Molecular cloud

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A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.

Contents

Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is carbon monoxide (CO). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. [1]

Within molecular clouds are regions with higher density, where much dust and many gas cores reside, called clumps. These clumps are the beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse. [2]

Research and discovery

Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals. Henk-van-de-hulst.jpg
Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals.

The history pertaining to the discovery of molecular clouds is closely related to the development of radio astronomy and astrochemistry. During World War II, at a small gathering of scientists, Henk van de Hulst first reported he had calculated the neutral hydrogen atom should transmit a detectable radio signal. [3] This discovery was an important step towards the research that would eventually lead to the detection of molecular clouds.

Jansky and his rotating directional radio antenna (early 1930s), the world's first radio telescope. JanskyatAntenna hi.tif
Jansky and his rotating directional radio antenna (early 1930s), the world's first radio telescope.

Once the war ended, and aware of the pioneering radio astronomical observations performed by Jansky and Reber in the US, the Dutch astronomers repurposed the dish-shaped antennas running along the Dutch coastline that was once used by the Germans as a warning radar system and modified into radio telescopes, initiating the search for the hydrogen signature in the depths of space. [3] [4]

The neutral hydrogen atom consists of a proton with an electron in its orbit. Both the proton and the electron have a spin property. When the spin state flips from a parallel condition to antiparallel, which contains less energy, the atom gets rid of the excess energy by radiating a spectral line at a frequency of 1420.405 MHz. [3]

This frequency is generally known as the 21 cm line, referring to its wavelength in the radio band. The 21 cm line is the signature of HI and makes the gas detectable to astronomers back on earth. The discovery of the 21 cm line was the first step towards the technology that would allow astronomers to detect compounds and molecules in interstellar space. [3]

Plaque commemorating the discovery of 21-cm radiation from the Milky Way Green Banks - Ewen-Purcell Horn Antenna info.jpg
Plaque commemorating the discovery of 21-cm radiation from the Milky Way

In 1951, two research groups nearly simultaneously discovered radio emission from interstellar neutral hydrogen. Ewen and Purcell reported the detection of the 21-cm line in March, 1951. Using the radio telescope at the Kootwijk Observatory, Muller and Oort reported the detection of the hydrogen emission line in May of that same year. [4]

Left to right: Jan Oort, Hendrik C. van de Hulst, Pieter Oosterhoff. Jan Oort had a pivotal role in the research that lead to the discovery of molecular clouds. LeidenObservatory1961c.jpg
Left to right: Jan Oort, Hendrik C. van de Hulst, Pieter Oosterhoff. Jan Oort had a pivotal role in the research that lead to the discovery of molecular clouds.

Once the 21-cm emission line was detected, radio astronomers began mapping the neutral hydrogen distribution of the Milky Way Galaxy. Van de Hulst, Muller, and Oort, aided by a team of astronomers from Australia, published the Leiden-Sydney map of neutral hydrogen in the galactic disk in 1958 on the Monthly Notices of the Royal Astronomical Society. This was the first neutral hydrogen map of the galactic disc and also the first map showing the spiral arm structure within it. [4]

Following the work on atomic hydrogen detection by van de Hulst, Oort and others, astronomers began to regularly use radio telescopes, this time looking for interstellar molecules. In 1963 Alan Barrett and Sander Weinred at MIT found the emission line of OH in the supernova remnant Cassiopeia A. This was the first radio detection of an interstellar molecule at radio wavelengths. [1] More interstellar OH detections quickly followed and in 1965, Harold Weaver and his team of radio astronomers at Berkeley, identified OH emissions lines coming from the direction of the Orion Nebula and in the constellation of Cassiopeia. [4]

In 1968, Cheung, Rank, Townes, Thornton and Welch detected NH₃ inversion line radiation in interstellar space. A year later, Lewis Snyder and his colleagues found interstellar formaldehyde. Also in the same year George Carruthers managed to identify molecular hydrogen. The numerous detections of molecules in interstellar space would help pave the way to the discovery of molecular clouds in 1970. [4]

Penzias and Wilson with the Holmdel horn antenna used to detect microwave emissions from the Big Bang Horn Antenna-in Holmdel, New Jersey - restoration1.jpg
Penzias and Wilson with the Holmdel horn antenna used to detect microwave emissions from the Big Bang

Hydrogen is the most abundant species of atom in molecular clouds, and under the right conditions it will form the H2 molecule. Despite its abundance, the detection of H2 proved difficult. Due to its symmetrical molecule, H2 molecules have a weak rotational and vibrational modes, making it virtually invisible to direct observation.

The solution to this problem came when Arno Penzias, Keith Jefferts, and Robert Wilson identified CO in the star-forming region in the Omega Nebula. Carbon monoxide is a lot easier to detect than H2 because of its rotational energy and asymmetrical structure. CO soon became the primary tracer of the clouds where star-formation occurs. [4]

In 1970, Penzias and his team quickly detected CO in other locations close to the galactic center, including the giant molecular cloud identified as Sagittarius B2, 390 light years from the galactic center, making it the first detection of a molecular cloud in history. [4] This team later would receive the Nobel prize of physics for their discovery of microwave emission from the Big Bang.

Due to their pivotal role, research about these structures have only increased over time. A paper published in 2022 reports over 10,000 molecular clouds detected since the discovery of Sagittarius B2. [5]

Occurrence

Molecular cloud Barnard 68, about 500 ly distant and 0.5 ly in diameter Barnard 68.jpg
Molecular cloud Barnard 68, about 500 ly distant and 0.5 ly in diameter

Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of it. The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years ) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). [6] Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. [7] That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region. [8]

Circinus molecular cloud has a mass around 250,000 times that of the Sun. Violent birth announcement from an infant star.jpg
Circinus molecular cloud has a mass around 250,000 times that of the Sun.

Perpendicularly to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of approximately 50 to 75 parsecs, much thinner than the warm atomic (Z from 130 to 400 parsecs) and warm ionized (Z around 1000 parsecs) gaseous components of the ISM. [10] The exceptions to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars; and as such they have approximately the same vertical distribution as the molecular gas.

This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of the gas is highly irregular, with most of it concentrated in discrete clouds and cloud complexes. [6]

General structure and chemistry of molecular clouds

Molecular clouds typically have interstellar medium densities of 10 to 30 cm-3, and constitute approximately 50% of the total interstellar gas in a galaxy. [11] Most of the gas is found in a molecular state. The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in a fast transition, forming “envelopes” of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary. [12]

Cosmic dust and ultraviolet radiation emitted by stars are key factors that determine not only gas and column density, but also the molecular composition of a cloud. The dust provides shielding to the molecular gas inside, preventing dissociation by the ultraviolet radiation. The dissociation caused by UV photons is the main mechanism for transforming molecular material back to the atomic state inside the cloud. [13] Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance. [14]

The star T Tauri with NGC 1555 cloud nearby. Ngc1555.jpg
The star T Tauri with NGC 1555 cloud nearby.

Most of the gas constituting a molecular cloud is molecular hydrogen, with carbon monoxide being the second most common compound. [11] Molecular clouds also usually contain other elements and compounds. Astronomers have observed the presence of long chain compounds such as methanol, ethanol and benzene rings and their several hydrides. Large molecules known as polycyclic aromatic hydrocarbons have also been detected. [13]

The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form the larger substructure of the cloud, having the average size of 1 pc. Clumps are the precursors of star clusters, though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are gravitationally bound and go through a collapse during star formation. [11]

In astronomical terms, molecular clouds are short-lived structures that are either destroyed or go through major structural and chemical changes approximately 10 million years into their existence. Their short life span can be inferred from the range in age of young stars associated with them, of 10 to 20 million years, matching molecular clouds’ internal timescales. [14]

Direct observation of T Tauri stars inside dark clouds and OB stars in star-forming regions match this predicted age span. The fact OB stars older than 10 million years don’t have a significant amount of cloud material about them, seems to suggest most of the cloud is dispersed after this time. The lack of large amounts of frozen molecules inside the clouds also suggest a short-lived structure. Some astronomers propose the molecules never froze in very large quantities due to turbulence and the fast transition between atomic and molecular gas. [14]

Cloud formation and destruction

Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating the rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars in approximately 3 M☉ per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 M☉ of gas being assembled in molecular clouds in the Milky Way per year. [14] [15]

The Elephant's Trunk Nebula is an elongated dark globule. The globule is a condensation of dense gas that is barely surviving the strong ionizing radiation from a nearby massive star. 54345main ic1396 highres.jpg
The Elephant's Trunk Nebula is an elongated dark globule. The globule is a condensation of dense gas that is barely surviving the strong ionizing radiation from a nearby massive star.

Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the collision theory have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures. [15] [14]

Gravitational instability is likely to be the main mechanism. Those regions with more gas will exert a greater gravitational force on their neighboring regions, and draw surrounding material. This extra material increases the density, increasing their gravitational attraction. Mathematical models of gravitational instability in the gas layer predict a formation time within the timescale for the estimated cloud formation time. [15] [14]

Once a molecular cloud assembles enough mass, the densest regions of the structure will start to collapse under gravity, creating star-forming clusters. This process is highly destructive to the cloud itself. Once stars are formed, they begin to ionize portions of the cloud around it due to their heat. The ionized gas then evaporates and is dispersed in formations called ‘champagne flows’. [16] This process begins when approximately 2% of the mass of the cloud has been converted into stars. Stellar winds are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability. [14]

Star Formation

Taurus Molecular Cloud. Located about 430 light-years from us, this vast complex of interstellar clouds is where a myriad of stars are being born, and is the closest large region of star formation. Herschel's view of the Taurus molecular cloud ESA384012.jpg
Taurus Molecular Cloud. Located about 430 light-years from us, this vast complex of interstellar clouds is where a myriad of stars are being born, and is the closest large region of star formation.

Star formation involves the collapse of the densest part of the molecular cloud, fragmenting the collapsed region in smaller clumps. These clumps aggregate more interstellar material, increasing in density by gravitational contraction. This process continues until the temperature reaches a point where the fusion of hydrogen can occur. [17] The burning of hydrogen then generates enough heat to push against gravity, creating hydrostatic equilibrium. At this stage, a protostar is formed and it will continue to aggregate gas and dust from the cloud around it.

One of the most studied star formation regions is the Taurus molecular cloud due to its close proximity to earth (140 pc or 430 ly away), making it an excellent object to collect data about the relationship between molecular clouds and star formation. Embedded in the Taurus molecular cloud there are T Tauri stars. These are a class of variable stars in an early stage of stellar development and still gathering gas and dust from the cloud around them. Observation of star forming regions have helped astronomers develop theories about stellar evolution. Many O and B type stars have been observed in or very near molecular clouds. Since these star types belong to population I (some are less than 1 million years old), they cannot have moved far from their birth place. Many of these young stars are found embedded in cloud clusters, suggesting stars are formed inside it. [17]

Types of molecular cloud

Giant molecular clouds

Within a few million years the light from bright stars will have boiled away this molecular cloud of gas and dust. The cloud has broken off from the Carina Nebula. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light-years and was taken by the Hubble Space Telescope in 1999. "Finger of God" Bok globule in the Carina Nebula.jpg
Within a few million years the light from bright stars will have boiled away this molecular cloud of gas and dust. The cloud has broken off from the Carina Nebula. Newly formed stars are visible nearby, their images reddened by blue light being preferentially scattered by the pervasive dust. This image spans about two light-years and was taken by the Hubble Space Telescope in 1999.
Part of the Taurus molecular cloud Part of the Taurus Molecular Cloud.jpg
Part of the Taurus molecular cloud

A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun [19] is called a giant molecular cloud (GMC). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses. [20] Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps. [8]

Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control the detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width. [21] A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation. [22]

The densest parts of the filaments and clumps are called molecular cores, while the densest molecular cores are called dense molecular cores and have densities in excess of 104 to 106 particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae. [23]

GMCs are so large that local ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion molecular cloud (OMC) or the Taurus molecular cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt. [24] The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space. [25]

Distribution of molecular gas in 30 merging galaxies. Distribution of molecular gas in 30 merging galaxies.jpg
Distribution of molecular gas in 30 merging galaxies.

Small molecular clouds

Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.

High-latitude diffuse molecular clouds

In 1984 IRAS [ clarification needed ] identified a new type of diffuse molecular cloud. [27] These were diffuse filamentary clouds that are visible at high galactic latitudes. These clouds have a typical density of 30 particles per cubic centimetre. [28]

The Serpens South star cluster is embedded in a filamentary molecular cloud, seen as a dark ribbon passing vertically through the cluster. This cloud has served as a testbed for studies of molecular cloud stability. Serpens south.jpg
The Serpens South star cluster is embedded in a filamentary molecular cloud, seen as a dark ribbon passing vertically through the cluster. This cloud has served as a testbed for studies of molecular cloud stability.

List of molecular cloud complexes

See also

Related Research Articles

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.

<span class="mw-page-title-main">Nebula</span> Body of interstellar clouds

A nebula is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.

<span class="mw-page-title-main">Open cluster</span> Large group of stars less bound than globular clusters

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 many more are thought to exist. They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the Galactic Center. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.

<span class="mw-page-title-main">Star</span> Large self-illuminated object in space

A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.

<span class="mw-page-title-main">Star formation</span> Process by which dense regions of molecular clouds in interstellar space collapse to form stars

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", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

<span class="mw-page-title-main">Astronomy</span> Scientific study of celestial objects

Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include planets, moons, stars, nebulae, galaxies, meteoroids, asteroids, and comets. Relevant phenomena include supernova explosions, gamma ray bursts, quasars, blazars, pulsars, and cosmic microwave background radiation. More generally, astronomy studies everything that originates beyond Earth's atmosphere. Cosmology is a branch of astronomy that studies the universe as a whole.

<span class="mw-page-title-main">Interstellar medium</span> Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field. Although the density of atoms in the ISM is usually far below that in the best laboratory vacuums, the mean free path between collisions is short compared to typical interstellar lengths, so on these scales the ISM behaves as a gas (more precisely, as a plasma: it is everywhere at least slightly ionized), responding to pressure forces, and not as a collection of non-interacting particles.

<span class="mw-page-title-main">Astrochemistry</span> Study of molecules in the Universe and their reactions

Astrochemistry is the study of the abundance and reactions of molecules in the universe, and their interaction with radiation. The discipline is an overlap of astronomy and chemistry. The word "astrochemistry" may be applied to both the Solar System and the interstellar medium. The study of the abundance of elements and isotope ratios in Solar System objects, such as meteorites, is also called cosmochemistry, while the study of interstellar atoms and molecules and their interaction with radiation is sometimes called molecular astrophysics. The formation, atomic and chemical composition, evolution and fate of molecular gas clouds is of special interest, because it is from these clouds that solar systems form.

<span class="mw-page-title-main">H II region</span> Large, low-density interstellar cloud of partially ionized gas

An H II region or HII 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 years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

In physical cosmology, a protogalaxy, which could also be called a "primeval galaxy", is a cloud of gas which is forming into a galaxy. It is believed that the rate of star formation during this period of galactic evolution will determine whether a galaxy is a spiral or elliptical galaxy; a slower star formation tends to produce a spiral galaxy. The smaller clumps of gas in a protogalaxy form into stars.

<span class="mw-page-title-main">Herbig–Haro object</span> Small patches of nebulosity associated with newly born stars

Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second. Herbig–Haro objects are commonly found in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over timescales of a few years as they move rapidly away from their parent star into the gas clouds of interstellar space. Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.

<span class="mw-page-title-main">Trihydrogen cation</span> Polyatomic ion (H₃, charge +1)

The trihydrogen cation or protonated molecular hydrogen is a cation with formula H+
3, consisting of three hydrogen nuclei (protons) sharing two electrons.

<span class="mw-page-title-main">Ethynyl radical</span> Hydrocarbon compound (•CCH)

The ethynyl radical (systematically named λ3-ethyne and hydridodicarbon(C—C)) is an organic compound with the chemical formula C≡CH (also written [CCH] or C
2H). It is a simple molecule that does not occur naturally on Earth but is abundant in the interstellar medium. It was first observed by electron spin resonance isolated in a solid argon matrix at liquid helium temperatures in 1963 by Cochran and coworkers at the Johns Hopkins Applied Physics Laboratory. It was first observed in the gas phase by Tucker and coworkers in November 1973 toward the Orion Nebula, using the NRAO 11-meter radio telescope. It has since been detected in a large variety of interstellar environments, including dense molecular clouds, bok globules, star forming regions, the shells around carbon-rich evolved stars, and even in other galaxies.

Hydrogen isocyanide is a chemical with the molecular formula HNC. It is a minor tautomer of hydrogen cyanide (HCN). Its importance in the field of astrochemistry is linked to its ubiquity in the interstellar medium.

<span class="mw-page-title-main">Elephant trunk (astronomy)</span>

Elephant trunks are a type of interstellar matter formations found in molecular clouds. They are located in the neighborhood of massive O type and B type stars, which, through their intense radiation, can create expanding regions of ionized gas known as H II regions. Elephant trunks resemble massive pillars or columns of gas and dust, but they come in various shapes, lengths, and colors. Astronomers study elephant trunks because of their unique formation process and use 2-D and 3-D simulations to try to understand how this phenomenon occurs.

High-velocity clouds (HVCs) are large collections of gas found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have velocities which are measured in excess of 70–90 km s−1. These clouds of gas can be massive in size, some on the order of millions of times the mass of the Sun, and cover large portions of the sky. They have been observed in the Milky Way's halo and within other nearby galaxies.

<span class="mw-page-title-main">Westerhout 40</span> Star-forming region in the constellation Serpens

Westerhout 40 or W40 is a star-forming region in the Milky Way located in the constellation Serpens. In this region, interstellar gas forming a diffuse nebula surrounds a cluster of several hundred new-born stars. The distance to W40 is 436 ± 9 pc, making it one of the closest sites of formation of high-mass O-type and B-type stars. The ionizing radiation from the massive OB stars has created an H II region, which has an hour-glass morphology.

<span class="mw-page-title-main">RCW 36</span> Emission nebula in the constellation of Vela

RCW 36 is an emission nebula containing an open cluster in the constellation Vela. This H II region is part of a larger-scale star-forming complex known as the Vela Molecular Ridge (VMR), a collection of molecular clouds in the Milky Way 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.

<span class="mw-page-title-main">Serpens–Aquila Rift</span> Region located in the constellations Serpens and Aquila that contains dark interstellar clouds

The Serpens–Aquila Rift (also known as the Aquila Rift) is a region of the sky in the constellations Aquila, Serpens Cauda, and eastern Ophiuchus containing dark interstellar clouds. The region forms part of the Great Rift, the nearby dark cloud of cosmic dust that obscures the middle of the galactic plane of the Milky Way, looking inwards and towards its other radial sectors. The clouds that form this structure are called "molecular clouds", constituting a phase of the interstellar medium which is cold and dense enough for molecules to form, particularly molecular hydrogen (H2). These clouds are opaque to light in the optical part of the spectrum due to the presence of interstellar dust grains mixed with the gaseous component of the clouds. Therefore, the clouds in the Serpens-Aquila Rift block light from background stars in the disk of the Galaxy, forming the dark rift. The complex is located in a direction towards the inner Galaxy, where molecular clouds are common, so it is possible that not all components of the rift are at the same distance and physically associated with each other.

<span class="mw-page-title-main">Argonium</span> Chemical compound

Argonium (also called the argon hydride cation, the hydridoargon(1+) ion, or protonated argon; chemical formula ArH+) is a cation combining a proton and an argon atom. It can be made in an electric discharge, and was the first noble gas molecular ion to be found in interstellar space.

References

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  10. ↑ Cox, D. (2005). "The Three-Phase Interstellar Medium Revisited". Annual Review of Astronomy and Astrophysics. 43 (1): 337–385. Bibcode:2005ARA&A..43..337C. doi:10.1146/annurev.astro.43.072103.150615.
  11. 1 2 3 Dominik, Carsten; Inga, Kamp (November 2023). "Star and Planet Formation" (PDF). University of Amsterdam.
  12. ↑ Williams, J. P.; Blitz, L.; McKee, C. F. (2000). "The Structure and Evolution of Molecular Clouds: from Clumps to Cores to the IMF". Protostars and Planets IV. Tucson: University of Arizona Press. p. 97. arXiv: astro-ph/9902246 . Bibcode:2000prpl.conf...97W.
  13. 1 2 Ward-Thompson, Derek; Whitworth, Anthony P. (2015). An introduction to star formation (1 ed.). Cambridge: Cambridge University Press. ISBN   978-0-521-63030-6.
  14. 1 2 3 4 5 6 7 Wilson, Thomas L.; Johnston, Kenneth J., eds. (1994). The Structure and Content of Molecular Clouds 25 Years of Molecular Radioastronomy: Proceedings of a Conference Held at Schloss Ringberg, Tegernsee, Germany 14–16 April 1993. Lecture Notes in Physics. Vol. 439. Berlin, Heidelberg: Springer Berlin Heidelberg. doi:10.1007/3-540-58621-0. ISBN   978-3-540-58621-0.
  15. 1 2 3 Lada, Charles J., ed. (1991). The physics of star formation and early stellar evolution: proceedings of the NATO Advanced Study Institute on the Physics of Star Formation and Early Stellar Evolution, Agia Pelagia, Crete, Greece, May 27 - June 8, 1990. NATO ASI series Series C, Mathematical and physical sciences. Dordrecht: Kluwer. ISBN   978-0-7923-1349-6.
  16. ↑ Tenorio-Tagle, G. (1979-01-01). "The gas dynamics of H II regions. I. The champagne model". Astronomy and Astrophysics. 71: 59–65. Bibcode:1979A&A....71...59T. ISSN   0004-6361.
  17. 1 2 Salaris, Maurizio; Cassisi, Santi (2005). Evolution of stars and stellar populations. Chichester: Wiley. ISBN   978-0-470-09220-0.
  18. ↑ "APEX Turns its Eye to Dark Clouds in Taurus". ESO Press Release. Retrieved 17 February 2012.
  19. ↑ See, e.g., Fukui, Y.; Kawamura, A. (2010). "Molecular Clouds in Nearby Galaxies". Annual Review of Astronomy and Astrophysics. 48: 547–580. Bibcode:2010ARA&A..48..547F. doi:10.1146/annurev-astro-081309-130854.
  20. ↑ Murray, N. (2011). "Star Formation Efficiencies and Lifetimes of Giant Molecular Clouds in the Milky Way". The Astrophysical Journal. 729 (2): 133. arXiv: 1007.3270 . Bibcode:2011ApJ...729..133M. doi:10.1088/0004-637X/729/2/133. S2CID   118627665.
  21. ↑ Zhang, Guo-Yin; André, Ph.; Men'shchikov, A.; Wang, Ke (1 October 2020). "Fragmentation of star-forming filaments in the X-shaped nebula of the California molecular cloud". Astronomy and Astrophysics. 642: A76. arXiv: 2002.05984 . Bibcode:2020A&A...642A..76Z. doi:10.1051/0004-6361/202037721. ISSN   0004-6361. S2CID   211126855.
  22. ↑ Li, Xue-Mei; Zhang, Guo-Yin; Men’shchikov, Alexander; Li, Jin-Zeng; Zhang, Chang; Wu, Zhong-Zu (June 2023). "Properties of the dense cores and filamentary structures in the Vela C molecular cloud". Astronomy & Astrophysics. 674: A225. arXiv: 2304.10863 . Bibcode:2023A&A...674A.225L. doi:10.1051/0004-6361/202345846. S2CID   258291496.
  23. ↑ Di Francesco, J.; et al. (2006). "An Observational Perspective of Low-Mass Dense Cores I: Internal Physical and Chemical Properties". Protostars and Planets V. arXiv: astro-ph/0602379 . Bibcode:2007prpl.conf...17D.
  24. ↑ Grenier (2004). "The Gould Belt, star formation, and the local interstellar medium". The Young Universe. arXiv: astro-ph/0409096 . Bibcode:2004astro.ph..9096G. Electronic preprint
  25. ↑ Sagittarius B2 and its Line of Sight Archived 2007-03-12 at the Wayback Machine
  26. ↑ "Violent Origins of Disc Galaxies Probed by ALMA". www.eso.org. European Southern Observatory . Retrieved 17 September 2014.
  27. ↑ Low; et al. (1984). "Infrared cirrus – New components of the extended infrared emission". Astrophysical Journal. 278: L19. Bibcode:1984ApJ...278L..19L. doi:10.1086/184213.
  28. ↑ Gillmon, K. & Shull, J.M. (2006). "Molecular Hydrogen in Infrared Cirrus". Astrophysical Journal. 636 (2): 908–915. arXiv: astro-ph/0507587 . Bibcode:2006ApJ...636..908G. doi:10.1086/498055. S2CID   18995587.
  29. ↑ Friesen, R. K.; Bourke, T. L.; Francesco, J. Di; Gutermuth, R.; Myers, P. C. (2016). "The Fragmentation and Stability of Hierarchical Structure in Serpens South". The Astrophysical Journal. 833 (2): 204. arXiv: 1610.10066 . Bibcode:2016ApJ...833..204F. doi: 10.3847/1538-4357/833/2/204 . ISSN   1538-4357. S2CID   118594849.
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