Robert G. Smith

Absorption features in the 3 micron spectra of protostars

Low-resolution spectra have been obtained for a selection of infrared protostars and one object located behind the Taurus dark cloud. Most of the differences in the spectra can be attributed to different H{sub 2}O ice temperatures combined with additional broad absorption between 3.3 and 3.5 microns plus another absorption in the 2.8-2.9 micron range. An NH{sub 3}-H{sub 2}O ice mixture, scattering by H{sub 2}O ice-coated grains, and hydrated silicates are ruled out as explanations for the last type of absorption. The most plausible explanation is still some form of hydrocarbon in the grain mantles. 59 refs.

High-resolution spectra of the 3.29 micron interstellar emission feature - A summary

High spectral resolution observations of the 3.29-micron interstellar emission feature show two types of profiles. Type 1 has a central wavelength of 3.289-micron and is observed in extended objects such as planetary nebulae and H II regions. Type 2 has a central wavelength of 3.296 microns and is observed around a small number of stellar sources. Type 2 has a full width at half-maximum of 0.020 micron; Type 1 has a broader FWHM, perhaps as much as 0.042 micron, but this is uncertain because of contamination by Pf(delta) emission. These profiles are tabulated for comparison to laboratory data. It is found that no proposed identification for the 3.29-micron emission feature definitely matches the observational spectra, although amorphous aromatic materials and heated polycyclic aromatic hydrocarbons tend to fit the best.

Volatiles on Triton: the infrared spectral evidence, 2.0-2.5 μm

Abstract We report new infraredspectra of Triton in which the methane band at 2.3 μm is shown in greater detail than in previous data. The spectral band at 2.15 μm reported by D. P. Cruikshank, R. H. Brown, and R. N. Clark (1984, Icarus 58, 293–305) is also present. Comparisons are made with methane ice spectra and with laboratory spectra of methane dissolved in liquid nitrogen. Based on these comparisons, we conclude that the 2.3-μm band in Tritons spectrum cannot be attributed entirely to methane gas; the methane also exists either in the solid state or possibly as a weak solute nitrogen. If the band is caused by methane frost, the grain size must be exceedingly small. The strength of the 2.3-μm band does not show an obvious correlation with the satellites orbital position in the new data set, but it is notably weaker than it was in data obtained at lower spectral resolution in 1980. The new spectral data support the earlier evidence for both methane and molecular nitrogen on Triton, but the physical state of these constituents remains uncertain.

Search for volatiles on icy satellites: I. Europa

Abstract New reflectance spectra have been obtained for both the leading and trailing sides of Europa, using the Cooled Grating Array Spectrometer (CGAS) of the NASA Infrared Telescope Facility (IRTF). The spectra are of higher precision than any yet obtained. Spectra of Europas trailing side (central meridian longitude ≈300°) obtained in 1985 show two weak absorptions near 2.2 and 2.3 μm. Both of these features as well as others are seen in spectra obtained by R. N. Clark, R. B. Singer, P. D. Owensby, and F.P. Fanale (1980a, Bull. Amer. Astron. Soc. 12, 713–714) at similar central meridian longitude. Data obtained with an improved detector array in 1986, however, do not show the absorptions seen in the 1980 and 1985 spectra. It is not clear why the newest data do not show the apparent absorptions seen in previous years, but the suggestion is that either the 1980 and 1985 data are spurious or that the material responsible for the weak absorptions is no longer detectable. Analysis of the 1980 and 1985 data did not reveal any obvious source of systematic error capable of introducing spurious features, but we are skeptical of any explanation that cites transient deposition, movement, and/or destruction of material on Europas trailing side to account for the nondetection of the features in the 1986 data. If the weak absorptions seen in the 1980 and 1985 data are real, they can be interpreted as indicating the transient spectroscopic presence of a molecular component on Europas trailing side different from the water ice that is known to be the dominant surface constituent. Further monitoring is required to determine if the apparent absorptions are real.

A study of H2O ice in the 3 micron spectrum of OH 231.8+4.2 (OH 0739-14)

Moderate resolution IR spectra of OH 231.8+4.2 in the 2.0-3.8-micron spectral region are presented. It is shown that the large-scale features of the observed spectra can only be modeled with an unusually large upper limit to the grain-size distribution (0.6-0.7 microns). Two new absorption features have been found in the wings of the 3.08-micron feature, near 2.95 and 3.2 microns. The results suggest that a mixture of amorphous and crystalline ice may have produced the 3.08-micron feature in this object. 37 references.

Detection of a new emission band at 2.8 µm in comet P/Halley

A 2.8- to 3.7-gm spectrum of comet Halley taken in May 1986 is presented. The spectrum shows (1) the 3.36-gm emission feature that most likely arises from carbonaceous material, (2) a previously unknown emission band at 2.8-2.9 gm, and (3) no evidence for the 3-gm water-ice band either in reflection or absorption. The 2.8- to 2.9-gm emission feature does not arise from infrared fluorescence from water or OH. Since LTE emission from water is unlikely, the nature of this feature is uncertain.

3 micron spectroscopy of comet Halley (1982i)

Near-infrared spectroscopy and photometry was obtained of comet Halley in December 1985 when its heliocentric distance was 1.19 AU. No water ice absorption or molecular band emission were observed. The interstellar 3.3-micron emission band was also not observed. The 3-micron spectrum of comet Halley consisted of scattered sunlight at wavelengths less than 3.1 microns and of thermal emission by dust at longer wavelengths. 29 references.

High Spectral Resolution Observation of the 3.3μm Emission Band and Comparison with Laboratory-Synthesized Quenched Carbonaceous Composite (QCC)

New high spectral resolution(λ/Δλ=1400) observations by a cooled grating array spectrometer are reported of the 3μm emission features of NGC7027, BD+303639, and HD44179. These data provide an accurate measurement of the feature’s center and width for the first time. The spectrum of laboratory-synthesized quenched carbonaceous composite (QCC) taken by the same spectrometer is also reported together with the comparison with the observed spectra.