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

TL;DR

This study investigates the wind-driving mechanism in giant stars, focusing on EU Del, and suggests pulsations may significantly influence mass loss, challenging the idea that dust radiation pressure alone drives stellar winds.

Contribution

It provides new observational data on EU Del's wind properties and proposes pulsations as a key factor in wind acceleration, contrasting with traditional dust-driven models.

Findings

EU Del has a wind velocity of 9.51 km/s.

Mass-loss rate is a few times 10^{-8} solar masses per year.

Pulsations may significantly contribute to wind driving.

Abstract

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 $^{+9}_{-4}$, 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.