Self-broadening of the hydrogen Balmerαline

Abstract

Context. Profiles of hydrogen lines in stellar spectra are determined by the properties of the hydrogen atom and the structure of the star's atmosphere. Hydrogen line profiles are therefore a very important diagnostic tool in stellar modeling. In particular they are widely used as effective temperature criterion for stellar atmospheres in the range T eff 5500-7000 K. Aims. In cool stars such as the Sun hydrogen is largely neutral and the electron density is low. The line center width at half maximum and the spectral energy distribution in the wings are determined primarily by collisions with hydrogen atoms due to their high relative density. This work aims to provide benchmark calculations of Balmer α based on recent H 2 potentials. Methods. For the first time an accurate determination of the broadening of Balmer α by atomic hydrogen is made in a unified theory of collisional line profiles using ab initio calculations of molecular hydrogen potential energies and transition matrix elements among singlet and triplet electronic states. Results. We computed the shape, width and shift of the Balmer α line perturbed by neutral hydrogen and studied their dependence on temperature. We present results over the full range of temperatures from 3000 to 12 000 K needed for stellar spectra models. Conclusions. Our calculations lead to larger values than those obtained with the commonly used Ali & Griem (1966, Phys. Rev. A, 144, 366) theory and are closer to the recent calculations of Barklem et al. (2000a, A&A, 355, L5;2000b, A&A, 363, 1091. We conclude that the line parameters are dependent on the sum of many contributing molecular transitions, each with a different temperature dependence, and we provide tables for Balmer α. The unified line shape theory with complete molecular potentials also predicts additional opacity in the far non-Lorentzian wing.

The metal-poor end of the Spite plateau

“…With the typical parameters of the stars in our sample, and using our fitting procedure, employing Ali & Griem (1966) self broadening leads to derived T eff estimates that are higher by about 150-200 K (a difference of about 0.1 dex in Li abundance) with respect to those derived by using the Barklem et al (2000a,b) theory. The theory by Allard et al (2008), on the other hand, leads to T eff within a few tens of K of the T eff estimates obtained when using the Barklem et al (2000a,b) theory. We thus restricted ourselves to using the selfbroadening theory of Barklem et al (2000a,b) (in BA and 3D temperatures) and the Ali & Griem (1966) self-broadening theory (ALI temperatures).…”

“…On the theoretical side, the uncertainties are due both to the atmosphere model structure and to the physics employed in the Hα synthesis. Hα-wing self broadening can be treated with different theories, most notably those of Ali & Griem (1966), Barklem et al (2000a,b), and Allard et al (2008). As a general rule, Ali & Griem (1966) theory leads to a significantly lower broadening coefficient with respect to the ones derived from Barklem et al (2000a,b) and Allard et al (2008).…”

“…Hα-wing self broadening can be treated with different theories, most notably those of Ali & Griem (1966), Barklem et al (2000a,b), and Allard et al (2008). As a general rule, Ali & Griem (1966) theory leads to a significantly lower broadening coefficient with respect to the ones derived from Barklem et al (2000a,b) and Allard et al (2008). A significantly higher T eff is required to reproduce a given observed profile when employing the Ali & Griem (1966) theory with respect to the other theories.…”

Review: progress in NLTE calculations and their application to large data-sets

“…To fit the solar H α flux profile using the MARCS models, one needs to reduce T eff of the model by 90 K compared with the solar temperature T eff = 5780 K (see Fig. 13 from Allard et al 2008). Applying the Allard et al (2008) theory and ATLAS9 model atmospheres (Kurucz 2009), Cayrel et al (2011) determined the LTE effective temperature from H α for the eleven stars with an accurate T eff derived from their apparent angular diameter and found that the two sets .…”

“…13 from Allard et al 2008). Applying the Allard et al (2008) theory and ATLAS9 model atmospheres (Kurucz 2009), Cayrel et al (2011) determined the LTE effective temperature from H α for the eleven stars with an accurate T eff derived from their apparent angular diameter and found that the two sets . In NLTE, the Ca abundance is log εCa = 2.34, and it is 0.55 dex higher in LTE.…”