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

1. Introduction

It was realized a few years ago (Anosova & Orlov 1991) that the multiple system ADS 6175, which contains three spectroscopic binaries (Castor A, Castor B and YY Gem), could be just the most prominent members of a moving group. This moving group would include about 18 stars, having spectral types between A1 V and M6 Ve, including one of the prototypes of the Pic stars.

The Pic type stars show IR excesses, which are associated the the presence of circumstellar dusty disks. The prototypes for this class of stars are Pic, Vega and Fomalhaut. As a result of their proximity and brightness, these three stars have been studied in great detail, and estimates of the mass, radial distribution and structure, dust-grain properties of their disks have been made (e.g., Backman & Paresce 1993). It is generally believed that these circumstellar dusty disks are either the direct descendents of T Tauri disks or the secondary products of the planet formation process. Knowledge of the ages of Pic stars is therefore one of the keys to understand the formation and evolution of their disks. However, there is only a handful of estimates of the ages of these systems, and no member of this class has yet been detected with certainty in an open cluster (although see Backman, Angelova & Stauffer 1998). All three prototypes are A stars, and estimates from their post-ZAMS evolution give approximate ages of 100, 200, and 400 Myr for Pic, Fomalhaut and Vega, respectively (Backman and Paresce 1993). Recently, Barrado y Navascués et al. (1997a) estimated an age of 200100 Myr for Fomalhaut, based on a number of properties (X-ray emission, rotation, lithium abundance, isochrones) of its physical companion GL879. Another relevant star belonging to this class, HR4796A, has been studied in a similar way by Stauffer et al. (1995). These authors derived an age of 82 Myr. Lately, Holland et al. (1998) have imaged the thermal emission from the disks of the three prototypes at submillimeter wavelengths. They concluded that any Earth-like planet must have already formed. Moreover, the central holes found by Holland et al. (1998) in the orbit of Fomalhaut and by Jayawardhana et al. (1998) and Koerner et al. (1998) in the case of HR4796A, support the idea of the early formation of planets.

In this paper, we examine the proper motions of several nearby stars. Based on Hipparcos data, we show that, indeed, Castor shares its Galactic motion with other stars, including Fomalhaut and Vega and several late spectral type stars. This fact allows us to derive the age of the group.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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