Photoevaporation
The term is typically used in 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] One of the most obvious manifestations of astrophysical photoevaporation is seen in the eroding structures of molecular clouds that luminous stars are born within.
[4] Evaporating gaseous globules or EGGs were first discovered in the Eagle Nebula.
These small cometary globules are being photoevaporated by the stars in the nearby cluster.
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.
Photoevaporation is the likely cause of the small planet radius gap.
Material from a possible evaporating planet around WD J0914+1914 might be responsible for the gaseous disk around this white dwarf.
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 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 (
Outside of the gravitational radius, particles can become sufficiently excited to escape the gravity of the disk, and evaporate.
Once an inner hole forms, the outer disk is very rapidly cleared.
is the ratio of specific heats (= 5/3 for a monatomic gas),
If the hydrogen atom is ionized, i.e., it is a proton, and is in a strong magnetic field then
This is summarised in the Table 1 , where we see that different gases may have different gravitational radii.
Table 1: Gravitational radius coefficient as a function of the degrees of freedom.
The most famous region containing photoevaporated protoplanetary disks is the Orion Nebula.
They were called bright proplyds and since then the term was used for other regions to describe photoevaporation of protoplanetary disks.
[8] There might even be a planetary-mass object in the Orion Nebula that is being photoevaporated by θ 1 Ori C.[9] Since then HST did observe other young star clusters and found bright proplyds in the Lagoon Nebula,[10] the Trifid Nebula,[11] Pismis 24[12] and NGC 1977.
[13] After the launch of the Spitzer Space Telescope additional observations revealed dusty cometary tails around young cluster members in NGC 2244, IC 1396 and NGC 2264.
These dusty tails are also explained by photoevaporation of the proto-planetary disk.
[17] Other bright proplyd candidates were found in the Carina Nebula with the CTIO 4m and near Sagittarius A* with the VLA.
[18][19] Follow-up observations of a proplyd candidate in the Carina Nebula with Hubble revealed that it is likely an evaporating gaseous globule.
[20] Objects in NGC 3603 and later in Cygnus OB2 were proposed as intermediate massive versions of the bright proplyds found in the Orion Nebula.
