The Hyades stream: evaporated cluster or intrusion from the inner disk?

Abstract

The nature of the Hyades stream, or Hyades moving group, is a long-standing question of Galactic Astronomy. While it has become widely recognized that the Hercules stream, an unbound group of stars lagging behind galactic rotation and moving outward in the galactic disk, is associated with the outer Lindblad resonance of the rotating galactic bar, there is still some debate about the nature of the more prominent low-velocity stream sharing the kinematics of the Hyades open cluster. Is this stream caused by additional non-axisymmetric perturbations of the galactic potential, such as transient or quasi-stationary spiral waves, or by the on-going evaporation of the Hyades cluster? Here, a simple observational test has been designed to determine whether the Hyades stream is primarily composed of coeval stars originating from the Hyades cluster, or of field stars. Using the Geneva-Copenhagen survey of F and G dwarfs, we compare the mass distribution and metallicity of the stream to those of field disk stars. If the Hyades stream is composed of stars trapped at resonance, its mass distribution should obey the present-day mass function (PDMF) of the disk, and its metallicity should reflect its origin in the inner regions of the Galaxy. On the other hand, if it is an evaporated cluster, we expect a different mass distribution, depending on the inital mass function (IMF) of the cluster, and on the proportion of evaporated stars as a function of mass. We find that extreme conditions have to be adopted for the selective evaporation and IMF of the cluster to make the observed mass distribution of the stream only roughly consistent (at a one-sigma level) with the coeval evaporated cluster scenario. The observed mass distribution is in much better agreement with the PDMF of the field. The peculiar metallicity of the stream is inconsistent with that of a field population from the solar neighbourhood trapped in the primordial cluster during its formation process and subsequently evaporated. These observations thus favour a resonant origin for the Hyades stream.