A VLBA Resolution of the Pleiades Distance Controversy

Because of its proximity and its youth, the Pleiades open cluster has been the subject of extensive observational and theoretical work. Thanks to the wealth of knowledge that exists for it, the Pleiades cluster stars are often used as a template with which to define the properties of other young stars. Such a role is called into question by the "Pleiades distance controversy" - the distance to the Pleiades from Hipparcos of about 120 pc is significantly different from the distance of 133 pc derived from other techniques (Figure 1). Although this amounts to a 10% difference in the distance, the resultant discrepancies as propagated into the Pleiades HR-diagram, and the necessary revisions of physical models to obtain agreement with the Hipparcos result, are quite significant.

To resolve this controversy we have been using the VLBA in combination with larger antennas (GBT, Arecibo, and Effelsberg) to derive a new, independent trigonometric parallax distance to the Pleiades. This project has followed the course outlined in the below list:

  • Preliminary observations with the VLA identified Pleiades cluster memebers with sufficiently strong radio emission to enable VLBA parallax measurements. From our VLA survey we identify ten candidates for follow-up with the VLBA. These stars have quasi-steady flux levels that range from 30-150 microJy.

  • Pilot VLBA observations of one Pleiad (HII 1136) showed our proposed program was viable.

  • A full-scale VLBI program targeting eight Pleiades star systems has been running over the course of the past three years.

The first parallax distances measured from this full-scale VLBI program are shown in Figure 2. From these, we calculate the Pleiades cluster distance shown in Figure 1. These results, although from only half of our Pleiades sample, already disagree strongly with the Hipparcos cluster distance and represent the most accurate and precise cluster distance measurement to date. These results appear in Melis et al. (2014, Science 345, 1029).

At the completion of this program in 2015, we will obtain at least four more Pleiades parallaxes and will be able to model orbits for two Pleaides binary star systems (e.g., Figure 3). In accordance with our approved Large VLBA program proposal, we report the measured flux densities from each epoch for each source.