New HST observations of the core of R Aquarii I. Imaging.

The inner region of the symbiotic system R Aquarii was imaged with the Faint Object Camera on the Hubble Space Telescope (HST) in order to determine the source and structure of the highly collimated bipolar jet near the central Mira variable. Coverage over both a wide wavelength and temporal range was ensured by exposures through narrow and medium band filters containing the [OIII] λ5007, Hβ λ4861, [OII] λ3727, MgII λ2800, [OII] λ2470, CII] λ2326, and CIII] λ1909 emission lines taken approximately a pulsation period apart on October 1991 and December 1992. The jet can be clearly detected in the UV down to at least 15 AU of the Mira, placing its source well within the expected binary orbit. From there, it flows towards the NE (position angle PA=40deg) in a narrow collimated stream with a width <15 AU for about 50 AU where it encounters a dense clump N2 that it shocks into emission and that deviates it in the direction of PA=55deg. N2's observed position is coincident with the center of the C1 radio continuum feature and it is, by far, the brightest object in the R Aqr core in the UV. Beyond N2, the flow manifests itself in a series of 3-4 prominent parallel features lying transverse to the flow direction at 170-250 AU from the Mira. The jet itself, however, proceeds beyond this area in a continuous and extremely well collimated flow with a length to width ratio of over 50 until it reaches a bright loop located at about 750 AU where it breaks up into separate pieces arcing out to several thousand AU towards the North. The counter jet seems more discontinuous and made up of separate knots roughly aligned in the SW direction. Line emission flux ratios in the most prominent features of the jet are all consistent with most, if not all, of the emission being due to a shock driven through material left over from an earlier episode of mass ejection and pre-ionized by the hot companion and/or an accretion disk. The observed intensity variations in the region within r<90AU of the Mira may be due to the Mira modulating the speed or direction of the jet thereby indicating that the source and collimation mechanisms are both intimately connected to the condition of the Mira wind. This favors the wind collision and nozzle theories for the origin of the jet in R Aqr.