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Stephan D. Price, Kathleen E. Kraemer
(Air Force Research Lab., Hanscom AFB)
Irene R. Little-Marenin (Wellesley and Colorado)
Paul D. LeVan (Air Force Research Lab., Kirtland AFB)
This project started with the large sample of infrared spectra obtained by the Infrared Astronomical Satellite (IRAS) in 1982 and 1983 and continued with spectra from the Infrared Space Observatory (ISO), which flew from 1996 to 1997. For a good review of our work with IRAS data, see Sloan, Little-Marenin, & Price (1996b). For the first report of the Silicate dust sequence, see Sloan et al. (1995).
Our use of the databases of infrared spectra produced by IRAS and ISO have revealed several important relations between the properties of the central star and the dust produced as that star ejects its outer layers into space. These correlations help constrain the poorly understood mechanisms responsible for mass loss and dust formation. Solving these mysteries are critical to understanding such fundamental questions as how much material enriched by nuclear fusion stars return to space as they die and their ultimate fate as white dwarfs and planetary nebulae, supernovae and neutron stars, or black holes. The IRAS and ISO are far from exhausted; they undoubtedly contain many more useful clues waiting to be uncovered.
As a graduate student at Wyoming, I collaborated with Paul LeVan of the Air Force Geophysics Lab. (AFGL), helping him develop his mid-infrared long-slit spectrometer (known as GLADYS) for use at the Wyoming Infrared Observatory (WIRO). One of his projects was an effort to confirm from the ground the existence of a 13 µm emission feature seen in the spectra of some oxygen-rich dust shells by the Infared Astronomical Satellite (IRAS), which had first been reported by Little-Marenin and Price (1986). When I joined Paul as a post-doc at Phillips Lab. (AFGL had been renamed), I took a careful look through the IRAS database of Low-Resolution Spectra (LRS) just to satisfy my curiosity about how many 13 µm sources there might be and what kind of sources they were.
After identifying several dozen spectra with 13 µm emission features, we decided to pursue the project, presenting our initial results at the Berkeley Meeting of the American Astronomical Society (LeVan et al. 1993). A more careful analysis required a comparison with other types of oxygen-rich dust, leading to a useful detour (next section), where we developed a new classification method for oxygen-rich dust spectra. We applied this method to the 13 µm sources and uncovered some important clues (Sloan, LeVan, & Little-Marenin 1996a).
To sum our results up in brief:
In order to compare the spectra from 13 µm sources with spectra from other oxygen-rich dust sources, we decided to expand the number of sources classified using the methods developed by Little-Marenin and Little (1988, 1990). Our expansion led to a new classification system, for which we presented early results at the Twentieth Anniversary Meeting of the Kuiper Airborne Observatory in California in 1994 (Sloan et al. 1995). The refereed paper came out soon after (Sloan and Price 1995).
Little-Marenin and Little (1988, 1990) used the novel approach of removing the stellar contribution to the spectrum and classifying only the contribution from the dust. The classic silicate emission feature at 10 µm had been known for nearly two decades at this point. It dominated most oxygen-rich dust spectra, so Little-Marenin and Little's discovery that a large fraction of oxygen-rich dust spectra showed other spectral features came as a surprise. These included the broad emission feature now believed to result from alumina dust that peaks ~11-12 µm, structured silicate emission, where an 11 µm feature accompanies the 10 µm feature, and as mentioned above, the 13 µm feature.
Our classification method used flux ratios (colors) from the dust component of the spectrum to order the various types of spectra seen. We found that in a plot of the ratios at 10, 11, and 12 µm (F10/F12 vs. F11/F12), the spectra lined up neatly in what have identified as the silicate dust sequence (see the figures in Sloan et al. 1996b).
The initial sample of Sloan and Price (1995) included only those sources classified as variable stars associated with the asymptotic giant branch (AGB). These include Mira variables, semi-regular variables of types SRb and SRa, and irregular variables of type Lb. Despite the differences in pulsational behavior of the central stars, there are surprisingly no obvious differences in the nature of the dust they produce (except for the 13 µm feature).
We have most recently published the individual classifications, along with the expansion of our database to include supergiants and S stars (Sloan and Price 1998). This paper showed that different samples of stars produce different classes of dust spectra. For example, supergiants usually have spectra with the classic narrow silicate feature. The S stars are much more likely to show broad emission (from alumina dust) than most other sources, and we think we know why. The atmospheres of most S stars have slightly more oxygen than carbon, so the formation of carbon monoxide will not leave much oxygen to form into dust. Normally, the oxygen will condense into alumina dust first, then the remaining oxygen will condense into silicate dust. Since there's a lot more silicon in stellar atmospheres than aluminum, the silicate dust usually dominates the alumina dust. But in S stars, there's such a shortage of free oxygen after the formation of CO, the alumina dust will leave no oxygen to bind with the silicon, so that's all you get. In other words, we think that the silicate dust sequence might simply reflect the chemical abundances of the atmosphere of the central star.
We have also expanded our sample of stars classified to include the carbon stars, which produce very different infrared dust spectra. Little-Marenin et al. (2000) presented initial results at the IAU Symposium on carbon stars in Turkey in 1996, and the final results (Sloan et al. 1998) were published in AJ in February, 1998.
The spectral properties of carbon-rich dust correlate well with the pulsational properties of the central star. Miras tend to produce thick dust shells which consist of both amorphous carbon and silicon carbide. Semi-regular variables, however, have much thinner shells without much amorphous carbon. Instead, the 11.3 µm feature from SiC is usually accompanied by additional spectral structure in the 8-9 µm region. In our paper, we suggested that this was an apparent emission feature ~8.5-9.0 µm, but more recent satellite data with better coverage at shorter wavelengths indicate that we were probably observing an absorption feature at 8 µm produced by carbon compounds in the stellar atmosphere.
LeVan, P.D., Sloan, G.C., & Little-Marenin, I.R. 1993, BAAS, 25, 877.
Little-Marenin, I.R., & Little, S.J. 1988, ApJ, 333, 305.
Little-Marenin, I.R., & Little, S.J. 1990, AJ, 99, 1173.
Little-Marenin, I.R., & Price, S.D. 1986, Summer School on Interstellar Processes, 137.
Little-Marenin, I.R., Sloan, G.C., & Price, S.D. 2000 , in IAU Symposium 177: The Carbon Star Phenomenon, ed. R.F. Wing, 559.
Sloan, G.C., et al. 2003, ApJ, 594,, 483.
Sloan, G.C., LeVan, P.D., & Little-Marenin, I.R. 1996a , ApJ, 463, 310.
Sloan, G.C., Little-Marenin, I.R., & Price, S.D. 1996b , in From Stardust to Planetesimals: Contributed Papers ed. M.E. Kress, A.G.G.M. Tielens, & Y. Pendleton (NASA-CP 3343), 65.
Sloan, G.C., Little-Marenin, I.R., & Price, S.D. 1998, AJ, 115, 809.
Sloan, G.C., Price, S.D., Little-Marenin, I.R., & LeVan, P.D., 1995, in Airborne Astronomy Symposium on the Galactic Ecosystem: From Gas to Stars to Dust, ed. M.R. Haas, J.A. Davidson, & E.F. Erickson (San Francisco: ASP), 425.
Sloan, G.C., & Price, S.D. 1995, ApJ, 451, 758.
Sloan, G.C., & Price, S.D. 1998, ApJS, 119, 141.
Last modified 27 May, 2008. © Gregory C. Sloan.