Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes:
* ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...
from hot
star
A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s acts on clouds of material such as
molecular cloud
A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
s,
protoplanetary disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...
s, or planetary
atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
s.
Molecular clouds

One of the most obvious manifestations of astrophysical photoevaporation is seen in the eroding structures of molecular clouds that luminous stars are born within.
Evaporating gaseous globules (EGGs)
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. EGGs are places of ongoing star-formation.
Planetary atmospheres
A
planet
A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
can be stripped of its atmosphere (or parts of the atmosphere) due to high energy
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
s and other
electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
. 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
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for an object to escape from contact with or orbit of a primary body, assuming:
* Ballistic trajectory – no other forces are acting on the object, such as ...
of the planet and "evaporate" into space. The lower the
mass number
The mass number (symbol ''A'', from the German word: ''Atomgewicht'', "atomic weight"), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It is appro ...
of the gas, the higher the velocity obtained by interaction with a photon. Thus
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
is the gas which is most prone to photoevaporation.
Photoevaporation is the likely cause of the
small planet radius gap.
Examples of exoplanets with an evaporating atmosphere are
HD 209458 b,
HD 189733 b and
Gliese 3470 b. Material from a possible evaporating planet around
WD J0914+1914 might be responsible for the gaseous disk around this white dwarf.
Protoplanetary disks
Protoplanetary disk
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...
s can be dispersed by
stellar wind
A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...
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
A protostar is a very young star that is still gathering mass from its parent molecular cloud. It is the earliest phase in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 500,000 years. The p ...
commences
nuclear fusion
Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutrons, neutron by-products. The difference in mass between the reactants and products is manifested as either the rele ...
.
The disk is composed of gas and dust. The gas, consisting mostly of light elements such as
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
and
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
, 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. After 10
6 – 10
7 years,
the viscous accretion rates fall below the photoevaporation rates at
.
A gap then opens around
, the inner disk drains onto the central star,
or spreads to
and evaporates. An inner hole extending to
is produced. Once an inner hole forms, the outer disk is very rapidly cleared.
The formula for the gravitational radius of the disk is
:
where
is the
ratio of specific heats (= 5/3 for a monatomic gas),
the universal
gravitational constant
The gravitational constant is an empirical physical constant involved in the calculation of gravitational effects in Sir Isaac Newton's law of universal gravitation and in Albert Einstein's general relativity, theory of general relativity. It ...
,
the mass of the central star,
the mass of the Sun,
the mean weight of the gas,
Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...
,
is the temperature of the gas and AU the
Astronomical Unit
The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
.
If we denote the coefficient in the above equation by the Greek letter
then
, .
where
is the number of
degrees of freedom
In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinite ...
and we have used the formula:
.
For an
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
, such as a
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
atom, then
, because an atom can move in three different, orthogonal directions. Consequently,
. If the hydrogen atom is ionized, i.e., it is a
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
, and is in a strong
magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
then
, because the proton can move along the magnetic field and
rotate around the field lines. In this case,
. A
diatomic molecule
Diatomic molecules () are molecules composed of only two atoms, of the same or different chemical elements. If a diatomic molecule consists of two atoms of the same element, such as hydrogen () or oxygen (), then it is said to be homonuclear mol ...
, e.g., a hydrogen molecule, has
and
. For a non-linear triatomic molecule, such as
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
,
and
. If ''
'' becomes very large, then
approaches zero. 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.
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.
Regions containing protoplanetary disks with clear signs of external photoevaporation
The most famous region containing photoevaporated protoplanetary disks is the
Orion Nebula
The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula in the Milky Way situated south of Orion's Belt in the Orion (constellation), constellation of Orion, and is known as the middle "star" in the "sword" of Orion. It ...
. They were called bright
proplyds and since then the term was used for other regions to describe photoevaporation of protoplanetary disks. They were discovered with the
Hubble Space Telescope
The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...
. There might even be a planetary-mass object in the Orion Nebula that is being photoevaporated by
''θ'' 1 Ori C. Since then HST did observe other young star clusters and found bright proplyds in the
Lagoon Nebula, the
Trifid Nebula,
Pismis 24 and
NGC 1977.
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. Later similar cometary tails were found with Spitzer in
W5. This study concluded that the tails have a likely lifetime of 5
Myrs or less. Additional tails were found with Spitzer in NGC 1977,
NGC 6193 and
Collinder 69. Other bright proplyd candidates were found in the
Carina Nebula with the
CTIO 4m and near
Sagittarius A* with the
VLA. Follow-up observations of a proplyd candidate in the Carina Nebula with Hubble revealed that it is likely an
evaporating gaseous globule.
Objects in
NGC 3603
NGC 3603 is a nebula situated in the Carina–Sagittarius Arm of the Milky Way around 20,000 light-years away from the Solar System. It is a massive H II region containing a very compact open cluster (probably a super star cluster) HD 97950.
Ob ...
and later in
Cygnus OB2 were proposed as intermediate massive versions of the bright proplyds found in the Orion Nebula.
References
{{Reflist
Concepts in stellar astronomy