We present moderate-resolution (lambda/delta lambda approximately 1200) observations of the solid CO band in a sample of protostars. The spectra reveal two independent solid CO components along most lines of sight. One produces a narrow (delta nu approximately 5 cm-1) band generally centered at about 2140 cm-1 and the other a broader (delta nu approximately 10 cm-1) one at about 2136 cm-1. Both the peak position and width of the narrow, and generally strongest, component vary from object to object. The relative strengths of the two components vary considerably in this sample. Laboratory studies of the shape and peak position of the solid CO banD in astrophysically relevant mixtures show that the narrow CO band occurs in mixtures dominated by non-polar molecules (e.g., CO itself, CO2, O2, N2), while the broad feature is due to more polar mixtures, such as H2O ice. Calculations show that for mixtures dominated by CO (CO concentration > 0.3), the peak position and shape of the CO fundamental are strongly influenced by "surface modes," while for lower concentrations the laboratory measured absorption spectra provide very accurate representations of the small particle extinction spectrum. The observed variations in peak position and width of the interstellar 2140 cm-1 component can be attributed to variations in composition and/or physical characteristics of the grains (i.e., shape). These observations show that many lines of sight contain (at least) two independent grain mantle components: a polar mixture (H2O-rich) responsible for the 3.08 and 6.0 micrometers ice bands and a nonpolar one dominating the solid CO spectrum. These two independent grain mantle components may reflect chemical variations during accretion. Around luminous protostars, differences in volatility of the nonpolar and H2O-rich ices also may play an important role in determining their relative abundances.

No abstract

No abstract

We present late‐time near‐infrared (NIR) and optical observations of the Type IIn SN 1998S. The NIR photometry spans 333–1242 d after explosion, while the NIR and optical spectra cover 333–1191 and 305–1093 d, respectively. The NIR photometry extends to the M′ band (4.7 μm), making SN 1998S only the second ever supernova for which such a long IR wavelength has been detected. The shape and evolution of the Hα and He i 1.083‐μm line profiles indicate a powerful interaction with a progenitor wind, as well as providing evidence of dust condensation within the ejecta. The latest optical spectrum suggests that the wind had been flowing for at least 430 yr. The intensity and rise of the HK continuum towards longer wavelengths together with the relatively bright L′ and M′ magnitudes show that the NIR emission was due to hot dust newly formed in the ejecta and/or pre‐existing dust in the progenitor circumstellar medium (CSM). The NIR spectral energy distribution (SED) at about 1 yr is well described by a single‐temperature blackbody spectrum at about 1200 K. The temperature declines over subsequent epochs. After ∼2 yr, the blackbody matches are less successful, probably indicating an increasing range of temperatures in the emission regions. Fits to the SEDs achieved with blackbodies weighted with λ−1 or λ−2 emissivity are almost always less successful. Possible origins for the NIR emission are considered. Significant radioactive heating of ejecta dust is ruled out, as is shock/X‐ray‐precursor heating of CSM dust. More plausible sources are (a) an IR echo from CSM dust driven by the ultraviolet/optical peak luminosity, and (b) emission from newly‐condensed dust which formed within a cool, dense shell produced by the ejecta shock/CSM interaction. We argue that the evidence favours the condensing dust hypothesis, although an IR echo is not ruled out. Within the condensing‐dust scenario, the IR luminosity indicates the presence of at least 10−3 M⊙ of dust in the ejecta, and probably considerably more. Finally, we show that the late‐time (K–L′)0 evolution of Type II supernovae may provide a useful tool for determining the presence or absence of a massive CSM around their progenitor stars.

No abstract

Spectra of 3 micrometers emission features have been obtained at several positions within the reflection nebulae NGC 1333 SVS3 and NGC 2023. Strong variations of the relative intensities of the 3.29 micrometers feature and its most prominent satellite band at 3.40 micrometers are found. It is shown that (i) the 3.40 micrometers band is too intense with respect to the 3.29 micrometers band at certain positions to arise from hot band emission alone, (ii) the 3.40 micrometers band can be reasonably well matched by new laboratory spectra of gas-phase polycyclic aromatic hydrocarbons (PAHs) with alkyl (-CH3) side groups, and (iii) the variations in the 3.40 micrometers to 3.29 micrometers band intensity ratios are consistent with the photochemical erosion of alkylated PAHs. We conclude that the 3.40 micrometers emission feature is attributable to -CH3 side groups on PAH molecules. We predict a value of 0.5 for the peak intensity ratio of the 3.40 and 3.29 micrometers emission bands from free PAHs in the diffuse interstellar medium, which would correspond to a proportion of one methyl group for four peripheral hydrogens. We also compare the 3 micrometers spectrum of the proto-planetary nebula IRAS 05341+0852 with the spectrum of the planetary nebula IRAS 21282+5050. We suggest that a photochemical evolution of the initial aliphatic and aromatic hydrocarbon mixture formed in the outflow is responsible for the changes observed in the 3 micrometers emission spectra of these objects.

No abstract

No abstract