A&A 603, A123 (2017) Astronomy DOI: 10.1051/0004-6361/201629696 & c ESO 2017 Astrophysics Dust in brown dwarfs and extrasolar planets V. Cloud formation in carbon- and oxygen-rich environments Ch. Helling, D. Tootill, P. Woitke, and G. Lee Centre for Exoplanet Science, SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK e-mail:
[email protected] Received 12 September 2016 / Accepted 6 December 2016 ABSTRACT Context. Recent observations indicate potentially carbon-rich (C/O > 1) exoplanet atmospheres. Spectral fitting methods for brown dwarfs and exoplanets have invoked the C/O ratio as additional parameter but carbon-rich cloud formation modeling is a challenge for the models applied. The determination of the habitable zone for exoplanets requires the treatment of cloud formation in chemically different regimes. Aims. We aim to model cloud formation processes for carbon-rich exoplanetary atmospheres. Disk models show that carbon-rich or near-carbon-rich niches may emerge and cool carbon planets may trace these particular stages of planetary evolution. Methods. We extended our kinetic cloud formation model by including carbon seed formation and the formation of C[s], TiC[s], SiC[s], KCl[s], and MgS[s] by gas-surface reactions. We solved a system of dust moment equations and element conservation for a prescribed Drift-Phoenix atmosphere structure to study how a cloud structure would change with changing initial C/O0 = 0:43 ::: 10:0. Results. The seed formation efficiency is lower in carbon-rich atmospheres than in oxygen-rich gases because carbon is a very effective growth species.