Photoevaporation

Photoevaporation denotes the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. This typically refers to an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.^[1][2][3]

Molecular clouds

Eagle nebula pillars being photoevaporated

One of the most obvious manifestations of astrophysical photoevaporation is seen in the eroding structures of molecular clouds as luminous stars are born within.^[4]

Planets and atmospheres

A planet can be stripped of its atmosphere (or parts of the atmosphere) due to high energy photons and other electromagnetic radiation. If a photon interacts with an atmospheric molecule, the molecule is accelerated and its temperature increased. If sufficient energy is provided, the molecule or atom may reach the escape velocity of the planet and "evaporate" into space. The lower the mass number of the gas, the higher the velocity obtained by interaction with a photon. Thus hydrogen is the gas which is most prone to photoevaporation.

Protoplanetary disks

Photoevaporation occurring to a protoplanetary disk due to the presence of a nearby O-type star

Protoplanetary disks can be dispersed by stellar wind and heating due to incident electromagnetic radiation. The radiation interacts with matter and thus accelerates it outwards. This effect is only noticeable when there is sufficient radiation strength, such as coming from nearby O and B type stars or when the central protostar commences nuclear fusion.

The disk is composed of gas and dust. The gas, consisting mostly of light elements such as hydrogen and helium, is mainly affected by the effect, causing the ratio between dust and gas to increase.

Radiation from the central star excites particles in the accretion disk. The irradiation of the disk gives rise to a stability length scale known as the gravitational radius ($r_{g}$). Outside of the gravitational radius, particles can become sufficiently excited to escape the gravity of the disk, and evaporate. After 10⁶ – 10⁷ years, the viscous accretion rates fall below the photoevaporation rates at $r_{g}$. A gap then opens around $r_{g}$, the inner disk drains onto the central star, or spreads to $r_{g}$ and evaporates. An inner hole extending to $r_{g}$ is produced. Once an inner hole forms, the outer disk is very rapidly cleared.

The formula for the gravitational radius of the disk is^[5]

$r_{g}={\frac {\left(\gamma -1\right)}{2\gamma }}{\frac {GM\mu }{k_{B}T}}\approx 1.4{\frac {\left(M/M_{\odot }\right)}{\left(T/10^{4}\ {{\rm {K}}}\right)}}\ {{\rm {AU}}},\!$

where $\gamma$ is the ratio of specific heats (= 5/3 for a monatomic gas), $G$ the universal gravitational constant, $M$ the mass of the central star, $M_\odot$ the mass of the Sun, $\mu$ the mean weight of the gas, $k_{B}$ Boltzmann constant, $T$ is the temperature of the gas and AU the Astronomical Unit.

Because of this effect, the presence of massive stars in a star-forming region is thought to have a great effect on planet formation from the disk around a young stellar object, though it is not yet clear if this effect decelerates or accelerates it.

References

1. Mellema, G.; Raga, A. C.; Canto, J.; Lundqvist, P.; Balick, B.; Steffen, W.; Noriega-Crespo, A. (1998). "Photo-evaporation of clumps in planetary nebulae". Astronomy and Astrophysics. 331: 335. arXiv:astro-ph/9710205. Bibcode:1998A&A...331..335M.
2. Owen, James E.; Ercolano, Barbara; Clarke, Cathie J. (2011). "Protoplanetary disc evolution and dispersal: The implications of X-ray photoevaporation". Monthly Notices of the Royal Astronomical Society. 412: 13. arXiv:1010.0826. Bibcode:2011MNRAS.412...13O. doi:10.1111/j.1365-2966.2010.17818.x.
3. Wu, Yanqin; Lithwick, Yoram (2013). "Density and Eccentricity of Kepler Planets". The Astrophysical Journal. 772: 74. arXiv:1210.7810. Bibcode:2013ApJ...772...74W. doi:10.1088/0004-637X/772/1/74.
4. Hester, J. J.; Scowen, P. A.; Sankrit, R.; Lauer, T. R.; Ajhar, E. A.; Baum, W. A.; Code, A.; Currie, D. G.; Danielson, G. E.; Ewald, S. P.; Faber, S. M.; Grillmair, C. J.; Groth, E. J.; Holtzman, J. A.; Hunter, D. A.; Kristian, J.; Light, R. M.; Lynds, C. R.; Monet, D. G.; O'Neil, E. J.; Shaya, E. J.; Seidelmann, P. K.; Westphal, J. A. (1996). "Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects". Astronomical Journal. 111: 2349. Bibcode:1996AJ....111.2349H. doi:10.1086/117968.
5. Liffman, Kurt (2003). "The Gravitational Radius of an Irradiated Disk". Publications of the Astronomical Society of Australia. 20 (4): 337. Bibcode:2003PASA...20..337L. doi:10.1071/AS03019.