Astron. Astrophys. 321, 907-920 (1997)

4. Distribution and kinematics of H in NGC 7027

NGC 7027 is a key object in the study of PNe because it is relatively young, with strong line and continuum emission. The dense inner regions of the extended circumstellar envelope have been ionized by the intense radiation field of the hot central star (, e.g., Robberto et al. 1993), whereas farther out the envelope is still molecular. The general properties of the neutral envelope have been investigated in detail by many authors (e.g., Jaminet et al. 1992 and references therein). NGC 7027 has also been well studied at arcsec and subarsec resolutions in the near-infrared (, , and emission of the dust feature at ) by Woodward et al. (1989, 1992), Graham et al. (1993a, b), and Kastner et al. (1994), and in the radio where both the continuum and hydrogen recombination lines have been observed (Roelfsema et al. 1991). These high spatial resolution studies delineate the inner parts of NGC 7027 including the ionized region and its immediate periphery abutting the molecular envelope. The morphology of the ionized gas is consistent with a prolate ellipsoidal shell, which is expanding radially away from the central, exciting star. The molecular hydrogen emission is found along a striking shell with fourfold symmetry which loops around the ionized gas (Graham et al. 1993a; Kastner et al. 1994, 1996) and traces the photodissociation region between the ionization front and the inner border of the expanding molecular envelope.

Images of NGC 7027 obtained with BEAR in the 1-0 S(1) transition of H₂ and in the lines of atomic hydrogen and helium (HeI and HeII) are shown together with the K'-band continuum emission in Fig. 4. The velocity field in the H₂ 1-0 S(1) transition derived from the observations with the narrow H₂ filter is shown in Fig. 5 as a set of velocity channel maps separated in velocity by 9.7 km s^-1. At the systemic velocity () the molecular hydrogen is distributed along a symmetrical structure showing four lobes encompassing the inner equatorial torus which is seen here for the first time in its entirety (Fig. 5). This confirms the results of Kastner et al. (1994) who found that the molecular hydrogen towards the HII region was confined along the waist of the nebula. However, we do not confirm the presence of a second inner H₂ loop bounding the ionized gas as found by Graham et al. (1993), which was a result of inadequate subtraction of the continuum and, to a lesser extent, to contamination by the nearby He I line at 4733.4 cm^-1. The inner regions are dominated by the continuum and line emission from the ionized gas including Br , and the helium lines. As seen in Fig. 4, low level Br emission exactly fills in the cavity delineated by the outer H₂ loops. We note that an unidentified feature at  4548.3 cm^-1, which was first reported by Smith et al. (1981), has a similar morphology as Br and is thus likely to be a line of an ionized element. This is in agreement with the conclusions reached by Geballe et al. (1991). A detailed analysis of the ionized gas will be given in a forthcoming paper.

Fig. 4. Images of NGC 7027 in the lines of Br at 4616.6 cm-1, HeI  - S at 4857.5 cm-1, HeII 10-7 at 4568.1 cm-1, and H2  1-0 S(1) after subtraction of the continuum. The continuum in the K'-band is shown in the middle right panel. For comparison, the 6 cm radio continuum (from Hajian et al. 1993) is displayed in the lower right panel. The lowest contour and contour intervals are 10% of the peak intensities which (in units of ) are: 1.1 10-2 (Br ), 4.3 10-3 (HeI), 1.8 10-3 (HeII), 1.1 10-3 (K' continuum) and 1.1 10-3 (H2  1-0 S(1)). The radio continuum peak is 0.29 Jy/beam. The dotted contours in the Br image show low-level Br emission at levels from 1 to 10% of the peak intensity

Fig. 5. Velocity channel maps of NGC 7027 in the H2  1-0 S(1) line. The velocity difference between each frame is 9.7 km s-1. The systemic velocity is 26 km s-1 and the velocity decreases from left to right and from top to bottom

In spite of the elliptical morphology of the ionized nebula in NGC 7027, the morphology in H₂ shares similarities with optical images of evolved "butterfly" type planetary nebulae. As described by Balick (1987), butterfly nebulae consist of a dense, equatorial waist formed by swept-up gas at low-latitudes with, above and below, two lobes defining the symmetry axis at higher latitudes. Typical examples of this class of planetary nebulae are NGC 650 or NGC 2440. The young planetary nebula He 3-1357 presents another striking optical counterpart to the H₂ structure observed in NGC 7027 (Bobrowsky 1994).

The velocity field of the H₂ emission in Fig. 5 shows with unprecedented detail the kinematics of the entire structure including both the outer loops and the equatorial torus for the first time. At blue-shifted velocities the brightest emission is seen along the southern lobe and the northern segment of the inner equatorial torus. Around the systemic velocity, the H₂ emission shows an almost complete ring-like structure tracing the equatorial torus and the two lobes around the central ionized cavity. At red-shifted velocities the brightest H₂ emission is observed along the northern lobe and the southern segment of the torus. The detailed shape of the H₂ contours and the limb brightening of the torus at the systemic velocity would seem to argue against the simple twin-loop model (like SN 1987A) for the H₂ emission, suggested by Kastner et al. (1994, 1996).

The outer H₂ loops are similar to those seen in the rotational transition of HCO , which trace the high-density photon-dominated region abutting the central ionized cavity (Cox et al. in preparation). The kinematics of the H₂ emission in the outer loops also match the velocity fields derived from high spatial resolution millimeter line studies in CO (Graham et al. 1993a) and HCO (Cox et al. in preparation). The near-infrared data thus confirm the general dynamical model derived from millimeter observations where the overall kinematics are explained by an inclined thin, dense, expanding torus with the outer loops towards the poles, tracing the warm, high-density gas of the PDR, which lies along the inner surface of the molecular envelope. But the velocity field of the H₂ emission in the equatorial torus introduces crucial and new constraints on the inclination angle of NGC 7027 to the line of sight. The fact that the blue-shifted gas is mainly observed along the northern parts of the ring, whereas the red-shifted velocities trace the southern parts suggests that the northern polar axis is tilted away from the observer, and not toward the observer as generally assumed in the literature (Atherton et al. 1979; Masson 1989; Roelfsema et al. 1991; Robberto et al. 1993). Such a projection is also implied by recent Plateau de Bure HCO measurements and the details will be discussed in a separate paper (Cox et al. in preparation).

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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