Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.387l&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 387
Other
5
Carbide, Carbon Stars, Interstellar Dust, Trace Elements
Scientific paper
Last year we reported major- and trace-element condensation chemistry in the circumstellar envelope (CSE) of the well known carbon-star IRC+10216 [1]. Here we present results of the most comprehensive study done to date for major- and trace-element chemistry in CSEs of C stars, considering wide ranges in pressure (P), temperature (T), and elemental abundances (s-process enhancements and variable C/O and C/N ratios). These calculations are helpful for interpreting astronomical observations of gas-phase abundances and dust formation in CSEs and the chemistry of graphite, TiC, and SiC grains found in meteorites. Parameters: The present results cover ranges of P = 10^-2 to 10^-15 bar and T < 3000 K. Carbon to oxygen (C/O) ratios of 1 to 10 are considered. Gow [2] reported C/O ratios of 1-10 in 61 C stars with a mean C/O ratio of 2. However, Lambert et al. [3] found C/O = 1.01-1.76 with a mean C/O ratio of 1.15 +/- 0.17 for 30 C stars that were also considered by Gow [2]. Major elements other than C have solar abundances [3], but s-process element abundances may be increased up to 100X solar [4]. Major-Element Condensates: Figure 1 illustrates graphite, TiC, and SiC condensation surfaces as a function of C/O ratio and P. The condensation sequence is very sensitive to C/O ratio and total pressure. At C/O > 2, over the whole pressure range considered, graphite condenses first. Then condensation temperatures of later condensates (e.g., TiC, SiC) are independent of the C/O ratio. However, at C/O = 2 and P < 3 X 10^-3 bar TiC condenses prior to graphite. At C/O = 1.05, the condensation sequence is more sensitive to pressure: At P < 3 X 10^-7 bar the sequence is C(sub)Gr, TiC, SiC, between 3 X 10^-7 < P < 3.4 X 10^-5 bar it changes to TiC, C(sub)Gr, SiC, and it becomes TiC, SiC, C(sub)Gr at P > 3.4 X 10^-5 bar. Trace-Element Condensation: At a given C/O ratio and P, the condensation temperatures of C(sub)Gr, TiC, and SiC provide boundaries for the classification of the condensation behavior of trace elements. For example, elements condensing prior to the first major condensate (at low C/O) or between the first and second major condensate behave coherently. In this respect Ta, Nb, W, Zr, and Hf are classified as extremely refractory. Highly refractory carbides (Mo, V) condense after the first condensate(s) but always prior to SiC. Refractory carbides (Y, Cr) condense after C(sub)Gr, TiC, and SiC. At constant C/O and P, condensation temperatures of s-process elements depend on the abundance enhancements (f(sub)abu) as: 1/T(sub)cond = A + B X log f(sub)abu. For example, an enhancement of 100X solar will increase the condensation temperatures of Zr, Mo, or Y by about 100-150 K (depending on total P). In this case, Mo behaves as an extremely refractory element and pure YC(sub)2 condenses closer to SiC. References: [1] Lodders K. and Fegley B. (1992) Meteoritics, 27, 250-251. [2] Gow C. E. (1977) Publ. Astron. Soc. Pac., 89, 510-518. [3] Lambert D. L. et al. (1986) Astrophys. J. Suppl., 62, 373-425. [4] Utsumi K. (1985) Proc. Japan Acad., 61B, 193-196. Fig. 1 appears here in the hard copy.
Fegley Bruce Jr.
Lodders Katharina
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