2 The zero-point issue

2.1 Drawing an extended sample

The zero-point issue is considered here for two reasons: a) the reliability of extant zero-point techniques does not seem to have been discussed comprehensively in the literature, and b) there appears to be a common misconception about which of those techniques is most often used. It is sometimes maintained that the predominant zero-point technique is "external zeroing,'' in which a published solar value of (Fe/H) is subtracted from a derived stellar value of (Fe/H) (see especially Kurucz 2002a, 2002b). In fact, that technique does not yield rigorous results, and its prevalence would almost certainly rule out the possibility of assembling a catalog with a reliable zero point. This is established in Appendix A, in which the zero-point reliability of techniques found in the literature is discussed at some length.

To establish a reliable picture of zero-point practice, an "extended sample'' of published papers is compiled. All papers included in this sample were published before the end of 2002 January. The stars whose spectra are analyzed in those papers

• are on or near the main sequence,

• have spectral types earlier than K5, and

• have effective temperatures $T_{\rm e} < 6833$ K.

In addition, the following relation applies for one or more of the stars considered:

$${\rm [Fe/H]} > 3.43 - 5\theta,$$ (1)

with $\theta \equiv 5040/T_{\rm e}$. Note that Eq. (1) admits only metal-rich and modestly metal-poor stars. HD 103095 and similar cool stars are included, but HD 140283 and stars with similar metallicity and temperature are not considered.

Restrictions are applied to the selection of analyses as well as the selection of stars. If two reports of an identical analysis appear in the literature, only one of those reports is included. In addition, there are restrictions based on the number of lines $N_{\rm L}$ that are used in the analyses. $N_{\rm L}$ is $\geq$9 for analyses based on photographic spectra and is usually $\geq$6 for analyses based on Reticon and CCD spectra. This rule will be required when a limited version of the extended sample is used to derive the catalog (see below). The rule reflects the fact that zero-point problems may arise when values of [Fe/H] are derived from small numbers of lines (see Sect. 5.2 of Taylor 1998b). Exceptions to the rule are allowed if the zero points of the pertinent values of [Fe/H] can be established extrinsically (see Sect. 3.2). (For further discussion of all but the last of these rules, Sect. 3.3 of Taylor (1994a, hereafter T94) should be consulted.)

2.2 Surveying the extended sample

The 182 papers in the extended sample are now sorted according to zero-point procedure. For each kind of analysis considered, the frequency of use may be expressed as a percentage. These percentages are listed in Table 1. As the entries in that table show, external zeroing is in fact a minority technique for the kinds of stars considered here. The majority technique is differential analysis relative to the Sun, with 82% of the papers in the extended sample being included in this class.

   

Table 1: Percentages for various kinds of high-dispersion analysis.

$\phantom{{\rm ^a}}$ID${\rm ^a}$	Description	Percentage
$\phantom{{\rm ^b}}1{\rm ^b}$	Differential analysis (own solar EWs)${\rm ^c}$	41
$\phantom{{\rm ^b}}3{\rm ^b}$	Differential analysis (solar EWs from atlas)${\rm ^c}$	41
$\phantom{{\rm ^b}}1, 3{\rm ^b}$	Differential analysis (all comparisons to Sun)	82
5, 6	External zeroing	7
7	Pseudo-absolute analysis	3
4	Star other than Sun used as a standard star	4
-	Zero-point procedure not adequately described	4

$^{{\rm a}}$

Number of example in Appendix A.

$^{{\rm b}}$

Analyses with and without the Holweger-Müller (1974) solar model are both counted.
"EWs'' are equivalent widths.