Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71

TL;DR

This study analyzes the spectroscopic and photometric variability of the LBV R71 during its outburst, revealing oscillatory behavior, dust formation, and atmospheric shocks, contributing to understanding LBV phenomena and their relation to other variable stars.

Contribution

It provides detailed multi-wavelength analysis of R71's outburst, identifying oscillatory variability and atmospheric shocks, and links LBV behavior to RV Tau variables.

Findings

R71's effective temperature during quiescence is 15,500 K.

Detected dust formation and grain evolution during outburst.

Observed semi-regular oscillatory variability with periods around 425/850 days.

Abstract

To better understand the LBV phenomenon, we analyze multi-epoch and multi-wavelength spectra and photometry of R71. Pre-outburst spectra are analyzed with the radiative transfer code CMFGEN to determine the star's fundamental stellar parameters. During quiescence, R71 has an effective temperature of $T_\mathrm{{eff}} = 15\,500~K$ and a luminosity of log$(L_*/L_{\odot})$ = 5.78 and is thus a classical LBV, but at the lower luminosity end of this group. We determine its mass-loss rate to $4.0 \times 10^{-6}~M_{\odot}~$yr$^{-1}$. We present R71's spectral energy distribution from the near-ultraviolet to the mid-infrared during its present outburst. Mid-infrared observations suggest that we are witnessing dust formation and grain evolution. Semi-regular oscillatory variability in the star's light curve is observed during the current outburst. Absorption lines develop a second blue component on a timescale twice that length. The variability may consist of one (quasi-)periodic component with P ~ 425/850 d with additional variations superimposed. During its current S Doradus outburst, R71 occupies a region in the HR diagram at the high-luminosity extension of the Cepheid instability strip and exhibits similar irregular variations as RV Tau variables. LBVs do not pass the Cepheid instability strip because of core evolution, but they develop comparable cool, low-mass, extended atmospheres in which convective instabilities may occur. As in the case of RV Tau variables, the occurrence of double absorption lines with an apparent regular cycle may be due to shocks within the atmosphere and period doubling may explain the factor of two in the lengths of the photometric and spectroscopic cycles.