2016MNRAS.456.2550R

We have performed new medium resolution spectroscopy, optical and near-infrared photometry to monitor the variability of the asymptotic giant branch (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 [Nii] doublet 6548-6584Å emission, strongly related to the simultaneous photometric fading. V381 Lac occupies regions of Two Micron All-Sky Survey and Wide-field Infrared Survey Explorer 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 characterizing 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 ∼ 2 M_☉, 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 (τ10 ∼ 0.22) by dust, mainly formed by amorphous carbon ( ∼ 80 percent) and SiC ( ∼ 20 percent), with dust grain dimensions around ∼ 0.2 and 0.08µm, respectively.