Evolution of Photoevaporating Protoplanetary Disks

Astronomy and Astrophysics – Astronomy

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Scientific paper

We model the evolution of protoplanetary disks under the influence of viscous accretion and photoevaporation by the central star. Previous studies are extended by considering the evolution of disks around different types of parent stars in which extrasolar planets have been discovered. We consider stellar masses in the range 1 to 4.7 solar masses, and extreme ultraviolet (EUV) fluxes in the range 1040 to 1043 photons/second. The disk evolves on the viscous diffusion time scale at the gravitational radius, the disk location inside which ionized hydrogen is gravitationally bound to the central star. Photoevaporation is initially powered by the diffuse EUV flux arising from recombinations in the gravitationally bound region. Following the analysis of Alexander et al. (2006), we include the direct contribution from the direct EUV flux as the gravitationally bound region is removed by the combination of viscous diffusion and photoevaporation. The disk is removed in ≈ 70 Myr for a 4.7 M&sun; central star and a EUV flux ≈ 1040. Increasing the EUV flux results in shorter disk life times, as expected. Reducing the stellar mass results in shorter disk lifetimes for two reasons. First, photoevaporation is more easily accomplished since material is less tightly bound to the central star (the gravitational radius moves inward). Second, the viscous diffusion time scale at the smaller gravitational radius decreases, speeding up the overall disk evolution. This work is supported by an NSF grant.

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