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Description
We report on high spatial resolution water maser observations, using the Australia Telescope Compact Array, towards water maser sites previously identified in the H2O southern Galactic Plane Survey (HOPS) within the area covering Galactic coordinates from l=290 to l=30 and b=-0.5 to b=+0.5. Of the 540 maser sites identified in the single-dish observations of Walsh et al. (2011MNRAS.416.1764W, Cat. J/MNRAS/416/1764), we detect emission in all but 31 fields. These maser sites together comprise 2790 individual spectral features (maser spots), with brightnesses ranging from 0.06Jy to 576Jy and with velocities ranging from -238.5 to +300.5km/s. Based on a definition of maser site size of 4-arcsec (except for G000.667+0.028, also known as Sgr B2, which we treat as a special case), we identify 631 maser sites. We have compared the positions of these sites to the literature to associate the sites with astrophysical objects. We identify 433 (69%) with star formation, 121 (19%) with evolved stars and 77 (12%) as unknown. Comparing the properties of maser sites of different origins, we find that those associated with evolved stars tend to have more maser spots and have smaller angular sizes than those associated with star formation. We present evidence that maser sites associated with evolved stars show an increased likelihood of having a velocity range between 15 and 35 km/s compared to other maser sites. We suggest this is because many of these maser sites are associated with the circumstellar shells of the evolved stars, which are expanding at these velocities. Of the 31 non-detections, we conclude they were not detected due to intrinsic variability and confirm previous results showing that such variable masers tend to be weaker and have simpler spectra with fewer peaks. Of the small number of maser sites showing linear features, we find evidence for lines that are both perpendicular and parallel to known outflows, suggesting that in star formation, H_2_O maser origins may be as varied and as complex as those of class II methanol masers.
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