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تسجيل مستخدم جديدA Lithium Age for the Young Cluster IC2391
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We have identified a large number of possible very low mass members of the cluster IC2391 based primarily on their location in an I versus (R-I)_C CM diagram. We have obtained new photometry and low resolution ($Delta lambda = 2.7$ AA) spectroscopy of 19 of these objects (14.9 $le$ I_C $le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based on their spectral types, radial velocity, EW(NaI8200AA), and H(alpha) emission strengths. One of these stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. We find the lithium depletion boundary in IC2391 is at I_C=16.2, which implies an age for IC2391 of 53$pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster (~35 Myr), the correction factor to the IC2391 age is comparable to those recently derived for the Pleiades and alpha Per clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.
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We have obtained new photometry and intermediate resolution ($Delta lambda = 2.7$ AA ) spectra of 19 of these objects (14.9 $le$ $I_c$ $le$ 17.5) in order to confirm cluster membership. We identify 15 of our targets as likely cluster members based
on their $VRI$ photometry, spectral types, radial velocity, and H$alpha$ emission strengths. Higher S/N spectra were obtained for 8 of these probable cluster members in order to measure the strength of the lithium 6708 AA doublet and thus obtain an estimate of the clusters age. One of these 8 stars has a definite lithium detection and two other (fainter) stars have possible lithium detections. A color-magnitude diagram for our program objects shows that the lithium depletion boundary in IC~2391 is at $I_c$=16.2. Using recent theoretical model predictions, we derive an age for IC~2391 of 53$pm$5 Myr. While this is considerably older than the age most commonly attributed for this cluster ($sim$35 Myr) this result for IC~2391 is comparable those recently derived for the Pleiades and Alpha Persei clusters and can be explained by new models for high mass stars that incorporate a modest amount of convective core overshooting.
High-dispersion spectra centered on the Li 6708 A line have been obtained for 70 potential members of the open cluster NGC 3680, with an emphasis on stars in the turnoff region. A measurable Li abundance has been derived for 53 stars, 39 of which hav
e radial velocities and proper motions consistent with cluster membership. After being transferred to common temperature and abundance scales, previous Li estimates have been combined to generate a sample of 49 members, 40 of which bracket the cluster Li-dip. Spectroscopic elemental analysis of 8 giants and 5 turnoff stars produces [Fe/H] = -0.17 +/- 0.07 (sd) and -0.07 +/- 0.02 (sd), respectively. We also report measurements of Ca, Si and Ni which are consistent with scaled-solar ratios within the errors. Adopting [Fe/H] = -0.08 (Sect. 3.6), Y^2 isochrone comparisons lead to an age of 1.75 +/- 0.10 Gyr and an apparent modulus of (m-M) = 10.30 +/- 0.15 for the cluster, placing the center of the Li-dip at 1.35 +/- 0.03 solar masses. Among the giants, 5 of 9 cluster members are now known to have measurable Li with A(Li) near 1.0. A combined sample of dwarfs in the Hyades and Praesepe is used to delineate the Li-dip profile at 0.7 Gyr and [Fe/H] = +0.15, establishing its center at 1.42 +/- 0.02 solar masses and noting the possible existence of secondary dip on its red boundary. When evolved to the typical age of the clusters NGC 752, IC 4651 and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces the observed morphology of the combined Li-dip within the CMDs of the intermediate-age clusters while implying a metallicity dependence for the central mass of the Li-dip given by Mass = (1.38 +/-0.04) + (0.4 +/- 0.2)[Fe/H]. The implications of the similarity of the Li-dichotomy among giants in NGC 752 and IC 4651 and the disagreement with the pattern among NGC 3680 giants are discussed.
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We review our the status of the photometric searches of brown dwarfs and vely low mass stars in several young open clusters and present some results on the spectroscopic follow-up, including new cluster ages based on the position of the lithium depletion boundary.
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