Is the semi-regular variable RU Vulpeculae undergoing a helium-shell flash?

TL;DR

This study investigates whether the period decline of the semi-regular variable star RU Vulpeculae is caused by a helium-shell flash, using photometric and spectroscopic data compared with stellar evolution models.

Contribution

The paper provides observational evidence supporting the helium-shell flash hypothesis for RU Vul's period decline through detailed photometry, spectroscopy, and comparison with AGB evolutionary models.

Findings

RU Vul is a metal-poor, old star with T_eff=3634K.

The current pulsation period is approximately 108 days.

The observed period decline matches predictions from AGB models with a thermal pulse.

Abstract

The semi-regular variable star RU Vulpeculae (RU Vul) is being observed visually since 1935. Its pulsation period and amplitude are declining since $\sim1954$. A leading hypothesis to explain the period decrease in asymptotic giant branch (AGB) stars such as RU Vul is an ongoing flash of the He-burning shell, also called a thermal pulse (TP), inside the star. In this paper, we present a CCD photometric light curve of RU Vul, derive its fundamental parameters, and test if the TP hypothesis can describe the observed period decline. We use CCD photometry to determine the present-day pulsation period and amplitude in three photometric bands, and high-resolution optical spectroscopy to derive the fundamental parameters. The period evolution of RU Vul is compared to predictions by evolutionary models of the AGB phase. We find that RU Vul is a metal-poor star with a metallicity $[{\rm M}/{\rm H}]=-1.59\pm0.05$ and an effective surface temperature of $T_{\rm eff}=3634\pm20$ K. The low metallicity of RU Vul and its kinematics indicate that it is an old, low-mass member of the thick disc or the halo population. The present day pulsation period determined from our photometry is $\sim108$ d, the semi-amplitude in the V-band is $0.39\pm0.03$ mag. The observed period decline is found to be well matched by an evolutionary AGB model with stellar parameters comparable to those of RU Vul. We conclude that the TP hypothesis is in good agreement with the observed period evolution of RU Vul.