A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. [1] Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth. [2]
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]
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. [7] [8] [9] [10]
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. [11] [12] [13] Most proplyds develop into a system with no planetesimal systems, or into one very large planetesimal system. [14] [15] [16] [17] [18] [19]
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. [20] 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. [21] 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. [22] Westerhout 5 is a region with many dusty proplyds, especially around HD 17505. [23] 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. [24]
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). [22] 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). [25] [26] 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. [27]
List is sorted after distance.
Star-Forming region (SFR) | example image | Distance | Age of SFR (Myrs) | Ionizing stars | spectral type of ionizing stars | Number of proplyds | type of proplyd | References |
---|---|---|---|---|---|---|---|---|
NGC 1977 | 1305 | 4 | 42 Orionis | B1V | 7 | gaseous + dusty tails | [28] [20] | |
Lambda Orionis Cluster | 1305 | 6 | Meissa | O8IIIf+B0.5V | 2 | dusty tails | [29] | |
Orion Nebula | 1344 | 1 | Theta1 Orionis C | O6Vp+B0V | 178 | gaseous + dark disks | [1] [28] | |
Messier 43 | 1300 | 1 | NU Orionis (HD 37061) | B0.5V | 3 | gaseous | [30] [31] | |
Flame Nebula | 1350 | 0.2 to 0.5 | IRS1, IRS2b | B0.5V, O8V | 4 or 8 | gaseous | [21] | |
NGC 2264 | 2609 | 4 | S Mon | O7Ve | 1 | dusty tails | [32] | |
IC 1396 | 2723 | 3 | HD 206267 | O6V | 1 | dusty tails | [32] | |
NGC 6193 | 3783 | 5 | HD 150136, CD-48 11071 | O3.5-4III(f*)+O6IV, B0V | 8 or 9 | dusty tails | [33] | |
Cygnus OB2 | 4566 | 3-5 | Cluster of O-stars | 11 | large "proplyd-like" objects + dusty tails | [34] [35] | ||
NGC 2244 | 4892 | 4 | HD 46150 | O5V | 1 | dusty tails | [32] | |
Trifid Nebula | 5479 | 8 | HD 164492A | O7.5 | 1 | gaseous | [28] [36] | |
Pismis 24 | 5544 | 1 | Pis 24-1, Pis 24-2 | O3I, O5.5 V(f) | 5 | gaseous | [37] | |
Lagoon Nebula | 5871 | 5 | Herschel 36 | O7V | 1 | gaseous | [28] [38] | |
Westerhout 5 | 7500 | 5 | HD 17505, HD 18326 | O6.5III((f))n+O8V, O7V | 4 | dusty tails | [39] [23] | |
Carina Nebula (disputed) [40] | 7501 | 3 | Cluster of O-stars | "dozens" | large "proplyd-like" objects + dark disks | [41] | ||
NGC 3603 | 19569 | 1 | Cluster of O-stars | 3 | large "proplyd-like" objects | [28] [42] | ||
Sgr A* | 26673 | unknown | Multiple O- and WR-stars | 34 | gaseous | [43] |
The Orion Nebula is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion,[b] and is known as the middle "star" in the "sword" of Orion. It is one of the brightest nebulae and is visible to the naked eye in the night sky with an apparent magnitude of 4.0. It is 1,344 ± 20 light-years (412.1 ± 6.1 pc) away and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light-years across. It has a mass of about 2,000 times that of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.
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 similar, an accretion disk is hotter and spins much faster. It is also found on black holes, not stars. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.
A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf.
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 radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.
The Orion molecular cloud complex is a star-forming region with stellar ages ranging up to 12 Myr. Two giant molecular clouds are a part of it, Orion A and Orion B. The stars currently forming within the complex are located within these clouds. A number of other somewhat older stars no longer associated with the molecular gas are also part of the complex, most notably the Orion's Belt, as well as the dispersed population north of it. Near the head of Orion there is also a population of young stars that is centered on Meissa. The complex is between 1 000 and 1 400 light-years away, and hundreds of light-years across.
Sh 2-279 is an HII region and bright nebulae that includes a reflection nebula located in the constellation Orion. It is the northernmost part of the asterism known as Orion's Sword, lying 0.6° north of the Orion Nebula. The reflection nebula embedded in Sh 2-279 is popularly known as the Running Man Nebula.
NGC 2547 is a southern open cluster in Vela, discovered by Nicolas Louis de Lacaille in 1751 from South Africa. The star cluster is young with an age of 20-30 million years.
The Flame Nebula, designated as NGC 2024 and Sh2-277, is an emission nebula in the constellation Orion. It is about 1350 light-years away. At that distance, the Flame Nebula lies within the Orion B cloud of the larger Orion Molecular Cloud Complex.
NGC 1333 is a reflection nebula located in the northern constellation Perseus, positioned next to the southern constellation border with Taurus and Aries. It was first discovered by German astronomer Eduard Schönfeld in 1855. The nebula is visible as a hazy patch in a small telescope, while a larger aperture will show a pair of dark nebulae designated Barnard 1 and Barnard 2. It is associated with a dark cloud L1450. Estimates of the distance to this nebula range from 980–1,140 ly (300–350 pc).
NGC 2244 is an open cluster in the Rosette Nebula, which is located in the constellation Monoceros. This cluster has several O-type stars, super hot stars that generate large amounts of radiation and stellar wind.
FU Orionis is a variable and binary star system in the constellation of Orion, that in 1937 rose in apparent visual magnitude from 16.5 to 9.6, and has since been around magnitude 9. The name FU Orionis is a variable star designation in the Argelander system, which are assigned sequentially as new variables are discovered. FU Orionis is about 1,360 light years distant and is associated with the molecular cloud Barnard 35.
Cygnus OB2 is an OB association that is home to some of the most massive and most luminous stars known, including suspected Luminous blue variable Cyg OB2 #12. It also includes one of the largest known stars, NML Cygni. The region is embedded within a wider one of star formation known as Cygnus X, which is one of the most luminous objects in the sky at radio wavelengths. The region is approximately 1,570 parsecs from Earth in the constellation of Cygnus.
AB Aurigae is a young Herbig Ae star in the Auriga constellation. It is located at a distance of approximately 509 light years from the Sun based on stellar parallax. This pre-main-sequence star has a stellar classification of A0Ve, matching an A-type main-sequence star with emission lines in the spectrum. It has 2.4 times the mass of the Sun and is radiating 38 times the Sun's luminosity from its photosphere at an effective temperature of 9,772 K. The radio emission from the system suggests the presence of a thermal jet originating from the star with a velocity of 300 km s−1. This is causing an estimated mass loss of 1.7×10−8 M☉ yr−1.
The Lambda Orionis Cluster is an open star cluster located north-west of the star Betelgeuse in the constellation of Orion. It is about five million years old and roughly 1,300 ly (400 pc) away from the Sun. Included within the cluster is a double star named Meissa. With the rest of Orion, it is visible from the middle of August in the morning sky, to late April before Orion becomes too close to the Sun to be seen well. It can be seen from both the northern hemisphere and the southern hemisphere.
Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula, the Ring Nebula, the Dumbbell Nebula, the Eskimo Nebula, and the Retina Nebula. They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System, with masses of around 0.00001 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
PDS 70 is a very young T Tauri star in the constellation Centaurus. Located 370 light-years from Earth, it has a mass of 0.76 M☉ and is approximately 5.4 million years old. The star has a protoplanetary disk containing two nascent exoplanets, named PDS 70b and PDS 70c, which have been directly imaged by the European Southern Observatory's Very Large Telescope. PDS 70b was the first confirmed protoplanet to be directly imaged.
A Peter Pan disk is a circumstellar disk around a star or brown dwarf that appears to have retained enough gas to form a gas giant planet for much longer than the typically assumed gas dispersal timescale of approximately 5 million years. Several examples of such disks have been observed to orbit stars with spectral types of M or later. The presence of gas around these disks has generally been inferred from the total amount of radiation emitted from the disk at infrared wavelengths, and/or spectroscopic signatures of hydrogen accreting onto the star. To fit one specific definition of a Peter Pan disk, the source needs to have an infrared "color" of , an age of >20 Myr and spectroscopic evidence of accretion.
Proplyd 114-426 is a large protoplanetary disk seen in absorption in front of the emission of the Orion Nebula. It is one of the largest protoplanetary disks in the Orion Nebula with a diameter of 950 AU.
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