We also mis-reported the temperature of the silicate and carbon grains in our fit to the HR 7012 IRS spectrum; the grains have a temperature 550 K, not 520 K as reported previously. Lastly, the composition of the enstatite used to fit the HR 7012 spectrum is Mg 0.7 Fe 0.3 SiO 3 , not Mg 0.7 Fe 0.3 SiO 4 . In addition, we noticed an error in the minimum blow-out size for silicate, carbon, and silica grains around HD 113766 and HR 7012; the blow-out sizes are smaller than previously estimated. For HD 113766, we originally estimated minimum silicate and carbon sizes of 1.4 and 1.9 m, respectively; we now estimate 0.35 and 0.46 m, respectively. With the exception of forsterite, all of the grains used to model the HD 113766 spectrum are larger than the minimum grain sizes. The forsterite grains (submicron) possess radii that are similar to the minimum silicate grain size. For HR 7012, we originally estimated minimum silicate, carbon, and silica sizes of 1.1, 1.4, and 1.6 m, respectively; we now estimate 0.9, 1.2, and 1.3 m, respectively. The enstatite and cristobalite grains used to model the infrared HR 7012 spectrum are still smaller than the minimum grain size. We had previously concluded that the minimum grain sizes (>1 m) were inconsistent with presence of submicron-sized grains inferred from the structure of the silicate emission features, suggesting that a recent massive collision must have occurred around HD 113766 and HR 7012. Our new minimum grain size estimates are more consistent with our models for the infrared spectra and do not require a recent massive collision around HD 113766. However, our models do indicate the presence of submicron-sized particles significantly smaller than the blow-out size around HR 7012, suggesting that a recent massive collision may have occurred in this system.

We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to obtain spectra of HD 100764, an apparently single carbon star with a circumstellar disk. The spectrum shows emission features from polycyclic aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than normally seen, as characteristic of "class C" systems in the classification scheme of Peeters et al. All seven of the known class C PAH sources are illuminated by radiation fields that are cooler than those which typically excite PAH emission features. The observed wavelength shifts are consistent with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We propose that the class C PAH spectra are distinctive because the carbonaceous material has not been subjected to a strong ultraviolet radiation field, allowing relatively fragile aliphatic materials to survive.

We characterize the crystalline silicate content and spatial distribution of small dust grains in a large sample of protoplanetary disks in the Taurus-Auriga young cluster,

Here we present the Spitzer IRS spectrum of CS Cha, a member of the ∼2 Myr old Chamaeleon star-forming region, which reveals an optically thick circumstellar disk truncated at ∼43 AU, the largest hole modeled in a transitional disk to date. Within this inner hole, ∼ lunar masses of dust are located in a small optically Ϫ5 5 # 10 thin inner region that extends from 0.1 to 1 AU. In addition, the disk of CS Cha has bigger grain sizes and more settling than the previously modeled transitional disks DM Tau, GM Aur, and CoKu Tau/4, suggesting that CS Cha is in a more advanced state of dust evolution. The Spitzer IRS spectrum also shows [Ne ii] 12.81 mm finestructure emission with a luminosity of ergs s Ϫ1 , indicating that optically thin gas is present in this 29 1.3 # 10 ∼43 AU hole, in agreement with Ha measurements and a UV excess that indicate that CS Cha is still accreting yr Ϫ1 . We do not find a correlation of the [Ne ii] flux with L X ; however, there is a possible Ϫ8 1.2 # 10 M , correlation with , which if confirmed would suggest that EUV fluxes due to accretion are the main agent foṙ M formation of the [Ne ii] line.

Mid-infrared spectra of 65 T Tauri stars (TTS) taken with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope are modeled using populations of optically thin dust at two temperatures to probe the radial variation in dust composition in the uppermost layers of protoplanetary disks. Most spectra with narrow emission features associated with crystalline silicates require Mgrich minerals and silica, but a very small number suggest other components. Spectra indicating large amounts of enstatite at higher temperatures (400-500 K) also require crystalline silicates, either enstatite or forsterite, at temperatures lower (100-200 K) than those required for spectra showing high abundance of other crystalline silicates. A few spectra show 10 µm complexes of very small equivalent width. They are fit well using abundant crystalline silicates but very few large grains, inconsistent with the expectation that low peak-to-continuum ratio of the 10 µm complex always indicates grain growth. Most spectra in our sample are fit well without using the opacities of large crystalline silicate grains. If large grains grow by agglomeration of submicron grains of all dust types, the amorphous silicate components of these aggregates must typically be more abundant than the crystalline silicate components. We also find that the more there is of one crystalline dust species, the more there is of the others. This suggests