Starlight as a Solvent: EUV and X-Ray Photoevaporation Rates, Launch Radii and Disc Dispersal Times

A young star irradiates its own disc with ultraviolet and X-ray photons energetic enough to heat surface gas until it escapes. This paper computes the launch geometry and the mass-loss rates for the two main drivers and converts them into dispersal times. Ionised gas at 10^4 K has a gravitational radius of 8.9 AU around a solar-mass star, inside which it stays bound and outside which it flows away; the EUV wind is launched mainly beyond about 0.9 AU. The EUV mass-loss rate at an ionising photon luminosity of 1e41 per second is 4.0e-10 solar masses per year, clearing a 0.01-solar-mass disc in 25 Myr acting alone — too slow. The X-ray rate at L_X = 1e30 erg per second is 6.0e-09 solar masses per year, fifteen times faster, clearing the same disc in 1.7 Myr and a 0.003-solar-mass disc in 0.5 Myr. Photoevaporation can therefore end disc lifetimes on the observed few-Myr scale only in its X-ray (or FUV) form and only once accretion has already thinned the disc; as a sole agent acting on a young massive disc it fails by an order of magnitude, which is why dispersal is a two-stage process. Computed from stated assumptions, not measured.