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

4. Distance to the companions

From the observed colours we can derive intrinsic colours by correcting for interstellar reddening. We adopt a value of E = 0.16, which is the value that produces the intrinsic colour expected for a hot star. = - (Johnson, 1966). This generalised "hot star" refers to O-B stars, thus temperatures between 20,000K to 40,000K. The central star is considerably hotter, as is shown below. It is possible therefore that the intrinsic V colour is somewhat bluer, although the effect is likely to be small. No I magnitudes are available for other PN central stars, but the effect can be seen in B colours. Whereas the standard intrinsic colour for O-B stars is = -0.31 (Johnson, 1966), bluer colours have been measured for very hot central stars such as those of NGC 7293 and NGC 1360. In fact Kaler & Feibelman (1985) recommend the use of = -0.38. If the intrinsic V colour is also bluer, the value of extinction would be somewhat larger. However the extinction may also be derived from the ratio of the H to radio continuum, and from the nebular Balmer decrement. Cahn et al. (1992) give E values of 0.12 and 0.14 from these two kinds of measurements. Thus there is no clear indication that the value of extinction we have found should be increased. The intrinsic colours are shown in Table 2. Also given is the spectral type which corresponds to the intrinsic colour (Johnson, 1966). It was assumed that the companions are main sequence stars (if they were giants this would lead to an improbably large distance). Companion No. 3 would then have a value of M 4.7 and a distance of d 6 kpc. Companion no. 2 would have M 4.4 and d 5 kpc. If this difference is significant, and if so what it is due to, is a matter of speculation. The image of companion no. 2 shows a slight indication of being widened, so that the "binary" could be a triple system, which could explain its being overly bright. On the other hand, companion no. 3 could be sub-luminous. But the difference is small, and the conclusion may be drawn that the companions are at a distance between 5 and 6 kpc.

Fig. 1. This combined F555/F814 HST/WFPC2 image shows the central and north-eastern part of NGC 650. Each of the three WF chips measures 125 on the side. The central star is centered in the PC chip, which measures 35 35. The central (boxed) part of this chip is shown enlarged in the insert. It includes the central star of the planetary nebula and its visual companions, clearly showing the binary nature of the latter. The object numbering corresponds to the entries in the Tables 1 and 2.

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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