Ultraviolet Observations of τ⁴ Serpentis (M5 IIb — IIIa)

Abstract

Ultraviolet observations of the late giant τ⁴ Ser = HD 139216 (M5 IIb — IIIa) were obtained with the International Ultraviolet Explorer (IUE) in the 1200 to 3200 A spectral range. Our data reveal strong Mg II resonance doublet emission that in high dispersion exhibits reversed asymmetric h and k line profile emission. High dispersion spectra of a number of K and M giants and supergiants obtained by Stencel and Mullan (1980) show similar emission profiles in the h and k lines that they attribute to a combination of interstellar and circumstellar absorption. Analysis of our data in context with recent ground-based observations of this M giant suggests that it most likely ejected material from its surface forming a low excitation expanding circumstellar shell. The tentative identification of Fe II emission in low and high spectral resolution also argues for a low extitation shell that surrounds τ⁴ Ser. Low dispersion UV spectra obtained with IUE of the Mira variable R Aquilae exhibit similar spectral properties when compared to the low resolution spectrum of τ⁴ Ser and suggests that a moderate excitation shell is also associated with long period variables. Our UV spectral observations and analysis are described below.