3. Trigonometric parallaxes
One star, RR Lyrae itself, has a reasonably well determined parallax, =4.38 0.59 mas which is equivalent to / =0.13. The remaining stars are at least a magnitude fainter with correspondingly smaller parallaxes and larger errors and values of / 0.30. These stars are clearly close to the measurement limit of the satellite and van Leeuwen and Evans (1997) have shown that in this case better solutions can be made using the Intermediate Astrometric Data, i.e. the individual position measurements. However, for our dataset no solution was possible (van Leeuwen 1997, Priv.Comm.). It should be noted that the solution for (RR) quoted in van Leeuwen and Evans (1997) is based on a dataset that is different from the present one in several respects. Firstly, their data are raw HIPPARCOS photometry, the magnitudes have not been either dereddened or transformed to the Johnson system. Secondly, they include many stars which we have rejected (see details in the Table 1) and in particular several of these stars are relatively bright, e.g. XZ Cet, V1719 Cyg and NSV5394.
For RR Lyrae itself, using the intensity mean Johnson magnitude, the reddening and the parallax listed in the Table 1 we obtain =0.78 0.29. RR Lyrae has [Fe/H]=-1.39. The question arises as to whether we need to make Lutz-Kelker corrections to this magnitude (Lutz and Kelker 1973, Hanson 1979). This correction arises when stars are selected according to parallax and compensates for a systematic bias in the sense of the observed parallaxes being overestimated. However our selection of RR Lyrae is not on the basis of its parallax (there are 13 other stars with parallaxes greater than that of RR Lyrae) but on the standard error of the parallax (RR Lyrae has =0.59 mas whereas all the other stars have 0.9 mas). Thus no L-K correction is required. In passing we note that given the value of / for RR Lyrae and given that our sample of stars has an N 2 proper motion distribution then from Hanson (1979) the correction would have been only 0.07 mags, in the sense of making brighter.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998