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Astron. Astrophys. 335, 277-280 (1998)

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1. Introduction

The planetary nebula (PN) NGC 650 is a large nebula showing a remarkable spatial structure. The brighter part looks like a bipolar outflow with a NE-SW axis, while the fainter parts also show a bipolar structure, but the symmetry axis is perpendicular to the axis of the brighter part. The catalogue of Manchado et al. (1996) shows a photograph of the PN, which clearly illustrates this. The nebular spectrum shows quite a high excitation, indicating a high temperature for the central star (Sect. 5 gives a more detailed discussion of the temperature). Analysis of the abundances shows a somewhat high He/H ratio (0.13 compared to 0.10 average PN value, Kwitter & Henry, 1996) and a higher than average N/O ratio (0.42 compared to 0.14 average PN value, Kwitter & Henry, 1996). These abundances may be indicative of a rather high mass star, and for this reason the nebula is classified as Type I. The central star, while faint, stands out against the nebula and was reported by Cudworth (1973) to be a double with a separation of 1. The northern component was found by Cudworth to be somewhat brighter, and probably bluer, indicating that this is the exciting star. For several other possible binary central stars, Cudworth was able to obtain proper motions, but for NGC 650 he only had single epoch plates so that it was impossible to say anything about a physical association of the stars. Bond et al. (1995) first reported that the secondary is itself a double star, using the same HST data available to us. The distance to NGC 650 is poorly known. There are statistical distances, which place it quite nearby. For example, Cahn et al. (1992) give 0.75 kpc. This might be expected for such a large, bright nebula. Acker (1978) gives a distance of 1.2 kpc on kinematic grounds. However, planetary nebulae, being older objects, often show considerable departure from circular rotation (e.g. Pottasch, 1984) so that this determination is also uncertain. If there were a physical association between the central star and its companions, it would be possible to obtain a distance to the planetary nebula from the spectral type of the companions. Because the distance is such an important parameter for discussing nebular evolution, we attempt in this paper to use the Hubble Space Telescope (HST) measurements of the central part of the nebula to determine the spectroscopic distance to the companion stars.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998