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Astron. Astrophys. 330, 515-520 (1998)

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2. The data

The data are listed in the Table 1 and described in the Notes thereto. Some further details are given below.

  1. V Photometry and Periods. HIPPARCOS obtained BV photometry on the Tycho system for the brighter stars and V photometry on the Hipparcos system for all the stars. In order to keep the photometry homogeneous we have used only the Hipparcos photometry. This covers the entire period of the mission (ie Aug'89 - Aug'93) and typically there were around 120 points per star. Table 1 lists the intensity mean V magnitudes on the Johnson system. These have been derived from the Hipparcos photometry as follows. Firstly the photometry was converted into fluxes and phased using periods from the GCVS (Kholopov et al. 1985). Using the program PULSAR (Skillen 1985) the period was optimised and this is the period listed in the Table 1. Secondly, the re-phased fluxes were fitted to a Fourier Series, again using the program PULSAR. The order of the fit varied from star to star depending on the quality of the photometry but was typically of order 6. The mean flux so found was then converted back into a magnitude. Finally this magnitude was transformed onto the Johnson system using the relations given by the HIPPARCOS project as a function of the (V-I) colour. Specifically we made the correction [FORMULA] = [FORMULA] - X where X = 0.09 (RRab Lyraes) and 0.06 (RRc Lyraes). Liu and Janes (1990a) list intensity mean [FORMULA] magnitudes for 11 RRab Lyraes and 2 RRc Lyraes and comparing our values with theirs shows a mean difference of 0.003 mags with an rms scatter of 0.007 mags.
  2. Reddenings. Using the Galactic latitudes and longitudes listed in the GCVS (Kholopov et al. 1985) we estimated E(B-V) values from the Burstein and Heiles (1982) maps. For the low latitude stars not covered by these maps the reddening was estimated by combining the (V-K) colour and [Fe/H] values (from the Table 1) with the intrinsic Period-(V-K)-[Fe/H] relations from Fernley (1993a) and the ratio E(B-V) = 0.35E(V-K) from the same source. Finally, to determine [FORMULA] we took a value of R = [FORMULA] /E(B-V) = 3.1.
  3. Mode Discrimination. Simon and Teays (1982) noted that the Fourier component [FORMULA] clearly discriminates between RRab (fundamental mode) and RRc (overtone mode) Lyraes. As a by-product of the Fourier fitting required to determine the intensity mean magnitudes (Sect. 2.1) we were thus able to separate the RRab and RRc Lyraes. In addition several stars gave a low value for [FORMULA], similar to an RRc Lyrae, but their periods were more than 0.50 day, like an RRab type. These stars were V363 Cas, XZ Cet and AR Ser. Simon and Teays similarly found XZ Cet to be unusual and from a more detailed study of it's photometric and spectroscopic parameters suggested it was an Anomalous Cepheid. Since Anomalous Cepheids could be significantly brighter than RR Lyraes we have excluded all three stars from the analysis.
  4. Binaries. Many of the RR Lyraes now have several estimates of their systemic velocities. Some stars showed more scatter than attributable purely to observational error and probably they are binaries. The stars are: CI And, TY Aps, TV Boo, RU Cet, XZ Cyg, BK Dra, ST Leo, BX Leo RW Tra and TU UMa. Only TU UMa has previously been reported as a binary (Saha and White 1990). Since any companion is likely to be either a low-mass main sequence star or an evolved star the photometric effect will be small and we have retained these stars in the analysis.
  5. Rejected Stars. Of the 180 stars listed in the HIPPARCOS Input Catalogue as RR Lyraes we have rejected 36 and these are listed and briefy discussed in the Table 1. The remaining 144 stars were used in the analyses described in the following two sections.
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© European Southern Observatory (ESO) 1998

Online publication: January 16, 1998
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