Temperature estimation with the LDR-method

 

The graph shows the temperature curve from February to mid-october 2018.
The data come from Anna Aret, Günter Gebhard, Indrek Kolka, Malin Moll, Christoph Quandt, Andrew Smith and Ulrich Waldschläger.

The data of the ratio Fe I 6431/Fe II 6433 are calibrated against our thermometer of the FGK supergiants. Since the second pair of lines contains two different metals, we have calibrated the data using our own spectra that are not affected by the emission phenomenon. This avoids discrepancies due to different metal abundances in different stars.

This temperature data was obtained using Line Depth Measurement (LDR). A second possibility is based on the Equivalent Width (EWR), which we only measure for comparison.
The LDR method is more robust against disturbances of the line by e.g. blends (Gray 1994).

The line depth is defined as:

ld = F_cont – F_line.

Where F_cont is the relative flux = 1 and F_line is the flux at the line minimum.
The ratio makes the measurement independent of the resolution. The two line pairs are close to each other so that errors during normalization do not have a strong effect.