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Context --- The classical method to infer microturbulent velocity in stellarspectra requires that the abundances of the iron lines are not correlated withthe observed equivalent widths. An alternative method, requiring the use of theexpected line strength, is often used to by-pass the risk of spurious slopesdue to the correlation between the errors in abundance and equivalent width.Aims --- To compare the two methods and identify pros and cons andapplicability to the typical practical cases. Methods --- I performed a testwith a grid of synthetic spectra, including instrumental broadening andPoissonian noise. For all these spectra, microturbulent velocity has beenderived by using the two approaches and compared with the original value withwhich the synthetic spectra have been generated. Results --- The two methodsprovide similar results for spectra with SNR$ > 70, while for lower SNR bothapproaches underestimate the true microturbulent velocity, depending of the SNRand the possible selection of the lines based on the equivalent width errors.Basically, the values inferred by using the observed equivalent widths betteragree with those of the synthetic spectra. In fact, the method based on theexpected line strength is not totally free from a bias that can heavily affectthe determination of microturbulent velocity. Finally, I recommend to use theclassical approach (based on the observed equivalent widths) to infer thisparameter. In cases of full spectroscopical determination of all theatmospherical parameters, the difference between the two approaches is reduced,leading to an absolute difference in the derived iron abundances of less than0.1 dex.

Microturbulent velocity from stellar spectra: a comparison between different approaches (Research Note)