Temperature variations from Hubble Space Telescope imagery and spectroscopy of NGC 7009

We present new Hubble Space Telescope (HST)/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (T_e) and the fractional mean-square T_e variation (t²_A) across the nebula. The WFPC2 T_e map is rather uniform; almost all values are between 9000-11 000 K, with the higher T_e values closely coinciding with the inner He⁺⁺ zone. The results indicate very small values -≲0.01 - for t²_A throughout. Our STIS data allow an even more direct determination of Te and t²_A, albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5-arcsec square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10 139 K; t²_A is 0.0035. The measurements of T_e reported here are an average along each line of sight. Therefore, despite finding remarkably low t²_A, we cannot completely rule out temperature fluctuations along the line of sight as the cause of the large abundance discrepancy between heavy element abundances inferred from collisionally excited emission lines compared to those derived from recombination lines.