The variable V381 Lac and its possible connection with the R CrB phenomenon

TL;DR

This study monitors the variability of the AGB carbon star V381 Lac, revealing extreme spectral and photometric changes linked to dust formation and mass ejection, resembling the R Coronae Borealis phenomenon.

Contribution

It provides new spectroscopic and photometric observations of V381 Lac, connecting its variability to dust-driven episodes similar to RCB stars, and models its evolutionary status as a late AGB carbon star.

Findings

V381 Lac shows rapid spectral and photometric variability.

Spectral lines switch from absorption to emission during fading.

Dust composition is mainly amorphous carbon and SiC, with specific grain sizes.

Abstract

We have performed new medium resolution spectroscopy, optical and near infrared photometry to monitor the variability of the AGB carbon star V 381 Lac. Our observations revealed rapid and deep changes in the spectrum and extreme variability in the optical and near infrared bands. Most notably we observed the change of NaI D lines from deep absorption to emission, and the progressive growing of the [N II] doublet 6548-6584 A emission, strongly related to the simultaneous photometric fading. V381 Lac occupies regions of 2MASS and WISE colour-colour diagrams typical of stars with dust formation in the envelope. The general framework emerging from the observations of V381 Lac is that of a cool AGB carbon star undergoing episodes of high mass ejection and severe occultation of the stellar photosphere reminiscent of those characterising the RCB phenomenon. Comparing the Spectral Energy Distribution obtained with the theoretical model for AGB evolution with dust in the circumstellar envelope, we can identify V381 Lac as the descendant of a star of initial mass ~2M_sun, in the final AGB phases, evolved into a carbon star by repeated Third Dredge Up episodes. According to our model the star is moderately obscured (tau_10 ~0.22) by dust, mainly formed by amorphous carbon (~80%) and SiC (~20%), with dust grain dimensions around ~0.2 micron and 0.08 micron respectively.