Physics – Optics
Scientific paper
Aug 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt..........c&link_type=abstract
University Microfilms International, Source DAI-B 58/08, p. 4275, Feb 1998, 378 pages.
Physics
Optics
5
Dust, Long Period Variable Stars, Silicates, Mid-Infrared Spectra, Stellar Dust
Scientific paper
Silicate dust, found around oxygen-rich stars, produces a hallmark mid-infrared spectral feature resulting from the bend or stretch in the SiO_4 tetrahedron. Long Period Variable (LPV) stars on the Asymptotic Giant Branch produce copious quantities of silicate dust over this short stage of their stellar evolution. A study of 31 oxygen-rich LPVs was conducted over a sixteen month period using the University of Denver's TNTCAM. The study has been supplemented with spectral data from UKIRT CGS3 service observations and IRAS LRS archives. Observations of the silicate features in the circumstellar environment indicate a possible evolutionary sequence for the stars, inferred from the changes of the dust spectra. These new observations suggest a relationship between the dust spectral signature and the stage of the dust formation process. There is evidence that acoustic shocks from the LPV are a catalyst in the dust formation process. Follow-on work is already occuring in the form of ISO SWS data on a selected subgroup of LPVs. To enhance this study, a mid-infrared, cross-dispersed spectrometer, TGIRS, was built. TGIRS covers a wavelength range of 7 to 14 microns at a resolving power of 750. The instrument utilizes a Boeing-Rockwell Si:As BIB HFPA as its detector. It is cooled using a two-stage Gifford-McMahon cryocooler, eliminating the need for liquid cryogens. All of the optics in the system are aluminum, permitting ambient alignment and focusing with a laser. A description of the design and building phases of TGIRS is presented. Brief descriptions concerning the evolution of LPVs, theories of dust formation, and the signatures of the silicate dust are given. Data acquisition and reduction are described, including a new method for removing telluric attenuation from the data. Spectral energy distributions are shown, including graphs of the silicate features with respect to stellar phase. Finally, results of the statistical analysis of the sample and conclusions are drawn, including a new scenario for silicate dust growth.
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