- 3.1 Data that define the zero point
- 3.2 Expanding the data base
- 3.3 Special correction techniques
- 3.4 Data that are not added to the data base

The values of [Fe/H] for the zero-point stars may be thought of as an initial zero-point data base. To expand that data base, a search is made for literature results whose zero points appear to be reasonably secure. Acceptable results may be referred to the Sun directly or through the zero-point stars listed above. Data for which T91 has noted some special problem are set aside for the moment. Data are also set aside if they have been derived from the Holweger-Müller (1974) solar model and a stellar model from a published grid. This combination will be referred to here as a "non-uniform'' grid of model atmospheres. Data derived with non-uniform grids require corrections ranging from -0.05 dex to +0.25 dex (see Sect. 6 of Paper I).

There appears to be a belief among some astronomers that the history of [Fe/H]
analysis has been dominated by external zeroing (see Kuroczkin & Wisniewski
1977; Twarog & Anthony-Twarog
1996)^{}. If this were true, presumably it would
not be possible to find enough differential values of [Fe/H] to form an
adequate zero-point data base. In fact, the expanded version of that data
base contains results for 193 stars. Given the data for these stars, one
can produce a final set of averages that does not depend on an adopted solar
value of log (Fe/H).

Alternative ways of setting the zero point include use of the Hyades as well as external zeroing. One could assume that the Hyades giants and dwarfs have the same mean value of [Fe/H] (again see Twarog & Anthony-Twarog 1996). By contrast, the approach adopted here is to test this assumption instead of adopting it at the outset. It is found that at 95% confidence, no difference as large as 0.049 dex exists between the values of [Fe/H] for the Hyades dwarfs and giants (see Sect. 7.6 of Paper II).

When necessary, data are referred to the Sun through the Paper II standard stars. This number includes only HD stars, since only data for such stars are used to rezero results in subsequent steps in the analysis. From Williams 1971, 1972 and Gustafsson et al. 1974. No lower limits from Williams 1972 are used. The Williams data are re-scaled and corrected to the Gustafsson et al. zero point by using Eq. (1). T91 found that an overall zero-point correction for the collected photometric data was required. These data are based on a non-uniform grid of model atmospheres. T91 found that a zero-point correction was required. Before averages are formed, rms errors are set to 0.07 dex if they are less than 0.07 dex. T91 found that a B-V-based correction was required.
From Luck 1991, Luck & Wepfer 1995, and Luck & Challener 1995. These data are based on a non-uniform grid of model atmospheres. Results using spectroscopic values of log g are adopted.
Literature sources for these data will be given in a subsequent paper in this series. |

A search is now made for discrepant zero-point data. A few such data are found and deleted. In addition, a search is made for results that are added at stage 6 but should be included earlier. An example of such a data set is from Cottrell & Sneden (1986). Their results are initally included at stage 6 because they are based on a non-uniform grid. However, a zero-point correction derived for them turns out to be effectively zero, so they are included at stage 2 instead. Once any required editing is done, the analysis is repeated, and it is iterated until no further editing is necessary.

As each part of the analysis beyond stage 1 is performed, a number of values of [Fe/H] for the Paper II zero-point stars are encountered. Those results have not been used to calculate the mean values of [Fe/H] given in Paper II for the zero-point stars. As a result, one might conceivably find a discrepancy that could signal that the Paper II mean values should be modified. Reassuringly, no discrepancy as large as is found.

Sometimes data require more than a simple zero-point adjustment before being added to the data base. As noted in Table 1, four additional correction algorithms are required.

1) *U*-*B* *corrections.* Suppose that values of [Fe/H] from a given
paper have a systematic error that varies with temperature. If the cause is
continuum misplacement, the error will increase if temperature decreases or if
metallicity increases. By using a parameter that does the same things, one
can correct for the error. *U*-*B* fits these requirements, and since it also
has a large dynamic range, it should be a good choice of correction parameter.
The adopted correction equation then has the following form:

(1) |

with [Fe/H] and *S* and *Z* being constants.

This technique can be tested by applying it to the results of Gratton et al. (1982). Those authors derived high-dispersion values of [Fe/H] from the blue-violet spectral region. They give convincing evidence that their results suffer from continuum-placement error. The technique described above detects the results of this error at a confidence level exceeding 99.99%. (For more information about this test, see T91, Sect. 2.6, paragraphs 1, 2, and 4.)

2) *B*-*V* *corrections.* These corrections are applied to data from Brown
et al. (1989), using a counterpart to Eq. (1). The Brown
et al. equivalent widths were measured to the red of 6600 Å, so they are unlikely
to suffer from continuum misplacement. It seems more likely that in this
case, the systematic error depends on temperature alone. Ideally, one would
use some version of *R*-*I* or a similar low-blanketing color index in the
correction equation. *B*-*V* is chosen instead because it is readily available.
The resulting correction equation appears to be quite adequate. (See T91,
Sect. 3, entry for Brown et al.)

3) *Corrections based on* . These
corrections are applied to the data of Zacs (1994).
Values of derived by Zacs are the only convenient data on
which to base the corrections. Again, the correction equation is a
counterpart to Eq. (1).

4) *Rescaling*. Spectrum-synthesis results given by Williams
(1971, 1972) are included in the analysis. Those
results are known to suffer from a scale-factor error. The adopted
procedure for correcting that error is to transform the Williams data so
that they correspond to spectrum-synthesis results from Gustafsson et al.
(1974). The correction equation applied here is

(2) |

It may be noted that when Eq. (1) and its counterparts are derived, one
must find out whether the resulting values of *S* differ significantly from
zero. If they do not, it is appropriate to assume that *S*=0 and derive a
zero-point correction *Z* alone. To solve this problem, a least-squares
routine that returns rms errors for derived coefficients is employed. The
ratio of *S* to its derived error is then calculated. This ratio is the
*t* statistic, and it may be used with standard tables to find out whether
*S* differs from zero at 95% confidence or better. A similar procedure
may be used if *S*=0 is assumed and one suspects that *Z* does not differ
significantly from zero.

Some published metallicities for evolved K stars are not included in the data base. The number of such data is not disturbingly large; one finds that even without them, the final catalog includes results for 1118 stars. Data may be set aside because of continuum-placement problems or other good reasons to suspect that they suffer from systematic errors. Other reasons for setting data aside include a lack of pertinent information about the analyses used to produce them. A complete list of omitted data bases and the reasons for omitting them will be given with the catalog (see Taylor 1999).

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