Lithium in Young Solar-Type Stars in the Orion Nebula Region

Lithium abundances have been determined for 24 late-type pre-main-sequence stars in the Orion Nebula region with V=12-14 and B-V=0.5-1.4. All of the stars are within 20 arc minutes of the Trapezium; they range from approximately 1.0 to 1.6 solar mass and from 106 to 107 years old. Approximate chronological ordering of the stars is possible from their location in the color-magnitude diagram, but they show no obvious evolutionary sequence of lithium depletion. In particular, the trend seen in the Pleiades, that faster rotators tend to have more lithium, is not seen here. There is no obvious correlation in the Orion stars between Li and v sin i. On the average, stars in the present sample have greater Li abundances than similar stars in the older Pleiades cluster. One can infer that the process which depletes Li in these late-type stars requires more time to act than the age of this Orion sample. The status of Parenago 1799 as an ultra-fast fotator (UFR) (Walker 1990) is qualitatively confirmed; it is shown that the star is rotating at an appreciable fraction of breakup velocity. This is the only UFR in the present sample, fewer than in a Pleiades sample of similar size. The two T Tauri stars in the binary system Parenago 1540 have similar lithium abundances, suggesting similar depletion histories, and consistent with the findings of Lee, et al. (1994). Parenago 1643 and 1929 are shown to have incorrect apparent colors via newly-determined spectral types; their observed colors lead to very high Li abundances, but their corrected colors suggest more typical lithium abundances. Similarly, we have used spectra types and the spectral type-color relation from Duncan (1993) to refine teh colors and thus improve our estimates of Li abundances for P1477 and P1518.