The first test was to remove one by one each reference star of the library,
treat it like an anonymous object, then compare the estimated parameters and the
true ones. This test was performed iteratively to adjust the atmospheric parameters of the reference
star as described in paper II. The overall resulting standard deviation of the estimated versus ``true''
parameters for this high S/N sample
is respectively 86 K, 0.28 dex, 0.16 dex for , and [Fe/H].
Several phenomenae influence the results. In metal-rich stars,
the amount of
spectral information is greater than in almost line free stars.
In the same way, stars cooler than
5000 K are losing the Stark profile of the wings of their hydrogen lines.
Therefore the uncertainities
on the determination of the parameters is greater for the cool and
metal-deficient stars than for the
hot and metal-rich stars. To take this effect into account,
the library was divided in four
zones (from A to D, see figure 7) and the mean error was calculated
as well as the dispersion in
each zone and for each parameter. The results are listed in Table 1.
Because our ultimate goal is to use low S/N ( 10)
spectra, the spectra
of the library were degraded at S/N = 10 by adding a
random (Poisson) noise.
The mean error and the dispersion in each zone were recalculated. The results are listed in table 2.
To be sure that no bias has been introduced with the artificial noise (such as correlation between pixels), the software was tested with 30 stars of well known parameters (some of them already belonging to the library), observed willingly at low S/N. Figure 8 shows the difference between the true and the estimated parameters as a function of the S/N of the object star.
Zone | sum | A | B | C | D |
number of star | 211 | 87 | 63 | 33 | 28 |
17 | 11 | 9 | 54 | 11 | |
86 | 74 | 63 | 131 | 87 | |
0.04 | 0.03 | 0.03 | 0.06 | 0.06 | |
0.28 | 0.25 | 0.20 | 0.43 | 0.28 | |
[Fe/H]> | 0.02 | 0.00 | 0.00 | 0.07 | 0.04 |
0.16 | 0.14 | 0.14 | 0.20 | 0.19 |
Zone | sum | A | B | C | D |
number of star | 211 | 87 | 63 | 33 | 28 |
10 | 12 | -4 | 50 | -9 | |
102 | 84 | 78 | 158 | 99 | |
0.03 | 0.00 | 0.04 | 0.08 | 0.05 | |
0.29 | 0.24 | 0.21 | 0.45 | 0.31 | |
[Fe/H]> | 0.02 | 0.00 | 0.00 | 0.07 | 0.08 |
0.17 | 0.14 | 0.14 | 0.22 | 0.18 |
The computing time for TGMET is about 10 min, when run in parallel with the aquisition and reduction softwares, on the SUN-sparc 20 station devoted to the observations with ELODIE.