Broad-band Spectroscopy of the Ongoing Large Eruption of the Luminous Blue Variable R71

TL;DR

This study presents spectroscopic observations of the LBV R71 during its ongoing eruption, revealing unprecedented low temperatures and high mass-loss rates, and compares these with previous outbursts to understand LBV eruption physics.

Contribution

First detailed spectral analysis of R71's current eruption, showing unique low temperature and high mass-loss features compared to past eruptions.

Findings

R71's apparent temperature during eruption is ~6650 K, resembling a late F supergiant.

Spectra lack typical P Cyg profiles seen in previous outbursts.

Mass-loss rate during eruption is 1000 times higher than in quiescence.

Abstract

The Luminous Blue Variable (LBV) R71 is currently undergoing an eruption, which differs photometrically and spectroscopically from its last outburst in the 1970s. Valuable information on the physics of LBV eruptions can be gained by analyzing the spectral evolution during this eruption and by comparing R71's present appearance to its previous outburst and its quiescent state. Here we present the first spectra of an ongoing monitoring program with VLT/X-shooter and compare them to archival VLT/UVES and MPG/ESO-2.2m/FEROS spectra from 2002-2011. The discussed data include pre-eruption spectra in 2002 and 2005, a spectrum of the transitionary phase between quiescent and eruptive state in 2007, and spectra of the eruption in 2011-2012. The 2011-2012 spectra are dominated by strong neutral and singly ionized metal absorption lines likely formed in a large "pseudo-photosphere." We find an unusually low apparent temperature of R71 of only ~6650 K; the star resembles a late F supergiant. R71's visual lightcurve had a maximum in 2012 with mV ~ 8.7 mag. Given the uncertainty in the extinction towards R71, this corresponds to a bolometric luminosity of -9.8 mag to -10.3 mag. R71's 2011-2012 spectra do not show H I and Fe II P Cyg profiles, which were present during its last outburst in the 1970s and which are normally observed during LBV outbursts. The rise in R71's visual magnitude and the low apparent temperature of its pseudo-photosphere during the current eruption are unprecedented for this star. R71 most likely increased its bolometric luminosity by 0.4-1.3 mag compared to its quiescent state. The very low temperature of its pseudo-photosphere implies a very high-mass loss rate, a factor of 1000 higher than during the quiescence state. No fast-moving material indicative of an explosion is observed (abridged).