In 1979 observations with the
Lallemand electronic camera at the
Pic-du-Midi Observatory showed six unresolved high-ionization sources near the
Trapezium Cluster. These sources were not interpreted as proplyds, but as partly ionized globules (PIGs). The idea was that these objects are being ionized from the outside by
M42.[3] Later observations with the
Very Large Array showed solar-system-sized condensations associated with these sources. Here the idea appeared that these objects might be low-mass stars surrounded by an evaporating protostellar accretion disk.[4]
Proplyds were clearly resolved in 1993 using images of the
Hubble Space Telescope Wide Field Camera and the term "proplyd" was used.[5]
Characteristics
In the Orion Nebula the proplyds observed are usually one of two types. Some proplyds
glow around
luminous stars, in cases where the disk is found close to the star, glowing from the star's luminosity. Other proplyds are found at a greater distance from the host star and instead show up as dark silhouettes due to the self-obscuration of cooler dust and gases from the disk itself. Some proplyds show signs of movement from
solar irradiance shock waves pushing the proplyds. The Orion Nebula is approximately 1,500
light-years from the
Sun with very active
star formation. The Orion Nebula and the Sun are in the same
spiral arm of the
Milky Way galaxy.[6][7][8][9]
A proplyd may form new
planets and
planetesimal systems. Current models show that the
metallicity of the star and proplyd, along with the correct planetary system temperature and distance from the star, are keys to planet and
planetesimal formation. To date, the
Solar System, with 8 planets, 5
dwarf planets and 5 planetesimal systems, is the largest
planetary system found.[10][11][12] Most proplyds develop into a system with no planetesimal systems, or into one very large planetesimal system.[13][14][15][16][17][18]
Proplyds in other star-forming regions
Photoevaporating proplyds in other star forming regions were found with the
Hubble Space Telescope.
NGC 1977 currently represents the star-forming region with the largest number of proplyds outside of the Orion Nebula, with 7 confirmed proplyds. It was also the first instance where a B-type star,
42 Orionis is responsible for the photoevaporation.[19] In addition, 4 clear and 4 candidate proplyds were discovered in the very young region
NGC 2024, two of which have been photoevaporated by a B star.[20] The
NGC 2024 proplyds are significant because they imply that external photoevaporation of protoplanetary disks could compete even with very early planet formation (within the first half a million years).
Another type of photoevaporating proplyd was discovered with the
Spitzer Space Telescope. These cometary tails represent
dust being pulled away from the disks.[21]Westerhout 5 is a region with many dusty proplyds, especially around
HD 17505.[22] These dusty proplyds are depleted of any gas in the outer regions of the disk, but the photoevaporation could leave an inner, more robust, and possibly gas-rich disk component of radius 5-10
astronomical units.[23]
The proplyds in the Orion Nebula and other star-forming regions represent proto-planetary disks around
low-mass stars being externally photoevaporated. These low-mass proplyds are usually found within 0.3
parsec (60,000
astronomical units) of the massive OB star and the dusty proplyds have tails with a length of 0.1 to 0.2 parsec (20,000 to 40,000 au).[21] There is a proposed type of intermediate massive counterpart, called
proplyd-like objects. Objects in
NGC 3603 and later in
Cygnus OB2 were proposed as intermediate massive versions of the bright proplyds found in the Orion Nebula. The proplyd-like objects in Cygnus OB2 for example are 6 to 14 parsec distant to a large collection of
OB stars and have tail lengths of 0.11 to 0.55 parsec (24,000 to 113,000 au).[24][25] The nature of proplyd-like objects as intermediate massive proplyds is partly supported by a spectrum for one object, which showed that the mass loss rate is higher than the mass accretion rate. Another object did not show any outflow, but accretion.[26]
Very bright proplyd 181-825 in the Orion Nebula, from Hubble Space Telescope
Dark proplyd 132-1832 in the Orion Nebula, from Hubble Space Telescope
Bright proplyd 170-249 in the Orion Nebula, from Hubble Space Telescope. The upward tail is a jet of dust and gas blowing away from the excited proplyd
A so-called proplyd-like object, which is being illuminated by massive stars of the
Cygnus OB2 association
ESO
VLTMUSE image of proplyd 244-440 in the Orion Nebula. The young object is ejecting a jet (red color) and is surrounded by a blue halo of unknown origin.[41]
^Laques, P.; Vidal, J. L. (March 1979). "Detection of a new kind of condensations in the center of the Orion Nebula, by means of S 20 photocathodes associated with a Lallemand electronic camera". Astronomy & Astrophysics. 73: 97–106.
Bibcode:
1979A&A....73...97L.
ISSN0004-6361.
^Wang, Ji; Fischer, Debra A. (January 1, 2015). "Revealing A Universal Planet-Metallicity Correlation For Planets of Different Sizes Around Solar-Type Stars". The Astronomical Journal. 149 (1): 14.
arXiv:1310.7830.
doi:
10.1088/0004-6256/149/1/14.
S2CID118415186.
^From Lithium to Uranium (IAU S228): Elemental Tracers of Early Cosmic Evolution
By International Astronomical Union. Symposium, by Vanessa Hill, Patrick Francois,
Francesca Primas, page 509-511, "the G star problem"
^Brandner, Wolfgang; Grebel, Eva K.; Chu, You-Hua; Dottori, Horacio; Brandl, Bernhard; Richling, Sabine; Yorke, Harold W.; Points, Sean D.; Zinnecker, Hans (January 2000). "HST/WFPC2 and VLT/ISAAC Observations of Proplyds in the Giant H II Region NGC 3603*". The Astronomical Journal. 119 (1): 292.
arXiv:astro-ph/9910074.
Bibcode:
2000AJ....119..292B.
doi:
10.1086/301192.
ISSN1538-3881.
S2CID15502401.