RCW 36
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RCW 36 (also designated Gum 20) is an
emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emission n ...
containing an
open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...
in the constellation Vela. This
H II region An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
is part of a larger-scale star-forming complex known as the
Vela Molecular Ridge Vela Molecular Ridge is a molecular cloud complex in the constellations Vela (constellation), Vela and Puppis. Radio 12CO observations of the region showed the ridge to be composed of several clouds, each with masses 100,000–1,000,000 Mass of th ...
(VMR), a collection of
molecular clouds 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, ...
in the
Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
that contain multiple sites of ongoing star-formation activity. The VMR is made up of several distinct clouds, and RCW 36 is embedded in the VMR Cloud C. RCW 36 is one of the sites of massive-star formation closest to the Solar System, whose distance of approximately 700
parsecs The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...
(2300
light-years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...
). The most massive stars in the star cluster are two stars with late-O or early-B spectral types, but the cluster also contains hundreds of lower-mass stars. This region is also home to objects with Herbig–Haro jets, HH 1042 and HH 1043. It is about 1.1
million years Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used ...
old.


Star formation in RCW 36

Like most star-forming regions, the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
around RCW 36 contains both the gas from which stars form and some newly formed young stars. Here, young stellar clusters form in
giant molecular clouds 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 ...
. Molecular clouds are the coldest, densest form of interstellar gas and are composed mostly of
molecular 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 ...
(H2), but also include more complex molecules,
cosmic dust Cosmic dustalso called extraterrestrial dust, space dust, or star dustis dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and , such as micrometeoroids (30 μm). Cosmic dust can ...
, and atomic helium. Stars form when the mass gas in part of a cloud becomes too great, causing it to collapse due to the
Jeans instability The Jeans instability is a concept in astrophysics that describes an instability that leads to the gravitational collapse of a cloud of gas or dust. It causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when ...
. Most stars do not form alone, but in groups containing hundreds or thousands of other stars. RCW 36 is an example of this type of "clustered" star formation.


Molecular cloud and H II region

The Vela Molecular Ridge can be subdivided into several smaller clouds, each of which in turn can be subdivided into cloud "clumps". The molecular cloud clump from which the RCW 36 stars are forming is Clump 6 in the VMR C cloud. Early maps of the region were produced by
radio telescopes A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...
that traced emission from several types of molecules found in the clouds, including CO, OH, and H2CO. More detailed CO maps were produced in the 1990s by a team of Japanese astronomers using the NANTEN millimeter-wavelength telescope. Using emission from C18O, they estimated the total mass of Cloud C to be 44,000 . The cloud maps suggest that Cloud C is the youngest component of the VMR because of an ultra-compact H II region associated with RCW 36 and several sites of embedded protostars, while H II regions in other VMR clouds are more evolved. Observations from the Herschel Space Telescope show that the material within the cloud is organized into filaments and RCW 36 sits near the south end of a 10-parsec long filament. Star formation in RCW 36 is currently ongoing. In the dense gas at the western edge of RCW 36, where the far-infrared emission is greatest, are found protostellar cores, the Herbig Haro objects, and an ultra-compact H II region. However, more deeply embedded star-formation is obscured by dust, so radiation can only escape from the cloud surface and not from the embedded objects themselves. The H II region is an area around the cluster in which hydrogen atoms in the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
have been ionized by ultraviolet light from O- and B-type stars. The H II region in RCW 36 has an hourglass morphology, similar to the shape of H II regions around other young stellar clusters like W40 or Sh2-106. In addition, an ultra-compact H II region surrounds IRAS source 08576−4333.


Star cluster

Due to the youth of RCW 36, most of the stars in the cluster are at an early stage of
stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is consi ...
where they are known as
young stellar object Young stellar object (YSO) denotes a star in its early stage of evolution. This class consists of two groups of objects: protostars and pre-main-sequence stars. Classification by spectral energy distribution A star forms by accumulation of mate ...
s or
pre-main-sequence stars A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope o ...
. These stars are still in the process of contraction before they reach the
main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...
, and they may still have gas accreting onto them from either a
circumstellar disk A Circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the res ...
or
envelope An envelope is a common packaging item, usually made of thin, flat material. It is designed to contain a flat object, such as a letter (message), letter or Greeting card, card. Traditional envelopes are made from sheets of paper cut to one o ...
. Cluster members in RCW 36 have been identified through both infrared and X-ray observations. Bright infrared sources, attributed to massive stars, were first identified by the TIFR 100-cm balloon-born telescope from the National Balloon Facility in Hyderabad, India. In the early 2000s, infrared images in the J, H, and Ks bands have suggested at least 350 cluster members. Observations by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
's
Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003, that was deactivated when operations ended on 30 January 2020. Spitzer was the third space telescope dedicate ...
and
Chandra X-ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...
were used to identify cluster members as part of the MYStIX survey of nearby star-forming regions. In the MYStIX catalog of 384 probable young stellar members of RCW 36, more than 300 of the stars are detected by X-ray sources. Modeling of the brightnesses of stars at various infrared wavelengths has shown 132 young stellar objects to have infrared excess consistent with
circumstellar disks A Circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the rese ...
or envelopes. The cluster has been noted by Baba et al. for having a high density of stars, with
star count Star counts are census counts of stars and the statistical and geometrical methods used to correct the corresponding data for bias. The surveys are most often made of nearby stars in the Milky Way galaxy. The total number of stars counted in a p ...
s (the number of stars within an angular area of the sky) exceeding 3000 stars per square parsec at the center of the cluster. A measurement of central
area density The area density (also known as areal density, surface density, superficial density, areic density, column density, or density thickness) of a two-dimensional object is calculated as the mass per unit area. The SI derived unit is the "kilogram p ...
using the MYStIX catalog suggested approximately 10,000 stars per square parsec at the cluster center, but this study also suggested that such densities are not unusual for massive star-forming regions. The spatial distribution of stars has been described as a King profile or alternatively as a "core-halo" structure. Stellar
density Density (volumetric mass density or specific mass) is the ratio of a substance's mass to its volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' (or ''d'') can also be u ...
near the center of RCW 36 has been estimated to be approximately 300,000 stars per cubic parsec (or 10,000 stars per cubic light year). In contrast, the density of stars in the Solar neighborhood is only 0.14 star per cubic parsec, so the density of stars at the center of RCW 36 is about 2 million times greater. It has been calculated that for young stellar clusters with more than 104 stars pc.−3 close encounters between stars can lead to interactions between protoplanetary disks that affect developing planetary systems.


Young stellar objects

Several special types of young stellar object have been identified in RCW 36, and are described in more detail below. The properties of these stars are related to their extreme youth. Two stars in RCW 36 have Herbig-Haro jets (HH 1042 and HH 1043). Jets of gas flowing out from young stars can be produced by accretion onto a star. In RCW 36 these jets were seen in a number of spectral lines, including lines from hydrogen, helium, oxygen, nitrogen, sulfur, nickel, calcium, and iron. Mass loss rates from the jets have been estimated to be on the order of 10−7 solar masses per year. Inhomogeneities in the jets have been attributed to variable accretion rate on timescales of approximately 100 years. The young star 2MASS J08592851-4346029 has been classified as a Herbig Ae star. Stars in this class are pre-main-sequence, intermediate-mass stars (spectral type A) with
emission lines A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...
in their spectra from hydrogen. Observations indicate that 2MASS J08592851-4346029 has a bloated radius as would be expected for a young star that is still contracting. Some of the lines in its spectrum have a P-Cygni Profile indicating the presence of a stellar wind. The young star CXOANC J085932.2−434602 was observed by the Chandra X-ray Observatory to have produced a large
flare A flare, also sometimes called a fusée, fusee, or bengala, bengalo in several European countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illuminatio ...
with a peak temperature greater than 100 million
kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...
s. Such "super hot" flares from young stars have been seen in other star-forming regions like 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 ...
.


See also

*
H II Region An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...
*
Star Formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...
*
Gum Catalog The Gum catalog is an astronomical catalog of 84 emission nebulae in the southern sky. It was made by the Australian astronomer Colin Stanley Gum (1924-1960) at Mount Stromlo Observatory using wide field photography. Gum published his findings i ...
* RCW Catalog


References

{{Stars of Vela H II regions Open clusters Vela (constellation) Star-forming regions