## 3. Distances## 3.1. HIPPARCOS distancesDistances to planetary nebulae are very difficult to obtain with HIPPARCOS due to their large distances (the nearest PN are located at about 100 pc). Table 3 (col.2) gives the distances deduced from the HIPPARCOS results; we give the mean value, and between brackets the minimal and maximal values derived from the parallaxes uncertainty. Good parallaxes have been measured for 4 objects with the usual first order approximation valid to within about 40%: A 35 ( = 0.2), NGC 1514 and PHL 932 ( = 0.3), SaSt 2-12 ( = 0.4). Furthermore, for most of these objects, the large proper motion supports the large parallax. Positive values are given for A 36, He 1-5 and Hu 2-1 ( = 0.6), NGC 1360 and SwSt 1 ( = 0.7). There is some indication of a positive measurement for LoTr 5 ( = 1.4), NGC 246 ( = 2.), NGC 2392 and He 2-138 ( = 5.), and M 2-54 ( = 8.).
_{H}, is shown for each distance scale. The value D of the adopted distance is given in col. 11 (see Sect. 3.3).Brown et al. (1997) discuss A set of 5 PN shows a
The error associated with the parallax of these objects is much higher than expected for stars of this magnitude, indicating that the problems that we just quoted, may play an important role. We just want to recommend further investigations. ## 3.2. Comparison with other distancesComparison with distances from the literature allows to test the reliability of the methods used. A recent discussion about distances is given in Pottasch (1996, 1997) and Terzian (1997).
Table 3 is separated in 3 parts: the first part indicates the 9 PN with fairly large diameters; then the 4 PN with a diameter smaller than 5 arcsec are listed; finally we show variable and binary stars, 4 of them presenting a negative trig. parallax. For the different distance methods, we calculate the ratio ; each ratio is weighted according to the errors on HIPPARCOS distances: for 0.4, we attribute a weight of 3 (the 4 first objects in Table 3); for 0.5 0.7, a weight of 2 is given, and a weight of 1 for all other PN with positive parallax measurements. It appears that all distance scales are overestimated. For our
sample, the individual distances which are in closest accordance with
HIPPARCOS trigonometric parallaxes are the spectroscopic estimates for
binary systems. Concerning the statistical scales, the nearest values
to D ## 3.3. Adopted distancesThe adopted distances (shown in Table 3, col. 12) are those deduced from HIPPARCOS parallaxes for the large PN. For the others, the distance (between brackets) is the mean value of all available distances (excepting He 2-36 kinematic distance), weighted by the ratios calculated for each distance scale (Table 3). For LoTr5 with very uncertain parallax, we select a value of the distance inside the HIPPARCOS distance-range, in agreement with the best literature value (based on spectroscopic classification). We give the adopted distances with a number of significant decimals corresponding to the uncertainty. © European Southern Observatory (ESO) 1998 Online publication: August 6, 1998 |