Astrophysical maser

From Wikipedia, the free encyclopedia
  (Redirected from Interstellar masers)
Jump to: navigation, search



Main article: maser

Discrete Transition Energy[edit]



Due to the differences between engineered and naturally occurring masers, it is often stated [1] that astrophysical masers are not "true" masers because they lack an oscillation cavity. The distinction between oscillator-based losers and single-pass lasers was intentionally disregarded by the laser community in the early years of the technology.[2]


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.[3] Townes advocated that the m stand for molecule since energy states of molecules generally provide the masing transition.[4] Along these lines, some will use the term laser to describe any system which exploits an electronic transition and the term maser to describe a system which 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,[5] even though the optics community has similar sources which they call lasers. The term taser has been used to describe laboratory masers in the terahertz regime[6] 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 roughly between 1 GHz and 300 GHz.

Astrophysical conditions[edit]

The simple existence of a pumped population inversion is not sufficient for the observation of a maser. For example, there must be velocity coherence along the line of sight so that Doppler shifting does not prevent inverted states in different parts of the gain medium from radiatively coupling. Also, while polarisation in laboratory lasers and masers may be achieved by selectively oscillating the desired modes, polarisation in natural masers will only arise 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 major use of maser study is that they give valuable information on the conditions in space, such as temperature, number density, magnetic field, and velocity, in the most interesting of environments — including stellar birth and death, and the centres of galaxies containing black holes.[1][2] The conditions involved in these events still need more accurate measuring so that theoretical models can be refined or revised.


Historical background[edit]

In 1965 an unexpected discovery was made by Weaver et al.[3]: 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, and so the emission was at first attributed to an unknown interstellar species named Mysterium, but the emission was soon identified as line emission from OH molecules in compact sources within molecular clouds[4]. More discoveries followed, with H2O emission in 1969[5], CH3OH emission in 1970[6] and SiO emission in 1974[7], all coming 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 OH emission in 1968[8], then H2O emission in 1969[9] and SiO emission in 1974[10]. Masers were also discovered in external galaxies in 1973[11], and in our own 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[12]. This was termed a megamaser because of its great luminosity; many more megamasers have since been discovered.




Known Interstellar Species[edit]

The following species have been observed in stimulated emission from astronomical environments:[citation needed]

Characteristics of Maser Radiation[edit]

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



Rapid variability[edit]


Line narrowing[edit]




High brightness[edit]

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 directly indicative of the kinetic temperature of the maser gas but is nevertheless useful in describing maser emission. Masers have huge effective temperatures, many around 109K, but some of up to 1012K and even 1014K.



It should be noted that many of the characteristics of megamaser emission are different.

Maser environments[edit]


Comets are small bodies (5–15 km diameter) of frozen volatiles (H2O, CO2, NH3, CH4) embedded in a crusty silicate filler. They orbit the sun in eccentric orbits and 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.[8] Despite the apparent rarity of these events, observation of the intense maser emission has been suggested as a detection scheme for extrasolar planets.[9]

Ultraviolet light from the sun breaks down some H2O molecules forming OH molecules that can mase. In 1997, 1667-MHz maser emission from the OH molecule was observed from comet Hale-Bopp.[10]

Planetary Atmospheres[edit]


Planetary Systems[edit]

In 2009, S. V. Pogrebenko et al.[11] reported [14] the detection of water masers in the plumes of water associated with the Saturnian moons Hyperion, Titan, Enceladus, and Atlas.

Stellar atmospheres[edit]

The conditions in the atmospheres of late-type stars support the pumping of different maser species at different distances from the star. Hydroxyl masers occur at a distance of about 1000 to 10000 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.[12]

Star-forming regions[edit]

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.[13]

Supernova remnants[edit]

The 1720 MHz maser transition of the OH molecule is known to be associated with supernova remnants that interact with molecular clouds.[14]

Extragalactic sources[edit]


Ongoing research[edit]




Distance determinations[edit]



Open issues[edit]



See also[edit]


  • ^ Snyder L.E., Buhl D. 1974 Astrophys. J. 189 L31-3
  • ^ Wilson W.J., Darrett A.H. 1968 Science 161 778-9
  • ^ Whiteoak J.B., Gardner F.F. 1973 Astrophys. Lett. 15 211-5
  • ^ Baan W.A., Wood P.A.D., Haschick A.D. 1982 Astrophys. J. 260 L49-52
  • ^ Cohen R.J. Rep. Prog. Phys. 1989 52 881-943
  • ^ Elitzur M. Annu. Rev. Astron. Astrophys. 1992 30 75-112


  4. ^ C. H. Townes Nobel Prize lecture
  5. ^ An ISO survey of possible water and hydroxyl IRASER transitions towards the star-forming regions W49, W3(OH) and Sgr B2M, M. D. Gray and J. A. Yates 1999 Monthly Notices of the Royal Astronomical Society 310, 1153
  7. ^ McGuire et al. (2003), "Interstellar Carbodiimide (HNCNH) - A New Astronomical Detection from the GBT PRIMOS Survey via Maser Emission Features." The Astrophysical Journal Letters 758 (2): L33 arXiv:
  11. ^ S. V. Pogrebenko et al., A&A, 494, L1-L4 (2009)
  12. ^ Vlemmings; Diamond; van Langevelde; M Torrelles (2006). "The Magnetic Field in the Star-forming Region Cepheus a from Water Maser Polarization Observations". Astronomy and Astrophysics 448 (2): 597–611. arXiv:astro-ph/0510452. Bibcode:2006A&A...448..597V. doi:10.1051/0004-6361:20054275. 
  13. ^ Fish; Reid; Argon; Xing-Wu Zheng (2005). "Full-Polarization Observations of OH Masers in Massive Star-Forming Regions: I. Data". The Astrophysical Journal Supplement Series 160: 220–271. arXiv:astro-ph/0505148. Bibcode:2005ApJS..160..220F. doi:10.1086/431669. 
  14. ^ Wardle, M.; Yusef-Zadeh, F (2002). "Supernova Remnant OH Masers: Signposts of Cosmic Collision". Science 296 (5577): 2350–2354. Bibcode:2002Sci...296.2350W. doi:10.1126/science.1068168. PMID 12089433. 
  15. ^ Lo, K.Y. (2005). "MEGA-MASERS AND GALAXIES". Annual Review of Astronomy and Astrophysics 43: 625–676. Bibcode:2005ARA&A..43..625L. doi:10.1146/annurev.astro.41.011802.094927. 
  16. ^ Herrnstein; Moran; Greenhill; Diamond; Inoue; Nakai; Miyoshi; Henkel; Riess (1999). "A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk". Nature 400 (6744): 539–541. arXiv:astro-ph/9907013. Bibcode:1999Natur.400..539H. doi:10.1038/22972. 
  18. ^ Hoffman; Goss; Patrick Palmer; Richards (2003). "The Formaldehyde Masers in NGC 7538 and G29.96-0.02: VLBA, MERLIN, and VLA Observations". The Astrophysical Journal 598 (2): 1061–1075. arXiv:astro-ph/0308256. Bibcode:2003ApJ...598.1061H. doi:10.1086/379062. 
  19. ^ Menon; Anish Roshi; Rajendra Prasad (2005). "A search for 53 MHz OH line near G48.4$-$1.4 using the National MST Radar Facility". Monthly Notices of the Royal Astronomical Society 356 (3): 958–962. arXiv:astro-ph/0501649. Bibcode:2005MNRAS.356..958M. doi:10.1111/j.1365-2966.2004.08517.x.