One month convection timescale on the surface of a giant evolved star.

The transport of energy through convection is important during many stages of stellar evolution 1,2 , and is best studied in our Sun 3 or giant evolved stars 4 . Features that are attributed to convection are found on the surface of massive red supergiant stars 5-8 . Also for lower-mass evolved stars, indications of convection are found 9-13 , but convective timescales and sizes remain poorly constrained. Models indicate that convective motions are crucial to produce strong winds that return the products of stellar nucleosynthesis into the interstellar medium 14 . Here we report a series of reconstructed interferometric images of the surface of the evolved giant star R Doradus. The images reveal a stellar disk with prominent small-scale features that provide the structure and motions of convection on the stellar surface. We find that the dominant structure size of the features on the stellar disk is 0.72 ± 0.05 astronomical units. We measure the velocity of the surface motions to vary between -18 and +20 km s -1 , which means that the convective timescale is approximately one month. This indicates a possible difference between the convection properties of low-mass and high-mass evolved stars.