Astron. Astrophys. 339, 831-839 (1998)

The Castor moving group

The age of Fomalhaut and Vega ^*

D. Barrado y Navascués <sup>1, 2

1</sup> MEC/Fulbright Fellow at the Smithsonian Astrophysical Observatory, 60 Garden St, Cambridge, MA 02138, USA
² Real Colegio Complutense at Harvard University, Trowbridge St, Cambridge, MA 02138, USA

Received 22 June 1998 / Accepted 8 September 1998

Abstract

We have recomputed the kinematic properties of several of dozens nearby stars, to try to verify if Castor ( Gem) indeed has a cohort of stars sharing the same space motion and age. We used kinematics, the location of the stars in Color-Magnitude Diagrams, their lithium abundances, and their activity, to establish that the moving group seems to be real and to reject several stars which were believed to be part of this group. Of the initial 26 stars, we show that probably only 16 stars are physically associated.

The moving group contains several A spectral type stars. Among them, Vega and Fomalhaut, two of the prototypes of the Pic type stars. If these stars are coeval, their different levels of IR emission suggest that the time scale for the formation of planets is not universal. Due to the age of the group, these IR excesses would appear as a consequence of collisions and sublimation of larger bodies and they would not arise from protoplanetary structures.

Since this association includes several late spectral type stars, we used their properties to estimate their age and, therefore, the age of the group and that of Vega and Fomalhaut. Our estimate for that age is 200100 Myr.

Key words: stars: individual: HD 216956 stars: individual: HD 172167 Galaxy: open clusters and associations: general

* Based on observations collected by the Hipparcos satellite

Present address: Max-Plank-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

Send offprint requests to: D. Barrado y Navascués (dbarrado@cfa.harvard.edu)

SIMBAD Objects

Contents

• 1. Introduction
• 2. The selection of the sample
• 3. Membership in the moving group
  • 3.1. The kinematic criterion
  • 3.2. Color-magnitude diagrams
  • 3.3. Stellar activity
  • 3.4. A list of members
  • 3.5. Are these stars physically associated?
• 4. Estimating the age of the moving group
  • 4.1. Age and kinematics
  • 4.2. Age from the isochrone fitting
  • 4.3. The lithium-Teff plane
  • 4.4. The age of the association
• 5. Summary
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
helpdesk.link@springer.de