Space Observatory Short Wavelength Spectrometer Research Articles

Detection of PAH Absorption and Determination of the Mid-infrared Diffuse Interstellar Extinction Curve from the Sight Line toward Cyg OB2-12

Abstract The sight line toward the luminous blue hypergiant Cyg OB2-12 is widely used to study interstellar dust on account of its large extinction (A V ≃ 10 mag) and the fact that this extinction appears to be dominated by dust typical of the diffuse interstellar medium. We present a new analysis of archival Infrared Space Observatory Short Wavelength Spectrometer and Spitzer IRS observations of Cyg OB2-12 using a model of the emission from the star and its stellar wind to determine the total extinction A λ from 2.4 to 37 μm. In addition to the prominent 9.7 and 18 μm silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons, including the first identification of the 7.7 μm feature in absorption. The 3.3 μm aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 μm aliphatic hydrocarbon features are observed with relative strengths that are consistent with observations of these features on sight lines toward the Galactic center. We identify and characterize more than 60 spectral lines in this wavelength range, which may be useful in constraining models of the star and its stellar wind. Based on this analysis, we present an extinction curve that extrapolates smoothly to determinations of the mean Galactic extinction curve at shorter wavelengths and to dust opacities inferred from emission at longer wavelengths, providing a new constraint on models of interstellar dust in the mid-infrared.

An infrared study of Be stars based on ISO SWS01 spectra

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) spectra of 10 Be stars are presented. It can be seen that the Be stars show a diversity in their ISO SWS01 spectral classifications by Kraemer et al., from naked stars, stars associated with dust, stars with warm dust shells, stars with cool dust shells to very red sources. In addition, the Bra/HI(14–6) line flux ratio derived for the sample stars is compared with that of P Cyg, and it is found that the line ratio of Be stars which were investigated show not only lower values as suggested by Waters et al., but also larger values. Therefore, the line ratio cannot be used to judge whether a star is a Be star or not.

The 3.3-μm PAH emission band of the Red Rectangle

A new analysis of long-slit CGS4 United Kingdom Infrared Telescope spectra of the 3.3-μm feature of the Red Rectangle and its evolution with offset along the NW whisker of the nebula is presented. The results support a proposed two-component interpretation for the 3.3-μm feature with peak wavelengths near 3.28 and 3.30 μm. Both components exhibit a small shift to shorter wavelength with increasing offset from the central star which, by comparison with laboratory studies, is consistent with a decrease in the temperature of the carriers with distance from HD 44179. The two-component approach is also applied to 3.3-μm data for the Red Rectangle, Orion Bar D2 and Orion Bar H2S1 from Infrared Space Observatory Short Wavelength Spectrometer (SWS) studies.

Analysis of ISO SWS01 spectra of S stars

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS01) plays an important role in studying properties of S stars. We reduce and analyze the SWS01 spectra of 17 S stars, and identify the candidate carriers of molecular absorption features. The ISO Spectral Analysis Package (ISAP) developed by the LWS and SWS Instrument Teams and Data Centers is used to process and analyze the SWS01 spectra of 17 S stars. The ISO archives of 17 S stars are obtained from the ISO database. Of 17 S stars, two stars are extrinsic S stars, the others are intrinsic S stars. The 15 intrinsic S stars can be divided into three groups (6 stars for Group I, 7 stars for Group II, and 2 stars for Group III) according to their Infrared Astronomical Satellite (IRAS) low-resolution spectra (LRS) and dust mass-loss rate. M-dust, where. M-dust increases from Group I to II and III gradually. 17 S stars show the following properties: 1. two extrinsic S stars and 15 intrinsic S stars among different groups have different infrared properties; 2. two extrinsic S stars and 6 intrinsic S stars in Group I have similar ISO SWS01 spectra and their continua can be approximately described by a single blackbody representing the stellar photosphere, while some intrinsic S stars in Group I have 60 mu m infrared excess; 3. for intrinsic S stars in Groups I, II, and III, their continua peak shifts toward longer wavelength from Groups I to II to III; 4. S stars in Groups II and III show obvious dust emission features in which the similar to 10 mu m dust features seem to display two different shapes. Moreover, two S stars (IRAS 00192-2020 & IRAS 15492 + 4837) present the 13 mu m feature; 5. molecules H2O, CO, and CO2 greatly affect the ISO SWS01 spectra of S stars. It is noted that the absorption features of molecules CS and HCN usually thought to exist only in C stars may appear in the S stars.

Challenging the identification of nitride dust in extreme carbon star spectra

Nitride dust is predicted to form in small amounts around carbon stars, but the most likely candidate species such as aluminium nitride (A1N) have not yet been detected. Recently, α-Si 3 N 4 was inferred to be the main carrier of the 8.5-12.5 μm absorption band(s) of an extreme carbon star (AFGL 5625), based on comparison with laboratory KBr dispersion spectra. However, this absorption band has also been attributed to silicon carbide (SiC) and C 3 . To investigate whether or not nitride dust has truly been detected and if it is present in other extreme carbon stars, we (i) gathered new laboratory infrared (IR) absorbance spectra from a suite of nitride compounds, including Si 3 N 4 , using the thin film technique which provides correct relative intensities of weak and strong peaks, and (ii) compared these data to Infrared Space Observatory Short Wavelength Spectrometer (ISO SWS) spectra of seven different extreme carbon stars which also show broad absorption features around ∼11 μm. The astronomical data show an apparent continuum of feature shapes ranging from full width at half-maximum (FWHM) of 2 to 2.7 μm, with peak barycenters ranging from 10.3 to 11 μm. The previous possible match between the extreme carbon star and Si 3 N 4 was also based on many minor peaks longward of the 8.5-12.5 μm feature. We show that the fit to AFGL 5625 and other extreme carbon stars is unconvincing, because the reported 13-25 μm features in the observed spectrum are essentially noise. Features due to other nitrides are also not obvious. The apparent continuum of absorption features in the 8.5-12.5 μm band is better represented by previously published fits of a combination of crystalline (β polytype) and amorphous SiC, where the apparent broadening and shift of the peak barycenter are due to increasing contributions from the amorphous solid.

The Disk Atmospheres of Three Herbig Ae/Be Stars

We present infrared (IR) spectrophotometry (R 180) of three Herbig Ae/Be stars surrounded by possible protoplanetary disks: HD 150193, HD 100546, and HD 179218. We construct a mid-IR spectral energy distributions (SEDs) for each object by using 7.6-13.2 μm HIFOGS spectra, 2.4-45 μm spectrophotometry from the Infrared Space Observatory Short-Wavelength Spectrometer, the 12, 25, 60, and 100 μm photometric points from IRAS, and for HD 179218, photometric bolometric data points from the Mount Lemmon Observing Facility. The SEDs are modeled by using an expanded version of the Chiang & Goldreich two-layer, radiative and hydrostatic equilibrium, passive disk. This expanded version includes the emission from Mg-pure crystalline olivine (forsterite) grains in the disk surface layer. Each of the three objects studied vary in the amount of crystals evident from their spectrophotometry. HD 150193 contains no crystals, while HD 100546 and HD 179218, respectively, show evidence of having crystalline silicates in the surface layers of their disks. We find that the inner region of HD 100546 has a 37% higher crystalline-to-amorphous silicate ratio in its inner disk region (≤5 AU) than in the outer disk region, while the inner disk region of HD 179218 has a 84% higher crystalline-to-amorphous silicate ratio in its inner disk region (≤5 AU) than in the outer region. All three objects are best fitted using a grain-size distribution power law that falls as a-3.5. HD 150193 is best fitted by a small disk (~5 AU in radius), while HD 100546 and HD 179218 are best fitted by larger disks (~150 AU in radius). Furthermore, HD 100546's disk flares larger than those of HD 150193 (25% more at 5 AU) and HD 179218 (80% more at 5 AU). We discuss the implications of our results and compare them with other modeling efforts.

The Profiles of the 3–12 Micron Polycyclic Aromatic Hydrocarbon Features

We present spectra of the 3.3 μm and 11.2 μm polycyclic aromatic hydrocarbon (PAH) features of a large number of stellar sources, planetary nebulae, reflection nebulae, H II regions, and galaxies, obtained with Infrared Space Observatory Short Wavelength Spectrometer. Clear variations are present in the profiles of these features. Most of the sources show a symmetric 3.3 μm feature peaking at ~3.290 μm, while only very few show an asymmetric 3.3 μm feature peaking at a slightly longer wavelength. The profiles of the 11.2 μm feature are distinctly asymmetric. The majority of the sources has a 11.2 μm feature peaking between 11.20 and 11.24 μm, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 μm feature with a peak position of ~11.25 μm, a less steep blue rise, and a high tail-to-top ratio. The sources are classified independently on the basis of the 3.3 and 11.2 μm feature profiles and peak positions. Correlations between these classes and those based on the 6-9 μm features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 μm region also show the most common 3.3 and 11.2 μm feature profiles. However, the uncommon profiles do not correlate with each other. Also, these classifications depend on the type of object. In general, H II regions, nonisolated Herbig AeBe stars and young stellar objects show the same profiles for all 3-12 μm features. Many planetary nebulae and post-asymptotic giant branch stars show uncommon feature profiles. The three galaxies in our sample show the same profiles as the H II regions for all but the 11.2 μm feature, being similar to that of evolved stars. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 μm bands) versus the CC modes (6.2, 7.7, and 8.6 μm bands) is striking: the peak wavelengths of the features attributed to CC modes vary by ~15-80 cm-1, while for the CH modes the variations are ~4-6.5 cm-1. We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 μm region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 μm components, which are attributed to PAH cations, the 3.3 μm feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these IR emission features to the composition of the PAH family. The variations in FWHM of the 3.3 μm feature remains an enigma, while those of the 11.2 μm can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.

Polycyclic Aromatic Hydrocarbons and Crystalline Silicates in the Bipolar Post–Asymptotic Giant Branch Star IRAS 16279−4757

IRAS 16279-4757 belongs to a group of post-asymptotic giant branch (post-AGB) stars showing both polycyclic aromatic hydrocarbon (PAH) bands and crystalline silicates. We present mid-infrared images that resolve the object for the first time. The morphology is similar to that of the Red Rectangle (HD 44179), the prototype object with PAHs and crystalline silicates. A two-component model and images suggest a dense oxygen-rich torus, an inner, low-density, carbon-rich region, and a carbon-rich bipolar outflow. The PAH bands are enhanced at the outflow, while the continuum emission is concentrated toward the center. Our findings support the suggestion that mixed chemistry and morphology are closely related. We discuss the Infrared Space Observatory Short-Wavelength Spectrometer (ISO SWS) spectra of IRAS 16279-4757. Several bands in the ISO SWS spectrum show a match with anorthite: this would be the first detection of this mineral outside the solar system. Compared to HD 44179, the shapes of PAH bands are closer to those of planetary nebulae, possibly related to a population of small PAHs present in HD 44179 but absent around IRAS 16279-4757. Detailed examination of the spectra shows the individual character of these two objects. The comparison suggests that the torus found in IRAS 16279-4757 may have formed more recently than that in HD 44179.

Molecular and dust features of 29 SiC carbon AGB stars

We have reduced and analyzed the Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) spectra of 29 infrared carbon stars with a silicon carbide feature at 11.30 mum, 17 of which have not been previously published. Absorption or emission features of C-2, HCN, C2H2, C-3 and silicon carbide (SiC) have been identified in all 17 unpublished carbon stars. In addition, two unidentified absorption features at 3.50 and 3.65 mum are listed for the first time in this paper. We classify these 29 carbon stars into groups A, B, C and D according to the shapes of their spectral energy distribution, and this classification seems to show an evolutionary sequence of carbon stars with an SiC feature. Moreover we have found the following results for the different groups: on average, the relative integrated flux of the 3.05 mum C2H2+HCN absorption feature increases gradually from group A to B and C; that of the 5.20 mum C-3 absorption feature becomes gradually weaker from group A to B and C; that of the 11.30 mum SiC emission feature increases gradually from group A to B and C but weakens in group D; and in contrast, that of the 13.70 mum C2H2 absorption feature weakens gradually from group A to B and C but becomes stronger in group D. We suggest that the evolution of the IR spectra of carbon stars along the sequence A to D is a result of the following phenomena: as the near-IR black-body temperature (T-nir) decreases, the circumstellar envelope becomes thicker; also the effective temperature (T-eff) of the photosphere of the central star decreases gradually and the C/O ratio increases from A to B.

Abundance Anomalies in CP Crucis (Nova Crux 1996)

We present spectroscopic observations of the classical nova CP Crucis (Nova Crux 1996) obtained with the Infrared Space Observatory Short Wavelength Spectrometer (ISO SWS) and the Anglo-Australian Telescope using both the Infrared Imaging Spectrograph and the Royal Greenwich Observatory Spectrograph. From the expansion parallax, we find that CP Crucis lies at a distance of 2.6 ± 0.5 kpc and reached a maximum MV of -8.7 at 0.96 days after discovery. We find abundance enhancements versus solar by mass of 75, 17, and 27 for N, O, and Ne, respectively. Additionally, we constrain the Mg abundance in the ejecta to be approximately solar. Abundance analysis suggests CP Crucis is an old Population I binary system. Combining the strong N and Ne abundances with the relatively low Mg abundance and a Ne/O ratio of 0.5, we propose that CP Crucis is an example of the missing link between CO and ONeMg novae.

ISO–SWS spectra of [WR] planetary nebulae

We have analyzed Infrared Space Observatory Short Wavelength Spectrometer (ISO–SWS) observations for 16 Wolf–Rayet ([WR]) planetary nebulae (PNe) with the aim of identifying the dust features present in this group of objects. We have found that Polycyclic Aromatic Hydrocarbon (PAH) molecular bands (which are present in most of the observed [WR] PNe) are more frequent than crystalline silicate features.

Infrared Space ObservatoryShort Wavelength Spectrometer Observations of HiiRegions in NGC 6822 and I Zwicky 36: Sulfur Abundances and Temperature Fluctuations

We report Infrared Space Observatory (ISO) Short Wavelength Spectrometer infrared spectroscopy of the H II region Hubble V in NGC 6822 and the blue compact dwarf galaxy I Zw 36. Observations of Br?, [S III] at 18.7 and 33.5 ?m, and [S IV] at 10.5 ?m are used to determine ionic sulfur abundances in these H II regions. There is relatively good agreement between our observations and predictions of S+3 abundances based on photoionization calculations, although there is an offset in the sense that the models overpredict the S+3 abundances. We emphasize a need for more observations of this type in order to place nebular sulfur abundance determinations on firmer ground. The S/O ratios derived using the ISO observations in combination with optical data are consistent with values of S/O, derived from optical measurements of other metal-poor galaxies. We present a new formalism for the simultaneous determination of the temperature, temperature fluctuations, and abundances in a nebula, given a mix of optical and infrared observed line ratios. The uncertainties in our ISO measurements and the lack of observations of [S III] ?9532 or ?9069 do not allow an accurate determination of the amplitude of temperature fluctuations for Hubble V and I Zw 36. Finally, using synthetic data, we illustrate the diagnostic power and limitations of our new method.

Neon Abundances in the HiiRegions of M33

We present neon abundances for 25 H II regions of M33, measured from line profiles of the mid-infrared transitions of [Ne II] and [Ne III] taken with the Infrared Space Observatory Short-Wavelength Spectrometer. The distribution of neon abundances as a function of galactocentric radius is best described as a step, -0.15 dex relative to the solar neon abundance from 0.7 to 4.0 kpc and -0.35 dex from 4.0 to 6.7 kpc, with estimated intrinsic scatter of 0.07 dex. The nearly flat neon abundance distribution differs from the steep oxygen abundance gradient found by previous investigators. Unless the oxygen abundance determinations are wrong, the chemical evolution of the galaxy has been radially dependent.

The Excitation and Metallicity of Galactic HiiRegions fromInfrared Space ObservatorySWS Observations of Mid‐Infrared Fine‐Structure Lines

We present mid-infrared Infrared Space Observatory Short-Wavelength Spectrometer (ISO-SWS) observations of the fine-structure emissions lines [Ne II] 12.8 ?m, [Ne III] 15.6 ?m, [Ne III] 36.0 ?m, [Ar II] 6.99 ?m, [Ar III] 8.99 ?m, [S III] 18.7 ?m, [S III] 33.5 ?m, and [S IV] 10.5 ?m and the recombination lines Br? and Br? in a sample of 112 Galactic H II regions and 37 nearby extra-Galactic H II regions in the LMC, SMC, and M33. We selected our sources from archival ISO-SWS data as those showing prominent [Ne II] 12.8 ?m or [Ne III] 15.6 ?m emissions. The Galactic sources have a wide range in galactocentric distance (0 kpc Rgal 18 kpc), which enables us to study excitation and metallicity variations over large Galactic scales. We detect a steep rise in the [Ne III] 15.6 ?m/[Ne II] 12.8 ?m, [Ar III] 8.99 ?m/[Ar II] 6.99 ?m, and [S IV] 10.5 ?m/[S III] 33.5 ?m excitation ratios from the inner Galaxy outward, and a moderate decrease in metallicity, from ~2 Z? in the inner Galaxy to ~1 Z? in the outer disk. The extra-Galactic sources in our sample show low gas density, low metallicity, and high excitation. We find a good correlation between [Ne III] 15.6 ?m/[Ne II] 12.8 ?m and [Ar III] 8.99 ?m/[Ar II] 6.99 ?m excitation ratios in our sample. The observed correlation is well reproduced by theoretical nebular models that incorporate new-generation wind-driven non-LTE model stellar atmospheres for the photoionizing stars. In particular, the non-LTE atmospheres can account for the production of [Ne III] emission in the H II regions. We have computed self-consistent nebular and stellar atmosphere models for a range of metallicities (0.5-2 Z?). We conclude that the increase in nebular excitation with galactocentric radius is due to an increase in stellar effective temperature (as opposed to a hardening of the stellar spectral energy distributions due to the metallicity gradient). We estimate an integrated [Ne III] 15.6 ?m/[Ne II] 12.8 ?m ratio for the Galaxy of 0.8, which puts it well inside the range of values for starburst galaxies. The good fit between observations and our models support the conclusion of Thornley and coworkers that the low [Ne III] 15.6 ?m/[Ne II] 12.8 ?m ratios observed in extra-Galactic sources are due to global aging effects.

The complete ISO spectrum of NGC 6302

We present the combined Infrared Space Observatory Short-Wavelength Spectrometer and Long- Wavelength Spectrometer 2.4{197 m spectrum of the Planetary Nebula NGC 6302 which contains in addition to strong atomic lines, a series of emission features due to solid state components. The broad wavelength coverage enables us to more accurately identify and determine the properties of both oxygen- and carbon-rich circumstellar dust. A simple model t was made to determine the abundance and typical temperature of the amorphous silicates, enstatite and forsterite. Forsterite and enstatite do have roughly the same abundance and temperature. The origin and location of the dust in a toroidal disk around the central star are discussed.

PAH features in the ISO SWS01 spectra of [WR] planetary nebulae

We present Infrared Space Observatory (ISO)Short Wavelength Spectrometer (SWS) observations for 16Wolf–Rayet ([WR]) planetary nebulae (PNe) in the range from 2.4 to16.5 μm with the aim of analyzing the dust features present inthis group of objects. We have found that Policyclic AromaticHydrocarbon (PAH) molecular bands are present in most of the observed[WR] planetary nebulae with clear exception for K 2–16 among latetype [WC] stars.

A Broad 22 Micron Emission Feature in the Carina Nebula H ii Region.

We report the detection of a broad 22 µm emission feature in the Carina Nebula H ii region by the Infrared Space Observatory (ISO) short-wavelength spectrometer. The feature shape is similar to that of the 22 µm emission feature of newly synthesized dust observed in the Cassiopeia A supernova remnant. This finding suggests that both of the features are arising from the same carrier and that supernovae are probably the dominant production sources of this new interstellar grain. A similar broad emission dust feature is also found in the spectra of two starburst galaxies from the ISO archival data. This new dust grain could be an abundant component of interstellar grains and can be used to trace the supernova rate or star formation rate in external galaxies. The existence of the broad 22 µm emission feature complicates the dust model for starburst galaxies and must be taken into account correctly in the derivation of dust color temperature. Mg protosilicate has been suggested as the carrier of the 22 µm emission dust feature observed in Cassiopeia A. The present results provide useful information in studies on the chemical composition and emission mechanism of the carrier.

ISO-SWS Observations of Jupiter: Measurement of the Ammonia Tropospheric Profile and of the 15N/ 14N Isotopic Ratio

We present the results of the Infrared Space Observatory Short Wavelength Spectrometer (ISO-SWS) observations of Jupiter related to ammonia. We focus on two spectral regions; the first one (the 10-μm region), ranging from 9.5 to 11.5 μm, probes atmospheric levels between 1 and 0.2 bar, while the second one (the 5-μm window), ranging from 4.8 to 5.5 μm, sounds the atmosphere between 8 and 2 bar. The two spectral windows cannot be fitted with the same ammonia vertical distribution. From the 10-μm region we infer an ammonia distribution of about half the saturation profile above the 1-bar level, where the N/H ratio is roughly solar. A totally different picture is derived from the 5-μm window, where we determine an upper limit of 3.7×10 −5 at 1 bar and find an increasing NH 3 abundance at least down to 4 bar. This profile is similar to that measured by the Galileo probe. The discrepancy between the two spectral regions most likely arises from the spatial heterogeneity of Jupiter, the 5-μm window sounding dry areas unveiled by a locally thin cloud cover (the 5-μm hot spots), and the 10-μm region probing the mean jovian atmosphere above 1 bar. The 15NH 3 mixing ratio is measured around 400 mbar from ν 2 band absorptions in the 10-μm region. We find the atmosphere of Jupiter highly depleted in 15N at this pressure level [( 15N/ 14N) [formula]=1.9 +0.9 −1.0)×10 −3, while ( 15N/ 14N) ⊕=3.68×10 −3]. It is not clear whether this depletion reveals the global jovian 15N/ 14N ratio. Instead an isotopic fractionation process, taking place during the ammonia cloud condensation, is indicated as a possible mechanism. A fractionation coefficient α higher than 1.08 would explain the observed isotopic ratio, but the lack of laboratory data does not allow us to decide unambiguously on the origin of the observed low 15N/ 14N ratio.

Mid‐InfraredInfrared Space ObservatorySpectroscopy of the Prototypical LINER NGC 1052: Shocks Reconsidered

We have observed the prototypical LINER NGC 1052 with the Infrared Space Observatory Short-Wavelength Spectrometer. While we have detected the [Ne II] 12.8 μm line, we have obtained a strict upper limit for the [Ne III] 15.6 μm line. A large [Ne II]/[Ne III] line ratio and small [Ne III]/[O III] λ5007 and [Ne II]/[OIII] line ratios are consistent with slow-shock models with ~130 km s-1 shock velocity. Our results support the original suggestions of shock excitation as a dominant source for NGC 1052. We have fitted the infrared and optical line ratios with a combination of shock and photoionization models. The best combination among the simple models we considered suggests that the contribution of shock and photoionization to [O III] λ5007 emission is four-fifths and one-fifth, respectively.

Mg-Rich Silicate Crystals in Comet Hale–Bopp: ISM Relics or Solar Nebula Condensates?

We compare the HIFOGS 10-μm spectra of Comet C/1995 (O1) (Hale–Bopp) to the mid-IR spectra of interplanetary dust particles (IDPs). The four epochs of Hale–Bopp silicate features, at 2.8 AU (pre-perihelion), 1.2 AU (pre-perihelion), 0.93 AU (10 days after perihelion), and 1.7 AU (post-perihelion) have different spectral shapes. At smaller heliocentric distances, r h≤1.7 AU, there appears to be enhanced mid-IR emission (i.e., 9.3- and 10.5-μm peaks) from Mg-rich pyroxene crystals. A two-temperature model of warm amorphous and crystalline olivines, warm amorphous pyroxenes, and ∼165 K cooler pyroxene crystals is successful with laboratory silicate minerals (D. H. Wooden et al. 1999, Astrophys. J. 517, 1034–1058). A two-temperature model using IDPs as cometary dust particle analogs also fits Comet Hale–Bopp's silicate features, with the pyroxene IDP ∼50 K cooler than the olivine and layer–lattice silicate IDPs. The warm layer–lattice silicate IDP contributes a broad 20-μm feature which is not in the Infrared Space Observatory Short Wavelength Spectrometer (ISO SWS) spectrum of Hale–Bopp at 2.8 AU, making it a questionable cometary component. On the other hand, members of the ``cluster IDP" subclass of pyroxene IDPs fit Hale–Bopp's 10-μm spectra better than other pyroxene IDPs. Cluster IDPs are highly fragile and contain significant deuterium enrichments, indicating their presolar origin. By spectroscopic analogy, Comet Hale–Bopp's Mg-rich pyroxene crystals may also be relic interstellar grains. The IDP and mineral models for Comet Hale–Bopp indicate the importance of multiple grain components at different temperatures to explaining the temporal evolution of the observed infrared features. Relative abundances of minerals depend on their relative temperatures. Cooler grains may be more abundant than warmer grains and yet may remain hidden from detection in the 10-μm silicate feature because of their cooler temperatures. The deduced larger abundance of cooler, pyroxene-dominated grains in Comet Hale–Bopp affects the interpretation of the origin of cometary dust, and dust evolution during protoplanetesimal accumulation and disk dissipation.