What is the fine-grained topic of the following text: We obtained time series of high resolution spectra around 1.6 µm for 15 short period semiregular variables to study the velocity variations in their atmospheres with the help of second overtone vibration-rotational lines of CO. All objects of our sample clearly show velocity variations of a few km s-1. Only return the topic.
Studying velocity variations in atmospheres of short period semiregular variables using high resolution spectra