Photoevaporation of Disks around Newly Formed Stars

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Session 37 -- Interferometry
Display presentation, Tuesday, 9:30-6:30, Pauley Room

[37.04] Photoevaporation of Disks around Newly Formed Stars

D.Johnstone (UCB), D. Hollenbach (NASA Ames), F.H.Shu (UCB)

Newly formed stars produce sufficient Lyman continuum luminosity $\phi$ to significantly alter the structure and evolution of the accretion disk surrounding them. In the absence of a stellar wind, a nearly static, photoionized, $10^4$ K, disk atmosphere, with a scale height that increases with $r^{3/2}$, forms inside the gravitational radius $r_g \approx 10^{14} (M_* / M_{\sun})$ cm where $M_*$ is the mass of the central star. This ionized atmosphere is maintained by both the direct radiation from the central star and the diffuse field produced in the disk atmosphere by the significant fraction of hydrogen recombinations directly to the ground state. Beyond $r_g$ the material evaporated from the disk is capable of escaping from the system and produces an ionized disk wind. The mass-loss due to this disk wind peaks at $r_g$. The inclusion of a stellar wind into the basic picture reduces the height of the inner disk atmosphere and introduces a new scale radius $r_w$ where the thermal pressure of the material evaporated from the disk balances the ram pressure in the wind. In this case the mass-loss due to the disk wind peaks at $r_w$ and is enhanced over the no-wind case.

The photoevaporation of disks around newly formed stars has significance to both the UCHII problem and the dispersal of solar-type nebulae. High mass stars are intrinsically hot and thus yield sufficient Lyman luminosity to create disk mass-loss rates of order $2 \times 10^{-5} \phi_{49}^{1/2} M_{\sun} {\rm yr}^{-1}$, where $\phi_{49} = \phi/(10^{49} $ Lyman continuum photons $s^{-1})$ even without a stellar wind. This wind which will last for $\sim 10^{5}$ yrs if the disk mass is $M_d \sim 0.3 M_*$, yields sizes, emission measures and ages consistent with observations of UCHIIs. On the other end of the stellar scale, many newly formed low-mass stars are known to have enhanced extreme ultraviolet luminosity suggested to be due to boundary layer accretion. Assuming that the sun had such an enhanced Lyman luminosity $\phi \approx 10^{41}$ s$^{-1}$, for $\sim 3\times 10^7$ yrs it is possible to remove all of the gas beyond the orbit of Saturn, $r_g$ for the sun, associated with the minimum solar nebula. This process also has implications for the formation of the giant planets.

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