Angela Karen Speck

Massive-Star Supernovae as Major Dust Factories

We present late-time optical and mid-infrared observations of the Type II supernova 2003gd in the galaxy NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499 to 678 days after outburst and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae could have been major dust producers throughout the history of the universe.

Water Ice, Silicate, and Polycyclic Aromatic Hydrocarbon Emission Featuresin the Infrared Space Observatory Spectrum of the Carbon-richPlanetary Nebula CPD –56°8032*

Combined Infrared Space Observatory Short-Wavelength Spectrometer and Long-Wavelength Spectrometer spectroscopy is presented of the late WC-type planetary nebula nucleus CPD -56°8032 and its carbon-rich nebula. The extremely broad coverage (2.4-197 μm) enables us to recognize the clear and simultaneous presence of emission features from both oxygen- and carbon-rich circumstellar materials. Removing a smooth continuum highlights bright emission bands characteristic of polycyclic aromatic hydrocarbons in the 3-14 μm region, bands from crystalline silicates longward of 18 μm, and the 43 and 62 μm bands of crystalline water ice. We discuss the probable evolutionary state and history of this unusual object in terms of (a) a recent transition from an O-rich to a C-rich outflow following a helium shell flash or (b) a carbon-rich nebular outflow encountering an O-rich comet cloud.

SILICON CARBIDE ABSORPTION FEATURES: DUST FORMATION IN THE OUTFLOWS OF EXTREME CARBON STARS

Infrared carbon stars without visible counterparts are generally known as extreme carbon stars. We have selected a subset of these stars with absorption features in the 10-13 μm range, which has been tentatively attributed to silicon carbide (SiC). We add three new objects meeting these criterion to the seven previously known, bringing our total sample to ten sources. We also present the result of radiative transfer modeling for these stars, comparing these results with those of previous studies. In order to constrain model parameters, we use published mass-loss rates, expansion velocities, and theoretical dust condensation models to determine the dust condensation temperature. These show that the inner dust temperatures of the dust shells for these sources are significantly higher than previously assumed. This also implies that the dominant dust species should be graphite instead of amorphous carbon. In combination with the higher condensation temperature we show that this results in a much higher acceleration of the dust grains than would be expected from previous work. Our model results suggest that the very optically thick stage of evolution does not coincide with the timescales for the superwind, but rather, that this is a very short-lived phase. Additionally, we compare model and observational parameters in an attempt to find any correlations. Finally, we show that the spectrum of one source, IRAS 17534–3030, strongly implies that the 10-13 μm feature is due to a solid state rather than a molecular species.

The Effect of Stellar Evolution on SiC Dust Grain Sizes

Stars on the asymptotic giant branch (AGB) produce dust in their circumstellar shells. The nature of the dust-forming environment is influenced by the evolution of the stars, in terms of both chemistry and density, leading to an evolution in the nature of the dust that is produced. Carbon-rich AGB stars are known to produce silicon carbide (SiC). Furthermore, observations of the ~11 μm SiC feature show that the spectral features change in a sequence that correlates with stellar evolution. We present new infrared spectra of amorphous SiC and show that the ~9 μm feature seen in both emission and absorption, and correlated with trends in the ~11 μm feature, may be due to either amorphous SiC or nanocrystalline diamond with a high proportion of Si substituting for C. Furthermore, we identify SiC absorption in three ISO spectra of extreme carbon stars, in addition to the four presented by Speck and coworkers. An accurate description of the sequence in the IR spectra of carbon stars requires accounting for both SiC emission and absorption features. This level of detail is needed to infer the role of dust in the evolution of carbon stars. Previous attempts to find a sequence in the infrared spectra of carbon stars considered SiC emission features while neglecting SiC absorption features, leading to an interpretation of the sequence that inadequately describes the role of dust. We show that the evolutionary sequence in carbon star spectra is consistent with a grain size evolution such that dust grains get progressively smaller as the star evolves. The evolution of the grain sizes provides a natural explanation for the shift of the ~11 μm SiC feature in emission and in absorption. Further evidence for this scenario is seen in both post-AGB star spectra and in meteoritic studies of presolar grains.

PROCESSING OF PRESOLAR GRAINS AROUND POST-ASYMPTOTIC GIANT BRANCH STARS: SILICON CARBIDE AS THE CARRIER OF THE 21 MICRON FEATURE

Some proto–planetary nebulae (PPNs) exhibit an enigmatic feature in their infrared spectra at � 21 m. This feature is not seen in the spectra of either the precursors to PPNs, the asymptotic giant branch (AGB) stars, or the successors of PPNs, ‘‘normal’’ planetary nebulae (PNs). However, the 21 m feature has been seen in the spectra of PNs with Wolf-Rayet central stars. Therefore, the carrier of this feature is unlikely to be a transient species that only exists in the PPN phase. This feature has been attributed to various molecular and solid-state species, none of which satisfy all constraints, although titanium carbide (TiC) and polycyclic aromatic hydrocarbons (PAHs) have seemed the most viable. We present new laboratory data for silicon carbide (SiC) and show that it has a spectral feature that is a good candidate for the carrier of the 21 m feature. The SiC spectral feature appears at approximately the same wavelength (depending on the polytype/grain size) and has the same asymmetric profile as the observed astronomical feature. We suggest that processing and cooling of the SiC grains known to exist around carbon-rich AGB stars are responsible for the emergence of the enigmatic 21 m feature. The emergence of this feature in the spectra of post-AGB stars demonstrates the processing of dust due to the changing physical environments around evolving stars.

Large-Scale Extended Emission around the Helix Nebula: Dust, Molecules, Atoms, and Ions

We present new observations of the ionized gas, molecular gas, and cool dust in the Helix Nebula (NGC 7293). The ionized gas is observed in the form of an Himage, which is constructed using images from the Southern HSky Survey Atlas. The molecular emission was mapped using the H2 v =1 ! 0 S(1) line at 2.122 lm. The far-infrared (FIR) observations were obtained using ISOPHOT on the Infrared Space Observ- atory. The Hobservations are more sensitive than previous measurements and show the huge extent of the Helix, confirming it as a density-bounded nebula and showing previously unseen point-symmetric structures. The H2 observations show that the molecular gas follows the distribution of molecular material shown in pre- vious work. The molecular emission is confined to that part of the nebula seen in the classic optical image. Furthermore, comparison of the H2 emission strength with time-dependent models for photodissociation regions (PDRs) shows that the emission arises from thermal excitation of the hydrogen molecules in PDRs and not from shocks. The FIR observations, at 90 and 160 lm, represent mostly contributions from thermal dust emission from cool dust grains but include a small contribution from ionized atomic lines. Comparison of the FIR emission with the Hobservation shows that the dust and ionized gas are coincident and extend to � 1100 00 radius. This equates to a spatial radial extent of more than 1 pc (assuming a distance to the Helix of � 200 pc). Assuming that the outer layers of the circumstellar shell have spherical symmetry, radiative transfer modeling of the emission in Hgives a shell mass of � 1.5 M� . However, the modeling does not cover the outermost part of the shell (beyond � 600 00 radius), and therefore this is a lower limit for the shell mass. Moreover, the models suggest the need for very large dust grains, with � 80% of the dust mass in grains larger than 3.5 lm. Comparison of these new observations with previous observations shows the large-scale stratifi- cation of the Helix in terms of ionized gas and dust, as well as the coexistence of molecular species inside the ionized zones, where molecules survive in dense condensations and cometary knots.

Optical properties of silicon carbide for astrophysical applications I. New laboratory infrared reflectance spectra and optical constants

Aims. The SiC optical constants are fundamental inputs for radiat ive transfer (RT) models of astrophysical dust environments. However, previously published values contain errors and do not adequately represent the bulk physical properties of th e cubic (β) SiC polytype usually found around carbon stars. We provide new, uncompromised optical constants forβ- andα-SiC derived from single-crystal reflectance spectra ⋆ and investigate quantitatively (i) whether there is any difference betweenα- andβ-SiC that can be seen in infrared (IR) spectra and optical functions and (ii) whether weak features fromλ∼ 12.5‐13.0� m need to be fitted. Methods. We measured mid- and far-IR reflectance spectra for two sampl es of 3C (β-)SiC and four samples of 6H (α-)SiC. For the latter group, we acquired polarized data ( E⊥c, Ek c orientations). We calculated the real and imaginary parts o f the complex refractive index (n(λ) + ik(λ)) and the ideal absorption coeffi cients via classical dispersion fits to our reflectance spect ra. Results. We find that β-SiC and E⊥cα-SiC have almost identical optical functions but that n(λ) and k(λ) for Ek cα-SiC are shifted to lower frequency. Peak positions determined for both 3C (β-) and 6H (α-)SiC polytypes agree with Raman measurements and show that a systematic error of 4 cm −1 exists in previously published IR analyses, attributable t o inadequate resolution of older instruments for the steep, sharp modes of SiC. Weak modes are present for samples with impurities. Our calculated absorption coeffi cients are much higher than laboratory measurements. Whereas astrophysical dust grain sizes remain fairly unconstrained, SiC gr ains larger than about 1� m in diameter will be opaque at frequencies near the peak center. Conclusions. Previous optical constants for SiC do not reflect the true bul k properties, and they are only valid for a narrow grain size range. The new optical constants presented here will allow narrow constraints to be placed on the grain size and shape dis tribution that dominate in astrophysical environments.

Two Subclasses of Proto-Planetary Nebulae: Model Calculations

We use detailed radiative transfer models to investigate the differences between the star-obvious low-level elongated proto-planetary nebulae (SOLE PPNs) and dust-prominent longitudinally extended proto-planetary nebulae (DUPLEX PPNs), which are two subclasses of PPNs suggested by Ueta, Meixner, & Bobrowsky. We select one SOLE PPN, HD 161796, and one DUPLEX PPN, IRAS 17150-3224, both of which are well studied and representative of their PPN classes. Using an axisymmetric dust shell radiative transfer code, we model these two sources in detail and constrain their mass-loss histories, inclination angles, and dust composition. The physical parameters derived for HD 161796 and IRAS 17150-3224 demonstrate that they are physically quite different and that their observed differences cannot be attributed to inclination-angle effects. Both HD 161796 and IRAS 17150-3224 are viewed nearly edge-on. However, the more intensive axisymmetric superwind mass loss experienced by IRAS 17150-3224 (8.5 × 10-3 M☉ yr-1 and an equator/pole = 160) has created a high optical depth dust torus (AV = 37) that obscures its central star. In contrast, HD 161796, which underwent a lower rate superwind ( = 1.2 × 10-4 M☉ yr-1 and an equator/pole = 9), has an optically thinner dust shell that allows the penetration of direct starlight. Based on our analysis of the dust composition, which is constrained by dust optical constants derived from laboratory measurements, both objects contain oxygen-rich dust, mainly amorphous silicates, but with some significant differences. IRAS 17150-3224 contains only amorphous silicates with sizes ranging from 0.001 to larger than ~200 μm. HD 161796 contains amorphous silicates, crystalline silicates (enstatite and forsterite), and crystalline water ice with sizes ranging from 0.2 to larger than ~10 μm. If these calculations reflect a more general truth about SOLE versus DUPLEX PPNs, then these two subclasses of PPNs are physically distinct, with the SOLE PPNs derived from low-mass progenitors and DUPLEX PPNs derived from high-mass progenitors.

Discovery of Extreme Carbon Stars in the Large Magellanic Cloud

Using Spitzer IRAC and MIPS observations of the Large Magellanic Cloud, we have identified 13 objects that have extremely red mid-IR colors. Follow-up Spitzer IRS observations of seven of these sources reveal varying amounts of SiC and C2H2 absorption as well as the presence of a broad MgS feature in at least two cases, indicating that these are extreme carbon stars. Preliminary estimates find these objects have luminosities of (4-11) ? 103 -->L? and preliminary model fitting gives mass-loss rates between 4 ? 10 -->?5 and 2 ? 10 -->?4 -->M? yr -->?1, higher than any known carbon-rich AGB star in the LMC. These spectral and physical properties require careful reconsideration of dust condensation and mass-loss processes for carbon stars in low-metallicity environments.

Astromineralogy of the 13 μm feature in the spectra of oxygen-rich asymptotic giant branch stars. I. Corundum and spinel

Asymptotic giant branch (AGB) stars have several interesting infrared spectral features. Approximately half the oxygen-rich AGB stars to be investigated spectroscopically exhibit a feature at ~13 μm. The carrier of this feature has not yet been unequivocally identified but has been attributed to various dust species, including corundum (α-Al2O 3), spinel (MgAl2O4), and silica (SiO 2). In order to constrain the carrier of the 13 μm feature, we have used the one-dimensional radiative transfer code DUSTY to model the effects of composition and optical depth on the shape and strength of the emerging 13 μm feature from corundum and spinel grains. We have modeled various corundum, spinel, corundum-silicate, and spinel-silicate mixtures in dust shells surrounding O-rich AGB stars. These models demonstrate that (1) if corundum is present in these circumstellar dust shells, even at very low relative abundances, a ~13 μm feature should be observed; (2) corundums weak ~21 μm feature will not be observed, even if it is responsible for the ~13 μm feature; (3) even at low relative abundances, spinel exhibits a feature at 16.8 μm that is not found in observations; and (4) the grains must be spherical. Other grain shapes (spheroids, ellipsoids, and hollow spheres) shift the features to longer wavelengths for both spinel and corundum. Our models show that spinel is unlikely to be the carrier of the 13 μm feature. The case for corundum as the carrier is strengthened but not yet proven.