111 L

Figure 6: Map of the distribution of the H20 masers near W3f OH), (a) Measured positions and proper motion based on a distance of 2.2 kpc. (b) Top view, with the assumption of constant outflow velocity. The motions clearly indicate a well collimated bipolar outflow [Alcolea et al., 1994].

Figure 7: Map of the distribution of the SiO masers at 43 GHz towards the late-type star TX Cam. The circle in the center represents the stellar photosphere of diameter 3.6 A U [Diamond et al., 1994],

3.3 Supernova remnants

The association of OH masers in the 1720 MHz transition with supernovaremnants was postulated long ago [BallandStaelin, 1968]. Recent VLA observations show an example of extensive maser emission in the envelope of a supernova remnant (see Figure 8). This type of maser may prove useful for tracing the extent of shock fronts, measuring gas motions and determining the magnetic field strength.

Figure 8: The distribution of masers superimposed upon the continuum image of the supernova remnant W28. Representative spectra are also shown [Frail et al., 1994a],

3.4 Active galactic nuclei

OH and H20 masers have been discovered in the nuclei of many galaxies with active nuclei. OH masers have been found in galaxies with distances up to 100 Mpc and they may prove to be useful cosmological probes. About 5 percent of active galaxies show detectable H20 maser emission. About 15 examples are currently known. The most interesting case of this phenomenon is in the galaxy NGC4258, a relatively nearby spiral galaxy that is a low luminosity type 2 Seyfert galaxy [Urry and Padovani, 1995]. The spectrum is shown in Figure 9. An unusual characteristic of the spectrum is the isolated groups of spectral features separated by ± 1000 kms-1 from the group of features near the systemic velocity of the galaxy (470 kms-1). Observations with the VLBA [Miyoshi et al., 1995] at high angular resolution (300 |Xas) and high spectral resolution (0.2 kms-1) show that the emission comes from an elongated region (Figure 10). Furthermore, there is a monotonic progression of velocity with position among the maser spots along the major axis, as shown in Figure 11. This dependence is exactly the form expected for Keplerian motion of a disk orbiting a central mass, i.e., V = <gm/r, where g is the gravitational constant, m is the central mass, and r is the radius from the center of rotation of a maser with velocity v. The required binding mass is at least 3.5 x 107 Mq, and the inner part of the disk takes 800 yrs to complete one orbit. Furthermore, this mass must be contained within the inner edge of the disk, which has a radius of 0.13 pc. The corresponding mass density is at least 4 x 109 Mope5. This mass density is 109 times the stellar density in the vicinity of the Sun and 104 times the density of the densest known star cluster. The mostly likely origin of the central mass is a black hole [Maoz, 1995; Kormendy and Richstone, 1995],

Because of the precision of the VLBA measurements, the parameters of the disk can be measured with great accuracy. For example, the disk is unresolvable in the vertical direction and has a height of less than 10 |J.as. This may be the first direct detection of the outer part of a thin accretion disk, long thought to accompany a black hole. A thermally supported disk in hydrostatic equilibrium should obey the relation H/R = cJV where H is the thickness of the disk and cs is the sound speed. For the maser disk, H/R < 0.0025 so that the sound speed must be less than about 2.5 kms-1 and the temperature must be less than about 1000K. For such a disk the inward drift velocity is expected to be about 1 ms_1 [Frank, King and Raine, 1992] and the accretion rate is about 10"4 M©pc-3. This rate is sufficient to power the observed X-ray source (Lx = 4 x 1040 erg s_1) [Moran etal., 1995].

4 Magnetic fields

The line profile of a maser can be split by the Zeeman effect, which provides a mechanism for estimating the magnetic field strength. OH is a paramagnetic molecule and the splitting of the 1665 MHz transition is about 3.2 kHz/mG. Hence, a field of 1.7 mG is required for the splitting to equal the width of a typical line of 1 kms-1 or 5 kHz.

The Zeeman effect has proven to be an effective tool for measuring the magnetic fields in star-forming regions when interferometric measurements have been made to convincingly identify Zeeman pairs. Magnetic fields have been found to have strengths of about 5 mG [Reid and Moran, 1988]. Water is nonparamagnetic and the splitting of

Ngc4258 Water Maser

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Observed maser spectrum £

Figure 9: (bottom) Spectrum of the H20 maser (1.35 cm wavelength) towards NGC4258. (top)

Cartoon illustrating the geometry of the maser [Moran et al., 1995], Hence, a magnetic field of 50 Gauss is required for a Zeeman slitting equal to the linewidth of typically 1 kms"1 or 74 kHz. However, the splitting can be detected by careful measurement of the difference in right and left circularly polarized spectra (see Figure 12). The field strength in H20 masers are about 50 mG [Fiebig and Glisten, 1989]. For molecular clouds that collapse to form stars, flux freezing suggests that the magnetic field scales as the density squared. This gives a density ratio for gas in H20 and OH masers of about 100, as expected on theoretical grounds for maser conditions. Efforts to measure magnetic fields in NGC4258 have only given limits on field strengths.

Figure 10: Distribution of maser towards NGC4258 which is attributed to emission from a disk viewed nearly edge-on. A slight warp is evident [Miyoshi et al., 1995].

Figure 10: Distribution of maser towards NGC4258 which is attributed to emission from a disk viewed nearly edge-on. A slight warp is evident [Miyoshi et al., 1995].

Figure 11: Distribution of line-of-sight velocities relative to the velocity of the galaxy [Miyoshi et al., 1995, Moran et al., 1995].

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