Fluid catalytic cracking

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Fluid_catalytic_cracking 

A typical fluid catalytic cracking unit in a petroleum refinery.

Fluid Catalytic Cracking (FCC) is the conversion process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, alkene gases, and other petroleum products. The cracking of petroleum hydrocarbons was originally done by thermal cracking, now virtually replaced by catalytic cracking, which yields greater volumes of high octane rating gasoline; and produces by-product gases, with more carbon-carbon double bonds (i.e. alkenes), that are of greater economic value than the gases produced by thermal cracking.

The feedstock to the FCC conversion process usually is heavy gas oil (HGO), which is that portion of the petroleum (crude oil) that has an initial boiling-point temperature of 340 °C (644 °F) or higher, at atmospheric pressure, and that has an average molecular weight that ranges from about 200 to 600 or higher; heavy gas oil also is known as “heavy vacuum gas oil” (HVGO). In the fluid catalytic cracking process, the HGO feedstock is heated to a high temperature and to a moderate pressure, and then is placed in contact with a hot, powdered catalyst, which breaks the long-chain molecules of the high-boiling-point hydrocarbon liquids into short-chain molecules, which then are collected as a vapor.

Economics

Oil refineries use fluid catalytic cracking to correct the imbalance between the market demand for gasoline and the excess of heavy, high boiling range products resulting from the distillation of crude oil.

As of 2006, FCC units were in operation at 400 petroleum refineries worldwide, and about one-third of the crude oil refined in those refineries is processed in an FCC to produce high-octane gasoline and fuel oils. During 2007, the FCC units in the United States processed a total of 5,300,000 barrels (840,000 m³) of feedstock per day, and FCC units worldwide processed about twice that amount.

FCC units are less common in Europe, the Middle East and Africa (EMEA) because those regions have high demand for diesel and kerosene, which can be satisfied with hydrocracking. In the US, fluid catalytic cracking is more common because the demand for gasoline is higher.

Flow diagram and process description

The modern FCC units are all continuous processes which operate 24 hours a day for as long as 3 to 5 years between scheduled shutdowns for routine maintenance.

There are several different proprietary designs that have been developed for modern FCC units. Each design is available under a license that must be purchased from the design developer by any petroleum refining company desiring to construct and operate an FCC of a given design.

There are two different configurations for an FCC unit: the "stacked" type where the reactor and the catalyst regenerator are contained in two separate vessels, with the reactor above the regenerator, with a skirt between these vessels allowing the regenerator off-gas piping to connect to the top of the regenerator vessel, and the "side-by-side" type where the reactor and catalyst regenerator are in two separate vessels. The stacked configuration occupies less physical space of the refinery area. These are the major FCC designers and licensors:

Side-by-side configuration:

• Lummus Technology
• ExxonMobil Research and Engineering (EMRE)
• Shell Global Solutions
• Axens / Stone & Webster Process Technology — currently owned by Technip
• UOP LLC - A Honeywell Company

Stacked configuration:

• Kellogg Brown & Root (KBR)

Each of the proprietary design licensors claims to have unique features and advantages. A complete discussion of the relative advantages of each of the processes is beyond the scope of this article.

Reactor and regenerator

The reactor and regenerator are considered to be the heart of the fluid catalytic cracking unit. The schematic flow diagram of a typical modern FCC unit in Figure 1 below is based upon the "side-by-side" configuration. The preheated high-boiling petroleum feedstock (at about 315 to 430 °C) consisting of long-chain hydrocarbon molecules is combined with recycle slurry oil from the bottom of the distillation column and injected into the catalyst riser where it is vaporised and cracked into smaller molecules of vapour by contact and mixing with the very hot powdered catalyst from the regenerator. All of the cracking reactions take place in the catalyst riser within a period of 2–4 seconds. The hydrocarbon vapours "fluidize" the powdered catalyst and the mixture of hydrocarbon vapors and catalyst flows upward to enter the reactor at a temperature of about 535 °C and a pressure of about 1.72 bar.

The reactor is a vessel in which the cracked product vapors are: (a) separated from the spent catalyst by flowing through a set of two-stage cyclones within the reactor and (b) the spent catalyst flows downward through a steam stripping section to remove any hydrocarbon vapors before the spent catalyst returns to the catalyst regenerator. The flow of spent catalyst to the regenerator is regulated by a slide valve in the spent catalyst line.

Since the cracking reactions produce some carbonaceous material (referred to as catalyst coke) that deposits on the catalyst and very quickly reduces the catalyst activity, the catalyst is regenerated by burning off the deposited coke with air blown into the regenerator. The regenerator operates at a temperature of about 715 °C and a pressure of about 2.41 bar, hence the regenerator operates at about 0.7 bar higher pressure than the reactor. The combustion of the coke is exothermic and it produces a large amount of heat that is partially absorbed by the regenerated catalyst and provides the heat required for the vaporization of the feedstock and the endothermic cracking reactions that take place in the catalyst riser. For that reason, FCC units are often referred to as being 'heat balanced'.

The hot catalyst (at about 715 °C) leaving the regenerator flows into a catalyst withdrawal well where any entrained combustion flue gases are allowed to escape and flow back into the upper part to the regenerator. The flow of regenerated catalyst to the feedstock injection point below the catalyst riser is regulated by a slide valve in the regenerated catalyst line. The hot flue gas exits the regenerator after passing through multiple sets of two-stage cyclones that remove entrained catalyst from the flue gas.

The amount of catalyst circulating between the regenerator and the reactor amounts to about 5 kg per kg of feedstock, which is equivalent to about 4.66 kg per litre of feedstock. Thus, an FCC unit processing 75,000 barrels per day (11,900 m³/d) will circulate about 55,900 tonnes per day of catalyst.

Figure 1: A schematic flow diagram of a Fluid Catalytic Cracking unit as used in petroleum refineries

Main column

The reaction product vapors (at 535 °C and a pressure of 1.72 bar) flow from the top of the reactor to the bottom section of the main column (commonly referred to as the main fractionator where feed splitting takes place) where they are distilled into the FCC end products of cracked petroleum naphtha, fuel oil, and offgas. After further processing for removal of sulfur compounds, the cracked naphtha becomes a high-octane component of the refinery's blended gasolines.

The main fractionator offgas is sent to what is called a gas recovery unit where it is separated into butanes and butylenes, propane and propylene, and lower molecular weight gases (hydrogen, methane, ethylene and ethane). Some FCC gas recovery units may also separate out some of the ethane and ethylene.

Although the schematic flow diagram above depicts the main fractionator as having only one sidecut stripper and one fuel oil product, many FCC main fractionators have two sidecut strippers and produce a light fuel oil and a heavy fuel oil. Likewise, many FCC main fractionators produce a light cracked naphtha and a heavy cracked naphtha. The terminology light and heavy in this context refers to the product boiling ranges, with light products having a lower boiling range than heavy products.

The bottom product oil from the main fractionator contains residual catalyst particles which were not completely removed by the cyclones in the top of the reactor. For that reason, the bottom product oil is referred to as a slurry oil. Part of that slurry oil is recycled back into the main fractionator above the entry point of the hot reaction product vapors so as to cool and partially condense the reaction product vapors as they enter the main fractionator. The remainder of the slurry oil is pumped through a slurry settler. The bottom oil from the slurry settler contains most of the slurry oil catalyst particles and is recycled back into the catalyst riser by combining it with the FCC feedstock oil. The clarified slurry oil or decant oil is withdrawn from the top of slurry settler for use elsewhere in the refinery, as a heavy fuel oil blending component, or as carbon black feedstock.

Regenerator flue gas

Depending on the choice of FCC design, the combustion in the regenerator of the coke on the spent catalyst may or may not be complete combustion to carbon dioxide CO₂. The combustion air flow is controlled so as to provide the desired ratio of carbon monoxide (CO) to carbon dioxide for each specific FCC design.

In the design shown in Figure 1, the coke has only been partially combusted to CO₂. The combustion flue gas (containing CO and CO₂) at 715 °C and at a pressure of 2.41 bar is routed through a secondary catalyst separator containing swirl tubes designed to remove 70 to 90 percent of the particulates in the flue gas leaving the regenerator. This is required to prevent erosion damage to the blades in the turbo-expander that the flue gas is next routed through.

The expansion of flue gas through a turbo-expander provides sufficient power to drive the regenerator's combustion air compressor. The electrical motor–generator can consume or produce electrical power. If the expansion of the flue gas does not provide enough power to drive the air compressor, the electric motor–generator provides the needed additional power. If the flue gas expansion provides more power than needed to drive the air compressor, then the electric motor–generator converts the excess power into electric power and exports it to the refinery's electrical system.

The expanded flue gas is then routed through a steam-generating boiler (referred to as a CO boiler) where the carbon monoxide in the flue gas is burned as fuel to provide steam for use in the refinery as well as to comply with any applicable environmental regulatory limits on carbon monoxide emissions.

The flue gas is finally processed through an electrostatic precipitator (ESP) to remove residual particulate matter to comply with any applicable environmental regulations regarding particulate emissions. The ESP removes particulates in the size range of 2 to 20 µm from the flue gas. Particulate filter systems, known as Fourth Stage Separators (FSS) are sometimes required to meet particulate emission limits. These can replace the ESP when particulate emissions are the only concern.

The steam turbine in the flue gas processing system (shown in the above diagram) is used to drive the regenerator's combustion air compressor during start-ups of the FCC unit until there is sufficient combustion flue gas to take over that task.

Mechanism and products of catalytic cracking

Figure 2: Diagrammatic example of the catalytic cracking of petroleum hydrocarbons

The fluid catalytic cracking process breaks large hydrocarbons by their conversion to carbocations, which undergo myriad rearrangements.

Figure 2 is a very simplified schematic diagram that exemplifies how the process breaks high boiling, straight-chain alkane (paraffin) hydrocarbons into smaller straight-chain alkanes as well as branched-chain alkanes, branched alkenes (olefins) and cycloalkanes (naphthenes). The breaking of the large hydrocarbon molecules into smaller molecules is more technically referred to by organic chemists as scission of the carbon-to-carbon bonds.

As depicted in Figure 2, some of the smaller alkanes are then broken and converted into even smaller alkenes and branched alkenes such as the gases ethylene, propylene, butylenes, and isobutylenes. Those olefinic gases are valuable for use as petrochemical feedstocks. The propylene, butylene and isobutylene are also valuable feedstocks for certain petroleum refining processes that convert them into high-octane gasoline blending components.

As also depicted in Figure 2, the cycloalkanes (naphthenes) formed by the initial breakup of the large molecules are further converted to aromatics such as benzene, toluene, and xylenes, which boil in the gasoline boiling range and have much higher octane ratings than alkanes.

In the cracking process carbon is also produced which gets deposited on the catalyst (catalyst coke). The carbon formation tendency or amount of carbon in a crude or FCC feed is measured with methods such as Micro carbon residue, Conradson carbon residue, or Ramsbottom carbon residue.

Catalysts

FCC units continuously withdraw and replace some of the catalyst in order to maintain a steady level of activity. Modern FCC catalysts are fine powders with a bulk density of 0.80 to 0.96 g/cm³ and having a particle size distribution ranging from 10 to 150 µm and an average particle size of 60 to 100 μm. The design and operation of an FCC unit is largely dependent upon the chemical and physical properties of the catalyst. The desirable properties of an FCC catalyst are:

• Good stability to high temperature and to steam
• High activity
• Large pore sizes
• Good resistance to attrition
• Low coke production

Structure of aluminosilicate cage in faujasite. Vertices are occupied by aluminium or silicon, the connecting struts are occupied by oxide (O^2-) or hydroxide (OH⁻) centers. Special modifications of faujesite are strong solid acids, which at high temperatures induce the rearrangements of C-C bonds that occur in FCC units.

A modern FCC catalyst has four major components: crystalline zeolite, matrix, binder, and filler. Zeolite is the active component and can comprise from about 15% to 50%, by weight, of the catalyst. Faujasite (aka Type Y) is the zeolite used in FCC units. The zeolites are strong solid acids (equivalent to 90% sulfuric acid). The alumina matrix component of an FCC catalyst also contributes to catalytic activity sites. The binder and filler components provide the physical strength and integrity of the catalyst. The binder is usually silica sol and the filler is usually a clay (kaolin). The predominant suppliers of FCC catalysts worldwide are Albemarle Corporation, W.R. Grace Company, and BASF Catalysts (formerly Engelhard).

History

The first commercial use of catalytic cracking occurred in 1915 when Almer M. McAfee of Gulf Refining Company developed a batch process using aluminium chloride (a Friedel–Crafts catalyst known since 1877) to catalytically crack heavy petroleum oils. However, the prohibitive cost of the catalyst prevented the widespread use of McAfee's process at that time.

In 1922, a French mechanical engineer named Eugene Jules Houdry and a French pharmacist named E. A. Prudhomme set up a laboratory near Paris to develop a catalytic process for converting lignite coal to gasoline. Supported by the French government, they built a small demonstration plant in 1929 that processed about 60 tons per day of lignite coal. The results indicated that the process was not economically viable and it was subsequently shut down.

Houdry had found that Fuller's earth, a clay mineral containing aluminosilicates, could convert oil derived from the lignite to gasoline. He then began to study the catalysis of petroleum oils and had some success in converting vaporized petroleum oil to gasoline. In 1930, the Vacuum Oil Company invited him to come to the United States and he moved his laboratory to Paulsboro, New Jersey.

In 1931, the Vacuum Oil Company merged with Standard Oil of New York (Socony) to form the Socony-Vacuum Oil Company. In 1933, a small Houdry unit processed 200 barrels per day (32 m³/d) of petroleum oil. Because of the economic depression of the early 1930s, Socony-Vacuum was no longer able to support Houdry's work and gave him permission to seek help elsewhere.

In 1933, Houdry and Socony-Vacuum joined with Sun Oil Company in developing the Houdry process. Three years later, in 1936, Socony-Vacuum converted an older thermal cracking unit in their Paulsboro refinery in New Jersey to a small demonstration unit using the Houdry process to catalytically crack 2,000 barrels per day (320 m³/d) of petroleum oil.

In 1937, Sun Oil began operation of a new Houdry unit processing 12,000 barrels per day (1,900 m³/d) at their Marcus Hook refinery in Pennsylvania. The Houdry process at that time used reactors with a fixed bed of catalyst and was a semi-batch operation involving multiple reactors with some of the reactors in operation while other reactors were in various stages of regenerating the catalyst. Motor-driven valves were used to switch the reactors between online operation and offline regeneration and a cycle timer managed the switching. Almost 50 percent of the cracked product was gasoline as compared with about 25 percent from the thermal cracking processes.

By 1938, when the Houdry process was publicly announced, Socony-Vacuum had eight additional units under construction. Licensing the process to other companies also began and by 1940 there were 14 Houdry units in operation processing 140,000 barrels per day (22,000 m³/d).

The next major step was to develop a continuous process rather than the semi-batch Houdry process. That step was implemented by advent of the moving-bed process known as the Thermofor Catalytic Cracking (TCC) process which used a bucket conveyor-elevator to move the catalyst from the regeneration kiln to the separate reactor section. A small semi-commercial demonstration TCC unit was built in Socony-Vacuum's Paulsboro refinery in 1941 and operated successfully, producing 500 barrels per day (79 m³/d). Then a full-scale commercial TCC unit processing 10,000 barrels per day (1,600 m³/d) began operation in 1943 at the Beaumont, Texas refinery of Magnolia Oil Company, an affiliate of Socony-Vacuum. By the end of World War II in 1945, the processing capacity of the TCC units in operation was about 300,000 barrels per day (48,000 m³/d).

It is said that the Houdry and TCC units were a major factor in the winning of World War II by supplying the high-octane gasoline needed by the air forces of Great Britain and the United States for the more efficient higher compression ratio engines of the Spitfire and the Mustang.

In the years immediately after World War II, the Houdriflow process and the air-lift TCC process were developed as improved variations on the moving-bed theme. Just like Houdry's fixed-bed reactors, the moving-bed designs were prime examples of good engineering by developing a method of continuously moving the catalyst between the reactor and regeneration sections. The first air-lift TCC unit began operation in October 1950 at the Beaumont, Texas refinery.

This fluid catalytic cracking process had first been investigated in the 1920s by Standard Oil of New Jersey, but research on it was abandoned during the economic depression years of 1929 to 1939. In 1938, when the success of Houdry's process had become apparent, Standard Oil of New Jersey resumed the project, hopefully in competition with Houdry, as part of a consortium of that include five oil companies (Standard Oil of New Jersey, Standard Oil of Indiana, Anglo-Iranian Oil, Texas Oil and Royal Dutch Shell), two engineering-construction companies (M. W. Kellogg Limited and Universal Oil Products) and a German chemical company (I.G. Farben). The consortium was called Catalytic Research Associates (CRA) and its purpose was to develop a catalytic cracking process which would not impinge on Houdry's patents.

Chemical engineering professors Warren K. Lewis and Edwin R. Gilliland of the Massachusetts Institute of Technology (MIT) suggested to the CRA researchers that a low velocity gas flow through a powder might "lift" it enough to cause it to flow in a manner similar to a liquid. Focused on that idea of a fluidized catalyst, researchers Donald Campbell, Homer Martin, Eger Murphree and Charles Tyson of the Standard Oil of New Jersey (now Exxon-Mobil Company) developed the first fluidized catalytic cracking unit. Their U.S. Patent No. 2,451,804, A Method of and Apparatus for Contacting Solids and Gases, describes their milestone invention. Based on their work, M. W. Kellogg Company constructed a large pilot plant in the Baton Rouge, Louisiana refinery of the Standard Oil of New Jersey. The pilot plant began operation in May 1940.

Based on the success of the pilot plant, the first commercial fluid catalytic cracking plant (known as the Model I FCC) began processing 13,000 barrels per day (2,100 m³/d) of petroleum oil in the Baton Rouge refinery on May 25, 1942, just four years after the CRA consortium was formed and in the midst of World War II. A little more than a month later, in July 1942, it was processing 17,000 barrels per day (2,700 m³/d). In 1963, that first Model I FCC unit was shut down after 21 years of operation and subsequently dismantled.

In the many decades since the Model I FCC unit began operation, the fixed bed Houdry units have all been shut down as have most of the moving bed units (such as the TCC units) while hundreds of FCC units have been built. During those decades, many improved FCC designs have evolved and cracking catalysts have been greatly improved, but the modern FCC units are essentially the same as that first Model I FCC unit.

Molecular cloud

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Molecular_cloud

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.

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.

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.

History

The form of molecular clouds by interstellar dust and hydrogen gas traces its links to the formation of the Solar System, approximately 4.6 billion years ago.

Occurrence

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 the medium, comprising roughly half of the total gas mass interior to the Sun's galactic orbit. 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). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. 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.

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. 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.

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.

Part of the Taurus molecular cloud

A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun 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. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times lower. 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.

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. A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation.

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 centimetre. Observationally, 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.

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. 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.

Distribution of molecular gas in 30 merging galaxies.

Small molecular clouds

Main article: Bok globule

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

Main article: Infrared cirrus

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

Processes

Young stars in and around molecular cloud Cepheus B. Radiation from one bright, massive star is destroying the cloud (from top to bottom in this image) while simultaneously triggering the formation of new stars.

Star formation

Main article: Star formation

The formation of stars occurs exclusively within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse. There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity (like stars, planets, and galaxies) rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity (a virial relation).

Physics

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.

The physics of molecular clouds is poorly understood and much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are highly supersonic but comparable to the speeds of magnetic disturbances. This state is thought to lose energy rapidly, requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most likely the effects of massive stars—before a significant fraction of their mass has become stars.

Molecular clouds, and especially GMCs, are often the home of astronomical masers.

List of molecular cloud complexes

For apparent groups of dark nebulae, see Dark cloud constellation.

The Milky Way as seen by Gaia, with prominent dark nebulae many of which are molecular cloud complex (labeled in white), as well as prominent star clouds (labeled in black).

• Great Rift
  • Serpens-Aquila Rift
• Rho Ophiuchi cloud complex
• Corona Australis molecular cloud
• Musca–Chamaeleonis molecular cloud
• Vela Molecular Ridge
• Orion molecular cloud complex
• Taurus molecular cloud
• Perseus molecular cloud

Megamaser

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Megamaser

A megamaser acts as an astronomical laser that beams out microwave emission rather than visible light (hence the ‘m’ replacing the ‘l’).

A megamaser is a type of astrophysical maser, which is a naturally occurring source of stimulated spectral line emission. Megamasers are distinguished from other astrophysical masers by their large isotropic luminosity. Megamasers have typical luminosities of 10³ solar luminosities (L_☉), which is 100 million times brighter than masers in the Milky Way, hence the prefix mega. Likewise, the term kilomaser is used to describe masers outside the Milky Way that have luminosities of order L_☉, or thousands of times stronger than the average maser in the Milky Way, gigamaser is used to describe masers billions of times stronger than the average maser in the Milky Way, and extragalactic maser encompasses all masers found outside the Milky Way. Most known extragalactic masers are megamasers, and the majority of megamasers are hydroxyl (OH) megamasers, meaning the spectral line being amplified is one due to a transition in the hydroxyl molecule. There are known megamasers for three other molecules: water (H₂O), formaldehyde (H₂CO), and methine (CH).

Water megamasers were the first type of megamaser discovered. The first water megamaser was found in 1979 in NGC 4945, a galaxy in the nearby Centaurus A/M83 Group. The first hydroxyl megamaser was found in 1982 in Arp 220, which is the nearest ultraluminous infrared galaxy to the Milky Way. All subsequent OH megamasers that have been discovered are also in luminous infrared galaxies, and there are a small number of OH kilomasers hosted in galaxies with lower infrared luminosities. Most luminous infrared galaxies have recently merged or interacted with another galaxy, and are undergoing a burst of star formation. Many of the characteristics of the emission in hydroxyl megamasers are distinct from that of hydroxyl masers within the Milky Way, including the amplification of background radiation and the ratio of hydroxyl lines at different frequencies. The population inversion in hydroxyl molecules is produced by far infrared radiation that results from absorption and re-emission of light from forming stars by surrounding interstellar dust. Zeeman splitting of hydroxyl megamaser lines may be used to measure magnetic fields in the masing regions, and this application represents the first detection of Zeeman splitting in a galaxy other than the Milky Way.

Water megamasers and kilomasers are found primarily associated with active galactic nuclei, while galactic and weaker extragalactic water masers are found in star forming regions. Despite different environments, the circumstances that produce extragalactic water masers do not seem to be very different from those that produce galactic water masers. Observations of water megamasers have been used to make accurate measurements of distances to galaxies in order to provide constraints on the Hubble constant.

Background

Masers

Main article: maser

Diagram showing the process of stimulated emission

The word maser derives from the acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation". The maser is a predecessor to lasers, which operate at optical wavelengths, and is named by the replacement of "microwave" with "light". Given a system of atoms or molecules, each with different energy states, an atom or molecule may absorb a photon and move to a higher energy level, or the photon may stimulate emission of another photon of the same energy and cause a transition to a lower energy level. Producing a maser requires population inversion, which is when a system has more members in a higher energy level relative to a lower energy level. In such a situation, more photons will be produced by stimulated emission than will be absorbed. Such a system is not in thermal equilibrium, and as such requires special conditions to occur. Specifically, it must have some energy source that can pump the atoms or molecules to the excited state. Once population inversion occurs, a photon with a photon energy corresponding to the energy difference between two states can then produce stimulated emission of another photon of the same energy. The atom or molecule will drop to the lower energy level, and there will be two photons of the same energy, where before there was only one. The repetition of this process is what leads to amplification, and since all of the photons are the same energy, the light produced is monochromatic.

Astrophysical masers

Main article: astrophysical maser

Masers and lasers built on Earth and masers that occur in space both require population inversion in order to operate, but the conditions under which population inversion occurs are very different in the two cases. Masers in laboratories have systems with high densities, which limits the transitions that may be used for masing, and requires using a resonant cavity in order to bounce light back and forth many times. Astrophysical masers are at low densities, and naturally have very long path lengths. At low densities, being out of thermal equilibrium is more easily achieved because thermal equilibrium is maintained by collisions, meaning population inversion can occur. Long path lengths provide photons traveling through the medium many opportunities to stimulate emission, and produce amplification of a background source of radiation. These factors accumulate to "make interstellar space a natural environment for maser operation." Astrophysical masers may be pumped either radiatively or collisionally. In radiative pumping, infrared photons with higher energies than the maser transition photons preferentially excite atoms and molecules to the upper state in the maser in order to produce population inversion. In collisional pumping, this population inversion is instead produced by collisions that excite molecules to energy levels above that of the upper maser level, and then the molecule decays to the upper maser level by emitting photons.

History

In 1965, twelve years after the first maser was built in a laboratory, a hydroxyl (OH) maser was discovered in the plane of the Milky Way. Masers of other molecules were discovered in the Milky Way in the following years, including water (H₂O), silicon monoxide (SiO), and methanol (CH₃OH). The typical isotropic luminosity for these galactic masers is 10⁻⁶–10⁻³ L_☉. The first evidence for extragalactic masing was detection of the hydroxyl molecule in NGC 253 in 1973, and was roughly ten times more luminous than galactic masers.

In 1982, the first megamaser was discovered in the ultraluminous infrared galaxy Arp 220. The luminosity of the source, assuming it emits isotropically, is roughly 10³ L_☉. This luminosity is roughly one hundred million times stronger than the typical maser found in the Milky Way, and so the maser source in Arp 220 was called a megamaser. At this time, extragalactic water (H₂O) masers were already known. In 1984, water maser emission was discovered in NGC 4258 and NGC 1068 that was of comparable strength to the hydroxyl maser in Arp 220, and are as such considered water megamasers.

Over the next decade, megamasers were also discovered for formaldehyde (H₂CO) and methine (CH). Galactic formaldehyde masers are relatively rare, and more formaldehyde megamasers are known than are galactic formaldehyde masers. Methine masers, on the other hand, are quite common in the Milky Way. Both types of megamaser were found in galaxies in which hydroxyl had been detected. Methine is seen in galaxies with hydroxyl absorption, while formaldehyde is found in galaxies with hydroxyl absorption as well as those with hydroxyl megamaser emission.

As of 2007, 109 hydroxyl megamaser sources were known, up to a redshift of ${\displaystyle z\approx 0.27}$. Over 100 extragalactic water masers are known, and of these, 65 are bright enough to be considered megamasers.

General requirements

Galaxies MCG+01-38-004 (upper) and MCG+01-38-005 (lower) – the microwave emissions from MCG+01-38-005 were used to calculate a refined value for the Hubble constant.

Regardless of the masing molecule, there are a few requirements that must be met for a strong maser source to exist. One requirement is a radio continuum background source to provide the radiation amplified by the maser, as all maser transitions take place at radio wavelengths. The masing molecule must have a pumping mechanism to create the population inversion, and sufficient density and path length for significant amplification to take place. These combine to constrain when and where megamaser emission for a given molecule will take place. The specific conditions for each molecule known to produce megamasers are different, as exemplified by the fact that there is no known galaxy that hosts both of the two most common megamaser species, hydroxyl and water. As such, the different molecules with known megamasers will be addressed individually.

Hydroxyl megamasers

Arp 220 hosts the first megamaser discovered, is the nearest ultraluminous infrared galaxy, and has been studied in great detail at many wavelengths. For this reason, it is the prototype of hydroxyl megamaser host galaxies, and is often used as a guide for interpreting other hydroxyl megamasers and their hosts.

Hosts and environment

Arp 220, the prototypical hydroxyl megamaser host galaxy (Hubble Space Telescope)

Hydroxyl megamasers are found in the nuclear region of a class of galaxies called luminous infrared galaxies (LIRGs), with far infrared luminosities in excess of one hundred billion solar luminosities, or L_FIR > 10¹¹ L_☉, and ultra-luminous infrared galaxies (ULIRGs), with L_FIR > 10¹² L_☉ are favored. These infrared luminosities are very large, but in many cases LIRGs are not particularly luminous in visible light. For instance, the ratio of infrared luminosity to luminosity in blue light is roughly 80 for Arp 220, the first source in which a megamaser was observed.

The majority of the LIRGs show evidence of interaction with other galaxies or having recently experienced a galaxy merger, and the same holds true for the LIRGs that host hydroxyl megamasers. Megamaser hosts are rich in molecular gas compared to spiral galaxies, with molecular hydrogen masses in excess of one billion solar masses, or H₂ > 10⁹ M_☉. Mergers help funnel molecular gas to the nuclear region of the LIRG, producing high molecular densities and stimulating high star formation rates characteristic of LIRGs. The starlight in turn heats dust, which re-radiates in the far infrared and produces the high L_FIR observed in hydroxyl megamaser hosts. The dust temperatures derived from far infrared fluxes are warm relative to spirals, ranging from 40–90 K.

The far infrared luminosity and dust temperature of a LIRG both affect the likelihood of hosting an hydroxyl megamaser, through correlations between the dust temperature and far infrared luminosity, so it is unclear from observations alone what the role of each is in producing hydroxyl megamasers. LIRGs with warmer dust are more likely to host hydroxyl megamasers, as are ULIRGs, with L_FIR > 10¹² L_☉. At least one out of three ULIRGs hosts an hydroxyl megamaser, as compared with roughly one out of six LIRGs. Early observations of hydroxyl megamasers indicated a correlation between the isotropic hydroxyl luminosity and far infrared luminosity, with L_OH ${\displaystyle \propto }$ L_FIR². As more hydroxyl megamasers were discovered, and care was taken to account for the Malmquist bias, this observed relationship was found to be flatter, with L_OH ${\displaystyle \propto }$ L_FIR^{1.2${\displaystyle \pm }$0.1}.

Early spectral classification of the nuclei of the LIRGs that host hydroxyl megamasers indicated that the properties of LIRGs that host hydroxyl megamasers cannot be distinguished from the overall population of LIRGs. Roughly one third of megamaser hosts are classified as starburst galaxies, one quarter are classified as Seyfert 2 galaxies, and the remainder are classified as low-ionization nuclear emission-line regions, or LINERs. The optical properties of hydroxyl megamaser hosts and non-hosts are not significantly different. Recent infrared observations using the Spitzer Space Telescope are, however, able to distinguish hydroxyl megamaser hosts galaxies from non-masing LIRGs, as 10–25% of hydroxyl megamaser hosts show evidence for an active galactic nucleus, compared to 50–95% for non-masing LIRGs.

The LIRGs that host hydroxyl megamasers may be distinguished from the general population of LIRGs by their molecular gas content. The majority of molecular gas is molecular hydrogen, and typical hydroxyl megamaser hosts have molecular gas densities greater than 1000 cm⁻³. These densities are among the highest mean densities of molecular gas among LIRGs. The LIRGs that host hydroxyl megamasers also have high fractions of dense gas relative to typical LIRGs. The dense gas fraction is measured by the ratio of the luminosity produced by hydrogen cyanide (HCN) relative to the luminosity of carbon monoxide (CO).

Line characteristics

The 1665 and 1667 MHz maser lines in Arp 220, which have been redshifted to lower frequencies (Arecibo Observatory data)

The emission of hydroxyl megamasers occurs predominantly in the so-called "main lines" at 1665 and 1667 MHz. The hydroxyl molecule also has two "satellite lines" that emit at 1612 and 1720 MHz, but few hydroxyl megamasers have had satellite lines detected. Emission in all known hydroxyl megamasers is stronger in the 1667 MHz line; typical ratios of the flux in the 1667 MHz line to the 1665 MHz line, called the hyperfine ratio, range from a minimum of 2 to greater than 20. For hydroxyl emitting in thermodynamic equilibrium, this ratio will range from 1.8 to 1, depending upon the optical depth, so line ratios greater than 2 are indicative of a population out of thermal equilibrium. This may be compared with galactic hydroxyl masers in star-forming regions, where the 1665 MHz line is typically strongest, and hydroxyl masers around evolved stars, in which the 1612 MHz line is often strongest, and of the main lines, 1667 MHz emission is frequently stronger than 1612 MHz. The total width of emission at a given frequency is typically many hundreds of kilometers per second, and individual features that make up the total emission profile have widths ranging from tens to hundreds of kilometers per second. These may also be compared with galactic hydroxyl masers, which typically have linewidths of order a kilometer per second or narrower, and are spread over a velocity of a few to tens of kilometers per second.

The radiation amplified by hydroxyl masers is the radio continuum of its host. This continuum is primarily composed of synchrotron radiation produced by Type II supernovae. Amplification of this background is low, with amplification factors, or gains, ranging from a few percent to a few hundred percent, and sources with larger hyperfine ratios typically exhibiting larger gains. Sources with higher gains typically have narrower emission lines. This is expected if the pre-gain linewidths are all roughly the same, as line centers are amplified more than the wings, leading to line narrowing.

A few hydroxyl megamasers, including Arp 220, have been observed with very long baseline interferometry (VLBI), which allows sources to be studied at higher angular resolution. VLBI observations indicate that hydroxyl megamaser emission is composed of two components, one diffuse and one compact. The diffuse component displays gains of less than a factor of one and linewidths of order hundreds of kilometers per second. These characteristics are similar to those seen with single dish observations of hydroxyl megamasers that are unable to resolve individual masing components. The compact components have high gains, ranging from tens to hundreds, high ratios of flux at 1667 MHz to flux at 1665 MHz, and linewidths are of order a few kilometers per second. These general features have been explained by a narrow circumnuclear ring of material from which the diffuse emission arises, and individual masing clouds with sizes of order one parsec that give rise to the compact emission. The hydroxyl masers observed in the Milky Way more closely resemble the compact hydroxyl megamaser components. There are, however, some regions of extended galactic maser emission from other molecules that resemble the diffuse component of hydroxyl megamasers.

Pumping mechanism

The observed relationship between the luminosity of the hydroxyl line and the far infrared suggests that hydroxyl megamasers are radiatively pumped. Initial VLBI measurements of nearby hydroxyl megamasers seemed to present a problem with this model for compact emission components of hydroxyl megamasers, as they required a very high fraction of infrared photons to be absorbed by hydroxyl and lead to a maser photon being emitted, making collisional excitation a more plausible pumping mechanism. However, a model of maser emission with a clumpy masing medium appear to be able to reproduce the observed properties of compact and diffuse hydroxyl emission. A recent detailed treatment finds that photons with a wavelength of 53 micrometres are the primary pump for main line maser emission, and applies to all hydroxyl masers. In order to provide enough photons at this wavelength, the interstellar dust that reprocesses stellar radiation to infrared wavelengths must have a temperature of at least 45 kelvins. Recent observations with the Spitzer Space Telescope confirm this basic picture, but there are still some discrepancies between details of the model and observations of hydroxyl megamaser host galaxies such as the required dust opacity for megamaser emission.

Applications

Hydroxyl megamasers occur in the nuclear regions of LIRGs, and appear to be a marker in the stage of the formation of galaxies. As hydroxyl emission is not subject to extinction by interstellar dust in its host LIRG, hydroxyl masers may be useful probes of the conditions where star formation in LIRGs takes place. At redshifts of z ~ 2, there are LIRG-like galaxies more luminous than the ones in the nearby universe. The observed relationship between the hydroxyl luminosity and far infrared luminosity suggests that hydroxyl megamasers in such galaxies may be tens to hundreds of times more luminous than observed hydroxyl megamasers. Detection of hydroxyl megamasers in such galaxies would allow precise determination of the redshift, and aid understanding of star formation in these objects.

The first detection of the Zeeman effect in another galaxy was made through observations of hydroxyl megamasers. The Zeeman effect is the splitting of a spectral line due to the presence of a magnetic field, and the size of the splitting is linearly proportional to the line-of-sight magnetic field strength. Zeeman splitting has been detected in five hydroxyl megamasers, and the typical strength of a detected field is of order a few milligauss, similar to the field strengths measured in galactic hydroxyl masers.

Water megamasers

Whereas hydroxyl megamasers seem to be fundamentally distinct in some ways from galactic hydroxyl masers, water megamasers do not seem to require conditions too dissimilar from galactic water masers. Water masers stronger than galactic water masers, some of which are strong enough to be classified "mega" masers, may be described by the same luminosity function as galactic water masers. Some extragalactic water masers occur in star forming regions, like galactic water masers, while stronger water masers are found in the circumnuclear regions around active galactic nuclei (AGN). The isotropic luminosities of these span a range of order one to a few hundred L_☉, and are found in nearby galaxies like Messier 51 (0.8 L_☉) and more distant galaxies like NGC 4258 (120 L_☉).

Line characteristics and pumping mechanism

Water maser emission is observed primarily at 22 GHz, due to a transition between rotational energy levels in the water molecule. The upper state is at an energy corresponding to 643 kelvins about the ground state, and populating this upper maser level requires number densities of molecular hydrogen of order 10⁸ cm⁻³ or greater and temperatures of at least 300 kelvins. The water molecule comes into thermal equilibrium at molecular hydrogen number densities of roughly 10¹¹ cm⁻³, so this places an upper limit on the number density in a water masing region. Water masers emission has been successfully modelled by masers occurring behind shock waves propagating through dense regions in the interstellar medium. These shocks produce the high number densities and temperatures (relative to typical conditions in the interstellar medium) required for maser emission, and are successful in explaining observed masers.

Applications

Water megamasers may be used to provide accurate distance determinations to distant galaxies. Assuming a Keplerian orbit, measuring the centripetal acceleration and velocity of water maser spots yields the physical diameter subtended by the maser spots. By then comparing the physical radius to the angular diameter measured on the sky, the distance to the maser may be determined. This method is effective with water megamasers because they occur in a small region around an AGN, and have narrow linewidths. This method of measuring distances is being used to provide an independent measure of the Hubble constant that does not rely upon use of standard candles. The method is limited, however, by the small number of water megamasers known at distances within the Hubble flow. This distance measurement also provides a measurement of the mass of the central object, which in this case is a supermassive black hole. Black hole mass measurements using water megamasers is the most accurate method of mass determination for black holes in galaxies other than the Milky Way. The black hole masses that are measured are consistent with the M–sigma relation, an empirical correlation between stellar velocity dispersion in galactic bulges and the mass of the central supermassive black hole.

Astrophysical maser

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Astrophysical_maser

Aurorae on the north pole of Jupiter generate cyclotron masers (Hubble)

An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar atmospheres, or various other conditions in interstellar space.

Background

Main article: maser

Discrete transition energy

Like a laser, the emission from a maser is stimulated (or seeded) and monochromatic, having the frequency corresponding to the energy difference between two quantum-mechanical energy levels of the species in the gain medium which have been pumped into a non-thermal population distribution. However, naturally occurring masers lack the resonant cavity engineered for terrestrial laboratory masers. The emission from an astrophysical maser is due to a single pass through the gain medium and therefore generally lacks the spatial coherence and mode purity expected from a laboratory maser.

Nomenclature

Due to the differences between engineered and naturally occurring masers, it is often stated that astrophysical masers are not "true" masers because they lack oscillation cavities. However, the distinction between oscillator-based lasers and single-pass lasers was intentionally disregarded by the laser community in the early years of the technology.

This fundamental incongruency in language has resulted in the use of other paradoxical definitions in the field. For example, if the gain medium of a (misaligned) laser is emission-seeded but non-oscillating radiation, it is said to emit amplified spontaneous emission or ASE. This ASE is regarded as unwanted or parasitic (some researchers would add to this definition the presence of insufficient feedback or unmet lasing threshold): that is, the users wish the system to behave as a laser. The emission from astrophysical masers is, in fact, ASE but is sometimes termed superradiant emission to differentiate it from the laboratory phenomenon. This simply adds to the confusion, since both sources are superradiant. In some laboratory lasers, such as a single pass through a regeneratively amplified Ti:Sapph stage, the physics is directly analogous to an amplified ray in an astrophysical maser.

Furthermore, the practical limits of the use of the m to stand for microwave in maser are variously employed. For example, when lasers were initially developed in the visible portion of the spectrum, they were called optical masers. Charles Townes advocated that the m stand for molecule, since energy states of molecules generally provide the masing transition. Along these lines, some use the term laser to describe any system that exploits an electronic transition and the term maser to describe a system that exploits a rotational or vibrational transition, regardless of the output frequency. Some astrophysicists use the term iraser to describe a maser emitting at a wavelength of a few micrometres, even though the optics community terms similar sources lasers. The term taser has been used to describe laboratory masers in the terahertz regime, although astronomers might call these sub-millimeter masers and laboratory physicists generally call these gas lasers or specifically alcohol lasers in reference to the gain species. The electrical engineering community typically limits the use of the word microwave to frequencies between roughly 1 GHz and 300 GHz; that is, wavelengths between 30 cm and 1 mm, respectively.

Astrophysical conditions

The simple existence of a pumped population inversion is not sufficient for the observation of a maser. For example, there must be velocity coherence (light) along the line of sight so that Doppler shifting does not prevent inverted states in different parts of the gain medium from radiatively coupling. While polarisation in laboratory lasers and masers may be achieved by selectively oscillating the desired modes, polarisation in natural masers will arise only in the presence of a polarisation-state–dependent pump or of a magnetic field in the gain medium. Finally, the radiation from astrophysical masers can be quite weak and may escape detection due to the limited sensitivity (and relative remoteness) of astronomical observatories and due to the sometimes overwhelming spectral absorption from unpumped molecules of the maser species in the surrounding space. This latter obstacle may be partially surmounted through the judicious use of the spatial filtering inherent in interferometric techniques, especially very long baseline interferometry (VLBI).

The study of masers provides valuable information on the conditions—temperature, density, magnetic field, and velocity—in environments of stellar birth and death and the centres of galaxies containing black holes, leading to refinements in existing theoretical models.

Discovery

Historical background

In 1965 an unexpected discovery was made by Weaver et al.: emission lines in space, of unknown origin, at a frequency of 1665 MHz. At this time many researchers still thought that molecules could not exist in space, even though they had been discovered by McKellar in the 1940s, and so the emission was at first attributed to an hypothetical form of interstellar matter named "mysterium"; but the emission was soon identified as line emission from hydroxide molecules in compact sources within molecular clouds. More discoveries followed, with water emission in 1969, methanol emission in 1970, and silicon monoxide emission in 1974, all emanating from within molecular clouds. These were termed masers, as from their narrow line widths and high effective temperatures it became clear that these sources were amplifying microwave radiation.

Masers were then discovered around highly evolved late-type stars (named OH/IR stars). First was hydroxide emission in 1968, then water emission in 1969 and silicon monoxide emission in 1974. Masers were also discovered in external galaxies in 1973, and in the Solar System in comet halos.

Another unexpected discovery was made in 1982 with the discovery of emission from an extra-galactic source with an unrivalled luminosity about 10⁶ times larger than any previous source. This was termed a megamaser because of its great luminosity; many more megamasers have since been discovered.

A weak disk maser was discovered in 1995 emanating from the star MWC 349A, using NASA's Kuiper Airborne Observatory.

Evidence for an anti-pumped (dasar) sub-thermal population in the 4830 MHz transition of formaldehyde (H₂CO) was observed in 1969 by Palmer et al.

Detection

The connections of maser activity with far infrared (FIR) emission has been used to conduct searches of the sky with optical telescopes (because optical telescopes are easier to use for searches of this kind), and likely objects are then checked in the radio spectrum. Particularly targeted are molecular clouds, OH-IR stars, and FIR active galaxies.

Known interstellar species

The following species have been observed in stimulated emission from astronomical environments:

• OH
• CH
• H₂CO
• H₂O
• NH₃, ¹⁵NH₃
• CH₃OH
• HNCNH
• SiS
• HC₃N
• SiO, ²⁹SiO, ³⁰SiO
• HCN, H¹³CN
• H (in MWC 349)
• CS

Characteristics of maser radiation

The amplification or gain of radiation passing through a maser cloud is exponential. This has consequences for the radiation it produces:

Beaming

Small path differences across the irregularly shaped maser cloud become greatly distorted by exponential gain. Part of the cloud that has a slightly longer path length than the rest will appear much brighter (as it is the exponent of the path length that is relevant), and so maser spots are typically much smaller than their parent clouds. The majority of the radiation will emerge along this line of greatest path length in a "beam"; this is termed beaming.

Rapid variability

As the gain of a maser depends exponentially on the population inversion and the velocity-coherent path length, any variation of either will itself result in exponential change of the maser output.

Line narrowing

Exponential gain also amplifies the centre of the line shape (Gaussian or Lorentzian, etc.) more than the edges or wings. This results in an emission line shape that is much taller but not much wider. This makes the line appear narrower relative to the unamplified line.

Saturation

The exponential growth in intensity of radiation passing through a maser cloud continues as long as pumping processes can maintain the population inversion against the growing losses by stimulated emission. While this is so the maser is said to be unsaturated. However, after a point, the population inversion cannot be maintained any longer and the maser becomes saturated. In a saturated maser, amplification of radiation depends linearly on the size of population inversion and the path length. Saturation of one transition in a maser can affect the degree of inversion in other transitions in the same maser, an effect known as competitive gain.

High brightness

The brightness temperature of a maser is the temperature a black body would have if producing the same emission brightness at the wavelength of the maser. That is, if an object had a temperature of about 10⁹K it would produce as much 1665-MHz radiation as a strong interstellar OH maser. Of course, at 10⁹K the OH molecule would dissociate (kT is greater than the bond energy), so the brightness temperature is not indicative of the kinetic temperature of the maser gas but is nevertheless useful in describing maser emission. Masers have incredible effective temperatures, many around 10⁹K, but some of up to 10¹²K and even 10¹⁴K.

Polarisation

An important aspect of maser study is polarisation of the emission. Astronomical masers are often very highly polarised, sometimes 100% (in the case of some OH masers) in a circular fashion, and to a lesser degree in a linear fashion. This polarisation is due to some combination of the Zeeman effect, magnetic beaming of the maser radiation, and anisotropic pumping which favours certain magnetic-state transitions.

Many of the characteristics of megamaser emission are different.

Maser environments

Comets

Comets are small bodies (5 to 15 km diameter) of frozen volatiles (e.g., water, carbon dioxide, ammonia, and methane) embedded in a crusty silicate filler that orbit the Sun in eccentric orbits. As they approach the Sun, the volatiles vaporise to form a halo and later a tail around the nucleus. Once vaporised, these molecules can form inversions and mase.

The impact of comet Shoemaker-Levy 9 with Jupiter in 1994 resulted in maser emissions in the 22 GHz region from the water molecule. Despite the apparent rarity of these events, observation of the intense maser emission has been suggested as a detection scheme for extrasolar planets.

Ultraviolet light from the Sun breaks down some water molecules to form hydroxides that can mase. In 1997, 1667-MHz maser emission characteristic of hydroxide was observed from comet Hale-Bopp.

Planetary atmospheres

It is predicted that masers exist in the atmospheres of gas giant planets. Such masers would be highly variable due to planetary rotation (10-hour period for Jovian planets). Cyclotron masers have been detected at the north pole of Jupiter.

Planetary systems

In 2009, S. V. Pogrebenko et al. reported the detection of water masers in the plumes of water associated with the Saturnian moons Hyperion, Titan, Enceladus, and Atlas.

Stellar atmospheres

Pulsations of the Mira variable S Orionis, showing dust production and masers (ESO)

The conditions in the atmospheres of late-type stars support the pumping of different maser species at different distances from the star. Due to instabilities within the nuclear burning sections of the star, the star experiences periods of increased energy release. These pulses produce a shockwave that forces the atmosphere outward. Hydroxyl masers occur at a distance of about 1,000 to 10,000 astronomical units (AU), water masers at a distance of about 100 to 400 AU, and silicon monoxide masers at a distance of about 5 to 10 AU. Both radiative and collisional pumping resulting from the shockwave have been suggested as the pumping mechanism for the silicon monoxide masers. These masers diminish for larger radii as the gaseous silicon monoxide condenses into dust, depleting the available maser molecules. For the water masers, the inner and outer radii limits roughly correspond to the density limits for maser operation. At the inner boundary, the collisions between molecules are enough to remove a population inversion. At the outer boundary, the density and optical depth is low enough that the gain of the maser is diminished. Additionally, the hydroxyl masers are supported chemical pumping. At the distances where these masers are found water molecules are disassociated by UV radiation.

Star-forming regions

Main article: Star formation

Young stellar objects and (ultra)compact H II regions embedded in molecular clouds and giant molecular clouds, support the bulk of astrophysical masers. Various pumping schemes – both radiative and collisional and combinations thereof – result in the maser emission of multiple transitions of many species. For example, the OH molecule has been observed to mase at 1612, 1665, 1667, 1720, 4660, 4750, 4765, 6031, 6035, and 13441 MHz. Water and methanol masers are also typical of these environments. Relatively rare masers such as ammonia and formaldehyde may also be found in star-forming regions.

Supernova remnants

WISE image of IC 443, a supernova remnant with maser emission

The 1720 MHz maser transition of hydroxide is known to be associated with supernova remnants that interact with molecular clouds.

Extragalactic sources

While some of the masers in star forming regions can achieve luminosities sufficient for detection from external galaxies (such as the nearby Magellanic Clouds), masers observed from distant galaxies generally arise in wholly different conditions. Some galaxies possess central black holes into which a disk of molecular material (about 0.5 parsec in size) is falling. Excitations of these molecules in the disk or in a jet can result in megamasers with large luminosities. Hydroxyl, water, and formaldehyde masers are known to exist in these conditions.

Ongoing research

Astronomical masers remain an active field of research in radio astronomy and laboratory astrophysics due, in part, to the fact that they are valuable diagnostic tools for astrophysical environments which may otherwise elude rigorous quantitative study and because they may facilitate the study of conditions which are inaccessible in terrestrial laboratories.

Variability

Maser variability is generally understood to mean the change in apparent brightness to the observer. Intensity variations can occur on timescales from days to years indicating limits on maser size and excitation scheme. However, masers change in various ways over various timescales.

Distance determinations

Masers in star-forming regions are known to move across the sky along with the material that is flowing out from the forming star(s). Also, since the emission is a narrow spectral line, line-of-sight velocity can be determined from the Doppler shift variation of the observed frequency of the maser, permitting a three-dimensional mapping of the dynamics of the maser environment. Perhaps the most spectacular success of this technique is the dynamical determination of the distance to the galaxy NGC 4258 from the analysis of the motion of the masers in the black-hole disk. Also, water masers have been used to estimate the distance and proper motion of galaxies in the Local Group, including that of the Triangulum Galaxy.

VLBI observations of maser sources in late type stars and star forming regions provide determinations of their trigonometric parallax and therefore their distance. This method is much more accurate than other distance determinations, and gives us information about the galactic distance scale (e.g. the distance of spiral arms).

Open issues

Unlike terrestrial lasers and masers for which the excitation mechanism is known and engineered, the reverse is true for astrophysical masers. In general, astrophysical masers are discovered empirically then studied further in order to develop plausible suggestions about possible pumping schemes. Quantification of the transverse size, spatial and temporal variations, and polarisation state (typically requiring VLBI telemetry) are all useful in the development of a pump theory. Galactic formaldehyde masing is one such example that remains problematic.

On the other hand, some masers have been predicted to occur theoretically but have yet to be observed in nature. For example, the magnetic dipole transitions of the OH molecule near 53 MHz are expected to occur but have yet to be observed, perhaps due to a lack of sensitive equipment.