The nature of anomalous period increase in the pulsating variable V725 Sgr

TL;DR

This study models stellar evolution and pulsations to explain the unusual period increase in V725 Sgr, attributing it to a helium shell thermal flash in an early post-AGB star with specific mass and envelope characteristics.

Contribution

It provides the first hydrodynamic models linking a helium shell flash to the observed pulsation period increase in V725 Sgr.

Findings

Identified models showing an eightfold period increase.

Linked period growth to a helium shell thermal flash.

Demonstrated transformation from regular to semi-regular pulsations.

Abstract

Evolutionary tracks of stars with masses on the main sequence $0.84M_\odot\le M_\textrm{ZAMS}\le 0.95M_\odot$ and initial metal abundances $Z=0.006$ and $Z=0.01$ were computed under various assumptions about the mass loss rate at the red giant stage as well as at the AGB and the post--AGB stages. Among 160 evolutionary sequences we selected nearly 30 sequences where the final thermal flash of the helium shell source occurs in the early post--AGB stage when the ratio of the hydrogen envelope mass to the stellar mass ranges from 0.01 to 0.08. Selected evolutionary sequences were used for calculation of initial and inner boundary conditions used in solution of the equations of radiation hydrodynamics and turbulent convection describing evolution of stellar pulsations after the helium flash. Among about three dozen hydrodynamic models we found the three ones demonstrating almost eightfold increase of the pulsation period observed in V725 Sgr during the last century as well as the gradual transformation of fairly regular pulsations with period $\Pi\approx 12$ day to semi--regular non--linear oscillations with period $80~\textrm{day} \lesssim\Pi\lesssim 90~\textrm{day}$. We conclude that the anomalous growth of the pulsation period in V725 Sgr is due to the final thermal flash of the helium shell source that occurred in the early post--AGB star with mass $M\approx 0.53M_\odot$ and the mass of the hydrogen envelope ranging from $0.013M_\odot$ to $0.019M_\odot$.