The Composition of Interstellar Grains Toward Zeta Ophiuchi: Constraining the Elemental Budget Near the Diffuse-Dense Cloud Transition

0.78 MB·English

by  Charles A. Poteet

Tobe publishedin the Astrophysical Journal: received 2014August18;accepted 2015January12
PreprinttypesetusingLATEXstyleemulateapjv.05/12/14
THE COMPOSITION OF INTERSTELLAR GRAINS TOWARD ζ OPHIUCHI: CONSTRAINING THE
ELEMENTAL BUDGET NEAR THE DIFFUSE-DENSE CLOUD TRANSITION
Charles A. Poteet1, Douglas C. B. Whittet1, and Bruce T. Draine2
1NewYorkCenter forAstrobiology,DepartmentofPhysics,AppliedPhysicsandAstronomy,
RensselaerPolytechnicInstitute, 110EighthStreet, Troy,NY12180, USA;[email protected]
2PrincetonUniversityObservatory,PeytonHall,Princeton,NJ08544,USA
Tobe published inthe Astrophysical Journal: received 2014 August 18; accepted 2015 January 12
5
1
ABSTRACT
0
We investigatethe compositionofinterstellargrainsalongthe line ofsighttowardζ Ophiuchi,awell-
2
studiedenvironmentnearthe diffuse-dense cloudtransition. Aspectraldecompositionanalysisof the
n solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space
a Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-
J
sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing
2 evidence of H O ice mantles. However, tentative evidence for thick H O ice mantles on large (a ≈
2 2
1
2.8 µm) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from

ouranalysisandcomparedtostandardreferenceabundances. WefindthatnearlyallofelementalMg
]
A and Si along the line of sight are present in amorphous silicate grains, while a substantial fraction of
elementalFeresidesincompoundsotherthansilicates. Moreover,wefindthatthetotalabundanceof
G
elementalOislargelyinconsistentwiththeadoptedreferenceabundances,indicatingthatasmuchas
. ∼156ppmofinterstellarOismissingalongthelineofsight. Aftertakingintoaccountadditionallimits
h
onthe abundance of elemental O in other O-bearingsolids, we conclude that any missing reservoirof
p
elemental O must reside on large grains that are nearly opaque to infrared radiation.
-
o Keywords: dust, extinction — localinterstellar matter — infrared: ISM — ISM: abundances — ISM:
r molecules — stars: individual (ζ Oph, HD 149757)
t
s
a
[ 1. INTRODUCTION are re-analyzed using a consistent system of oscillator

strengths for the various transitions used to derive gas-
1 The interchange of gas and dust plays an essential
v role in the evolution of the interstellar medium (ISM). phase column densities. Independent of the adoptedref-
0 The surfaces of the dust grains act as both reposito- erenceabundance,Jenkins(2009)findsthatelementalO
1 ries for atoms and molecules adsorbed from the gas- (asobservedinitsdominantneutralatomicform,OI,in
8 the diffuse ISM) is depleted fromthe gas-phaseata rate
phaseandactivesitesformoleculeformationoverawide
2 that far exceeds the rate at which it can be sequestered
range of interstellar environments. The apparent under-
0 into silicate and metallic oxide particles.
abundances or “depletions” of many heavy elements de-
. As later explored in detail by Whittet (2010), this re-
1 tected in the gas-phase are interpreted as evidence for
sult implies a crisis in our understanding of interstel-
0 their inclusion on or in interstellar grains. Forty years
lar dust. At the high end of the density range covered
5 ago, Greenberg (1974) concluded that for the cosmi-
1 cally abundant elements of carbon, nitrogen, and oxy- by the depletion measurements (i.e., near the transition
: gen, significantly larger amounts were missing from the between diffuse neutral and dense molecular phases of
v the ISM), as much as 25%–30%of the total elemental O
gas-phasethan could be accountedfor by accretiononto
i
X interstellar dust grains. In view of this critical finding, budget is unaccounted for in any known observed form.
Otherthansilicatesandoxides,possibleO-bearingreser-
r Greenberg (1974) hypothesized that “there is more hid-
a den mass thanthereis mass in thedust”,withsuchmass voirs in common grain materials include organic refrac-
tory matter (Li & Greenberg 1997) and interstellar ices
possiblyexistingintheformofintermediate-sized“snow-
of H O, CO, and CO . However, ice mantles have only
balls” or more complicated interstellar molecules that 2 2
been detected in the denser regionsof the ISM (Whittet
had yet to be detected in the solid-state.
2010, and references therein), and a failure to detect the
With major advancements in the sensitivity and
∼6 µm C O carbonyl feature in diffuse ISM dust ap-
resolution of ultraviolet spectroscopy, as well as de-
pearstoplacestrongconstraintsontheabundanceofO-
creases in solar and stellar abundance estimates, ap-
bearingorganicmatter(Pendleton & Allamandola2002;
parent inconsistencies between the expected (approxi-
Chiar et al. 2013).
mately solar) and observed (gas and dust) interstellar
A further possibility is that O-bearing gaseous
abundances have largely subsided over the years (e.g.,
molecules, such as CO and O , might be important con-
Cardelli et al.1994;Sofia et al.1994;Snow & Witt1996; 2
tributors. However, in the case of CO, the observed
Sofia & Meyer 2001). However,the depletion of elemen-
abundances toward diffuse molecular clouds are well-
tal O remains problematic in the more heavily depleted
constrained, and clearly too small to account for a sig-
regions of the ISM. In a recent comprehensive study,
nificant fraction of the total elemental O abundance
Jenkins (2009) presents a unified representation of de-
(Liszt et al. 2009). Abundances of O are less well-
pletions, in which previous results from the literature 2
constrained, but are unlikely to exceed those of CO. Fi-
2
380 1984; Bowey et al. 1998). However, the high signal-to-
noise ratio of the Spitzer observations toward ζ Oph al-
lowsus,forthefirsttime,tocharacterizethecomposition
ofthe silicates in a diffuse interstellarcloud near the HI
360 → H transition.
2
In Section 2, an overview of the archival ISO and
Spitzer observations is presented. In Section 3, spectral
decomposition analyses of the infrared spectra toward
340 ζ Oph are employed to investigate the presence of solid-
]
2m state absorptionalong the line of sight. In Section 4, we
µ use our results to quantify the elemental abundances of
y
J Mg, Si, O, and Fe carried by amorphous silicate grains,
[
F ν and also place strict limits on the abundance of elemen-
λ2 320 tal O in other O-bearing solids. Finally, implications of
ourresultswithregardtothe “missingoxygen”problem
are discussed in Section 5.
300 2. ARCHIVAL OBSERVATIONS
Observations toward ζ Oph are assembled using
archivalspectra obtained by the Spitzer Space Telescope
Infrared Spectrograph (IRS; Houck et al. 2004) and the
280 ShortWavelengthSpectrometer(SWS;de Graauw et al.
2 3 4 5 6 7 8 910 15 20 25 3035 1996)onboardISO.AspartoftheSpitzer AtlasofStel-
Wavelength [µm] lar Spectra (PID 485; Ardila et al. 2010), the Spitzer-
IRS observations (Spitzer AOR key 27582976)were car-
Figure 1. CombinedarchivalISO-SWSandSpitzer-IRSspectra
(blacklines)oftheζ Ophlineofsightand3.4µmbroadbandpho- ried out with the low-resolution (R = 60–120) mod-
tometry(magenta circle)fromWISE,shownwiththeirrespective ules, Short-Low (SL; 5.2–14.0 µm) and Long-Low (LL;
uncertainties (gray lines). For comparison, the expected photo- 14.0-36.1 µm), on 2008 September 10. The observa-
spheric continuum emission by an O9.5 V star (blue line), corre-
tions utilized the standard two-position nod configura-
spondingtoaT =34300Kblackbody(Howarth&Smith2001),is
alsoindicated. NotethatintheRayleigh-Jeanslimit,ablackbody tion, yielding a total on-source exposure time of 12 s for
spectrumhasλ2Fν equal toaconstant. each module. The extracted 5–36 µm Spitzer-IRS spec-
trumwasretrievedfromtheCornellAtlasofSpitzer IRS
nally, the possible existence of large, micron-sized inter- Sources (CASSIS v5.2; Lebouteiller et al. 2011), which
stellargrainshavealsobeenproposedasviableO-bearing is based on the Advanced Optimal Extraction of the
repositories (Greenberg 1974; Jenkins 2009), in view of Spitzer ScienceCenter S18.18.0pipeline basiccalibrated
the known difficulty in detecting such materials. data (Lebouteiller et al. 2010). Post-extraction process-
In this paper, we re-examine the line of sight toward ing was performed in order to scale the flux density of
the bright O9.5 V star ζ Ophiuchi (ζ Oph, HD 149757; the SL module to that of the LL module at 14 µm. The
Herbig 1968). The low-velocity (v⊙ = −15 km s−1) uncertainties in the flux density are estimated to be half
component toward this well-studied star is regarded as thedifferencebetweenthetwoindependentspectrafrom
a prototypical “cool diffuse” cloud with high elemen- each nod position.
tal depletions and partial H I → H conversion, and is Coveringthe nominalspectralrangeof2.4–45µm, the
2
representative of an environment near the diffuse-dense ISO-SWSfullgratingscanswereperformedon1997Au-
ISM transition (Savage & Sembach 1996; Jenkins 2009; gust5(ISO TDT62803102)withtheSWS01Astronom-
Liszt et al. 2009). We utilize archival infrared spectra ical Observing Template at speed 1. The observations
from the Spitzer Space Telescope (Werner et al. 2004) result in a mean spectral resolving power of R ≈ 200–
and the Infrared Space Observatory (ISO; Kessler et al. 300 and a total exposure time of 1140 s. The extracted
1996) to quantify the solid-state absorption by refrac- ISO-SWS spectrum was retrieved from the SWS Atlas
tory and molecular compounds along the line of sight, of fully processed spectra (Sloan et al. 2003). Utilizing
and combine our results with previously published data broadbandphotometryfromtheWide-fieldInfraredSur-
fromtheliteraturetoassesstheoverallelementalbudget vey Explorer (WISE; Wright et al. 2010) All-Sky Data
toward ζ Oph. Release, the ISO-SWS spectrum was scaled by a factor
In general, ∼20–29% of the elemental O abundance of1.01to matchthe flux density derivedfromthe WISE
in the ISM is expected to be present in amorphous sil- 3.4 µm passband.
icate grains. The chemical and physical properties of The combined 2–36 µm spectra are shown in Figure
amorphoussilicates canbe determined by detailedmod- 1. However,duetotherelativelypoorsignal-to-noisera-
eling of their observed 10 and 20 µm resonance fea- tio in the ISO-SWS spectrum at wavelengths longward
tures. Revealedthroughinfraredspectroscopy,itiswell- of 4 µm, only the 2.4–4 µm spectral range is retained
known that the profile of the 10 µm silicate emission to investigate the presence of H O ice along the line of
2
feature detected toward the Trapezium (Forrest et al. sight. For comparison, the expected photospheric con-
1975) is similar to the silicate absorption feature ob- tinuum emission by an O9.5 V star, corresponding to
servedtowarddensemolecularclouds(Gillett et al.1975; a T = 34300 K blackbody (Howarth & Smith 2001), is
Roche & Aitken1984),whereassilicatesindiffuseclouds also indicated. Note that in the Rayleigh-Jeans limit, a
possess a narrower absorption profile (Roche & Aitken blackbody spectrum has λ2F equal to a constant. Dif-
ν
3
370
360
]
2m 350
µ
y
J
[ν 340
F
λ2
330
320
0.00
0.01
h
pt
e
D
al 0.02
c
pti
O
0.03
0.04
5 6 7 8 9 10 15 20 25 30 35 5 6 7 8 9 10 15 20 25 30 35
Wavelength [µm] Wavelength [µm]
Figure 2. Spitzer-IRSspectrum andbest-fit spectral decomposition models forthe ζ Ophsightline. Spectral decomposition models are
shownforasecond-(leftpanels)andfourth-order(rightpanels)polynomialfluxcontinuum;thethird-orderspectraldecompositionmodel
results are identical to those of the fourth-order spectral decomposition model. Top panels: observed spectrum (black line), model flux
continuum (grayline), andbest-fit modelspectrum (blueline). Bottom panels: observed optical depth spectrum (blackline)andbest-fit
modelopticaldepth(blueline). ThetotalabsorptionismodeledasalinearcombinationofamorphoussilicateswithMg2SiO4 (greenline),
MgFeSiO4 (violetline),andMgSiO3 (redline)stoichiometries.
ferences between the observed spectrum and that of a In our approach, which is based on simulating the
pure stellar photosphere are attributed to extinction by emergent spectral energy distribution (SED), the flux
intervening dust, consisting of amorphous silicates and continuumandextinctionopacityaresimultaneouslyde-
possibly other dust species lacking spectral features in termined in the logarithmic domain by
the mid-infraredwavelengthrange(e.g., amorphouscar-
bon or metallic iron). λFλobs =λFλconte−Piαiτλ,i, (1)
where Fobs and Fcont are the observed and continuum
3. SPECTRAL DECOMPOSITION ANALYSIS λ λ
flux densities, respectively, τ is the normalized optical
λ,i
Inordertocharacterizetheinterstellarabsorbersalong depth of an individual absorbing component, and α is
i
thelineofsighttowardζ Oph,weapproximatetheatten- its corresponding non-negative scaling parameter. The
uationbydustasanintermediatescreenagainstabright flux continuum is approximated as a low-order polyno-
infraredcontinuum. Historically,infraredabsorptionfea- mial, log(λFcont) = a (logλ)i, with a maximum de-
λ Pi i
tures have been studied by modeling optical depth spec- gree ranging from two to four, and represents the stellar
tra with laboratory-derived spectra of candidate mate- photospheric contribution by ζ Oph, as well as any un-
rials. However, in this classical approach, optical depth knownfeaturelessdustextinction. Theextinctionoptical
spectra are often inferred through the subjective pro- depth is modeled using a linear combination of amor-
cessoflocal-continuumfitting. Inconsequence,this pro- phous silicates and H O ice absorption spectra. We as-
2
cess may artificially set the underlying optical depth to sume that these individual absorbing components have
be zero over a pre-defined spectral region, which can fixed profiles, but their relative contribution to the total
strongly affect the subsequent modeling of the absorp- optical depth may vary.
tion profiles. In light of this concern, we choose to carry We adopt amorphous silicate optical constants with
out a different method in extracting information on the stoichiometric compositions from the melting and
absorbers. quenching experimental studies of Dorschner et al.
4
Table 1
SummaryofBest-fitOpticalDepthParametersTowardζ Oph
Poly.a τsil τsil τsil τsil τH2O τFe3O4b τPOMc χ2ν ν
Degree (total) (Mg2SiO4) (MgFeSiO4) (MgSiO3)
Spitzer-IRSspectraldecompositionmodels
2 .......... 0.032±0.009 0.014±0.006 0.010±0.006 0.009±0.004 ··· ≤0.006 ≤0.002 1.21 327
3 .......... 0.031±0.011 0.013±0.007 0.011±0.007 0.008±0.004 ··· ≤0.006 ≤0.002 1.21 326
4 .......... 0.031±0.011 0.013±0.007 0.011±0.007 0.008±0.004 ··· ≤0.006 ≤0.002 1.21 325
ISO-SWSspectraldecompositionmodels
2 .......... ··· ··· ··· ··· ≤0.008d ··· ··· ··· ···
2 .......... ··· ··· ··· ··· ≤0.009e ··· ··· 2.34 1957
Note. —Uncertainties arebasedonstatistical errorsintheSpitzer-IRSandISO-SWSspectraonly.
a Degreeofthepolynomialfluxcontinuum.
b Opticaldepthupperlimitestimateofthe16µmFe3O4 feature.
c Opticaldepthupperlimitestimateofthe8.9µmPOMfeature.
d Opticaldepthupperlimitestimateofthe3.1µmH2Oicefeatureforsub-micron-sizedmantledgrains.
e Derivedfromthe95%upperconfidence limitofthe3.53µmH2Oicefeatureforthickmantleswithcentralradiusofa=2.8µm.
(1995) and Henning & Stognienko (1996). In particu- scaling parameter of each absorbing component are ran-
lar, we use Fe-free and Fe-bearing amorphous silicates domized,withparameterconvergencetypicallyoccurring
with pyroxene (MgSiO and MgFeSi O ) and olivine after 30 or fewer iterations. Adopting a detection limit
3 2 6
(Mg SiO and MgFeSiO ) compositions. Following the criterionof ≥1σ, the best-fit model parameters are eval-
2 4 4
statistical approach of Min et al. (2003), the optical uated for significance upon convergence. If absorbing
properties of the amorphous silicate species are mod- components are detected with a significance less than
eled using a distribution of hollow spheres (DHS) in 1σ, their corresponding scaling parameters are set to
the Rayleigh limit (λ ≫ 2πa, where a is the radius of zero and the fitting procedure is repeated until all re-
the largest grains). In DHS theory, the optical proper- maining components satisfy the detection criterion. The
ties of hollow spherical particles are uniformly averaged formal uncertainties reported by the fitting procedure
from 0 < f < f , where f is the fraction of the total are derived from the statistical uncertainties within the
max
volume occupied by the central vacuum inclusion. The Spitzer-IRS spectrum. If the resultant model is of good
value of f indicates the degree of irregularity of the quality (i.e., the chi-squared per degree of freedom, χ2,
max ν
particles, with small values of f representing nearly is oforder unity), the estimated parameteruncertainties
max
sphericalparticles. Adoptingthemaximumvolumefrac- are subsequently scaled by the square root of χ2.
tion of fmax = 0.7 determined for the Sgr A∗ line of The best-fit spectral decomposition model reνsults to-
sight (Min et al. 2007), the resulting ensemble of parti- ward ζ Oph are presented in Figure 2 for a second- and
cles is representative of small, irregularly-shaped amor- fourth-order polynomial flux continuum. The Spitzer-
phous silicate grains. IRS spectrum and best-fit model spectrum are shown in
For the H2O ice component, we adopt optical con- the top panels, while the contributions by sightline ab-
stants derived from the laboratory spectrum of pure, sorbers to the observed optical depth spectrum are pre-
amorphous H2O ice (T = 10 K; Hudgins et al. 1993). sented in the bottom panels. The best-fit optical depth
Because irregularly shaped particle models have been parametersarelistedinTable1formodelsusingsecond-,
largely successful at simulating the profile shape of in- third-, and fourth-order polynomial flux continua.
terstellaricefeatures(e.g.,Pontoppidan et al.2003),the
optical properties of H2O ice are modeled using a con- 3.1. Amorphous Silicate Composition
tinuous distribution of ellipsoids (CDE) in the Rayleigh
In general, we find that the 5.3–34 µm optical depth
limit (Bohren & Huffman 1983). However, we note that
spectrum towardζ Oph is adequately (χ2 = 1.21)simu-
the calculated absorptioncross section of H2O ice is not ν
latedbyalinearcombinationofamorphoussilicateswith
verysensitivetothe choiceoftheadoptedparticleshape
Mg SiO ,MgFeSiO ,andMgSiO compositions. Specif-
model (i.e., only small differences in the profile shapes 2 4 4 3
ically,wefindidenticalresultsforspectraldecomposition
of the absorption features are found if we instead adopt
models utilizing a third- or fourth-order polynomial flux
small, homogenous spherical mantles).
continuum. In these cases, the Fe-free and Fe-bearing
Utilizing the model described in Equation (1) and
amorphous silicate species are detected with a signifi-
the aforementioned solid-state absorption spectra, we
cancerangingfrom1.6σ–2σ. Furthermore,wefindthata
employ a nonlinear least-squares fitting technique
fitofequalqualitymaybeobtainedforaspectraldecom-
(Markwardt 2009) to simulate the mid-infrared SED of
position model consisting of a second-order polynomial
ζ Oph. When present, blended silicate and H O ice ab-
2
flux continuum. In this case, the Fe-free and Fe-bearing
sorption features occupy nearly all of the mid-infrared
spectralregion,andforthisreasontheSpitzer-IRSspec- amorphous silicate components are detected at a simi-
lar significance level ranging from 1.7σ–2.3σ. The con-
trum is modeled over the entire 5.3–34 µm wavelength
sistency between the best-fit optical depth results sug-
range. In the fitting procedure, the initial values of the
gests that the amorphous silicate grain composition is
polynomial coefficients of the flux continuum and the
not sensitive to the adopted polynomial flux continuum.
5
Finally,wefindnoevidenceofFe-bearingamorphoussil- 3.50
29 ppm
icates with pyroxene composition (MgFeSi O ) or H O 000...000333000 3.00
2 6 2
ice along the line of sight. 2.50
Depending on the choice of the adopted polynomial 25 ppm 2.00
flux continuum, ouranalysisindicates that the peak sili- 000...000222555 1.90
cateopticaldepthtowardζ Ophisτ =0.031±0.011or 1.80
sil
0.032± 0.009. In the local-diffuseISM, the peak silicate 1.70
20 ppm
optical depth is correlated with the extinction along the 000...000222000 1.60
lineofsightaccordingtoAV/τsil≈18.5(Roche & Aitken O)4 18 ppm 1.50
1984). Adopting a total-to-selective extinction ratio of Si 16 ppm 1.40
e
RV = AV/EB−V = 2.55 ± 0.24 and a color excess of gF 000...000111555 14 ppm 1.30
EB−V =0.32±0.04fromValencic et al.(2004),wepre- M 1.29
dict a peak silicate optical depth of τ = 0.044 ± 0.007 τ( 12 ppm
sil 1.28
for the ζ Oph sightline, which is a factor of ∼1.4 greater 10 ppm
000...000111000 1.27
thanourmeasuredvalues. However,giventheuncertain-
1.26
ties associated with the measured and predicted values,
1.25
we conclude that reasonable agreement is found among
000...000000555 1.24
thesequantities. Ifthesmalldifferencebetweenthemea-
sured and predicted values is real, then it might be ex- 1.23
plained by partial masking of the observed silicate ab- 1.22
000...000000000 1.21
sorption by emission from a warm dust component near
ζ Oph. 000...000000000 000...000000555 000...000111000 τ(000M...000g111555SiO000)...000222000 000...000222555 000...000333000 χν2
2 4
3.1.1. Degeneracy Test for Silicate Components Figure 3. Reduced chi-squared (χ2ν) space of the fourth-order
The robustness of the best-fit results was investigated polynomial spectral decomposition model results. Contours of χ2ν
by randomizing the initial values of the parameters 103 ffoorr tvhaeluτes(MragnFgeiSnigOf4r)omand1.2τ1(Mtog23S.5iO0.4)Tphaerasmhaepteersofarteheillχu2νstmraitneid-
times. On each occasion, we find that the spectral de- mum is characterized by an ellipse with a slope of approximately
composition models converge to the same results. To −0.8,indicatingthatthelinearcombinationoftheMgFeSiO4 and
further explore the possibility of degeneracy among the Mg2SiO4 components iswell determined. Additionally, the abun-
dance of elemental Fe (N(Fe)/N(H); black horizontal lines) con-
best-fit amorphous silicate components, the correlation
tainedinamorphoussilicategrainsisalsoillustratedforeachmodel
coefficient, r, for each off-diagonal element is computed (see Section 4 for details of the calculation). Finally, the best-fit
using (χ2ν = 1.21) optical depth values and formal uncertainties from
covij Table1(whitecircle)areindicatedforreference.
r = , (2)
ij
σ σ
i j
slope of approximately −0.8. (3) The orientation of the
where σ and σ are the formal parameter uncertainties
i j χ2 minimum demonstrates that a linear combination
of the ith and jth components, respectively, and cov ν
ij of both components is required to accurately simulate
is the covariance between the component pairs. Highly
the absorption profile by amorphous silicates toward ζ
degeneratecomponentpairsare anti-correlatedandpos-
Oph. (4)AlthoughtheopticaldepthoftheMg SiO and
sess a correlation coefficient near r = −1. Similarly, 2 4
MgFeSiO components cannot be precisely disentangled
non-degenerate component pairs are uncorrelated and 4
fromthe total line ofsightcontribution, anindicationof
exhibit a correlation coefficient near r = 0. The most
their dependence on one another is well determined. As
significant degeneracy is found between the Mg SiO
2 4 such,increasingthe opticaldepth ofone componentand
and MgFeSiO components of the third- and fourth-
4 decreasing the other yields a goodness-of-fit of similar
order polynomial spectral decomposition models. For
quality (χ2 = 1.22). Hence, the two parameter χ2 space
thiscomponentpair,wecalculateacorrelationcoefficient ν ν
is instrumentalin revealingboth qualitative and quanti-
ofr=−0.84. Conversely,we findthatonlysmalldegen-
tativeinformationthatcouldnototherwisebe extracted
eracies,correspondingtor =−0.16andr =−0.32,exist
from the formal 1σ parameter uncertainties.
between the MgSiO :Mg SiO and MgSiO :MgFeSiO
3 2 4 3 4
component pairs, respectively.
3.2. H O Ice Mantles
In order to visualize the degeneracy between the Fe- 2
free andFe-bearingamorphoussilicate components with 3.2.1. Sub-Micron-Sized Grains
olivine stoichometry, we recompute the fourth-order Our model simulations of the Spitzer-IRS spectrum
polynomial spectral decomposition model for 33 × 33 toward ζ Oph do not require a contribution from the
combinations of the Mg2SiO4 and MgFeSiO4 compo- 12.6µmH2Oicelibrationfeature. However,becausethis
nents. All model parameters are held at their best-fit feature is blended with the strong vibrational modes of
values except for the Mg2SiO4 and MgFeSiO4 parame- amorphous silicates, it may be difficult to detect along
ters, which are varied over an optical depth range of 0 lines of sight where H O ice is not abundant. In con-
2
≤ τ ≤ 0.032. Contours of the two parameter χ2 space trast, the 3.1 µm O H stretching mode is intrinsically
ν
are presented in Figure 3 and indicate the following ob- stronger than the libration mode (τ /τ ≈ 4.5) and
3.1 12.6
servables: (1) the best-fit (χ2 = 1.21) component pair far less affected by overlap with other absorption fea-
ν
from Table 1 is situated within the innermost contour, tures, and thus is a more reliable and sensitive tracer of
which is representative of the true global minimum. (2) H O ice mantles on sub-micron-sized grains.
2
The χ2 minimum is characterized by an ellipse with a Following a similar methodology as before, a spectral
ν
6
332200
1.5
331100
]
−1O ice) [µm] 1.0 λ22F [Jy µmν 232390900000
H2
(
V
C/ext 0.5 228800
00..0000
0.0
h
2.5 3.0 3.5 4.0 4.5 pt
Wavelength [µm] De 00..0022
Figure 4. Extinction cross sections for grain size distributions cal
of thick H2O icemantles on largegrains. The weighted sum pro- pti
files are presented for central grain radii of a = 2.0 (black line), O
2.2 (green line), 2.4 (blue line), 2.6 (red line), 2.8 (magenta line),
00..0044
3.0 (cyan line), and 3.2 µm (violet line). The individual scatter-
ing (dotted violet line) and absorption (dashed violet line) cross
sections are indicated for a grain size distribution with a central
radiusofa=3.2µm.
00..0066
decomposition analysis of the ISO-SWS spectrum was 22..44 22..66 22..88 33..00 33..22 33..44 33..66 33..88
Wavelength [µm]
performed over the 2.8–3.8 µm wavelength range using
only the calculated absorption cross sections of H2O ice Figure 5. ISO-SWS 2.4–3.9 µm spectrum and best-fit second-
from Section 3. However, we find no evidence of the orderpolynomialspectraldecompositionmodelfortheζOphsight-
3.1 µm H O ice feature at the ≥1σ significance level. line. Toppanel: observedspectrum(blackline),modelfluxcontin-
2 uum(gray line),andbest-fit model spectrum (blueline). Bottom
Consequently,from the standarderrorestimate between
panel: observed optical depth spectrum (black line) and best-fit
the observed optical depth spectrum and the second- model optical depth (blue line). The total extinction is modeled
order polynomialspectral decompositionmodel over the byagrainsizedistributionofthickH2Oicemantleswithacentral
2.8–3.3 µm spectral region, we deduce a limiting opti- radiusofa=2.8µm.
cal depth of τ ≤ 0.008 for the 3.1 µm H O ice fea-
2
ture. We note that this result corresponds to a limit-
sibly interstellar dust grains returned by the Stardust
ing optical depth of τ ≤ 0.002 for the 12.6 µm H O
2 spacecraftprovidestentativeevidenceforthepresenceof
ice libration mode, which is a factor of ∼2–3 less than
micron-sized grains in the ISM (Westphal et al. 2014).
the uncertainties reported for best-fit optical depth val-
Adopting amorphous silicate and H O ice (T =
2
ues ofthe amorphoussilicate components. Furthermore,
25 K) optical constants from Draine & Lee (1984) and
the non-detection of H O ice toward ζ Oph is in ac-
2 Mastrapa et al. (2009), respectively, the optical proper-
cordwiththe finding thatsurfacesofsmall(sub-micron-
ties of large, ice-coated spherical grains are modeled us-
sized) grains begin to accumulate H O ice mantles at
2 inganextensionofMietheory(Bohren & Huffman1983)
columndensitiescorrespondingto silicateopticaldepths
for particle radii in the range of 1 µm ≤ a ≤ 4 µm. We
of τ & 0.15 ± 0.03 toward several prototypical dark
sil assumethatone-eighthofthetotalparticlevolumeisoc-
clouds (Whittet et al. 2013, and references therein).
cupiedbytheamorphoussilicatecore. Inthisthickman-
tle limit, we note that the ice-coatedextinction profileis
3.2.2. Very Large Grains
similar to that of pure homogeneous H O ice spheres.
2
Having found no convincing evidence of sub-micron- Finally,to smoothoscillatorystructure inthe calculated
sized icy grains,we next consider the possibility of thick absorption cross sections, we sum over three grain radii
H Oicemantlesonverylargegrains(a&1µm). Theex- of a−0.2 µm, a, and a+0.2 µm, with relative weights
2
istence of micron-sized interstellar grains was proposed of 25%, 50%, and 25%, respectively.
by Greenberg (1974). More recently, possible evidence The extinction cross sections for grain size distribu-
for the presence of micron-sized interstellar grains have tions of thick H O ice mantles are presented in Figure 4
2
been found by Wang et al. (2014), who modeled the with central radii of a = 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, and
“flat” mid-infrared extinction toward a variety of inter- 3.2 µm. The contributions from absorption and scatter-
stellar environments using a population of micron-sized ing are separatelyshown for the largestgrainsize distri-
grains. Furthermore, the recent analysis of three pos- bution, and demonstrate that the extinction cross sec-
7
Table 2
Solid-StateColumnDensitiesTowardζ Oph
Poly.a Σ(sil) Σ(Mg2SiO4) Σ(MgFeSiO4) Σ(MgSiO3) Σ(Fe3O4)b N(H2O) N(POM)c X(oli)d
Degree (10−5 gcm−2) (10−6 gcm−2) (10−6 gcm−2) (10−6 gcm−2) (10−6 gcm−2) (1016 cm−2) (1015 cm−2) (%)
Spitzer-IRSspectraldecompositionmodels
2 ............ 1.1±0.3 5.6±2.4 3.8±2.3 1.9±0.8 ≤2.5 ··· ≤9.7 85
3 ............ 1.1±0.4 5.2±2.8 4.2±2.7 1.6±0.8 ≤2.5 ··· ≤9.7 85
4 ............ 1.1±0.4 5.2±2.8 4.2±2.7 1.6±0.8 ≤2.5 ··· ≤9.7 85
ISO-SWSspectraldecompositionmodels
2 ............ ··· ··· ··· ··· ··· ≤1.2e ··· ···
2 ............ ··· ··· ··· ··· ··· ≤3.2f ··· ···
Note. —Uncertaintiesarebasedonstatistical errorsintheSpitzer-IRSandISO-SWSspectraonly.
a Degreeofthepolynomialfluxcontinuum.
b Basedonthe16µmFe3O4 opticaldepth.
c Basedonthe8.9µmPOMopticaldepth.
d Massfractionofamorphoussilicateswitholivinecomposition.
e Basedonthe3.1µmH2Oiceopticaldepthofsub-micron-sizedmantledgrains.
f Basedonthe95%upperconfidence limitofthe3.53µmH2Oiceoptical depthforthickmantleswithcentralradiusofa=2.8µm.
tionoflargegrainsaredominatedbyscatteringatwave- dances of Mg, Si, O, and Fe relative to H, we adopt a
lengths shortward of 2.8 µm and longward of 3.3 µm. total hydrogen column density of N(H) = 1.42×1021
Moreover,we find that the extinction profile shifts long- cm−2 for the diffuse clouds toward ζ Oph (Savage et al.
ward and decreases in amplitude with increasing grain 1977). However, the gas-phase interstellar absorption
radius. along this line of sight is known to originate from two
Employing a similar approach as before, a spectral separate diffuse cloud components (e.g., Hobbs 1973;
decomposition analysis of the ISO-SWS spectrum was Snow & Meyers 1979; Savage et al. 1992). The high-
performed over the 2.5–3.9 µm wavelength range using velocitycomponent(v⊙ =−27kms−1)samplesonlythe
onlytheaforementionedextinctionspectraofverylarge, modest depletion associated with lower density clouds.
spherical H2O ice mantles. In general, we find that In contrast, the low-velocity component (v⊙ = −15 km
the ISO-SWS spectrum can be adequately (χ2ν = 2.34) s−1) samples the highly depleted, denser cloud medium,
simulated using a second-order polynomial flux contin-
andaccountsformostoftheobservedhydrogenalongthe
uum and grain size distributions with central radii in
lineofsight(Jenkins2009). Adoptingasynthetichydro-
the range of 2 µm ≤ a ≤ 3.2 µm. The most signif- gen column density of N(H)=3.39×1019 cm−2 for the
icant (3σ) evidence of large H O ice mantles is found
2 high-velocity component from Jenkins (2009), we esti-
for central radii of a = 2.6 and 2.8 µm, which result in
mate a hydrogen column density of N(H)= 1.39×1021
H O ice optical depths of τ = 0.006 ± 0.002 at 3.46 and
2 cm−2 for the low-velocity component, which accounts
3.53 µm, respectively. The best-fit spectral decomposi-
for 97.6% of the total hydrogen column density toward
tion model result is shown in Figure 5 for a grain size
ζ Oph.
distribution with a central radius of a = 2.8 µm. We
note that the 95% upper confidence limit of τ ≤ 0.009
would only weakly affect the extinction law near 3.5 µm 4.1. Amorphous Silicates
(i.e., ∆Aλ/EB−V = 1.086×(0.009/0.32) = 0.03). This Under the assumption that the absorption features in
result demonstrates that the presence of H2O ice in the the Spitzer-IRS spectrum areentirely attributed to dust
diffuse ISM may be revealedthrough scattering by thick located in the highly depleted, low-velocity component,
icy mantles on large grains. The notion that such mate- therelativeabundanceofelementxcontainedinsilicates
rial might form within dense molecular clouds and sur- isdeterminedfollowingtheprescriptionofWhittet et al.
vive subsequent dispersal to more diffuse regions of the (1990):
ISM may be supported by the longer lifetimes predicted
N(x) 1 f τ
for thick H2O ice mantles (e.g., O¨berg et al. 2009). N(H) = N(H)Xµxh,iκλ,ii, (3)
x λ,i
i
4. SOLID-STATE ELEMENTAL ABUNDANCES
where N(H) = 1.39×1021 cm−2, f is the mass frac-
x,i
The results from the spectral decomposition analysis
tion of element x in the ith silicate component, µ is
x
allow constraints to be placed on the abundances of ele-
the atomic mass of element x, τ is the observed peak
λ,i
mentscontainedwithinthe relevantsolidstowardζ Oph
optical depth of the ith silicate component, and
(i.e., those with vibrational modes in the spectral range
coveredbytheavailableobservations). Acomparisonbe- hC i
abs,i
hκ i= (4)
tween gas-phase and solid-state abundances in this line λ,i
V ρ
i i
ofsightmaybeinformative,particularlyinregardtothe
questionofanypotential“missing”reservoirofinterstel- is the shape-averaged peak mass absorption coeffi-
lar O, as discussed in Section 1. cient for the ith silicate component with a shape-
In order to estimate the solid-state elemental abun- averaged absorption cross section per unit volume
8
Table 3
Solid-StateElemental Abundances Towardζ Oph
AmorphousSilicates Fe3O4 H2OIceMantlesb POM
Poly.a N(Mg) N(Si) N(O) N(Fe) N(O) N(Fe) N(O) N(O) N(O)
N(H) N(H) N(H) N(H) N(H) N(H) N(H) N(H) N(H)
Degree (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
Spitzer-IRSspectraldecompositionmodels
2 ................ 52±16 34±10 130±39 10±6 ≤19 ≤14 ··· ··· ≤7
3 ................ 49±19 33±11 126±45 10±7 ≤19 ≤14 ··· ··· ≤7
4 ................ 49±19 33±11 126±45 10±7 ≤19 ≤14 ··· ··· ≤7
ISO-SWSspectraldecompositionmodels
2 ................ ··· ··· ··· ··· ··· ··· ≤9 ≤23 ···
Elemental abundances absentfromthegas-phasec
Protosolard 42±4 34±3 230±72 35±3 ··· ··· ··· ··· ···
65
Protosolar+GCEe 46±5 41±4 282±78 48±5 ··· ··· ··· ··· ···
4 71 4
FandGstarsf 42±27 39±29 282±207 41±32 ··· ··· ··· ··· ···
17 17 155 18
Bstarsg 34±4 30±4 268±72 33±2 ··· ··· ··· ··· ···
Note. — Uncertainties are based on statistical errors in the Spitzer-IRS and ISO-SWS spectra only. Abundances are calculated
assumingahydrogencolumndensityofN(H)=1.39×1021 cm−2 forthelow-velocity(v⊙=−15kms−1)component.
a Degreeofthepolynomialfluxcontinuum.
b AbundanceofelementalOinsub-micron-sizedandverylargeH2Oicemantles,respectively. Thelatterisderivedfromthe95%upper
confidence limitofthe3.53µmH2Oiceoptical depthforthickmantleswithcentralradiusofa=2.8µm.
c Based on the difference between reference abundances and the observed gas-phase abundances derived from Jenkins (2009): 1.9 ±
0.1ppm(Mg),1.6±0.1ppm(Si),307±30ppm(O),and0.18±0.01ppm(Fe).
d Asplundetal.(2009)
e Asplundetal.(2009);Chiappinietal.(2003)
f Bensbyetal.(2005);Loddersetal.(2009)
f Nieva&Przybilla(2012)
hC i/V andspecificdensityρ . Usingthelaboratory- sidering the average reference abundance in B stars, we
abs,i i i
measured specific densities of ρ = 3.2, 3.71, and note that the elemental abundance of Si is slightly over-
2.71 g cm−3 for Mg SiO , MgFeSiO , and MgSiO consumedby∼10%–13%.) Incontrasttothesubstantial
2 4 4 3
(Henning & Stognienko1996;Dorschner et al.1995), we fractionof elemental Mg and Si contained in amorphous
calculate shape-averaged peak mass absorption coeffi- silicates, we find that ∼70%–79% of the available ele-
cients of hκ i = 2510, 2624, and 4853 cm2 g−1, respec- mental Fe abundance is expected to be present in some
λ
tively. other solid-state species. Finally, our model results in-
The total and individual mass column densities, Σ = dicate that only ∼45%–57% of the elemental O that is
τ /hκ i, of amorphous silicates, as well as the mass missing from the gas-phase is present in amorphous sili-
λ λ
fraction of amorphous silicates with olivine composi- cate grains. Independent of the adopted reference abun-
tion, are presented in Table 2. The solid-state ele- dances,theseresultssuggestthatadditionalreservoirsof
mental abundances of Mg, Si, O, and Fe are listed interstellar Fe and O are needed along the line of sight
in Table 3. For comparison, reference elemental dust toward ζ Oph.
abundances, based on average abundances in young F
and G stars (Bensby et al. 2005; Lodders et al. 2009), 4.2. Additional Repositories of Interstellar Fe
protosolar abundances (Asplund et al. 2009), proto- 4.2.1. Fe-rich Amorphous Silicates
solar abundances with Galactic chemical enrichment
Similar to previous diffuse ISM studies (e.g.,
(GCE) over the past 4.56 Gyr (Asplund et al. 2009;
Kemper et al. 2004; Chiar & Tielens 2006; Min et al.
Chiappini et al. 2003), and average abundances in B
2007), Fe-bearing amorphous silicates with equal
stars(Nieva & Przybilla2012), arecalculatedusing pre-
amounts of elemental Mg and Fe have been utilized to
viously published and revised gas-phase column densi-
simulatetheabsorptionprofileofamorphoussilicatesto-
ties of the low-velocity cloud component toward ζ Oph
ward ζ Oph. To investigate whether a significant frac-
(Jenkins 2009, and references therein).
tion of interstellar Fe may be present in amorphous sil-
In general, we find that solid-state elemental abun-
icates with Fe-to-Mg ratios greater than unity (Fe/Mg
dances derived from the spectral decomposition models
> 1), the Spitzer-IRS spectrum towardζ Oph is further
are in good agreement with those predicted from stellar
modeled using the Mg Fe Si O species instead of the
and solar abundances. Specifically, we find that the ele- 4 6 5 20
MgFeSiO component. However,wefindthattheslightly
mental abundance of Mg is slightly-to-moderately over- 4
Fe-richcomponent is not utilized by the nonlinear least-
consumed, requiring ∼7%-53% more than the available
squaresfittingroutine;thisistrueforall(2nd-,3rd-,and
abundance. Similarly, our model results indicate that
4th-order) polynomial flux continua considered in Sec-
nearly all (∼80%–100%) of the available elemental Si
tion 3. This result is likely a consequence of the broad-
abundanceispresentinamorphoussilicates. (Whencon-
enedspectralprofilespossessedbyFe-richamorphoussil-
9
shells surrounding asymptotic giant branch stars (e.g.,
337700 Kemper et al. 2002), but may be unlikely to remain
pristine in the oxygen-rich environment of the diffuse
ISM (Jones 1990). In contrast, Fe-bearing oxides are
336600
considered potential constituents of interstellar grains
(Sofia et al. 1994). Depending on the precise compo-
]
2m 335500 sition of these species, Fe-bearing oxides exhibit single
µ
y or multiple vibrational resonances within the 17–28 µm
[Jν 334400 spectral region (Henning et al. 1995; Glotch et al. 2006;
F Glotch & Rossman 2009). However, because their spec-
λ2
tral features are blended with the strong bending mode
333300 of amorphous silicates, Fe-bearing oxides have not been
detectedinthediffuseISM,andthusarenotusuallycon-
sideredto be a significantcomponentof interstellardust
332200
(e.g., Chiar & Tielens 2006). More recently, Li et al.
(2013) found that nanoparticles of FeO are not likely
00..0000
the carrier species responsible for the 21 µm emission
observed toward post-asymptotic giant branch stars.
To examine whether Fe-bearing oxides are a major
00..0011 reservoir of elemental Fe toward ζ Oph, we consider ir-
h regularly shaped particles of FeO (wu¨stite) and Fe O
pt 3 4
e (magnetite) as potential line of sight absorbers. While
D
al 00..0022 γ-Fe2O3 (maghemite) is known to possesses broad reso-
c nancefeaturesnear17and27µm,itsabsorptionopacity
pti contributes primarily to the featureless dust continuum,
O
and a non-detection of the features does not provide a
00..0033
usefulupperlimitestimateontheabundanceofγ-Fe O .
2 3
Forthisreason,wedonotincludeγ-Fe O inourspectral
2 3
analysis.
00..0044 As a result of the low signal-to-noise ratio within the
19–35 µm region of the Spitzer-IRS optical depth spec-
55 66 77 88 99 1100 1155 2200 2255 3300 3355
trum, we are unable to accurately quantify the pres-
Wavelength [µm]
ence of Fe-bearing oxides along the sightline using the
Figure 6. Spitzer-IRS spectrum and best-fit fourth-order poly- spectral decomposition analysis from Section 3. Alter-
nomialspectraldecompositionmodelfortheζ Ophsightline. Top
natively, from the standard error estimate between the
panel: observedspectrum(blackline),modelfluxcontinuum(gray
line),andbest-fitmodelspectrumwith(solidblueline)andwith- observedandmodeledopticaldepthspectrumwithinthe
out (dotted blue line) Fe3O4. Bottom panel: observed optical 15–30µmwavelengthregion,wededucealimitingoptical
depthspectrum(blackline)andbest-fitmodelopticaldepthwith depth of τ ≤ 0.009 for the 22 µm FeO feature. Assum-
(solid blue line) and without (dotted blue line) Fe3O4. The to- ing a specific density of ρ= 5.70 g cm−3 (Henning et al.
tal absorption is modeled as a linear combination of Fe3O4 (ma-
genta line) and amorphous silicates with Mg2SiO4 (green line), 1995) and a DHS (fmax = 0.7) shape-averagedpeak ab-
MgFeSiO4 (violet line),and MgSiO3 (redline)stoichiometries. A sorptioncrosssectionof hCabsi/V = 13590cm−1, we set
limitingopticaldepthofτ ≤0.006forthe16µmFe3O4 featureis a limit of ≤23 ppm for the abundance of elemental Fe
assumed.
in FeO grains. (In this context, a limiting abundance of
≤23 ppm for elemental O in FeO is also deduced.) Nev-
icates (Dorschner et al. 1995), and further demonstrates
ertheless, we emphasize that the generous limit on the
that stoichiometries with Fe/Mg . 1 are required when
optical depth of FeO, which is likely a consequence of
simulatingthe absorptionprofileof diffuse ISM silicates.
the appreciable noise present in the 19–30 µm spectral
Finally, if we assume the unlikely notion that MgFeSiO
4 region,wouldconsiderablyaffecttheshapeoftheoptical
is the only olivine-like species present toward ζ Oph, we
depth spectrum toward ζ Oph, and therefore should be
find that the abundance of elemental Fe is increased by
considered with caution.
only ∼10 ppm, as indicated in Figure 3. Therefore, we
Similarly, from the standard error estimate between
concludethatsomeotherFe-bearingcompoundmustac-
theobservedandmodeledopticaldepthspectrumwithin
countfor the ∼23–38ppm ofelemental Fe missing along
the 13–18 µm wavelength region, we calculate a limit-
the line of sight.
ing optical depth of τ ≤ 0.006 for the 16 µm Fe O
3 4
feature. In contrast to the FeO investigation, we note
4.2.2. Fe Oxides and Metallic Fe
that the limit on the optical depth of Fe O does not
3 4
In addition to its presence in silicate grains, in- strongly affect the appearance of the 13–30 µm re-
terstellar Fe could exist in pure metallic Fe or Fe- gion of the best-fit models from Section 3.1, as illus-
bearing oxide particles of FeO, γ-Fe2O3, and Fe3O4 trated in Figure 6. Adopting a specific density of ρ =
(e.g., Draine & Hensley 2013, and references therein). 5.18gcm−3 (Draine & Hensley2013)andaDHSshape-
Metallic Fe possesses no active vibrational modes in the averaged peak absorption cross section of hC i/V =
abs
mid-infrared wavelength region and contributes only to 12260cm−1,wededucelimitingabundancesof≤14ppm
the continuum opacity of dust. Pure metallic Fe par- and≤19ppmforelementalFeandOcontainedinFe O ,
3 4
ticles have been found toward the circumstellar dust
10
respectively. In summary, these results establish that 30
Fe O may accountfor .37–61%of the missing elemen-
3 4
tal Fe abundance toward ζ Oph.
25
4.3. Missing Reservoirs of Interstellar O
As previously noted, the rate at which the observed
20
depletion of elemental O varies with mean line of sight
]
density in the diffuse ISM is more rapid than can be m
p
accountedfor by exchange of atoms with amorphous sil- p
[
icates and metallic oxide particles (Jenkins 2009). In H) 15
consequence, a substantial fraction of interstellar O is N(
thought to reside in an additional reservoir, presumably O)/
in combination with some other abundant element(s). N( 10
Given that hydrogen and carbon are the only reactive
elements with sufficiently large interstellar abundance
(and given the improbability of substantial amounts of 5
O existing in such environments), it follows that H O
2 2
iceandorganicrefractorycompoundsareattractivecan-
didates as carriers of the missing elemental O (Jenkins
0
2009; Whittet 2010).
1.5 2.0 2.5 3.0 3.5
4.3.1. H2O Ice a [µm]
TheabundanceofelementalOcontainedinsmallH2O Figure 7. Abundance of elemental O that may be contained in
icemantlesmaybe directlyinferredfromitsline ofsight thickH2OicemantlesonlargegrainsalongtheζOphsightlinefor
column density (Allamandola & Sandford 1988): grainsizedistributions withcentral radiiof a= 2.0, 2.2, 2.4, 2.6,
2.8,3.0,and3.2µm. Themagnitudeofeachdatapointrepresents
τ dν˜ the significance of the best-fit H2O ice optical depth parameters,
N(H O)≈ R ν˜ , (5) whichvariesoverthe1σ–3σ level.
2
A
whereAisthelaboratory-derivedintrinsicbandstrength grains toward ζ Oph. Although this result is more than
and τ dν˜ is the integrated optical depth in the a factor of two larger than the upper limit abundance
R ν˜
wavenumber domain, ν˜. Adopting an intrinsic band obtainedforsmallicygrains,itindicatesthatthickH2O
strengthofA=2.0×10−16cmmolecule−1(Hagen et al. mantles on large (a = 2.8 µm) grains can only account
1981) for the 3.1 µm stretching mode, we calculate a for 15%–23%of the missing elemental O abundance.
limiting column density of N(H O) . 1.2 × 1016 cm−2.
2 4.3.2. Organic Compounds
The abundance of elemental O contained in small H O
2
ice mantles towardthe low-velocitycomponentofζ Oph We nextconsider organicrefractorymatter asa possi-
is thus no more than 9 ppm (i.e., no more than 6%– ble repository of interstellar O toward ζ Oph. It is well-
9% of the missing elemental O abundance). We note known that laboratory experiments simulating the ener-
that an even stricter limit (<2 ppm) was reported to- getic processing (e.g., ultraviolet photolysis or ion bom-
ward the highly reddened (A ≈10) luminous star Cyg bardment) of interstellar ices lead to the production of
V
OB2no.12(Whittet et al.1997),asightlinealsothought organicrefractorymatterwithsignificantoxygencontent
to be dominated by diffuse ISM material at compara- (e.g., Greenberg et al. 1995; Pendleton & Allamandola
tivelyhighdensity(e.g.,Geballe et al.1999;Gredel et al. 2002; Mun˜oz Caro & Schutte 2003). These products in-
2001). These results demonstrate that H O ice mantles clude species with carbonyl molecular groups (esters,
2
onsub-micron-sizedgrainsarenota significantreservoir amides, and carboxylic acid salts) and species related
of elemental O near the diffuse-dense ISM transition. to polyoxymethylene (POM), a polycrystalline material
ByanalogywithEquation3,theabundanceofelemen- consisting of linear chains of formaldehyde molecules
tal O in thick H O ice mantles on large grains is given [( CH O ) ]. The ester and amide species possess
2 2 n
by minor C O stretching modes near 5.7 and 6.0 µm,
N(O) 1 τ ρV respectively, while carboxylic acid salts are known to
N(H) = N(H)σ Qλ 18Hm2O, (6) exhibit a strong COO− stretching mode at 6.3 µm
g ext,λ H
(Mun˜oz Caro & Schutte2003). However,apartfromthe
where σg is the geometric cross section of the grains detection of weak ∼6 µm absorption towardcarbon-rich
(πa2), V is the volume of the mantle, and Q is Wolf-Rayet stars (Schutte et al. 1998; Chiar & Tielens
H2O ext,λ
the peak extinction efficiency of H O ice. Assuming a 2001), evidence for the presence of these O-bearing or-
2
specific density of ρ = 1.1 g cm−3 for amorphous H O ganiccompoundsinthediffuseISMhasremainedelusive.
2
ice (Narten et al. 1976), the abundance of elemental O Thequalityofthe5–8µmregionoftheSpitzer-IRSop-
in thick H O ice mantles is presented in Figure 7 for tical depth spectrum towardζ Oph does not permit any
2
grainsize distributions with centralradii in the range of meaningfulprofilecomparisonwithlaboratoryspectraof
2 µm ≤ a ≤ 3.2 µm. Considering the 95% upper confi- carbonylmolecular groups. In consequence, we make no
dence limit for a grain size distribution central radius of attempt to constrain the possible fraction of interstellar
a=2.8µm,wefindthatasmuchas∼23ppmofelemen- Ocontainedincarbonylgroups,andfurtherconsiderthe
tal O may be stored in thick H O ice mantles on large possibility of POM-like species instead.
2