iSpec implements two approaches to derive atmospheric parameters: the synthetic spectral fitting technique and the equivalent width method.
Synthetic spectral fitting technique
This technique minimizes \(\chi^2\) between an observed spectrum and synthetic spectra computed on the fly or interpolate it from a pre-computed grid. It requires:
- Initial atmospheric parameters
- List of parameters that should be free (recommended: effective temperature, surface gravity, metallicity, microturbulence and resolution)
- Line masks with the spectral regions that are going to be used in the computation of the \(\chi^2\). A good selection is required for a good determination of parameters, iSpec includes a line selection for spectra of resolution ~47000 based on VALD and GES atomic line lists:
- SPECTRUM: 'input/regions/47000_VALD/spectrum_synth_good_for_params_all_extended.txt'
- SME: 'input/regions/47000_VALD/sme_synth_good_for_params_all_extended.txt'
- Turbospectrum: 'input/regions/47000_VALD/turbospectrum_synth_good_for_params_all_extended.txt'
- SYNTHE: 'input/regions/47000_VALD/synthe_synth_good_for_params_all_extended.txt'
- MOOG: 'input/regions/47000_VALD/moog_synth_good_for_params_all_extended.txt'
- SPECTRUM: 'input/regions/47000_GES/spectrum_synth_good_for_params_all_extended.txt'
- SME: 'input/regions/47000_GES/sme_synth_good_for_params_all_extended.txt'
- Turbospectrum: 'input/regions/47000_GES/turbospectrum_synth_good_for_params_all_extended.txt'
- SYNTHE: 'input/regions/47000_GES/synthe_synth_good_for_params_all_extended.txt'
- MOOG: 'input/regions/47000_GES/moog_synth_good_for_params_all_extended.txt'
- Segments that cover one or more line masks and for which the synthetic spectra will be computed (instead of the full spectrum, which would be slower)
The atmospheric parameters can be derived through the "Parameters - Determine astrophysical parameters with synthesis" or "Parameters - Determine astrophysical parameters with grid" option menu.
The radiative transfer codes, linelists, solar abundances and model atmospheres are the same as for the synthetic generation option but they are not required in case of using a pre-computed grid. The results are shown directly in the terminal.
Equivalent width method
This method uses the equivalent width from observed iron absorption lines to derive iron abundances and impose:
- Ionization balance: Average iron abundance is equal to average ionized iron abundance
- Excitation equilibrium: No trends are found when abundances are plot against line excitation potential
- Abundances not correlated with equivalent widths: No trends are found when abundances are plot against reduced equivalent width.
The combination of parameters that comply with the three previous points are the ones of the star.
The following steps are needed before selection "Parameters - Determine atmospheric parameters with equivalent widths":
- Load a spectrum and the line masks (corresponding to iron lines) to be used in the analysis
- Fit the continuum
- Fit the lines:
- A gaussian/voigt profile will be fitted to determine the equivalent width, central wavelength, etc.
- A cross-match with the some selected atomic data will be executed to assign atomic data for each line (it will be used to derive atmospheric parameters)
As radiative transfer codes for equivalent width, only MOOG and WIDTH9 are recommended (SPECTRUM and Turbospectrum are extremely slow for this kind of approach). Also, a good selection is required for a good determination of parameters, iSpec includes a line selection for spectra of resolution ~47000 based on VALD and GES atomic line lists:
- MOOG: 'input/regions/47000_VALD/moog_ew_ispec_good_for_params_all_extended.txt'
- WIDTH9: 'input/regions/47000_VALD/width_ew_ispec_good_for_params_all_extended.txt'
- MOOG: 'input/regions/47000_GES/moog_ew_ispec_good_for_params_all_extended.txt'
- WIDTH9: 'input/regions/47000_GES/width_ew_ispec_good_for_params_all_extended.txt'