Astron. Astrophys. 330, L41-L44 (1998)

4. Results and discussion

The photometric results for all stars are given in Table 1 along with the derived stellar classification and the resulting reddening values. All stars with useable results have been used to construct a reddening vs. distance relation. Additionally, photometric data and MK classification for HD 162740 (square at 1 kpc), located 16.4 arcminutes from Sakurai's Object, were taken from SIMBAD. The particularly red star (arrowed) was observed spectroscopically and classified as a Mira-like M giant in about the distance of the bulge. The resulting reddening vs. distance relation can be seen in Fig. 2. We derive a distance of 1.1 kpc with errors allowing a range of 0.9 to 1.5 kpc. Distances larger than 2 kpc can be excluded for the measured reddening of E = 0 54. Reddening as a function of distance rises steeply up to a distance of 2 kpc. Such a behaviour is well known from extragalactic studies and can nicely be explained in terms of the galactic structure. At the location of Sakurai's Object (l = 10 48, b = 4 41) the line of sight will first pass through the galactic disk resulting in a rather uniform increase in reddening for the first two kpc. Assuming a scale height of 100 pc for the dust in the galactic plane (Kimeswenger et al. 1993), the distance z from the galactic plane for the field exceeds 150 pc at about a distance of 2 kpc onwards, therefore only minimal additional reddening occurs farther out, as is observed. Due to the small galactic latitude of the field, stars can be found out to bulge distances. The unusually red star investigated spectroscopically and classified as a Mira-like star confirms the "asymptotic" reddening of ca. 1 0 out to 7 kpc. Most of the stars at large distances in Fig. 2 are assumed to be giants. This find is consistent with star count models (Cohen 1993 and references therein, Ruphy et al. 1997).

Table 1. Photometry, extinction and adopted spectral types from CCD photometry:

Fig. 2. Extinction vs. distance of all field stars (diamonds) classified. The filled square near 1 kpc indicates the position of HD 162740; the one beyond 6 kpc the position of the spectroscopically investigated red giant star (see text).

With the distance derived above we obtain an absolute magnitude range of M_V = -0 8 (-0 4 to -1 5). Assuming the same effective temperature T = 7250 as taken in Duerbeck & Benetti (1997) and for the post-AGB star HR 4049 (T = 7500, Bakker et al. 1996) and the typical absolute magnitude of an F2 supergiant we come up with the radius to be 9 to 15 times smaller than that one of such a F2 supergiant (4 to 6 times smaller than HR 4049). The old PN around Sakurai's Object has a diameter of 32", corresponding to 0.17 (0.14 - 0.23) pc at 1.1 kpc, which results in a kinematic age of 4200 (3400 - 5600) years assuming an expansion velocity of about 20 km s^-1. The distance found by us is significantly smaller than those reported so far. Duerbeck & Benetti (1996) obtain a value of 5.5 kpc on grounds of a statistical method using the 5 GHz flux derived from the observed H . In support of a large distance they state that field 154 from Neckel & Klare (1980) indicates distances larger than 2 kpc for the measured reddening of E = 0 54. Unfortunately there seems to be a mix-up here. The field closest to the position of Sakurai's Object (number 242) is very large ( 300 square degrees) which makes it highly vulnerable to variations in the reddening. More recently a distance of 8 kpc was given by Duerbeck et al. (1997) placing Sakurai's Object at bulge distance. This is the result of a pulsational analysis of the observed photometric variability which requires a high luminosity, high mass (0.8 M ) central star. Such a star implies a small age of the nebula, which is difficult to reconcile with the physical dimensions of the nebula at bulge distance, assuming a normal expansion velocity. Furthermore the distribution of bulge PNe shows a pronounced cut-off at diameters of 10 to 12", for a discussion of the properties of bulge PNe see Pottasch (1990), Stasinska et al. (1991) and Pottasch & Zijlstra (1992). While one might argue that the very low surface brightness of the nebula prevented it from being discovered, the nebula would still be unusually large (1.2 pc) for a bulge object. Furthermore it would rank among the faintest bulge PNe observed judging from the H flux. No such problem exists for a distance of 1.1 kpc.

© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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