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Astron. Astrophys. 361, 1112-1120 (2000)

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1. Introduction

The little studied, high excitation planetary nebula (PN) NGC 1501 (PNG144.5+06.5, Acker et al., 1992) is "a very irregular and patchy elliptical disk, about 56"x48" in P.A.=98o. The periphery shows traces of a broken ring formation; the brightest portions are the edges at the ends of the minor axis. Relatively faint" (Curtis, 1918).

At least a dozen of distance estimates of this nebula are contained in the literature. They span in the range 0.9 Kpc (Amnuel et al., 1984; statistical distance based on the surface radio-brightness) to 2.0 Kpc (Acker, 1978; individual distance based on the average extinction in the galactic disk) and roughly peak around 1.3 Kpc (the value we will adopt in the present paper).

A kinematical study by Sabbadin & Hamzaoglu (1982a) suggests that NGC 1501 is a prolate spheroid of moderate ellipticity.

The spectral type of the exciting star is WC4/ OVI (Aller, 1976, Tylenda et al., 1993, Gorny & Stasinska, 1995). The star has a high temperature, between [FORMULA] (Sabbadin, 1986, Stanghellini et al., 1994) and [FORMULA] (Koesterke & Hamann, 1997a). It is loosing mass at a rate of [FORMULA] (Koesterke & Hamann, 1997a) and terminal wind velocities of 1800 or 3300 [FORMULA] (Koesterke & Hamann, 1997a and Feibelman, 1998, respectively).

It is one of few PNe nuclei showing nonradial g-mode pulsations (Bond et al., 1993, 1996, and Ciardullo & Bond, 1996). Following the "born-again scenario" proposed by Iben et al. (1983; see also Blöcker, 1985 and Herwig et al., 1999), these extremely hot, hydrogen deficient stars suffer a late thermal pulse (ejecting the hydrogen rich layers and exposing the naked C/O core) after they reached the White Dwarf cooling sequence and are possible precursors of the short-period GW Vir (PG 1159-035) pulsating White Dwarfs. Hamann (1997) and Koesterke & Hamann (1997b) suggest the evolutionary sequence [WC-late] [FORMULA] [WC-early] [FORMULA] [WC-PG1159] [FORMULA] PG1159.

Recently we started a long-term observing program aimed to investigate the physical conditions and the spatial structure of selected PNe through direct imagery and long-slit Echelle spectra. This article concerns NGC 1501 and its plane is as follows: in Sect. 2 we present the observational material; Sect. 3 contains the expansion velocity field; the physical conditions (electron temperature, turbulence and electron density) of the ionized gas are analysed in Sect. 4 and the tomographic maps in Sect. 5; in Sect. 6 we describe and discuss the resulting spatial model; conclusions are drawn in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000