It is now well established that meteorites contain reduced presolar grains, such as graphite and silicon carbide (SiC), which are probably formed by condensation of dust in the circumstellar envelopes of carbon-rich AGB stars. Here we model condensation in envelopes of carbon stars, with an emphasis on trace elements. Since absolute elemental abundances in stellar atmospheres are generally not known, we assume solar abundances (Anders and Grevesse 1989), except for carbon. A C/O ratio of 2, consistent with the mean and median values of 2.1 and 1.8 respectively, for 61 carbon stars (Gow 1977) was used. The C/O ratio was increased by adding carbon because astrophysicists believe that carbon produced in helium-burning zones may be mixed to the surfaces of C stars (e.g. Lucy 1976). We used physical parameters for the circumstellar shell of the high mass-loss rate, prototypical carbon star IRC +10216 (e.g. Keady et al. 1988, Dominik et al. 1990) and theoretical considerations by Salpeter (1974a,b) to construct a P-T-model of the envelope (see Fig. 1). Thermodynamic equilibrium condensation calculations for a reduced gas include ~600 gaseous and solid compounds of the elements H, C, N, O, S, P, F, Cl, Fe, Mg, Al, Ti, Si, Ca, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and REE. Refractory oxides, sulfides, nitrides, and carbides were considered as condensates. The calculations were done from T = 800 to 2500 K, and P= 10^-5 to 10^-13 bars. The effects of nucleation on condensation temperatures were calculated using the nucleation model discussed by Salpeter (1974a,b) and Cameron and Fegley (1982). The temperature drop required for condensation depends on (P,T, density) in the expanding envelope and also on the abundance, density, and surface energy (Es) of the nucleating compound. The range of E(sub)s values for NaCl-type carbides are about 800-1700 erg/cm^2 (Livey & Murray 1956); however, these data are generally poorly known. Another important variable is the sticking coefficient (s), taken = 10^-3 here. Results of the equilibrium condensation calculations as a function of pressure at C/O = 2 are shown in Table 1 and Fig. 1. The initial major element condensates are graphite, TiC, SiC, Fe3C, AlN, and CaS (Table 1). The data for C(sub)GR TiC, and SiC are also shown in Fig. 1, together with the P-T profile for the carbon star IRC +10216. Also included are the condensation temperatures if nucleation constraints are applied (dotted lines). Neglecting nucleation effects, C(sub)Gr, TiC, and SiC would be present within 2-3 stellar radii from the photosphere (r/R = 1). With nucleation constraints, TiC and SiC form at lower T at a distance of about 5 stellar radii. The T-drop required for graphite condensation is only about 100 K lower than the equilibrium condensation temperatures at higher P. Therefore, graphite grains would be stable at r/R >1.5. We note that at r = 3-5 R there is observational evidence for SiC, graphite and amorphous carbon in the envelope of the C star IRC +10216 (e.g. Keady et al. 1988, Ridgway and Keady 1988). Of the nitrides, AlN is the only which forms initially. Because of its structural similarity to SiC and TiC one could expect formation of AlN solid solutions with NaCl-type carbides. Most trace elements initially form carbides. The most refractory carbides are TaC, WC, NbC, ZrC, and HfC, condensing about 100-250 K higher than TiC. E(sub)s data are available for TaC and ZrC. Nucleation constraints show that only ZrC would form prior to TiC. Other trace element carbides (Mo(sub)2C, MoC, VC(sub)0.88, YC(sub)2, Cr(sub)3C(sub)2) condense as pure compounds below the equilibrium condensation temperatures of C(sub)Gr, TiC, and SiC. However, they may condense in solid solution in TiC or SiC or in both if allowed by their crystal structures. In any case, nitrides are not initial condensates for these trace elements. However, because the carbides and nitrides can form solid solutions, one could expect carbide-nitride solutions. References: Anders E. and Grevesse N. (1989) Geochim. Cosmochim. Acta 53, 197-214. Cameron A.G.W. and M.B. Fegley (1982) Icarus 52, 1-13. Dominik C., Gail H.P., Sedlmayr E., and Winters J.M. (1990) Astron. Astrophys. 240, 365-375. Gow C.E. (1977) Pub. Astron. Soc. Pac. 89, 510-518. Keady J.J., Hall D.N.B. and Ridgway S. T. (1988) Ap. J. 326, 832-842. Livey D.T. and Murray P. (1956) J. Am. Cer. Soc. 39, 363-372. Lucy L. B. (1976) Ap. J. 205, 482-491. Ridgway S. T. and Keady J. J. (1988) Ap. J. 326, 843-858. Salpeter E. E. (1974a) Ap. J. 193, 579-584. Salpeter E. E (1974b) Ap.J. 193, 585-592. Figure 1, which in the hard copy appears here, shows condensation temperatures of graphite, TiC, and SiC at various pressures. Solid lines: equilibrium condensation; dotted lines: nucleation constraints considered. The P-T profile of the carbon star IRC +10216 and the radial pressure variation from the photosphere are also indicated.
- Pub Date:
- July 1992