3. The expansion velocity field
The peak separation, measured at the centre of each nebular emission, gives the expansion velocities contained in Table 2, where ions are put in order of increasing ionization potential. For all ions (but not HI) the estimated error in the expansion velocity is essentially anti-correlated to the emission intensity; the large uncertainty in H comes from the line broadening due to thermal motions.
Table 2. Expansion velocities
The very low (if any) expansion gradient present in NGC 1501 confirms that stratification effects are negligible and suggests that the main nebular emission occurs in a narrow shell (note, in particular, the large H expansion velocity, as large as for the other lines).
More precise informations can be obtained from the detailed analysis of the expansion velocity field at the four position angles. These are shown in Fig. 3 for H; the velocity and intensity distributions derived for 5007 Å of [OIII] coincide with the H ones; though the [NII] line at 6584 Å is too weak for an accurate study, its intensity and velocity trends appear very similar to those observed in H and [OIII].
The H radial velocity distributions of Fig. 3 present the classical bowed shape expected of an expanding shell, but they are so inhomogeneous and distorted that a simple model (like a triaxial ellipsoid) is decidedly inadequate to fit all the data. Note, in particular:
Moreover, the imagery of the nebula (taken a few months after the spectroscopic material) stands out a further complication: the direction of the apparent major axis of NGC 1501 seems closer to 110o than to 100o (the P.A. we used), or to 120o and 98o (the P.A. given by Pease, 1917, and Curtis, 1918, respectively).
To make easier the interpretation of the velocity maps of Fig. 3 in terms of an acceptable model, we believe convenient the introduction of a parting line at an apparent distance of about 15" from the central star, separating the "low latitude" ionized gas from the "high latitude" one.
The "low latitude" velocities are quite regular elliptical arcs (i.e. projections of a triaxial ellipsoid), whose tilt is maximum at P.A.=10o and P.A.=145o, and minimum (but not null) at P.A.=100o (i.e. close to the apparent major axis). To be noticed that the un-tilted velocity ellipse occurs at P.A.80o (corresponding to the line of nodes; this agrees with the foregoing indication that one of the axes of symmetry is projected at P.A.170o). The intensity distribution appears knotty and irregular at P.A.= 55o and 100o, while at P.A.=10o and 145o it presents two symmetric, extended condensations (as normally observed in bipolar PNe).
The "high latitude" velocity maps of Fig. 3 are characterized by an extreme variability: they are faint, extended and asymmetric at P.A.=10o and 145o, faint, extended and quite symmetric at P.A.=100o, knotty, bright, symmetric and small at P.A.=55o. The general impression is that they represent hemispheric bubbles protruding from the ellipsoidal region of NGC 1501; in some directions (as in the East sector of P.A.=100o) multiple structures appear, i.e. an external, smaller dome overlaps the internal, larger one.
A further, remarkable feature of the "high latitude" zones in Fig. 3 is the internal, weak, diffuse emission present at all the four position angles, indicating that a low density ionized gas completely fills these nebular regions.
In order to decipher the complex gas motion observed in NGC 1501, a detailed analysis of the nebular physical conditions will be performed in the next section.
© European Southern Observatory (ESO) 2000
Online publication: October 10, 2000