High-Resolution Spectroscopy of NGC 7026

Extract

Up to now, the geometrical and kinematical structure of the well-known bizarre nebula NGC 7026 has not been discussed in the literature. Using the large vertical Coude spectrograph of the 2.2 m telescope on Calar Alto, Spain, we obtained long-slit spectra covering the nebula at 5 different position angles, in the ranges from 4730 to 5050 Å and from 6470 to 6770 Å. The high spectral (up to 6 km s⁻¹ FWHM) and spatial resolution (seeing-limited ≲ 2″) reveals a rather complex structure in the lines of Hα, Hβ, (OIII) λλ 4959, 5007, HeI 6678, HeII 6560, (NII) λλ 6548, 6583, and (SII) λλ 6716, 6731. Generally, the lines exhibit a double “bowed” appearance; both components consist of several condensations of small angular extent. The velocity field suggests a non-spherical expansion of an elongated thin shell structure. The observations can be explained by an ovoidal or “bipolar” configuration of the nebula consisting of an expanding equatorial toroid (V_exp = 54 km s⁻¹ in (SII)) and two blobs moving at higher velocities outwards along the polar axis (inclination angle with respect to the line-of-sight: 75°). The geometrical and kinematical structure observed in the lines of various excitation degrees indicates a pronounced ionization stratification and allows to derive the dependence of the expansion velocities on the radial distance inside the nebulae. No noticeable extinction within the nebula has been found. The bipolar structure of NGC 7026 resembles that of some other planetary nebulae and might be caused by an equatorial concentration of the circumstellar material lost during the late phase by the progenitor asymptotic giant-branch star. Using distances and interstellar extinctions of 48 stars within 1° of the planetary, we determined a distance of 2180 (≈ ± 700) pc for the nebula.