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 -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 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.
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 ( 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
M
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).
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.
MASERS, INTERSTELLAR AND CIRCUMSTELLAR
Molecule
Frequency
(GHz)
Typical
transition
Characteristics*
OH
1.612
23/2,
J = 3/2, F = 1-2
M
1.665
23/2,
J = 3/2, F = 1-1
O, M
1.667
23/2,
J = 3/2, F = 2-2
O, M
1.720
23/2,
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.
Cohen, R.J. (1989). Compact maser sources.
Rep. Prog. Phys. 52 881.
Moran, J.M. and Ho, P.T.P., eds. (1988). Interstellar Matter.
Gordon and Breach, New York.
Reid, M.J. and Moran, J.M. (1981). Masers.
Ann. Rev. Astron. Ap. 19 231.
See also Interstellar Medium, Molecules; Masers, Interstellar
and Circumstellar, Theory; Stars, Evolved, Circumstellar Masers; Stars,
Long Period Variables.