EU Del: exploring the onset of pulsation-driven winds in giant stars

We explore the wind-driving mechanism of giant stars through the nearby (117 pc), intermediate-luminosity ($L\approx 1600$ L${}_{\odot }$) star EU Del (HIP 101810, HD 196610). APEX observations of the CO (3–2) and (2–1) transitions are used to derive a wind velocity of 9.51 $\pm$ 0.02 km s${}^{-1}$, a ${}^{12}$C/${}^{13}$C ratio of 14 ${}_{-4}^{+9}$, and a mass-loss rate of a few $\times$ 10${}^{-8}$ M${}_{\odot }$ yr${}^{-1}$. From published spectra, we estimate that the star has a metallicity of [Fe/H] = --0.27 $\pm$ $\sim$0.30 dex. The star’s dusty envelope lacks a clear 10-$\mu$m silicate feature, despite the star’s oxygen-rich nature. Radiative transfer modelling cannot fit a wind acceleration model which relies solely on radiation pressure on condensing dust. We compare our results to VY Leo (HIP 53449), a star with similar temperature and luminosity, but different pulsation properties. We suggest the much stronger mass loss from EU Del may be driven by long-period stellar pulsations, due to its potentially lower mass. We explore the implications for the mass-loss rate and wind velocities of other stars.