We present results of deep echelle spectrophotometry of the brightest knot of the Herbig–Haro object HH 202 in the Orion Nebula – HH 202‐S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 Å. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca+ and Cr+ abundances in the Orion Nebula. We also calculate the He+, C2+, O+ and O2+ abundances from recombination lines. The difference between the O2+ abundances determined from collisionally excited and recombination lines – the so‐called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t2, of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t2 values for both components from the analysis of the intensity ratios of He i lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202‐S, finding enough evidence to conclude that the flow of HH 202‐S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202‐S.

Abstract. We present adaptive optics spectro-imaging observations of the RW Aur jet in optical forbidden lines, at an angular resolution of 0.4 . Comparison with HST data taken 2 years later shows that proper motions in the blueshifted and redshifted lobes are in the same ratio as their radial velocities, a direct proof that the velocity asymmetry in this jet is real and not an emissivity effect. The inferred jet inclination to the line of sight is i = 46 ± 3• . The inner knot spacing appears best explained by time variability with at least two modes: one irregular and asymmetric (possibly random) on timescales of ≤3-10 yr, and another more regular with 20 yr period. We also report indirect evidence for correlated velocity and excitation gradients in the redshifted lobe, possibly related to the blue/red velocity and brightness asymmetry in this system.

We explain the H« emission of the cometary knots in the Helix Nebula (NGC 7293) with an analytical model that describes the emission of the head of the globules as a photoevaporated flow produced by the incident ionizing radiation of the central star. We compare these models with the H« emission obtained from the HST (Hubble Space Telescope) archival images of the Helix Nebula. From a comparison of the H« emission with the predictions of the analytical model we obtain a rate of ionizing photons from the central star of about 5 ¢10 45 s −1 , which is consistent with estimates based on the total H¬ flux of the nebula. We also model the tails of the cometary knots as a photoevaporated wind from a neutral shadow region produced by the diffuse ionizing photon field of the nebula. A comparison with the HST images allows us to obtain a direct determination of the value of the diffuse ionizing flux. We compare the ratio of diffuse to direct stellar flux as a function of radius inside an HII region with those obtained from the observational data through the analytical tail and head wind model. The agreement of this model with the values determined from the observations of the knots is excellent.