PROPERTIESOFOBSERVED15.2kmFEATURES
la
i *lb *lc
SOURCE (km) (cm~1) q (cm~i1) (cm~1) S/Nd COMMENTSe
i
W3IRS5 .............. 15.26 655.3 0.37^0.01 3.2 24.6 260 Weakgas,weakshoulder
15.14 660.7 0.34^0.01 2.5 [NeIII]emission
GL490................. 15.34 651.7 0.08^0.02 ... 22.2 87 Broad,singlepeak
GL4176................ 15.27 654.8 0.047^0.003 2.5 26.3 500 Stronggas,strongshoulder
15.14 660.7 0.044^0.003 3.0 FFT
Elias29................. 15.29 654.2 0.33^0.02 1.9 23.1 42 Strongshoulder
15.12 661.4 0.30^0.01 4.0
SgrA*.................. 15.28 654.5 0.077^0.005 2.1: 20.3 500 Broadsinglepeak,[NeIII]emission
15.1: 662: 0.08: 1.7: Possible2ndpeak
GCS4.................. 15.3: 652: 0.04^0.01 ... 19: 90 Verystrong[NeIII]emission
15.1: 662: 0.05^0.01 ...
W33A.................. 15.24 656.3 0.58^0.01 2.8 27.4 108 Mod.gas,strongshoulder
15.14 660.7 0.57^0.02 4.3
GL2136................ 15.24 656.0 0.30^0.01 2.9 24.5 500 Stronggas,strongshoulder
15.11 662.0 0.28^0.01 4.9
GL2591................ 15.24 656.0 0.073^0.005 2.8 25.1 307 Verystronggas
15.11 661.8 0.068^0.005 3.2 Strongshoulder,FFT
S140 .................... 15.25 655.8 0.21^0.01 3.1 16.3 136 Narrow,sharppeaks
15.09 662.9 0.24^0.02 6.6
NGC7538IRS1...... 15.24 656.1 0.21^0.01 3.5 24.3 139 Weakgas,weakshoulder
15.14 660.5 0.19^0.01 1.8 Weak[NeIII]emission,FFT
NGC7538IRS9...... 15.25 655.9 0.736^0.005 2.2 21.1 101 Weakgas
15.10 662.1 0.766^0.005 4.3
aValuesmarkedwithcolonshavehighdegreesofuncertainty.
bFWHMrelativetotheunderlyingbroadcomponent.
cWidthoftheentirefeature.
dMediansignal-to-noiseratioacrosstherangeofthefeature.
e(cid:147)(cid:147)FFTˇˇ indicates that fringing has been removed by clipping frequencies from the Fourier transform of the data (see (cid:176) 2). (cid:147)(cid:147)[Ne III]
emissionˇˇindicatesthepresenceofNe—ne-structureemissionat15.55km.
366 GERAKINES ET AL. Vol. 522
1.0
0.0
0.8
0.5
0.6
1.0
0.4
1.5 Elias 16
0.2
2.0
-0.020.0
0.0
0.00
0.1
0.2 0.02
h
0.3
pt GL 4176 0.04
e
D 0.4
0.06

l -0.2
a
c 0
ti -0.1
p
O
1 0.0
0.1
2
0.2
Elias 29
0.3
3
-0.02

0.0 0.00
0.02
0.2
0.04
0.4
0.06
Sgr A*
0.6
0.08
0.10
4.2 4.25 4.3 14.5 15 15.5 16

m
Wavelength [ m]
FIG. 7.¨Polar]annealedlaboratory—tstotheCO spectratowardElias16,GL4176,Elias29,andSgrA*.LinesareasinFig.6.
2
all ice mixtures studied, with the exception of pure CO , ice matrix (Ehrenfreund et al. 1997; Boogert et al. 1999).
2
whichisdoublypeaked.Thisvibrationisdoublydegenerate While it is the splitting of the 15.2 km band that indicates
and splits when the axial symmetry of the molecule is ice composition, both the peak position and width of the
broken.CO isknowntoformT-shapedcomplexeswitha 13CO stretchingmodearestronglydependentonicecom-
2 2
variety of molecules, leading to very broad bands (e.g., position; polar and nonpolar components may be easily
CO :CH OHcomplexes).Uponwarmupofsuchmixtures, distinguished.
2 3
multipeakedstructuresforminallCO absorptionfeatures
(Ehrenfreundetal.1999).Theexactpo2sitionsandwidthsof 3.3. LaboratoryFitstoObservationalData
these peaks vary according to ice matrix and thereby All spectra were matched to a suite of laboratory data
greatly constrain the composition of the ice mixture (see from the Leiden Observatory Laboratory (Ehrenfreund et
(cid:176)3.3). al. 1996, 1997, 1999), applying a s2 minimization method
l
The 13CO asymmetric stretching mode is, like the 15.2 similar to that used by Chiar et al. (1994, 1995) to —t solid-
2
km bending mode, an extremely sensitive diagnostic of the state CO features toward the Serpens and Taurus dark