Table 4.
Polynomial coefficients of the best fit for the strong-line metallicity diagnostics.
| Diagnostic | A | B | C | D | E |
|---|---|---|---|---|---|
| R3 | 0.158 | −1.977 | −1.629 | −0.329 | – |
| R2 | 0.290 | −1.202 | −4.104 | −3.167 | −0.812 |
| R23 | 0.515 | −1.614 | −1.421 | −0.286 | – |
 |
−0.055 | −3.049 | −3.168 | −0.807 | – |
| O32 | −0.537 | −2.832 | −1.200 | −0.002 | – |
| Ne3O2 | −1.679 | −2.034 | −0.458 | – | – |
| N2 | −0.910 | 0.486 | – | – | – |
| O3N2 | 0.619 | −3.794 | −2.561 | −0.382 | – |
Notes. The fitted functional form is f(x) = A + Bx + Cx2 + Dx3 + Ex4, where f is the line ratio diagnostic defined as in Sect. 1 and x represents the gas-phase metallicity, normalised to the solar value (12 + log(O/H) − 8.69). The polynomials are fitted over the range 12 + log(O/H) ∈ [7.0, 8.4] and include a regularisation at high and low metallicity based on the Curti et al. (2017) calibrations to stabilise the fit where direct Te measurements are not available. Caution is advised when extrapolating these relations beyond the fitted range.
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