Opacity Effects on Pulsations of Main-Sequence A-Type Stars

Abstract

Opacity enhancements for stellar interior conditions have been explored to explain observed pulsation frequencies and to extend the pulsation instability region for B-type main-sequence variable stars [see, e.g., 1-6]. For these stars, the pulsations are driven in the region of the opacity bump of Fe-group elements at ∼200,000 K in the stellar envelope. Here we explore effects of opacity enhancements for the somewhat cooler main-sequence A-type stars, in which p-mode pulsations are driven instead in the second helium ionization region at ∼50,000 K. We compare models using the new LANL OPLIB [7] vs. LLNL OPAL [8] opacities for the AGSS09 [9] solar mixture. For models of 2 solar masses and effective temperature 7600 K, opacity enhancements have only a mild effect on pulsations, shifting mode frequencies and/or slightly changing kinetic-energy growth rates. Increased opacity near the bump at 200,000 K can induce convection that may alter composition gradients created by diffusive settling and radiative levitation. Opacity increases around the hydrogen and 1st He ionization region (13,000 K) can cause additional higher-frequency p modes to be excited, raising the possibility that improved treatment of these layers may result in prediction of new modes that could be tested by observations. New or wider convective zones and higher convective velocities produced by opacity increases could also affect angular momentum transport during evolution. More work needs to be done to quantify the effects of opacity on the boundaries of the pulsation instability regions for A-type stars.

Asteroseismic Modeling of Gravity Modes in Slowly Rotating A/F Stars with Radiative Levitation

“…We do point out that Deal et al (2016) observe an iron convection zone around = T log 5.3 for stars .7 . Here, we only observe such a convection zone when the opacities are artificially increased with a factor of 5 around the Z-bump, in agreement with the study by Guzik et al (2018). It is not surprising that differences between model properties in the outer envelope resulting from different codes occur, as the computations by Deal et al (2016) rely on the SVP approximation while we used the opacity sampling method and treated the 10 −5 % of outer mass as a single cell for atomic diffusion.…”

Common Envelope Wind Tunnel: Range of Applicability and Self-Similarity in Realistic Stellar Envelopes

“…Notably, none of the envelopes shown could be described as perfectly polytropic. In fact, the outer envelope often contains one or more regions of highly compressible material interspersed with convective or radiative regions, including density inversions that correspond to hydrogen and helium opacity peaks at T ∼ 5500K and 13000K (Sanyal et al 2015;Guzik et al 2018). The differences in structure seen here affect key processes in CE evolution, namely orbital decay due to drag and the ability of released energy to escape the envelope.…”

“…In other profiles, some of the features visible are bands of convective and radiative regions within the same envelope, as well as spikes near the limb that represent density inversions in the outermost envelope. The bands generally do not appear in the characteristic curves, but the density inversions, which are a result of steep temperature gradients in zones of partial ionization (Harpaz 1984) that correspond to hydrogen and helium opacity "bumps" (Sanyal et al 2015;Guzik et al 2018), fall outside simulated parameters and force ρ values to be negative; thus models that have such density inversions are not appropriate for the drag formalism. It is worth noting that, due to mass loss during onset, the regions containing this feature may possibly be stripped from the star prior to CE, and envelope regions internal to this feature fit comfortably within the established parameter space.…”

An Expanded Set of Los Alamos OPLIB Tables in MESA: Type-1 Rosseland-mean Opacities and Solar Models

“…Helium ionization drives pulsations in RR Lyrae variables (Smith 2004;Ngeow et al 2022) and δ Scuti variables (Balona 2018;Bowman & Kurtz 2018;Guzik 2021;Murphy et al 2023), DBV white dwarf variables (Córsico et al 2019;Saumon et al 2022), and is furthermore responsible for acoustic glitches in solar-like oscillators (Gough 1990;Mazumdar et al 2014;Verma et al 2017Verma et al , 2019Saunders et al 2023). Other opacity increases from the iron group elements (Cr, Fe, Ni, and Cu) at temperatures of ;2 × 10 5 K and densities of ;10 −7 g cm −3 are likely the cause of pulsations in B-type stars (Townsend 2005;Aerts et al 2010;Guzik et al 2018;Shi et al 2023), and β-Cephei variables (e.g., β Centauri, γ Pegasi, and ν Eridani), where κ may account for differences between the observed and calculated pulsation periods (Daszyńska-Daszkiewicz & Walczak 2009Cugier 2012;Walczak et al 2015).…”