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VLBA antenna at Pie Town
  VLBA antenna at Pie Town, New Mexico

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.

Sub-milliarcsecond Polarization Observations of Water Masers in W51 M

The VLA array near Socorro (New Mexico) was used to calibrate the electric vector polarization angle.

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%).

Fig. a: The first VLBI polarization image of water masers in W51 M (the masers marked as circles). The lines show the direction of linear polarization; their lengths are proportional to the polarization degrees of the spots. The inset is an enlargement of the protostellar cocoon area; bluesifted (redshifted) spots are west (east) of the dotted line.   Fig. b: Proper motion vectors of the low-velocity water masers in W51 M.

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).

Inversion of Water Maser Observations to Shock Physics

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 spectra of W49 N observed with the narrow-band acousto-optical spectrometer. Water maser spectra of W49 N observed with the broad-band acousto-optical spectrometer.

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).

Vibrationally Excited Water Masers

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.

| Astrophysical Masers | Star Formation | Space Projects |

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Monday, 24-Oct-2011 10:49:49 EEST

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