4.4. Masers at Centimeter and Millimeter Wavelengths (~ 109 km; ~ 10-4 parsec)
Masers are small regions located in the compressed shell between the shock front and the ionization front around an expanding object in a star formation site. A pump mechanism is required to start and maintain the maser activity (Elitzur 1982; Elitzur 1992). Circumstellar masers are important signs indicating the late stages of stellar evolution, associated with stellar mass loss. Maser stars are often (i) Mira variables or semi-regular variables in the process of becoming planetary nebulae, or (ii) supergiant stars on their way to becoming Wolf-Rayet stars or supernovae.
Magnetic fields in masers or hot spots can be measured by the Zeeman method. Zeeman splitting at centimeter and millimeter wavelengths, acting on circularly polarized radiation emitted by neutral molecules (SiO, H2O), allows the study of magnetic fields on substellar hot spots at high thermal gas densities and small sizes (< 1 pc). Here the line splitting in frequency is proportional to the strength of the B-vector component parallel to the line-of-sight.
4.4.1. SiO masers (~ 10 Gauss)
SiO masers typically have a gas density ~ 1012 cm-3, a size ~ 3 × 108 km ~ 10-5 pc, and a magnetic field ~ 40 Gauss (e.g., Barvainis et al. 1987).
One of the best known SiO maser lines is at 43 GHz (7.0 mm). Maser images show in each case that maser spots lie in a ring of ~ 3 stellar radii in the extended atmosphere of the associated star, inside the dust-formation region. The ring structure suggests an ordered outflow (e.g., Cohen 1997b).
In one important model of SiO masers, the maser polarization depends on the geometric relation between the magnetic field direction and the propagation direction. Pumping is mainly by collisions. The circular polarization is generated by a large 10-100 Gauss magnetic field (e.g., McIntosh & Predmore 1996). Observed narrow maser lines are not saturated, in support of this model and in contrast to saturation models with radiative pumping (e.g., Nedohula & Watson 1994).
Advances in very long baseline interferometry (VLBI) now allow polarimetric imaging of the SiO masers in the extended atmosphere of late-type stars. Radio images made at 43 GHz (7 mm) of the Mira variable TX Cam located at 317 pc show an orderly magnetic field over the shell with diameter ~ 10 AU. A tangential polarization structure around the shell, with a mean magnetic field strength of 5 to 10 Gauss, is indicative of a global poloidal magnetic field in the shell (Kemball & Diamond 1997).
4.4.2. H2O masers (~ 0.1 Gauss)
H2O masers typically have a gas density ~ 1010 cm-3, a size ~ 3 × 109 km ~ 10-4 pc, and a magnetic field ~ 100 milliGauss (e.g., Fiebig & Gusten 1989).
One of the best known H2O maser lines is at 22 GHz (1.3 cm), and others reach all the way to 658 GHz (456 µm).
In Sagittarius B2, the twenty H2O maser features in the middle area exhibit an increase in velocity with an increasing angular distance from the center, suggesting that the features are located in/near an expanding envelope which has a radial velocity gradient (e.g., Elmegreen et al. 1980).
4.4.3. OH masers (~ 10-3 Gauss)
OH masers typically have a gas density ~ 107 cm-3, a size ~ 3 × 1010 km ~ 10-3 pc, and a magnetic field ~ 4 milliGauss (e.g., Bloemhof et al. 1992).
OH masers in starforming regions are pumped radiatively by the strong far IR radiation from the dust near the HII region. Two peaks are observed in a typical maser line near the shell center at 1612 MHz (18.6 cm), characteristic of an expanding shell (the peak separation indicates twice the shell expansion velocity).
A radio image of an expanding HII shell and of its increasing angular diameter across the line of sight can be combined with the maser's expansion velocity along the line of sight to give an estimate of the distance to the shell (with an accuracy of 5%). The picture that is emerging from proper motion studies of masers is that of clumpy mass loss, with discrete maser cloudlets moving through relatively stationary regions where maser action is excited (Cohen, 1997b).
OH masers at 1720 MHz (17.4 cm) in supernova remnants are pumped collisionnally by the H2 molecules in the postshocked gas just inside the remnant edges. Typical gas density ~ 3 × 105 cm-3, kinetic temperature ~ 80°, and magnetic field ~ 0.2 milliGauss are found in three nearby SNR shells (e.g., Claussen et al. 1997). For these SNR masers, the path of maximum amplification occurs when the acceleration from the shock is transverse to the line of sight - it is along this direction that the velocity gradient is small and the largest coherent path length large.