Period doubling and Blazhko modulation in BL Herculis hydrodynamic models

Abstract

We present hydrodynamic BL Herculis type models which display a long‐term modulation of pulsation amplitudes and phases. The modulation is either strictly periodic or quasi‐periodic, with the modulation period and modulation pattern varying from one cycle to another. Such behaviour has not been observed in any BL Herculis variable so far; however, it is a common property of their lower luminosity siblings – RR Lyrae variables showing the Blazhko effect. These models provide support for the recent mechanism proposed by Buchler & Kolláth to explain this still mysterious phenomenon. In their model, a half‐integer resonance that causes the period‐doubling effect, discovered recently in Blazhko RR Lyrae stars, is responsible for the modulation of pulsation as well. Although our models are more luminous than is appropriate for RR Lyrae stars, they clearly demonstrate, through direct hydrodynamic computation, that the mechanism can indeed be operational. Of great importance are models that show quasi‐periodic modulation – a phenomenon observed in Blazhko RR Lyrae stars. Our models coupled with the analysis of the amplitude equations show that such behaviour may be caused by the dynamical evolution occurring in the close proximity of the unstable single periodic saddle point.

New results in RR Lyrae modeling: Convective cycles, additional modes and more

“…This has been observed-along with Blazhko-like modulation-in models of BL Herculis-type stars by Smolec & Moskalik (2012) but not yet in RR Lyrae models.…”

“…The calculations revealed that the resonance is capable to create not only period doubling but also periodic and chaotic modulation. Though hydrodynamic RR Lyrae models have not been able to reproduce this modulation yet, the radial resonance model gained popularity, thanks to recent developments: the criticism of the Stothers-model (Section 2), the detection of additional modes and the broad range of possible resonances and mode interactions (Sections 3.2-3.4), and the detection of modulation in period-doubled BL Herculis models (Smolec & Moskalik 2012). …”

Diversity of dynamical phenomena in type II Cepheids of the OGLE collection

“…Two stars were already reported in the past, based on the analysis of OGLE-III data. T2CEP-279 was the first BL Her star with PD effect, reported by Soszyński et al (2011) and Smolec et al (2012). In the same papers, T2CEP-257 was announced as a candidate.…”

“…The existence of PD in BL Her star was predicted by Buchler & Moskalik (1992) based on radiative hydrodynamic models. However, only recently the phenomenon was detected in T2CEP-279 (firm detection) and in T2CEP-257 (candidate star) based on OGLE-III data (Soszyński et al 2011;Smolec et al 2012). Here we confirm both detections and add one more BL Her star to the sample, .…”

Blazhko effect in Cepheids and RR Lyrae stars

“…This result is a very important step towards the understanding of the Blazhko effect. The final step of proving the concept with full hydrodynamics models still remains to be done, but it is worth mentioning here that Smolec & Moskalik (2012) were able to produce modulated hydrodynamic models of BL Her stars in the presence of period doubling. The radial resonance paradigm is currently the most tenable, and the only one that is backed up by full hydrodynamic models.…”