The effect of pulsation on the near-ultraviolet spectrum of AGB stars

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 < φ < 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 < φ < 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)