Shuji Deguchi

Maser emission from astronomical objects, discovered in 1965, serves as a powerful probe for the physical and chemical conditions of the dense gas around protostars and late-type stars. The term ``maser'' stands for microwave amplification of stimulated emission of radiation and is applicable to these objects because millimeter and submillimeter wavelengths are involved. No optical or infrared ``maser'' has yet been observed in the sky; hence, use of the term laser (light amplification of stimulated emission of radiation) is less common in the astronomical community. Molecules in the interstellar gas normally radiate thermal line emission with an intensity that is comparable to or less than the intensity expected from a blackbody having the temperature of the gas. However, spectral line emission due to some molecules is observed to be much stronger than the intensity expected from the gas, assuming blackbody radiation. (For example, the ground state Lambda-doublet transition of OH at 1667 MHz from the W 49 molecular cloud is stronger than expected by approximately a factor of 1010.) It is now well-established that this phenomenon is a result of the amplification of the microwave radiation by stimulated emission, which is caused by the inversion of level populations of interstellar and circumstellar molecules.

Maser emission in astronomical objects is characterized by the following properties: Within a cloud the maser emitting region is confined to a region of size leq 1016 cm; the spectral-line profiles of masers consist of sharp peaks having widths comparable to the thermal width of the gas; the intensity of a line can vary on a time scale of several months to a year; occasionally, the radiation is linearly or circularly polarized. In general, masers are associated with protostellar objects in star-forming regions and with late-type stars that lose their mass into circumstellar space. In detail, the maser characteristics are somewhat different in the various objects and molecular transitions. Table 1 summarizes the molecules that exhibit maser action in celestial objects.

Table 1. Molecules exhibiting maser action

Molecule Frequency

OH 1.612 2Pi3/2, J = 3/2, F = 1-2 M
1.665 2Pi3/2, J = 3/2, F = 1-1 O, M
1.667 2Pi3/2, J = 3/2, F = 2-2 O, M
1.720 2Pi3/2, J = 3/2, F = 2-1 O
H2CO 4.829 110-111 O
CH3OH 12.178 20-3-1 E O
SiS 18.155 1-0 C
H2O 22.235 616-523 O, M
NH3 23.870 3,3-3,3 O
SiO 43.122 v = 1, J = 1-0 M, S, O
86.243 v = 1, J = 2-1 M, S
HCN 89.087 v = 2, J = 1-0 C

*O means that the maser emission is frequently found in star-forming regions; M, in M stars; S, in S stars; C, in carbon stars.

The power of astronomical masers is expressed in terms of the number of maser photons per second emitted from the source. For the usual H2O masers in star-forming regions (e.g., the Orion molecular cloud, W 3, W 51, etc.), this is about 1046 s-1, and for the H2O masers in W 49, about 1049 s-1 (if isotropic emission is assumed); W 49 is the strongest maser source in the Galaxy. OH and H2O masers have also been found in the nuclei of active galaxies, such as NGC 3079 and NGC 1068. The powers of these extragalactic masers are stronger [by about 106 for OH masers (``megamasers'') and by 103 for HO masers] than the known maser source's in our own galaxy.

The intensity of maser emission can vary dramatically on a time scale of a year. One of the most spectacular examples is the H2O outburst in Orion that occurred in late 1979 and which lasted for 8 years. The intensity of this one maser component in Orion suddenly increased by a factor of 1000 making it the brightest H2O maser source in the sky. The position of this flare was near IRc 4 in the Orion molecular cloud and the maser spot size was measured to be about 1013 cm. of

The distributions OH and H2O emission are studied using very long baseline interferometry (VLBI): Radio signals are recorded on magnetic tapes at several different telescopes located on different continents; these data are then correlated at a later time. By this technique, maser emission-in star-forming regions is found to come from clusters of maser spots in molecular clouds. The size of an individual H2O maser spot is about 1013 cm, and the size of the cluster is about 1016 cm. OH masers tend to be larger than H2O masers by a factor of about 10. It is not well-understood whether these spots are individual protostellar objects or just fragments formed by a protostellar outflow. Proper motions of individual maser spots have been measured by VLBI; these motions seem to be random except for the masers in Orion, where a systematic outflow is found. From the random or systematic motions of maser spots, the distance to a molecular cloud can be obtained by assuming that the average radial velocity is comparable to the mean motion tangential to the line of sight. For example, a distance of 7.1 ± 1.5 kpc is obtained for the Sgr B2 molecular cloud.

OH maser emission in star-forming regions is strongly circularly polarized due to the weak magnetic field in molecular clouds. This is partly due to Zeeman splitting and partly due to maser amplification. However, the Zeeman patterns (two circular components and one linear component) are not very clearly observed in the spectral line profiles, even with the spatial resolution of VLBI. The magnetic field strength deduced from the Zeeman splitting is several milligauss (mG). Linear polarization of a few percent up to 50% is observed occasionally in the spectra of H2O masers. However, the H2O masers in the W 49 molecular cloud (the most powerful H2O masers in our galaxy) exhibit negligible polarization.

Some transitions of the molecules NH3, H2CO, and CH3OH, are also found to produce maser emission; these are relatively weak and have not been observed as extensively as the OH or H2O masers. There are numerous transitions of NH3 at frequencies around 23 GHz associated with different energy levels; some of the transitions exhibit maser characteristics and have been determined to be masers by measurements of the size of the emitting region. Maser emission from the 110-111 line of H2CO at 4.8 GHz is found in the star-forming region NGC 7538. This transition of H2CO has been found previously in absorption against the cosmic microwave background in many dark clouds. The size of the H2CO maser cloud in NGC 7538 is known to be less than 1017 cm. The molecule CH3OH has many rotational transitions at microwave frequencies and some of them are found as weak masers. Maser emission from CH3OH is usually extended (~ 1018 cm), but emission from the 20-3-1 E transition at 12.2 GHz is found to be quite strong and to come from a compact cloud (leq 1015 cm).

In spite of large numbers of observations no simple picture has been established for the maser clouds in star-forming regions.

Maser emission has been also found in the circumstellar envelopes of late-type stars with optical spectral types of M, S, and C. These stars contain molecules in their atmospheres and expel them into circumstellar space at a rate of 10-7-10-5 Msmsun yr-1 due to radiation pressure on grains and due to the pulsation of the stars. Masers from the OH, H2O, SiO, SiS, and HCN molecules have been found. The species of masers found in circumstellar space are well-correlated with the spectral type of the associated stars. In M stars, where oxygen is more abundant than carbon, masers from oxygen-bearing molecules (such as OH, H20, and SiO) are found. It is well-established by interferometry that the OH 1612-MHz masers occur at the outer side of the stellar envelope (r about 1016 cm), where interstellar ultraviolet radiation dissociates H2O and forms OH. The line profile of OH 1612-MHz masers consists of characteristic double peaks that are interpreted to be emissions from the approaching and receding parts of the expanding shell of the star (Fig. 1).

Figure 1

Figure 1. Line profiles of circumstellar masers in VY CMa. Flux density is shown as a function of velocity with respect to the Local Standard of Rest (LSR). [Reid and Moran (1981), Annual review of Astronomy and Astrophysics.]

H2O and SiO masers occur closer to the star. From VLBI, the radius of an H2O emitting region is known to be about 1015 cm and that of SiO about 1014 cm. Intensities of these two types of maser vary on a time scale of a month to a year. The line profiles are also very complex and it is difficult to interpret them with a simple model for an expanding shell. H2O and SiO circumstellar masers emit typically 1043 photons per second.

In carbon stars, where carbon is more abundant than oxygen, masers from the carbon-bearing molecule HCN and the sulfur-containing molecule SiS have been found. Strong masers from the vibrationally excited state of HCN recently have been found in carbon stars such as IRC +10216 and CIT 6. HCN masers are used as a probe of the inner region of the envelope in carbon stars, whereas the SiS maser is a probe of the outer envelope. In S stars, where oxygen is as abundant as carbon, SiO masers frequently are found.

SiO masers have been found in late-type stars that are in a late stage of evolution. The only exception being the SiO masers found in Orion, a well-known star-forming region. It had been suspected that the SiO source in Orion might be an evolved object. However, SiO masers recently have been found in other star-forming regions (the W 51 and Sgr B2 molecular clouds) so that SiO masers are now recognized to be associated with young objects as well.

Masers due to molecules containing isotopically replaced atoms, 29SiO and H13CN, are also found in late-type stars. It is expected that these emissions will provide a clue to understanding the pump mechanism of astronomical masers.

Additional Reading
  1. Cohen, R.J. (1989). Compact maser sources. Rep. Prog. Phys. 52 881.
  2. Moran, J.M. and Ho, P.T.P., eds. (1988). Interstellar Matter. Gordon and Breach, New York.
  3. Reid, M.J. and Moran, J.M. (1981). Masers. Ann. Rev. Astron. Ap. 19 231.
  4. See also Interstellar Medium, Molecules; Masers, Interstellar and Circumstellar, Theory; Stars, Evolved, Circumstellar Masers; Stars, Long Period Variables.