MASERS, INTERSTELLAR AND CIRCUMSTELLAR

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 10**.) 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  *10** 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 H*O
masers in star-forming regions (e.g.,  the Orion molecular cloud, W 3,
W 51, etc.), this is about 10**  s**, and for the H*O  masers in W 49,
about  10** s** (if  isotropic  emission  is  assumed);  W 49 is   the
strongest maser source in the Galaxy. OH and H*O 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
10* for OH masers  ("megamasers") and by 10*  for H*O 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  H*O
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 H*O 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
10**cm.  of

  The distributions  OH and H*O  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 H*O
maser spot is about 10** cm, and the size of the cluster is about 10**
cm. OH masers tend to  be larger than H*O  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 H*O  masers. However, the H*O masers in
the W 49 molecular cloud (the most  powerful H*O masers in our galaxy)
exhibit negligible polarization.

 Some transitions of the  molecules  NH*, H*CO,  and CH*OH, are   also
found to produce maser emission;  these are  relatively weak and  have
not been observed as  extensively as the OH  or H*O masers.  There are
numerous  transitions of NH*  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 1**-1**
line of H*CO at 4.8 GHz is found  in the star-forming region NGC 7538.
This   transition  of H*CO has  been   found  previously in absorption
against the cosmic microwave background  in many dark clouds. The size
of the H*CO maser cloud in NGC 7538 is known  to be less than 10** cm.
The molecule   CH*OH  has many   rotational transitions  at  microwave
frequencies and some of them are found  as weak masers. Maser emission
from  CH*OH  is usually  extended (-10**  cm),  but emission  from the
2*-3**  E transition at  12.2 GHz is found  to be  quite strong and to
come from a compact cloud (*10** 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**-10**  M* yr** due  to radiation
pressure on grains and due to the  pulsation of the stars. Masers from
the OH, H*O, 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, H*0,  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 10** cm), where interstellar ultraviolet
radiation  dissociates  H*O  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).

  H*O and  SiO masers occur closer to  the star. From VLBI, the radius
of an H*O emitting region is known to be about 10** cm and that of SiO
about 10** 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. H*O and  SiO circumstellar masers emit typically 10**
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,
**SiO  and H** CN, 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

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.(1982). Masers.
   Masers. Ann. Rev. Astron. Ap. 19 231.