SpringerLink
Forum Springer Astron. Astrophys.
Forum Whats New Search Orders


Astron. Astrophys. 332, 875-876 (1998)

Previous Section Next Section Title Page Table of Contents

2. The data and the calibrations

The data used here have been exactly those of Fernley et al. (1997), thus guaranteeing the validity of the comparative study. A total of 144 stars had both the proper motions and radial velocities required by the algorithm.

Several calibrations have been performed as described hereafter. The RJH algorithm occasionally rejects stars on the basis of their individual space velocities as not belonging to the parent statistical population.

2.1. Calibrations with individual reddening corrections

  1. For a resulting global sample of 143 stars (rejection of RV Cap), [FORMULA] with [FORMULA]. This result is to be compared with Fernley et al. (1997)'s value of [FORMULA] ;
  2. For a resulting sample of 122 RRab stars (rejection of RV Cap), [FORMULA] with [FORMULA]. There is no equivalent in Fernley et al. (1997).
  3. For a resulting sample of 20 RRc stars (rejection of RZ Cep), [FORMULA] with [FORMULA]. The small size of this sample contributes of course to the poorer precision of the results. There is no equivalent calibration in Fernley et al. (1997).

2.2. Calibrations with an average reddening correction

The average reddening correction applied here has been the cosecant law introduced by Parenago (1940):

[EQUATION]

where r is the distance (in pc) and b the galactic latitude. There is no equivalent calibration in Fernley et al. (1997).

  1. For a resulting global sample of 142 stars (rejection of RV Cap and RZ Cep), [FORMULA] with [FORMULA].
  2. For a resulting sample of 122 RRab stars (rejection of RV Cap), [FORMULA] with [FORMULA].
  3. For a resulting sample of 20 RRc stars (rejection of RZ Cep), [FORMULA] with [FORMULA]. The small size of this sample contributes of course to the poorer precision of the results.

Compared to the previous one, this set of calibrations confirm that an average correction overestimates slightly the reddening as already pointed out in Heck (1973). It should be avoided as far as possible. Fernley et al. (1997) quite correctly used individual reddening corrections in their study.

2.3. Calibrations according to the metallicity

The total sample has been subdivided according to [Fe/H] following Fernley et al. (1997)'s structuring of their data.

Their intention was to consider two groups of stars, respectively with [Fe/H] [FORMULA] -1.3 and [Fe/H] [FORMULA] -0.8, but, because of the small size of the latter sample, they splitted their total sample at [Fe/H] = -1.3.

We followed suit, but also calibrated the sample of stars with [Fe/H] [FORMULA] -0.8. All the calibrations have been performed with individual corrections of reddening.

  1. For a resulting sample of 84 stars (no rejection), [FORMULA] with [FORMULA]. This is to be compared with [FORMULA] obtained by Fernley et al. (1997) for their group of 84 stars with an average [Fe/H] of -1.66.
  2. Starting with an initial sample of 60 stars with [Fe/H] [FORMULA] -1.3, the tests on the space velocities performed by the RJH algorithm rejected 18 stars out of which 17 had an [Fe/H] between -1.3 and -0.8, the 18th object being S Ara (see Calibration 2.3.4 below). This is typical of an heterogeneous sample and it is interesting how the RJH algorithm can detect it. For the remaining sample of 42 stars, [FORMULA] with [FORMULA]. Since the size of the sample decreased, the precision became poorer.
  3. Starting with the same sample, but with looser tests on the space velocities (factor 3.5 on the semi-axes instead of 3.0), only RY Col was rejected and, for the remaining sample of 59 stars, [FORMULA] with [FORMULA]. Clearly here, the larger size of the sample compensates for the dispersion resulting from the heterogeneity, but [FORMULA] remains large. This result is to be compared with the value of [FORMULA] given by Fernley et al. (1997) for their group of 60 stars with an average [Fe/H] of -0.85.
  4. For a resulting sample of 25 stars (S Ara rejected) with [Fe/H] [FORMULA] -0.8, [FORMULA] with [FORMULA]. This calibration has not been done by Fernley et al. (1997) and the small size of this sample contributes of course to the poorer precision of the results. The trend is however clearly towards a brighter magnitude for these metal-rich RR Lyrae stars. Notice also here that S Ara is not considered as belonging to the parent population (as in Calibration 2.3.2).
Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
helpdesk.link@springer.de