Effects of Metallicity on Graphite, TiC, and SiC Condensation in Carbon Stars

Abstract

From transmission electron microscopy and other laboratory studies of presolar grains, the implicit condensation sequence of carbon-bearing condensates in circumstellar envelopes of carbon stars is (from first to last) TiC-graphite-SiC. We use thermochemical equilibrium condensation calculations and show that the condensation sequence of TiC, graphite, and SiC depends on metallicity in addition to C/O ratio and total pressure. Calculations were performed for a characteristic carbon star ratio of C/O = 1.2 from 1E-10 to 1E-4 bars total pressure and for uniform metallicity variations ranging from 0.01 to 100 times solar elemental abundances. TiC always condenses before SiC, and the carbide condensation temperatures increase with increasing metallicity and total pressure. Graphite, however, can condense in a cooling circumstellar envelope before TiC, between TiC and SiC, or after SiC, depending on the carbon-bearing gas chemistry, which is dependent on metallicity and total pressure. Analytical expressions for the graphite, TiC, and SiC condensation temperatures as functions of metallicity and total pressure are presented. The inferred sequence from laboratory presolar grain studies, TiC-graphite-SiC, is favored under equilibrium conditions at solar and subsolar metallicities between ~1E-5 to 1E-8 bar total pressure within circumstellar envelopes of carbon stars with nominal C/O = 1.2. We also explored the dependence of the sequence at C/O ratios of 1.1 and 3.0 and found that as the C/O ratio increases, the TiC-graphite-SiC region shifts towards higher total pressures and lower metallicities.

arXiv comment33 pages, 9 figures, 2 tables. Accepted to The Astrophysical Journal, March 2025

References in corpus (35)

• Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)
• Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood
• New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
• Observational constraints on the origin of the elements. IV: The standard composition of the Sun
• The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
• s-Process Nucleosynthesis in Carbon Stars
• Presolar grains from meteorites: Remnants from the early times of the solar system
• Chemical equilibrium in AGB atmospheres: Successes, failures, and prospects for small molecules, clusters, and condensates
• The growth of carbon chains in IRC+10216 mapped with ALMA
• Synthetic photometry for carbon-rich giants. IV. An extensive grid of dynamic atmosphere and wind models
• Formation of SiC Grains in Pulsation-Enhanced Dust-Driven Wind Around Carbon Rich Asymptotic Giant Branch Stars
• Effects of Metallicity on the Chemical Composition of Carbon Stars
• Is there a metallicity ceiling to form carbon stars? - A novel technique reveals a scarcity of C stars in the inner M31 disk
• Resolving the extended atmosphere and the inner wind of Mira ($o$ Ceti) with long ALMA baselines
• Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER II. Time variations between pre-maximum and minimum light
• Unusual integrated metallicity profile of our Milky Way
• Synthetic photometry for carbon-rich giants II. The effects of pulsation and circumstellar dust
• Do meteoritic silicon carbide grains originate from asymptotic giant branch stars of super-solar metallicity?
• Nucleation of small silicon carbide dust clusters in AGB stars
• Aperture synthesis imaging of the carbon AGB star R Sculptoris: Detection of a complex structure and a dominating spot on the stellar disk
• The shock-heated atmosphere of an asymptotic giant branch star resolved by ALMA
• Resolving the extended stellar atmospheres of Asymptotic Giant Branch stars at (sub-)millimetre wavelengths
• High-resolution observations of gas and dust around Mira using ALMA and SPHERE/ZIMPOL
• The Role of Acetylene in the Chemical Evolution of Carbon Complexity
• Origin of large meteoritic SiC stardust grains in metal-rich AGB stars
• Dust Formation in Astrophysical Environments: The Importance of Kinetics
• Decoding the message from meteoritic stardust silicon carbide grains
• Synthetic photometry for carbon-rich giants. III. Tracing the sequence of mass-losing galactic C-type Miras
• On the mass and metallicity distribution of parent AGB stars of presolar SiC
• The weather report from IRC+10216: Evolving irregular clouds envelop carbon star
• ALMA observations of the vibrationally-excited rotational CO transition $v=1,J=3-2$ towards five AGB stars
• Exploring the innermost dust formation region of the oxygen-rich AGB star IK~Tau with VLT/SPHERE-ZIMPOL and VLTI/AMBER
• The carbon star mystery: forty years later
• Structure, stability and optical absorption spectra of small Ti${}_n$C${}_x$ clusters: a first-principles approach
• Charting Circumstellar Chemistry of Carbon-rich AGB Stars: I. ALMA 3 mm spectral surveys

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