The use of non-solar-scaled opacities to derive stellar parameters

SAFFE, CARLOS; FLORES, MATÍAS; MIQUELARENA, PAULA; LÓPEZ, F. M.; JAQUE ARANCIBIA, M.; COLLADO, ANA; JOFRÉ, EMILIANO; PETRUCCI, ROMINA

La Serena

Congreso; 2da Reunión Binacional AAA-SOCHIAS 2018; 2018

AAA y SOCHIAS

We implemented non-solar-scaled opacities in a simultaneous derivation of fundamental parameters and chemical abundances, in order to compare the results with the classical solar-scaled approach.Our sample is composed by 20 stars (10 main-sequence and 10 evolved objects), with six stars belonging to binary systems. The stellar parameters were determined by imposing ionization and  excitation equilibrium of Fe lines, with an updated version of the FUNDPAR program, together with plane-parallel ATLAS12 model atmospheres and the MOOG code. Opacities for an arbitrary composition and vmicro were calculated through the OS (Opacity Sampling) method. Detailed abundances were derived using equivalent widths and spectral synthesis with the MOOG program. We applied the full line-by-line differential technique using the Sun as reference star, both in the derivation of stellar parameters and in the abundance determination. We start using solar-scaled models in a first step, and then continue the process but scaling to the abundance values found in the previous step (i.e. non-solar-scaled). The process finish when the stellar parameters of one step are the same of the previous step, i.e. we use a doubly-iterated method. We obtained a small difference in stellar parameters derived with non-solar-scaled opacities compared to classical solar-scaled models. The differences in Teff, log g and [Fe/H] amount up to 26 K, 0.05 dex and 0.020 dex for the stars in our sample. These differences could be considered as the first estimation of the error due to the use of classical solar-scaled opacities to derive stellar parameters withsolar-type and evolved stars. We note that some chemical species could also show an individual variation higher than those of the [Fe/H] (up to 0.03 dex) and varying from ∼ one specie to another, obtaining a chemical pattern difference between both methods. This means that condensation temperature Tc trends could also present a variation. We include an example showing that using non-solar-scaled opacities, the solution found with the classical solar-scaled method indeed cannot always verify the excitation and ionization balance conditions required for a model atmosphere. We consider that the use of the non-solar-scaled opacities is not mandatory e.g. in every statistical study with large samples of stars. However, for those high-precision works whose results depend on the mutual comparison of different chemical species (such as the analysis of condensation temperature Tc trends), we consider that it is worthwhile its application. To date, this is probably one of the more precise spectroscopic methods of stellar parameters derivation.