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

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1. Introduction

This paper is the continuation of an ongoing project aimed at understanding pre-main sequence (PMS) Li burning. In previous works we have studied Li in different types of PMS stars: Martín et al. (1992) obtained Li abundances for 5 post T Tauri secondaries of early-type stars. Martín & Rebolo (1993) studied the PMS secondary of the eclipsing binary EK Cep and derived its Li abundance. Martín et al. (1994) presented Li abundances for 53 "weak" T Tauri stars (WTTS) in Taurus. They found a narrow peak in the Li distribution centered at log [FORMULA] =3.1, which is the initial Li content of newly formed stars. They also found that PMS Li depletion is a strong function of mass and luminosity. García López et al. (1994) derived Li abundances for 24 Pleiades stars and confirmed the presence of a Li-rotation connection (see also Soderblom et al. 1993) among the late-G and early K-type stars, but not among the late-K and early-M stars. Such connection has been studied theoretically by Martín & Claret (1996), who showed that rotation lowers the temperature at the base of the convection region and hence reduces the efficiency of PMS Li depletion. Finally, Zapatero-Osorio et al. (1996) performed a search for Li in very low-mass (M3-M6) [FORMULA]  Per stars with no positive Li detection.

The IC 4665 open cluster offers the possibility of testing our current ideas about PMS Li burning because it may have an age of only [FORMULA] 35 Myr (Mermilliod 1981, but see the discussion in Sect. 4.2). Its coordinates (17h40m, [FORMULA]) make it a suitable target from northern hemisphere observatories. Its distance ([FORMULA] 350pc) is almost a factor of 3 farther than the Pleiades, and thus the cluster members are relatively faint. Nevertheless, a list of reliable low-mass members with V-magnitudes in the range 12-14 has been released from proper motion and radial velocity studies (Prosser & Giampapa 1994). We selected for spectroscopic observations the 13 bona fide IC 4665 members with colors (B-V) [FORMULA] 0.75, i.e. spectral type G0 and later, given by Prosser & Giampapa. We added to this sample one early M-type photometric member from Prosser (1993).

Our spectra contain not only the Li I [FORMULA] 6708  feature, but also H [FORMULA]   . The H [FORMULA]   emission excess is a fairly good indicator of chromospheric activity (Pasquini & Pallavicini 1991, Montes et al. 1995a), which in turn is connected to rotation via the dynamo mechanism. In the Pleiades cluster the fast rotators present both high Li abundance and high level of chromospheric activity (Soderblom et al. 1993, García López et al. 1994). Hence, there is a Li-activity connection induced by rotation. We have tested if such connection also holds for the IC 4665 stars.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998