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3 Results

At the time of our observations the spectrum of the 22 GHz H2O masers around U Her did not show significant structure, as only a few features were detected. The averaged cross power spectrum is shown in an inlay in Fig. 1. We find that the shape of the spectrum is similar to previous observations, performed with MERLIN, the Very Large Array (VLA) and with the 100-m radio telescope in Effelsberg (Baines et al. in preparation; Yates & Cohen 1994; Colomer et al. 2000; Engels et al. 1988). In our spectrum we find that the strongest feature is located at -15.7 km s-1, and this is the feature which was used to determine the phase solutions. The stellar velocity of U Her is  $-14.5 \pm 0.5$ km s-1, which was determined from OH and SiO maser observations (Chapman et al. 1994). As the U Her H2O maser emission is located between -13 and -20 km s-1, the strongest feature is not the most blue-shifted feature.


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{Eg104_f1.ps}
\end{figure} Figure 1: The location of the U Her H2O masers with respect to the stellar positions determined from the Hipparcos observations, as determined from phase referencing to J1628+214. The star is denoted by two circles indicating the star itself and the stellar radio photo-sphere. The large cross indicates the position of the H2O maser, the error bars are due to the positional fitting. The errors on the stellar position (small cross) are due to the link to the radio-reference frame and due to the errors in the proper motion used to transpose the optical position. The triangle denotes the stellar position when using the phase referencing results on the J1619+2247 reference source. The inlay is the averaged cross-power spectrum of the H2O maser emission. The masers were mapped over the velocity range indicated in the spectrum.

After phase referencing and determining the position of J1628+214 with respect to the brightest H2O maser feature of U Her, we find that the position of J1628+214 is shifted by +83 mas in right ascension and +15 mas in declination with respect to the positions in vL00 ( $(\alpha,\delta)_{2000}=16^{\rm h}30^{\rm m} 11\hbox{$.\!\!^{\rm s}$ }23117$, $+21\hbox{$^\circ$ }31\hbox{$^\prime$ }34\hbox{$.\!\!^{\prime\prime}$ }3144$). A similar process for J1619+2247 results in a position shift of +76 mas in right ascension and +25 mas in declination with respect to the position in the VLBA calibrator list. As a consistency check we have also determined the position shift of the brightest maser spot after phase referencing with respect to J1628+214. We find that the maser spot is shifted -83 and -11 mas in right ascension and declination respectively with respect to the a priori assumed target coordinates.

The formal uncertainty in fitting a Gaussian profile to the reference source or maser spot is a fraction of the beam width, and depends on the SNR of the image. For the reference sources the formal position errors are of the order of 5 mas in each coordinate, for the U Her maser spot the errors are $\approx$1 mas. The best phase connection was made when phase referencing J1628+214 to the brightest maser spot, so we assume that the positions as determined with J1628+214 are the most reliable. The actual phase referencing errors can be estimated from the difference between the position of the brightest maser spot with respect to the two reference sources. From this, we conclude that our positions are accurate to within 10 mas, which is in agreement with the estimated errors due to the correlator model that are described above. For the brightest H2O maser spot around U Her we then find a position of  $(\alpha,\delta)_{2000}=16^{\rm h}25^{\rm m}47\hbox{$.\!\!^{\rm s}$ }468$, $+18\hbox{$^\circ$ }53\hbox{$^\prime$ }32\hbox{$.\!\!^{\prime\prime}$ }849$at the time of our observations.

To compare this position to the stellar position of U Her we have extrapolated the optical position found by the Hipparcos satellite at J1991.25 to our epoch of observation. We have used the proper motion and parallax determined by monitoring the position of the most blue-shifted OH maser spot, which was shown to be the stellar image. The first fit was performed in vL00 for 6 epochs of observations, a fit including additional epochs was presented in Vlemmings et al. (2000). As described in vL00, the OH maser proper motion is entirely consistent with the Hipparcos proper motion. The error in the transposed position is dominated by the error in proper motion. At our epoch of observation this error is $\approx$6 mas in each coordinate. Combined with the errors on the parallax and our position errors, we have been able to compare the radio and optical position with $\approx$18 mas accuracy. Figure 1 shows a map of the H2O maser features, covering the velocity range indicated in the spectrum, including the position of the star. Circles indicate the size of the star and the radio-photosphere. The size of the radio-photosphere can be estimated from SiO maser observations by Diamond et al. (1994). Their observations provide an upper limit of $\approx$20 mas if, as proposed by Reid & Menten (1997), the radio-photosphere extends to the edge of the SiO masing region. The triangle denotes the stellar position as determined when using the maser positions with J1635+1831 as the reference source.

Although the size of the brightest H2O maser spot ($\approx$50 mas) is larger than the expected size of the stellar radio-photosphere, most likely due to the blending of several weaker maser features, the position of peak intensity matches the predicted location of the star within the errors. This indicates that the H2O maser spot also coincides with the most blue-shifted OH maser spot which has been shown to be the amplified stellar image. Thus, also the brightest H2O maser spot seems to be emission from the stellar radio-photosphere amplified by the maser medium at the line of sight.


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