The Different Evolution of Gas and Dust in Disks around Sun-like and Cool Stars

Planet formation is profoundly impacted by the properties of protoplanetary disks and their central star. However, how disk properties vary with stellar parameters remain poorly known. Here we present the first comprehensive, comparative Spitzer/IRS study of the dust and gas properties of disks around young sun-like stars and cool stars/brown dwarfs. The comparison of these two large samples of over 60 sources reveal major differences in the evolution of both the dust and gas components. We report the first detection of organic molecules in disks around brown dwarfs. The detection rate statistics and the line flux ratios of HCN and C2H2 show a striking difference between the two samples, demonstrating a significant under-abundance of HCN relative to C2H2 in the disk surface of cool stars. We propose this to originate from the large difference in the UV-irradiation around the two types of sources. The statistical comparison of the 10μm silicate emission features also reveals a significant difference between the two samples. Cool stars and brown dwarfs show weaker features arising from more processed silicate grains in the disk atmosphere. These findings complement previous indications of flatter disk structures and longer disk lifetimes around cool stars. Our results highlight important differences in the chemical and physical evolution of protoplanetary disks as function of stellar mass, temperature, and radiation field which should be taken into account in planet formation models. We note that the different chemistry of pre-planetary materials in the disk may also influence the bulk composition and volatile content of the forming planets. In particular, if exogenous HCN has played a key role in the synthesis of prebiotic molecules on Earth as proposed, then prebiotic chemistry may unfold differently on planets around cool stars.