On the effect of rotation on populations of classical Cepheids

Abstract

Context. Classical Cepheids are among the most important variable star types due to their nature as standard candles and have a long history of modeling in terms of stellar evolution. The effects of rotation on Cepheids have not yet been discussed in detail in the literature, although some qualitative trends have already been mentioned. Aims. We aim to improve the understanding of Cepheids from an evolutionary perspective and establish the role of rotation in the Cepheid paradigm. In particular, we are interested in the contribution of rotation to the problem of Cepheid masses, and explore testable predictions of quantities that can be confronted with observations. Methods. Recently developed evolutionary models including a homogeneous and self-consistent treatment of axial rotation are studied in detail during the crossings of the classical instability strip (IS). The dependence of a suite of parameters on initial rotation is studied. These parameters include mass, luminosity, temperature, lifetimes, equatorial velocity, surface abundances, and rates of period change. Results. Several key results are obtained: i) mass−luminosity (M−L) relations depend on rotation, particularly during the blue loop phase; ii) luminosity increases between crossings of the IS. Hence, Cepheid M−L relations at fixed initial rotation rate depend on crossing number (the faster the rotation, the larger the luminosity difference between crossings); iii) the Cepheid mass discrepancy problem vanishes when rotation and crossing number are taken into account, without a need for high core overshooting values or enhanced mass loss; iv) rotation creates dispersion around average parameters predicted at fixed mass and metallicity. This is of particular importance for the period−luminosity relation, for which rotation is a source of intrinsic dispersion; v) enhanced surface abundances do not unambiguously distinguish Cepheids occupying the Hertzsprung gap from ones on blue loops (after dredgeup), since rotational mixing can lead to significantly enhanced main sequence (MS) abundances; vi) rotating models predict greater Cepheid ages than non-rotating models due to longer MS lifetimes. Conclusions. Rotation has a significant evolutionary impact on classical Cepheids and should no longer be neglected in their study.

NGC 6067: a young and massive open cluster with high metallicity

“…By using the Period-Mass relations from Anderson et al (2014) and Anderson et al (2016), we derive masses for the Cepheids: 6.1 ± 0.5 M for V340 Nor and 5.3 ± 0.4 M for QZ Nor (having taken into account that it is an overtone pulsator). According to the isochrone, the most evolved stars have masses very slightly below 6 M .…”

Evidence from K2 for Rapid Rotation in the Descendant of an Intermediate-mass Star

“…Therefore, we have few constraints on the past or future evolution of angular momentum in Cepheids; rotation can have a significant evolutionary impact on these standard candles (Anderson et al 2014).…”

How rotation affects masses and ages of classical Cepheids

“…Rotation leads to additional testable predictions, since rotation impacts stellar structure, evolution, mass-loss, and surface abundances ( [3]). To this end, we have carried out extensive comparisons with different kinds of observational data ( [13,14]) in the past and will continue to confront model predictions to observations.…”

“…Rotation significantly prolongs the duration over which a star can produce energy via fusion in its core by effectively increasing core size, leading to increased lifetimes for increasing levels of rotation. Note that period-age relations for models with average and fast rotation are comparable, in contrast to the effects of rotation on MLRs ( §2, [13]). Using Geneva models ( [14]), we have quantified this effect and find that typical levels of rotation imply Cepheids to be older by approximately 60 − 100% than if rotation is not taken into account (cf.…”

“…via enhanced mass-loss ( [18]), and b) increasing luminosity at fixed mass, e.g. by increasing effective stellar core sizes via enhanced convective core overshooting (e.g., [19]) or rotational mixing ( [13]). Despite growing observational evidence for circumstellar material in the vicinity of Cepheids ( [20][21][22]), pulsationally-enhanced mass-loss rates ( [18],Ṁ ∼ 10 −10 − 10 −8 M yr −1 ) do not appear sufficient to resolve the mass discrepancy given that the Cepheid evolutionary phase is relatively short-lived (10 4 − 10 6 yr).…”