NARROW-BAND IMAGING OF THE INNER R-AQUARII NEBULA - FURTHER EVIDENCE FOR SHOCK EXCITATION

The jet like nebulosity in the inner regions of the symbiotic variable R Aquarii was imaged through narrow-band interference filters centered on the lines of H-beta 4861 angstrom, [O III] 5007 angstrom, H-alpha 6563 angstrom, [N II] 6583 angstrom, and [S II] (6716 + 6731) angstrom. Most of the radiation from the Mira and the nebular core surrounding it was suppressed by an occulting wedge in the STScI coronograph placed at the focus of the ESO 2.2 m telescope. A high spatial resolution image (seeing FWHM = 0".9) in the [N II] 6583 angstrom filter shows that the relative fluxes of features B and D defined by Paresce, Burrows, & Horne have changed in the sense of a higher D/B brightness ratio at this line in little over a year with respect to that observed previously in similar seeing conditions. The overall morphology of the jet has remained stable in this period as no proper motion of any nebular feature in this band was detected at an accuracy of +/- 0".2. The second set of narrow-band images (second night) allows the determination of the following line ratios for feature B corrected for atmospheric absorption and reddening: [N II](6548 + 6583)/H-alpha = 0.9 +/- 0.4, [O III]5007/H-beta = 3.3 +/- 1.1, and [S II](6716 + 6731)/H-alpha = 0.2 +/- 0.1 and for feature D: [N II](6548 + 6583)/H-alpha = 0.9 +/- 0.4, [O III]5007/H-beta = 3.8 +/- 1.2, and [S II](6716 + 6731)/H-alpha = 0.2 +/- 0.1. These results are best understood in terms of excitation of gas clumps surrounding R Aqr by a moving shock, as first suggested by Solf & Ulrich that started exciting B about 15 yr ago and is now heating D. Comparison of the observed fluxes with theoretical expectations yields shock velocities of order 90-100 km s-1, a preshock gas density of almost-equal-to 10 cm-3, and a gas temperature of almost-equal-to 10(4) K.