Rotation of the Milky Way
Cepheids enable new precise determination of the rotation of our Galaxy
Since Shapley's discovery of the centre of our Galaxy, and our position within it, and later the Hubble's discovery of the expansion of the universe, the motion of Galactic Cepheids is misleading and sows discord among researchers. Indeed, Galactic Cepheids seem to fall on the sun with a systematic velocity of about 2 km/s.
A multi-decade debate was then to determine if this phenomenon was truly related to the space motion of the stars and, consequently, to a complicated kinematical structure of our Galaxy, or if it was the result of the dynamical structure of Cepheids' atmosphere.
Recently, astronomers, by observing eight Cepheids (see Fig. 1) with the high-precision HARPS spectroscope (originally used for the discovery of exoplanets) show almost unquestionably that this phenomenon, namely the "K-Term", stems from an intrinsic property of Cepheids.
Indeed, they found that residual line-of-sight velocity of Cepheids are significantly linked to the chemical species considered, and more specifically, to the line-forming region within the pulsating atmosphere. For the first time, it was possible to provide "physically calibrated" space motions of Cepheids. This result, generalized to all Cepheids would mean that the rotation of the Milky Way is more simple than expected, certainly axi-symmetric.
"Our new findings solve a puzzle which has existed in astronomy for decades", says Nicolas Nardetto.