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ASTROPHYSICAL MASERS
The 22 GHz water maser line is the brightest spectral line in the radio universe and highlights shocked star forming regions, dense circumstellar shells around evolved stars as well as circumnuclear disks around black holes of galactic nuclei. Most interestingly, this for life very important H2O molecule
is copiously produced already during the birth process of a star via
the powerful shocks associated with the star formation process.
The birth of a massive star generates strong stellar winds (with
velocities up to a few thousand km/s), which shock the ambient cloud
material. Shocks with velocities exceeding some 20 km/s
running into high-density magnetized material successfully
explain the water maser emission (Hollenbach & McKee 1989; Elitzur,
Hollenbach & McKee 1989). The shocked gas has both the high
temperature and density required for the water molecule production and
for the collisional excitation of the maser levels.
The first 22 GHz linear-polarization sensitive Very Long Baseline
Interferometry (VLBI) images of low-velocity water masers in the star
forming region W51 M were obtained with the Very Long Baseline Array
(VLBA) of NRAO (USA) using the new calibration technique developed by
K. Leppänen (1995, Thesis, Helsinki Univ. of Technology). These
2-epoch VLBA observations revealed two distinct features:
(1) a compact, slightly elongated water maser concentration near the
reference position (0,0), and (2) a NNE-SSW oriented 1200 AU long
structure of masers, which is roughly aligned along the large-scale
galactic magnetic field and the direction of the polarization vectors
of these masers (Fig. a). The compact maser concentration at the
reference position is identified as a protostellar cocoon with both
rotational and radial motions.
The inner and outer radii of this maser cocoon are 5 AU and 66 AU,
respectively. In contrast to the cocoon masers, which show a
mean linear polarization of only 3%, the masers in the large-scale
structure show higher degrees of linear polarization (mean 12%,
maximum 35%).
The proper motions (Fig. b), obtained from the two observing epochs, indicate that the masers in the large-scale structure move longitudinally with a median space velocity of 25 km/s relative to the protostar. This streamer cannot be explained as a bipolar outflow from the protostar. Most likely the maser stream is produced by shocks, caused by the nearby expanding HII region, which interacts with the dense molecular core of W51 M on its western side. The proximity to the protostar suggests that these shocks have affected, or even triggered, the star formation in W51 M. More details of this study can be found in Leppänen, Liljeström, & Diamond, (1998, ApJ, 507, 909-918). These first spectral line VLBI polarization observations of 22 GHz
water masers have shown that VLBI polarimetry provides
an effective tool for probing large scale magnetic field structures in
very dense star forming regions. Therefore, we have extended our maser
polarization observations (using VLBA) to a few other well-known water
maser sources (Liljeström et al., in preparation).
The Metsähovi radio telescope has been used for a long-term monitoring program of the 22 GHz water maser line. One of our main targets has been W49 N, the most powerful and populous water maser cluster in our Galaxy.
Water maser observations commonly show dramatic variations in flux density as function of time. Such short-lived (typically some 2 months) increases in flux density are termed as "outbursts". Combining the Metsähovi database of some 150 maser outbursts ( Liljeström 2000, Journal of Astron. Data, 6, 2) with simultaneously obtained VLBI data of W49 N (Gwinn 1994), notably obtained with the same velocity resolution, Liljeström & Gwinn (2000, ApJ, 354, 781-800) succeeded to fix the free parameters in the shock model of Hollenbach & McKee (1989) and the maser model of Elitzur, Hollenbach & McKee (1989). This enabled a straightforward determination of some 20 physical parameters of W49 N. The most important characteristic of the novel diagnostic method of
Liljeström & Gwinn (2000) is its capability to determine both preshock and
postshock magnetic field strengths in dense, shocked regions, as well as
the inclination angle of the mean field with respect to the line of sight.
The crucial observation is the measurement of the nonthermal velocity
variation of the line peak during maser outbursts. These velocity
fluctuations are caused by Alfvenic wave pressure, which is substantially
increased in the sudden compressions associated with shocks.
The fact that Alfvenic wave fluctuations are oriented perpendicular to the
magnetic field lines, enabled us to estimate the inclination of the mean
magnetic field from the observed dispersion of the Doppler velocity
fluctuations (see Liljeström & Gwinn 2000, ApJ, 354, 781-800).
A 96 GHz water maser survey programme, carried out towards protostars
and late-type stars, yielded the first discovery of vibrationally
excited water masers in two protostars. The maser line velocities
were close to the stellar velocity (or slightly redshifted).
Because the upper energy level of this vibrationally excited line
at 96 GHz is 3065 K, it is very likely that this maser
line originates within a few stellar radii from the central
protstar in the inner edge of the circumstellar envelope. In this
region the density and radiation field are yet sufficiently low
to allow nonthermal maser emission. Also the dust (which promotes
the outward expansion of the envelope) has not yet formed. Fore more
details, see Liljeström, Winnberg, & Booth (in
P. Piironen, A. Räisänen (eds.), URSI/IEEE/IRC XXI Convention on Radio
Science, Helsinki Univ. of Technology, Radio Laboratory Report, Vol.
S 222 (Espoo 1996), p. 96.
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