The Castor Moving Group: The age of Fomalhaut and Vega

We have recomputed the kinematic properties of several of dozens nearby stars, to try to verify if Castor has a cohort of stars sharing the same space motion and age. We used kinematics, the location of the stars in CM diagrams, their lithium abundances, and their activity, to establish that the moving group seems to be real and to reject several stars which were believed to be part of this group. Of the initial 26 stars, we show that probably only 16 stars are physically associated. The moving group contains several A spectral type stars. Among them, Vega and Fomalhaut, two of the prototypes of the beta Pic type stars. Since this association includes several late spectral type stars, we used their properties to estimate their age and, therefore, the age of the group and that of Vega and Fomalhaut. Our estimate for that age is 200$\pm$100 Myr.Comment: 6 pages plus 9 figures and 4 tables. 1998, A&A 339, 83

Chromospheric Emission from Red Giants in the Open Cluster NGC 6940

The observation of the Ca II H and K lines in red giants in NGC 6940 allows the strength of chromospheres and their behavior to be evaluated in a population whose evolution is well understood. Spectra in the Ca II lines have been obtained for giant stars in this cluster. Emission reversals are present in some objects. The absolute flux in the chromospheric emission is determined as a function of effective temperature for stars on the red giant branch. The stellar surface flux in the Ca II lines decreases smoothly with increasing $(B-V)$ in contradiction to model predictions.Comment: 4 pages, LATEX file. Mem.S.A.It style macro (included). Contribution to "Cool Stars in Clusters and Associations: Magnetic Activity and Age Indicators", Palermo, May 199

The low mas end of the young cluster IC2391

By collecting optical and infrared photometry, as well as medium resolution spectroscopy, we have discovered a sample of low mass stars and brwon dwarfs in the young cluster IC2391. Using the lithium depletion boundary near the substellar limit, we have estimated the age of the cluster as ~50 Myr. We have also estudied the H(alpha) emission in this sample.Comment: To appear in IAU Symp. 211 on "Brown Dwarfs" ed. E. Marti

Lithium and binarity

We present an analysis of the lithium abundances in late spectral type binaries of different ages. They belong to several open clusters (Pleiades, Hyades and M67), as well as to Chromospherically Active Binary Systems (CABS). All these binaries have reliable ages, since they are members of well-known open clusters or, in the case of the CABS, some stellar parameters such masses and radii are accurate enough to derive ages using theoretical isochrones. Their age span covers from barely one hundred million years to several gigayears. We have compared different stellar properties, such as lithium abundances, stellar masses, effective temperatures and orbital periods. It is shown that, in general, close binaries have lithium abundances larger than those characteristic of single stars or binaries with larger orbital periods. The largest difference between the abundances of binaries and single stars appears at Hyades' age. The origin of those overabundances are discussed in the context of the proposed mechanisms for the lithium depletion phenomenon and the stellar evolution.Comment: 9 pages, LATEX file. ASP Series style macro (included). Contribution to "10th Cambridge Workshop. Cool Stars, Stellar Systems and the Sun", Cambridge, 199

Why is the X-ray luminosity function different for the Hyades and Praesepe?

We have studied dF-dK and dM stars belonging to Praesepe in order to determine membership via radial velocities and to compare several stellar properties, such as X-ray luminosities, H$\alpha$ equivalent width, etc, between this cluster and Hyades, which have similar age. We show that all properties are analogous in both clusters except the distribution of Lx. This fact could be related to difficulties in the analysis of the Lx upper limits or to more fundamental reasons, such as binarity rates and differences in the properties of the binaries of each cluster.Comment: 4 pages, LATEX file. Mem.S.A.It style macro (Included). Contribution to "Cool Stars in Clusters and Associations: Magnetic Activity and Age Indicators", Palermo, May 199

Deep Photometric Survey of IC2391

We present a deep survey of the young southern cluster IC2391. We have covered 2 sq. degrees in the R,I filter. We have selected several dozens VLM stars and BD candidates based on the position of these objects in the Color-Magnitude Diagram.Comment: 4 pages. Contribution to the "Very Low Mass Stars and Brown Dwarfs in Stellar Clusters and Associations, La Palma, Spain, 19

Brown Dwarfs: Toward a New Age Scale for Young Open Cluster

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.Comment: 4 pages. "Proceedings of the III Scientific Meeting of the Spanish Astronomical Society", 1999, Astrophysics and Space Sciences, in pres

A Lithium Age for the Young Cluster IC2391

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(NaI8200\AA), 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.Comment: 4 pages. Oral contribution to "Stellar Clusters and Association: Convection, Rotation, and Dynamos", ASP Conf. Series, in pres

WIYN Open Cluster Study: Lithium in Cool Dwarfs of the M35 Open Cluste

We have obtained high resolution spectra of $\sim$40 members of M35, determined the Lithium-T$_{\rm{eff}}$ morphology and the distribution of the rotational velocity for G and K stars, and compared them to those of the Pleiades and other well-known open clusters.Comment: 4 Pages. Contribution to "Stellar Clusters and Association: Convection, Rotation, and Dynamos", ASP Conf. Series, in pres

Mass Function at the end of the Main Sequence: the M35 Open Cluster

We present very deep RIc photometry of the M35 open cluster. We have covered 30' \times 30', equivalent to about a fourth of the total area of the cluster. The data range from I_c=17.5 to 23.5 magnitudes, and the color interval is $0.5 \le (R-I)_c \le 2.5$. Roughly, these values correspond from $0.6 M_\odot$ to $0.1 M_\odot$ in the cases of members of the cluster. By using the location of the stars on a Color-Magnitude Diagram, we have selected possible members of this cluster and obtained the Luminosity and Mass Functions of the M35 open cluster and compared them with those from the Pleiades.Comment: 4 pages. "Proceedings of the III Scientific Meeting of the Spanish Astronomical Society", 1999, Astrophysics and Space Sciences, in pres