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Astron. Astrophys. 318, 805-811 (1997)

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4. Discussion

4.1. Chromospheric activity and Li depletion in IC 4665

We have found a large spread in H [FORMULA]   chromospheric fluxes and photospheric Li abundances among the IC 4665 stars. Fig. 3 displays the temperature dependence of H [FORMULA]   and Li. They show different behaviours: H [FORMULA]   emission tends to increase towards cooler [FORMULA]  , whereas Li decreases. Both effects can be qualitatively explained by the increase in the depth of the convection region towards cooler stars. Note that the spread both in H [FORMULA]   and Li appears to increase for [FORMULA]   [FORMULA] 5500 K.

[FIGURE] Fig. 3. Upper panel: Excess H [FORMULA]   emission fluxes for the IC 4665 stars versus [FORMULA]   . The star P146 is not plotted because it did not show a measurable H [FORMULA]   excess. Lower panel: Li abundances for the IC 4665 stars versus [FORMULA]   .

In order to place our results in a wider context we have compared the IC 4665 stars with the well-studied Pleiades cluster. Fig. 4 shows our Li abundances together with those of Soderblom et al. (1993) and García López et al. (1994) for the Pleiades. The patterns are very similar. Broadly speaking we can distinguish three groups of stars:

i) early G-type (6000 K-5300 K) stars have quite uniform abundances, with a mean value of log N(Li)=3.1, which is the same as in WTTS (Martín et al. 1994) and the interstellar medium. Hence, these stars have retained their initial Li abundance.

ii) late-G and early-K stars (5300K-4500K) present a broad range of abundances, which in the Pleiades was clearly shown to be related to chromospheric activity and rotation (Soderblom et al. 1993; García López et al. 1994). Such effect could be caused by the effect of rotation on the temperature at the base of the convection zone (Martín & Claret 1996). We only have six stars in this group: P12, P100 and P165 with high Li abundances and high H [FORMULA]   excesses, and P94, P146 and P166 with low abundances and low excesses. In Fig. 5, we have plotted the IC 4665 stars from this work, and the Pleiades stars from Soderblom et al. (1993) with [FORMULA]   in the range 5300K-4900K in a Li vs. H [FORMULA]   diagram. We converted our H [FORMULA]   fluxes to flux ratios ([FORMULA] = [FORMULA] / [FORMULA] [FORMULA]  4) in order to compare with Soderblom et al. In the stars of both clusters there is a similar correlation of Li with H [FORMULA]   excess. The stars P146 and P166 have very small H [FORMULA]   excesses (we could not measure any excess in the first one and it is not included in Fig. 5) and we could not detect Li. The comparison with the Pleiades suggests that such H [FORMULA]   activity and Li are very unusual and it may indicate that these stars are in fact not cluster members. However, we hesitate to discard P146 and P166 as cluster members because their radial velocities match the cluster mean (Prosser & Giampapa 1994). We note that if these stars were non-members, the correlation between Li and H [FORMULA]   flux does not disappear, but the specimens with low Li and low H [FORMULA]   excess are reduced to only one (P94). Hence, it is important to find more low-mass IC 4665 members in which we can test better the Li-activity connection in this cluster.

Prosser & Giampapa (1994) and Allain et al. (1996) obtained vsini and photometric rotation periods, respectively, for several of our programme stars. The three stars with low Li and H [FORMULA]   excess (P94, P146 and P166) have vsini [FORMULA] 10 km s [FORMULA]. Allain et al. could not detect any period in P94 and P146. On the other hand the three stars with high Li abundance and H [FORMULA]   excess have high vsini (see Table 2). Interestingly, Allain et al. (1996) were able to derive rotation periods for P12 (0.6 days) and P100 (2.27 days). Therefore, there is a real difference between the equatorial velocities of these two stars of about a factor 5. The H [FORMULA]   emission level of P100 is a bit higher than that of P12, indicating that there is not a good one to one relationship between H [FORMULA]   and rotation, but both stars do have higher activity than the slow rotators (P94, P146, P166). The Li abundance of P12 is higher than that of P100, but the difference is only 0.1 dex which is within our error bars. With only these two stars it is not yet possible to test the models of Martín & Claret (1996), which predict that fast rotating stars deplete less Li than slow rotating stars during their PMS evolution.

iii) the late-K and early M stars (4500K-3500K) do not present the Li-activity-rotation connection in the [FORMULA] Per and Pleiades clusters (García López et al. 1994). We have only observed one M-type star in IC 4665, and we could not detect Li in it. The inferred upper limit to the Li abundance is shown in Figs. 3 and 4. Our result is consistent with the high efficiency of PMS Li depletion found for the stars of this group. The possibility that such property could be used for a fine dating of young open clusters is discussed in the following section.

[FIGURE] Fig. 4. Li abundances versus [FORMULA]   for stars in IC 4665 (diagonal crosses) and the Pleiades (empty pentagons). The four upper limits at [FORMULA]   [FORMULA] 4000 K correspond to P309 (dotted trace) and 3 [FORMULA]  Per stars observed by García López et al. (1994).
[FIGURE] Fig. 5. Li abundances versus H [FORMULA]   excess emission ratios for stars in IC 4665 (diagonal crosses) and the Pleiades (empty pentagons) with [FORMULA]   in the range 5300 K - 4900 K.

4.2. The age of young clusters and the LL-clock

Mermilliod (1981) determined an age for IC 4665 of 36 Myr from empirical isochrone fitting of the upper main sequence, which is clearly younger than the ages he obtained for [FORMULA]  Per (51 Myr) and the Pleiades (78 Myr). However, some of the stars used by Mermilliod have been shown to be binaries in later works, and Prosser (1993) revised the upper main-sequence fitting to yield an age of [FORMULA] 50-70 Myr. Prosser concluded that IC 4665 is not younger than [FORMULA]  Per and it might be as old as the Pleiades.

The distribution of Li abundances in the low-mass stars of IC 4665 and the Pleiades are quite similar (Fig. 4). However, this does not necessarily imply that both clusters have the same age. The distribution of Li is quite similar also in [FORMULA] Per (Balachandran, Lambert & Stauffer 1988, García López et al. 1994). With respect to H [FORMULA]   activity, in Fig. 6 we compare the IC 4665 stars with the Pleiades. The IC 4665 stars present the same level of H [FORMULA]   activity as the Pleiades.

[FIGURE] Fig. 6. H [FORMULA]   excess emission ratios for stars in IC 4665 (diagonal crosses) and the Pleiades (empty pentagons) versus [FORMULA]   .

Stauffer et al. (1989) gave Li I   equivalent widths for 10 low-mass members of the IC 2391 cluster, but they did not derive Li abundances. This cluster is very interesting because according to Mermilliod (1981) it belongs to the same age group as IC 4665. We have applied the abundance analysis described in Sect. 3 to the IC 2391 stars. The effective temperatures were derived from the Arribas & Martinez-Roger relationship (1988) using the (B-V) colours given by Stauffer et al. (1989), dereddened by the mean E(B-V)=0.04 of the cluster. The Li abundances obtained are listed in Table 3.


Table 3. Li abundances for IC 2391 stars

The Li abundances of the IC 2391 stars fall within the range of abundances of the IC 4665 and Pleiades stars (Fig. 4). We can also compare with the post T Tauri stars (PTTS) studied by Martín et al. (1992) and Martín (1993). The estimated ages of PTTS are 20-50 Myr, and the Li abundances for 9 PTTS are in the range log N(Li)=3.3 - 1.8, their [FORMULA]   ranging from 5900 K to 4400 K. The degree of Li depletion among the PTTS is similar to that among the IC 2391, IC 4665, [FORMULA] Per and the Pleiades stars, indicating that either the PTTS and the 4 open clusters share similar age distributions, or Li is not sensitive for ages in the range [FORMULA]  20-100 Myr. The first interpretation would imply a rather large age spread within the low-mass population of open clusters. On the other hand, the strong correlation of Li with luminosity found by Martín et al. (1994) in WTTS, indicates that PMS Li burning is very efficient during a short timescale. It is conceivable that the observed pattern of Li abundances in PTTS and young clusters is formed quite rapidly through PMS convective mixing (timescale of a few Myr) at an age of [FORMULA] 20 Myr. Theoretical models are qualitatively consistent with this scenario, because they predict a sudden drop in Li abundance when the bottom of the convection region reaches high enough temperature for Li burning, but as the star approaches the ZAMS, convection becomes shallower and Li depletion should slow down. However, it has been noted that theoretical models do not give yet a quantitative good description of the observed Li abundances among PMS stars (Martín et al. 1994, García López et al. 1994).

In Fig. 4, it can be seen that below [FORMULA]   4000 K, Li has not been detected in any star of the [FORMULA]  Per and IC 4665 clusters. The upper limits imply very strong Li depletions. However, as we move to lower masses, the internal temperatures diminish and we should reach a point where they are not high enough for Li burning. Hence, we should see Li again in the very low-mass cluster members. This effect has been proposed as a test for distinguishing between stars and brown dwarfs (Rebolo et al. 1992; Magazzù et al. 1993). Very recently, Li has been detected in the coolest known members of the Pleiades (Basri et al. 1996, Rebolo et al. 1996). For the age of the Pleiades (70-120 Myr), theoretical models predict that only brown dwarfs preserve lithium (e.g. Magazzù et al. 1993), but for younger ages the very low-mass stars also start to preserve it because they require a long time to contract. Thus, we expect that in clusters younger than the Pleiades, the reappearance of Li should be shifted to higher masses. Such an effect provides a precision clock for dating clusters. D'Antona & Mazzitelli (1994) suggested on the basis of their theoretical isochrones of Li depletion that low luminosity stars could be used for dating open clusters. They also noted that the Li depletion is quite sensitive to input physics of the models which have considerable uncertainties (opacities, convection). Hence, it may be difficult to use Li to assign absolute ages to open clusters, but relative ages will probably be safe.

What we call in this paper "LL-clock" stands for Lithium-Luminosity clock. It is based on the high efficiency of Li depletion for fully convective low luminosity objects. The age of a young cluster will be given by the most massive objects among the very low-mass members in which Li is seen to re-emerge, after having been efficiently depleted by higher mass cluster members. In the Pleiades the Li destruction-preservation borderline has been found at luminosity around log L=-2.9 L [FORMULA] (Basri et al. 1996) according to the Li detection in the object PPl 15. The traditional method for dating open clusters, i.e. upper main-sequence turn off fitting, says that [FORMULA] Per is younger by [FORMULA] 20 Myr than the Pleiades (Mermilliod 1981). If PPl 15 were 20 Myr younger, it luminosity would be about 0.12 dex larger (D'Antona & Mazzitelli 1994), and it should be slightly hotter and have higher Li abundance. Zapatero-Osorio et al. (1996) have reported a negative Li detection in AP J0323+4853, which has a luminosity around log L=-2.6 L [FORMULA]. This result does not rule out that [FORMULA] Per is younger than the Pleiades, but it shows how close the observations are to telling us very interesting things about the relative ages of the young clusters. In IC 4665, it will be a little more difficult to investigate the very low-luminosity members, simply because of its larger distance, but if it is indeed younger than the Pleiades, we expect that Li should re-appear at higher luminosities.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998