Lithium Abundances in the Young PMS Stars FN Tau and V927 Tau: an Interplay of Observations and Theory
We present models of the Li 1 670.8 nm resonance doublet and the subordinate Li 1 812.6 nm doublet. These lines are important indicators of lithium abundance in pre-main sequence (PMS) stars. Model atmospheres from Allard & Hauschildt (1995) were used to synthesize the lines an to include LTE and NLTE effects. We used the models to determine lithium abundances in the PMS stars, FN Tau (M6.5 IV) and V927 Tau (M6 IV), for which we have Keck Observatory observations with the HIRES instrument. The observations have the extremely high resolution of 45000. The derived lithium abundances in the atmospheres of both stars are cosmic (log N(Li) ~3.2). Our procedure also permits the determination of each star's rotational velocity, Vr, and microturbulent velocity, Vt. These parameters can be determined independently using each of the two Li doublets separately. Thus two measurements per star can be made. For V927 Tau we find Vr = 22 +/- 2 km/s and Vt ~3.0 +/- 1 km/s for both lithium doublets. Interestingly, using the same classical approach for the younger PMS star FN Tau, we obtained different Vr from the 670.8 and the 812.6 nm doublets: 13 +/- 2 km/s and <= 6 km/s respectively for Vt=3.0 +/- 1 km/s. This discrepancy may be explained by including two other effects in the models. These effects are veiling, which can affect the continuum by up to 10%, and/or the presence of chromospheric-like features.
Cool Stars, Stellar Systems, and the Sun
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