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Session 58 - Stellar Modelling/Abundances.
Display session, Wednesday, June 12
Lithium abundances have been determined for 27 stars in the Orion Nebula region with V=12-14, and (B-V)=0.5-1.2. All of the stars are within 20 arc minutes of the Trapezium; they range from approximately 1.0 to 1.6 M_\sun and from 10^6 to 10^7 years old. Approximate chronological ordering of the stars is possible from their location in the HR 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 with (B-V)=0.8-1.4 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 ages of this Orion sample. The status of Parenago 1799 as an ultra-fast rotator (UFR) (Walker 1990) is confirmed; it is shown that the star is rotating near breakup velocity. However, there are fewer UFRs in the present sample that 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). The apparent very high Li abundances of the stars Parenago 1643 and 1929 are shown to be an artifact of their apparent colors; when newly-determined spectral types are used as temperature indicators, more typical lithium abundances are found. Similarly, we have used spectral types from Duncan (1993) to refine the colors and thus improve our estimates of Li abundances for P1477 and P1518.
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