Testing the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds

Abstract

The B‐type pulsators known as β Cephei and slowly pulsating B (SPB) stars present pulsations driven by the κ mechanism, which operates thanks to an opacity bump due to the iron‐group elements. In low‐metallicity environments such as the Magellanic Clouds, β Cep and SPB pulsations are not expected. Nevertheless, recent observations show evidence for the presence of B‐type pulsator candidates in both galaxies. We seek an explanation for the excitation of β Cep and SPB modes in those galaxies by examining basic input physics in stellar modelling: (i) the specific metal mixture of B‐type stars in the Magellanic Clouds and (ii) the role of a potential underestimation of stellar opacities. We first derive the present‐day chemical mixtures of B‐type stars in the Magellanic Clouds. Then, we compute stellar models for that metal mixture and perform a non‐adiabatic analysis of these models. In the second approach, we simulate parametric enhancements of stellar opacities due to different iron‐group elements. We then study their effects in models of B stars and their stability. We find that adopting a representative chemical mixture of B stars in the Small Magellanic Cloud cannot explain the presence of B‐type pulsators there. An increase of the opacity in the region of the iron‐group bump could drive B‐type pulsations, but only if this increase occurs at the temperature corresponding to the maximum contribution of Ni to this opacity bump. We recommend an accurate computation of the Ni opacity to understand B‐type pulsators in the Small Magellanic Cloud, as well as the frequency domain observed in some Galactic hybrid β Cep–SPB stars.

Interpretation of the BRITE oscillation data of the hybrid pulsator ν Eridani: a call for the modification of stellar opacities

“…4), we added also 100% opacity around log T 0 = 5.06. Within the Z-bump, we have at log T 0 = 5.3 the maximum contribution of iron to the opacity whereas at log T 0 = 5.46 the maximum contribution of nickel occurs (e.g., Salmon et al 2012). Adding 100% of the opacity only at log T 0 = 5.3 increases the instability of both p-and high order gmode frequencies whereas an additional increase of κ at the deeper temperature log T 0 = 5.46 raises the parameter η mostly for the low frequency modes.…”

A solar twin in the eclipsing binary LL Aquarii

“…Serenelli et al 2009), but also to improve the modelling of other pulsating stars, for example the B-type pulsators (e.g. Salmon et al 2012;Walczak et al 2013). It will also help to construct better models for LL Aqr.…”

Seismic models of O and B stars with Kurucz’s opacity data

“…In this context, variable B stars observed in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are interesting. Compilations of spectroscopic data listed in Brott et al (2011) and Salmon et al (2012) indicates that Z = 0.0047−0.0060 and Z = 0.0021−0.0024 for the LMC and SMC, respectively. Several candidates for β Cepheitype stars were, however, found by Pigulski & Kolaczkowski (2002), Kolaczkowski et al (2004Kolaczkowski et al ( , 2006, Diago et al (2008a,b), Karoff et al (2008), and Pigulski & Pojmanski (2008).…”

“…No unstable low-degree oscillation modes in the p-domain were obtained for models of B stars using Grevesse & Noels (1993) mixture of elements for Z < 0.01 (Pamyatnykh 1999;Miglio et al 2007). If a small contribution of C, N, O, and Ne to Z as listed in Asplund et al (2009), then Z ≥ 0.007 is needed for excitation of p-modes in B stars (Salmon et al 2012), because of the higher contribution of irongroup elements to Z. In this context, variable B stars observed in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are interesting.…”