2020MNRAS.491..680G

Pulsating and non-pulsating asymptotic giant branch (AGB) stars exhibit a variable near-ultraviolet (near-UV) spectrum, which suggest that mechanisms other than pulsation may affect their near-UV spectrum. In this work, we analyse the near-UV spectra of two groups of AGB stars: (1) regularly pulsating and (2) irregular, small-amplitude stars. Near-UV and blue spectra were obtained for 27 stars distributed into these two groups with the Isaac Newton Telescope (La Palma, Spain). Additional near-UV spectra were taken from the IUE library. The occurrence of Fe II lines depends mainly on the intensity of the stellar continuum: as it increases, Fe II lines are gradually hampered. Balmer emission lines are pulsation driven, as they appear only among the large-amplitude pulsating stars of our sample, between -0.10 < ph < 0.50. Among the regularly pulsating stars, the intensity of the Mg II λ2800 doublet is driven by pulsation, with its maximum between 0.20 < ph < 0.35. On the other hand, this feature is also highly variable among small-amplitude, irregularly pulsating stars. This suggests that, besides pulsation, other mechanisms may participate in the formation of this line. The spectral slope between 3000 < λ(Å)<3200 is approximately constant among small-amplitude stars, but it shows a strong correlation with the phase of the pulsation in the other group, which indicates that the origin of the continuum is chromospheric. Different phase lags between the Mg II λ2800 and the slope of the continuum suggests that this line and its neighbouring continuum might be formed in distinct places in the chromosphere or its surroundings.