4. Estimating the age of the moving group
4.1. Age and kinematics
Palous & Piskunov (1985) have shown that there is a relation between the kinematic properties and the age of B and A stars. Based on their Table 3, it is possible to constrain the age of the hot members of the Castor moving group. They have average values of the velocities of U=-10.73.5, V=-8.02.4 and W=-9.73.0 km/s, leading to ages of 100-400, 200-600 and 50-100 Myr, respectively. These ages are indicative, since the velocity dispersion components in the Palous & Piskunov (1985) study are quite large and, in the last case, the W component of the velocity of the Castor group is slightly out the range of velocities examined by Palous & Piskunov (1985). A combined age would be 100-400 Myr, in good agreement with the values derived for the late spectral type stars (see below).
4.2. Age from the isochrone fitting
Figs. 3a and 3b can provide an estimate of the age of the association. The final set of members lies below the 35 Myr isochrone and above the 70 Myr. Both the proximity to the ZAMS and the unknown metallicity of the group do not allow an accurate determination of the age. However, it seems that these stars are clearly above the ZAMS. Then, they would be Premain sequence (PMS) objects, still approaching at the MS. We believe that due to the errors in the conversion from the theoretical plane to observational plane, together with the errors associated with the observational data (very small in most cases), the two color-magnitude diagrams put only a lower limit to the age, of 35 Myr.
On the other hand, the hotter real members of the moving group should be close to the end of their lives in the Main Sequence (in particular, Castor A and B, and 2 Lib). This particular evolutionary status allows for the possibility of isochrone fit. Again, uncertainties in the individual actual distances -negligible, see Table 3, magnitudes and colors are too large to provide accurate ages. Backman & Paresce (1993), by fitting isochrones, derived ages close to 400 and 200 Myr for Vega and Fomalhaut, respectively, with uncertainties of 30%. For our data set, we use Meynet et al. (1993) set of evolutionary tracks. We find that Fomalhaut is too close to the MS to fit an isochrone. The location of Vega, hotter than Fomalhaut, in the CMD is completely compatible with the turn-off position of the open cluster NGC 6494, and provides an age close to 300 Myr. In any case, it seems that at least some stars, including the Castor system, YY Gem, Fomalhaut and Gl 879, are physically associated and, for this reason, they have the same age.
The common origin and age of these stars would have important implications. In particular, YY Gem, an eclipsing binary, is one of the two M dwarfs with accurate measurements of radii and temperatures. Chabrier & Baraffe (1995), using new evolutionary models, fit isochrones to both components of the system (either in the radii-mass or the -mass planes), and concluded that the system has to be on the late Pre-main Sequence contraction phase. They estimated an age of 100 Myr. We note that if it is really slightly a PMS system, then it should not be used as a calibrator of theoretical models (unless one can accurately correct for its PMS nature or accurately know its age).
4.3. The lithium-Teff plane
The lithium abundance of late spectral type stars can be used as a membership criterion, as well as a statistical method to estimate the age of an association of stars, since the lithium abundance depends on both age and mass (e.g., Balachandran 1994). Figs. 5a and 5b depict the abundance against the effective temperature for those stars with published values (crosses). In the first case (Fig. 5a), we have included data corresponding to Hyades stars (filled circles) and Pleiades stars (open circles). These two clusters have standard ages of 600-800 and 70-80 Myr, respectively (see Stauffer et al. 1998 for a new determination of the age of the Pleiades, which yields 1255 Myr, based on the position of the lithium boundary for very low mass stars and brown dwarfs). The second figure contains data from the M34 cluster (open circles) and the UMa moving group (filled circles), which are 200 and 300 Myr old, respectively. Fig. 5b also includes a 300 Myr lithium depletion isochrone from Chaboyer (1993). The sources of the lithium data for all four associations can be found in Barrado y Navascués et al. (1997a).
The comparison between the lithium abundances of cluster stars of different ages and GL879, a physical companion of Fomalhaut, was used by Barrado y Navascués et al. (1997a) to establish that these stars are younger than 300 Myr, probably in the range 100-300 Myr. Figs. 5a and 5b indicates that the lithium abundance of Lib is compatible with this age (see our caveats about the membership based on the kinematic, Sect. 3.5). GL848 seems too old to be part of the moving group, whereas GL896A is too young.
The case of YY Gem is more complex. It is well known that it is a very close SB2 eclipsing binary. Its rapid rotation (v sini=40 kms-1) causes significant for the important spectral line blending. Moreover, since the system is composed of 2 similar stars (M=0.62 and 0.57 , Bopp 1974), lines arising from both components appear in the spectrum. For the particular observation used to derive the lithium abundance (phase=0.21, Barrado y Navascués 1996), the LiI6708.8 Å doublets of each component were well detached. The final abundance is Log N(Li)=0.020.20, in the customary scale where Log N(H)=12. However, close binaries in clusters (Barrado y Navascués & Stauffer 1996 in the case of Hyades, and Ryan & Deliyannis 1995 in the case of M67) and chromospherically active binaries, such as YY Gem (Barrado y Navascués et al. 1997b) inhibit partially the lithium depletion, probably due to mixing related to rotation. Therefore, this binary is not a good candidate to be used to estimate the age of the Castor moving group, although it is very well established that there is a physical connection between these stars.
4.4. The age of the association
All these comparisons, which are compatible with the well determined age of GL879 and Fomalhaut, allows us to conclude that, indeed, the Castor moving group has an age of 200100 Myr. Note that at this point, we have assumed that the stellar kinematic group is real and that these stars are members of it. However, as mentioned before, the group, as found in open clusters, could contain spurious members, and even the possibility of having a sample with the same kinematic properties and compatible CMD and a different origin cannot be rejected.
If these stars do have a common origin, as it seems, the same age derived for them is a very relevant fact, since the group has several A stars with different infrared excesses (or none at all), indicating that, at least at this age, the evolution of the protoplanetary disks does not depend on their age.
© European Southern Observatory (ESO) 1998
Online publication: October 22, 1998