Electron conduction opacities at the transition between moderate and strong degeneracy: Uncertainties and impacts on stellar models

Abstract

Electron conduction opacities are one of the main physics inputs for the calculation of low- and intermediate-mass stellar models. A critical question considers how to devise a bridge when calculating both moderate and strong degeneracy, which are necessarily performed adopting different methods. The density-temperature regime at the boundary between moderate and strong degeneracy is, in fact, crucial for modelling the helium cores of red giant branch stars and the hydrogen-helium envelopes of white dwarfs. Prompted by newly published, improved calculations of electron thermal conductivities and opacities for moderate degeneracy, we study different, physically motivated prescriptions to bridge these new computations with well-established results in the regime of strong degeneracy. We find that these varied prescriptions have a sizable impact on the predicted He-core masses at the He-flash (up to 0.01 M⊙ for initial total masses far from the transition to non-degenerate He-cores and up to ∼0.04 M⊙ for masses around the transition), the tip of the red giant branch (up to ∼0.1 mag), and the zero-age horizontal branch luminosities (up to 0.03 dex for masses far from the transition and up to ∼0.2 dex around the transition), and white dwarf cooling times (up to 40–45% at high luminosities, and up to ∼25% at low luminosities). Current empirical constraints on the tip of the red giant branch and the zero age horizontal branch absolute magnitudes do not yet allow for the definitive exclusion of any of these alternative options for the conductive opacities. Tests against observations of slowly-cooling faint WDs in old stellar populations will need to be performed to see whether they are capable of setting some more stringent constraints on bridging calculations of conductive opacities for moderate and strong degeneracy.

New Theoretical Period–Luminosity–Metallicity Relations for RR Lyrae in the Rubin-LSST Filters

“…For each chemical composition, the higher stellar mass and the lowest luminosity level correspond to the predicted Zero Age Horizontal Branch (ZAHB) values (see, e.g., Pietrinferni et al 2013, and references therein). To account for possible uncertainties in the evolutionary luminosity prediction (see, e.g., Valle et al 2013;Cassisi et al 2021, and references therein), a slightly brighter (by 0.1 dex) luminosity level (than the ZAHB one) for the ZAHB stellar mass is also considered, while lower stellar mass (by about 10%) and still brighter models (0.2 dex more than the ZAHB level) were also computed for each Z and Y combination, in order to model possibly evolved RR Lyrae (see Marconi et al 2015, and references therein, for details). In particular, all the bolometric light curves of models with effective temperatures within the F or FO instability strip for the tabulated chemical compositions and masses have been transformed into the Rubin-LSST filters u LSST , g LSST , r LSST , i LSST , z LSST , and y LSST bands.…”

White Dwarfs as Physics Laboratories: Lights and Shadows

“…A key ingredient is the electron conductivity in the frontier between moderate and strong degeneracy. Cassisi et al, 2021 andSalaris et al, 2022 have computed two sets of models using the electron conductivities obtained by Cassisi et al, 2007 andby Blouin et al, 2020b and have found that models computed with the Blouin et al conductivities were predicting longer cooling times at bright luminosities and shorter cooling times at fainter luminosities as compared with the Cassisi et al…”

New calibrated models for the TRGB luminosity and a thorough analysis of theoretical uncertainties

“…Therefore, the effect of conductive opacities becomes secondary in this context. Before ending this section, it is worth mentioning that recently Cassisi et al (2021) found that the uncertainty on conductive opacity could be much larger than 5%, mainly in the regime of weak level of electronic degeneration. In our case, their results could affect mainly the uncertainties estimated for metal poor stars (i.e.…”

“…The theoretical uncertainties of the Tip luminosity has been discussed in several previous works. For the most recent we refer to Viaux et al (2013), Valle et al (2013a,c), Serenelli et al (2017) Straniero et al (2020) and Cassisi et al (2021). Some of these works focused on the effect on the predicted Tip luminosity (e.g.…”

“…Some of these works focused on the effect on the predicted Tip luminosity (e.g. Valle et al 2013a,c), while others on Tip magnitudes (Viaux et al 2013, Serenelli 2010, Straniero et al 2020, Cassisi et al 2021. In particular, Viaux et al (2013) studied in detail the impact of each individual quantity that could potentially affect the Tip magnitude, but they considered a tight range of mass (0.8-0.85M ) and metallicity (Z=0.001-0.0017).…”