SIMBAD references

We present the results of the highest-resolution 18-cm (L -band) radio continuum and optical kinematic studies to date of the young planetary nebula (PN), NGC 7027. We discuss the radio image of NGC 7027 made from combined data from the Multi-Element Radio-Linked Interferometer Network and the Very Large Array. This image is used to derive the distribution of the nebular brightness temperature at this frequency. By combining the 18-cm image with the 6-cm (C -band) image of Bryce et al., two-dimensional distributions of the spectral index between L - and C -bands, C -band optical depth and emission measure are constructed and analysed. In the optical regime, high dispersion, spatially resolved observations at high spectral resolution have been obtained of this PN. The resulting velocity ellipses confirm the inclination of the north of the PN towards the line of sight. A waist feature is seen in the equatorial plane and [SII]6716/6731 Å ratios indicate that a density enhancement exists in this region. The PN equatorial expansion velocity is deprojected and measured as 13±1 km/s from the [OIII]4959 Å emission line. This velocity is used to calculate a revised distance estimate of 650±100 pc. Using the electron density derived from the [SII] ratio, an ionized mass of 0.005 M_☉is determined. The nature of the bright knot situated to the north-west of the PN is investigated at optical and radio wavelengths. In optical emission lines it is found to be redshifted. It is located in the same region as the position of the peak flux density at 18 cm. It is shown that no evidence of a local density enhancement exists in its environs. It is suggested that the appearance of the knot is due to a temperature effect and its existence is interpreted in terms of a breach in the near side of the PN shell. The existence of a jet, at some stage in the evolution of the PN, is invoked to explain the mechanism by which such a breach may have been created.