FUV-Driven Photoevaporation of protoplanetary Disks
Final Report Abstract
The question of how planets are formed is one of the key unsolved questions of astronomy. We know that they are formed as a by-product of star formation. When a star is formed from the collapse of an interstellar cloud, the excess angular momentum of that cloud causes the newly formed star to be surrounded by a so-called protoplanetary disk consisting of gas and dust. It is in such a disk that rocky and gaseous planets are formed. Current theories of planets formation predict that it may take several millions of years for planets to form in such a disk. Since we know that most stars have planets, this implies that these disks must live at least a few millions of years before they are destroyed. The main destruction process is photoevaporation. Ultraviolet (UV) and X-ray photons from the star impinge on the surface of the disk, heating the gas to high temperatures (close to or above the virial temperature), and thus causing this gase to hydrodynamically flow out of the system. The next layer is then heated, and flows away, etc. In this “peeling off” manner, the disk can be rapidly destroyed. Two kinds of UV radiation play a role. The most studied is EUV radiation, causing the gas to be ionized. This mechanism of heating is relatively easy to implement in models, and the resulting photoevaporation is well undertood. The other kind is FUV radiation, which is less energetic than EUV radiation. This heats up the disk though complex physics, chemistry and radiative transfer. The physics and chemistry is similar to that of so-called “photon-dominated regions” (PDRs). The radiative transfer is, however, much more complex because of the complex geometry of the disk and the photons impinging on it under a very narrow angle (because they originate from the central star). Effects such as shadowing and 2-D radiative transfer, coupled to hydrodynamics, play a key role. This process has only been studied using simple 1+1D radiative transfer methods and with very simplified hydrodynamics. The goal of this project is/was to study the 2-D (i.e. axially symmetric 3-D) radiationhydrodynamics of this problem. This adds both the self-consistent radiative transfer as well as the self-consistent hydrodynamics to the problem. We then wish to see how this affects the disk evolution and how it is affected by (and how it affects) the dust evolution. The project has been succesful so far, but has not yet reached a conclusion. We have developed the required machinery and produced first results. We have managed to develop a fully 2-D 3-temperature radiation hydrodynamics code where the radiative transfer includes both dust continuum radiative transfer as well as the FUV PDR radiative transfer. The model involves a simplified fast PDR chemistry model developed by Simon Bruderer. This code is unique in that it is the first disk radiation hydrodynamics code with 3-temperature radiative transfer, with dust and PDR radiative transfer and with (simplified) PDR chemistry. The development of this code was a challenge, and cost us more time than initially planned. But its development is a success and a paper has been published on this method. The modeling of the FUV photoevaporation, using this code, is currently in progress. Initial results are shown in this report. Another success is the study of the effect of (and the effect on) dust growth within the disk. Although initially planned within the context of the full 2-D model, it was instead done within the 1+1D framework of Gorti, Dullemond & Hollenbach (2009). We implemented the dust growth & fragmentation model of Birnstiel, Ormel & Dullemond (2011) and its effect on the PDR physics. We find that the dust growth does not have a big effect on the FUV photoevaporation (and thus the disk life times), but reversely that the gas depletion affects the dust coagulation & fragmentation physics. It enhances the dust-to-gas ratio, which affects the ability of the disk to form planets. In conclusion: the project is well developed and first results have been achieved. Further work is, however, still necessary to fully achieve the goals. Dr. Ramsey (currently postdoc in Copenhagen) and I are currently working on this.
Publications
- “AZEuS: An Adaptive Zone Eulerian Scheme for Computational Magnetohydrodynamics”, 2012, ApJ Supplements, 199, 13
Ramsey, J.P., Clarke, D.A., Men’shchikov, A.B.
- “A quantification of hydrodynamical effects on protoplanetary dust growth”, 2013, A&A, 560, 96
Sellentin, E., Ramsey, J.P., Windmark, F., Dullemond, C.P.
(See online at https://doi.org/10.1051/0004-6361/201321587) - , “Radiation hydrodynamics including irradiation and adaptive mesh refinement with AZEuS. I. Methods”, 2014, A&A
Ramsey, J.P., Dullemond, C.P.
(See online at https://doi.org/10.1051/0004-6361/201424954) - “The Impact of Dust Evolution and Photoevaporation on Disk Dispersal”, ApJ The Astrophysical Journal, Volume 804, Number 1, S. 29. Apr 2015
Gorti, U., Hollenbach, D., Dullemond, C.P.
(See online at https://doi.org/10.1088/0004-637X/804/1/29)
