Nebula around R Corona Borealis

TL;DR

This paper reports the detection of nebular emission lines around R Corona Borealis during its deep minimum, indicating the presence of low-density gas and discussing its physical conditions and possible origins.

Contribution

It provides detailed spectroscopic observations of forbidden lines during deep minima, revealing nebular material around R CrB and similar stars, which is a new insight into their environment.

Findings

Forbidden lines show double peaks with 170 km/s separation.

Electron density of the nebula is about 100 cm$^{-3}$.

Forbidden lines are common in R CrB stars during minima.

Abstract

The star R Corona Borealis (R CrB) shows forbidden lines of [O II], [N II], and [S II] during the deep minimum when the star is fainter by about 8 to 9 magnitudes from normal brightness, suggesting the presence of nebular material around it. We present low and high spectral resolution observations of these lines during the ongoing deep minimum of R CrB, which started in July 2007. These emission lines show double peaks with a separation of about 170 km/s. The line ratios of [S II] and [O II] suggest an electron density of about 100 cm$^{-3}$. We discuss the physical conditions and possible origins of this low density gas. These forbidden lines have also been seen in other R Coronae Borealis stars during their deep light minima and this is a general characteristic of these stars, which might have some relevance to their origins.