Friday, February 10, 2017

The Role of Dust in Coud Formation in Dust in Brown Dwarfs & Exoplanets

Dust in brown dwarfs and extra-solar planets V. Cloud formation in carbon- and oxygen-rich environment


Hellig et al

Recent observations indicate potentially carbon-rich 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. 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. We extend 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. The seed formation efficiency is lower in carbon-rich atmospheres than in oxygen-rich gases due to carbon being a very effective growth species. The consequence is that less particles will make up a cloud for C/O_0>1. The cloud particles will be smaller in size than in an oxygen-rich atmosphere. An increasing initial C/O ratio does not revert this trend because a much greater abundance of condensible gas species exists in a rich carbon environment. Cloud particles are generally made of a mix of materials: carbon dominates if C/O_0>1 and silicates dominate if C/O_0<1 .="" 10.0.="" 80-90="" a="" an="" and="" are="" atmosphere.="" atmospheres="" become="" blockquote="" c2h2="" c="" carbon-rich="" carbon="" carbonaceous="" cases="" cloud="" clouds="" combination="" compositions="" continuum="" cover="" crystalline.="" depleted="" due="" extreme="" far="" featureless="" form="" gas-phase="" hcn="" height-dependent="" in="" is="" less="" made="" mixed="" molecules="" numbers.="" of="" only="" or="" oxygen-rich="" particles="" reached="" remaining="" sizes="" smooth="" than="" that="" the="" to="" tracer="" typical="" unless="" where="" with="" would="">

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