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|On the nature of lithium-rich giant stars. Constraints from beryllium abundances|
We have derived beryllium abundances for 7 Li-rich giant (A(Li) >1.5) stars and 10 other Li-normal giants with the aim of investigatingthe origin of the lithium in the Li-rich giants. In particular, we testthe predictions of the engulfment scenario proposed by Siess & Livio(1999, MNRAS, 308, 1133), where the engulfment of a brown dwarf or oneor more giant planets would lead to simultaneous enrichment of7Li and 9Be. We show that regardless of theirnature, none of the stars studied in this paper were found to havedetectable beryllium. Using simple dilution arguments we show thatengulfment of an external object as the sole source of Li enrichment isruled out by the Li and Be abundance data. The present results favor theidea that Li has been produced in the interior of the stars by aCameron-Fowler process and brought up to the surface by an extra mixingmechanism.
|Detailed analysis of a sample of Li-rich giants|
A detailed analysis has been carried out for a sample of 16 red giantsshowing a strong Li I 670.8 nm line. Ten of them were detected in asurvey by Castilho et al. (1998), and the other 6 stars are Li-richgiants selected from the literature. Element abundances in the sampleLi-rich giants are similar to those in normal red giants, differing onlyby their high Li abundance and infrared excess. This suggests thatLi-rich giants may correspond to a phase of stellar evolution of normalred giants, when Li is produced and transported to the atmosphere.
|On a Rapid Lithium Enrichment and Depletion of K Giant Stars|
A model scenario has recently been introduced by de la Reza andcolleagues to explain the presence of very strong Li lines in thespectra of some low-mass K giant stars. In this scenario all ordinary,Li-poor, K giants become Li rich during a short time (~105 yr) whencompared to the red giant phase of 5 x 107 yr. In this "Li period," alarge number of the stars are associated with an expanding thincircumstellar shell supposedly triggered by an abrupt internal mixingmechanism resulting in a surface new 7Li enrichment. This Letterpresents nearly 40 Li-rich K giants known up to now. The distribution ofthese Li-rich giants, along with 41 other observed K giants that haveshells but are not Li rich, in a color-color IRAS diagram confirms thisscenario, which indicates, also as a new result, that a rapid Lidepletion takes place on a timescale of between ~103 and 105 yr. Thismodel explains the problem of the presence of K giants with far-infraredexcesses presented by Zuckerman and colleagues. Other present and futuretests of this scenario are briefly discussed.
|Lithium Enrichment--Mass-Loss Connection in K Giant Stars|
Based on observed far-infrared properties of K giant stars, we propose ascenario linking the high Li abundances of some of these stars to theevolution of circumstellar shells. In this model, every K giant withmasses between 1.0 and 2.5 Mȯ become Li rich during the red giantbranch stage, and the internal mechanism responsible for the Lienrichment will initiate a prompt mass-loss event. The evolutionarypaths of the detached shells are compatible with observations for lowexpansion velocities of the order of 2 km s-1 and mass loss of (2--5) x10-8 Mȯ yr-1. This modest mass loss is, however, 2 orders ofmagnitude larger than those of normal, Li-poor K giants. A "Li time" ofthe order of 80,000 yr or somewhat larger is possible. This Li phase is,nevertheless, not related to the 12C/13C ratio which appears to evolve,for these low-mass stars, over a much longer time. This model requires arapid internal process of Li enrichment and depletion. New argumentsappearing in the literature concerning this internal process arediscussed. Reference is also made to the importance of these stars tothe Galactic Li evolution.
|The Li K giant stars|
|Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.|
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
|Lithium and rapid rotation in chromospherically active single giants|
The rotational velocities presently obtained via spectroscopicobservations of a group of moderately rapidly rotating, chomosphericallyactive single giants indicate that Gray's (1989) rotostat hypothesisrequires modification. Their rapid rotation appears to be due to high Liabundances, and results in increased chromospheric activity. A scenariois projected in which the surface convection zone reaches the rapidlyrotating core just as a star begins its first ascent of the giantbranch, and dredges both high angular momentum material and freshlysynthesized Li to the surface.
|Lithium in red giants|
The Li abundances in Pop-I red giants are reviewed and discussed interms of the depletion mechanism and theoretical problems of theanomalous Li-rich red giants. The Li measurements are compared to thevalues predicted by the standard evolutionary model, and the objects areconsidered in five distinct groups. CNO abundances and C-12/C-13 ratiosare listed for several types of red giants to consider the Li content,Li production, and the Be-7 transport mechanism. The general pattern ofLi abundances agrees with the depletion of Li related to thepre-main-sequence phase. The red giants that show evidence of deepmixing are the S- and C-stars, weak G-band stars, and normal K-giants,in which the original Li has been destroyed. The Be-7 transportmechanism accounts for the high Li content in the S- and C-stars. Someexplanations are given for the Li production in red giants that are lessluminous.
|Another lithium-rich K giant - HD 30238|
The K giant HD 30238 is reported to show a strong feature due to Li I6707 A, corresponding to a lithium abundance of log epsilon(Li) = 1.2 +or - 0.05. This star appears to be another example of a rare class ofotherwise normal, low-mass K giants with unusually high lithiumabundances. The C-12/C-13 isotope ratio in HD 30238 is approximately 26+ or - 4; this low value suggests that the star has developed a normal Kgiant convective envelope. The observed abundance of lithium in thespectrum of the star may be inconsistent with this low carbon isotoperatio. The abundance of lithium in low-mass stars on the red giantbranch as a function of effective temperature suggests that lithiumcontinues to be depleted from the convective envelope during evolutionup the red giant branch.
|Late M stars found in a hemispheric survey|
An objective-prism survey of somewhat more than half the sky, being theportion more than 10 deg from the Galactic plane and north ofdeclination -25 deg, has been completed, using the visual-red spectralregion. From this survey 583 late M stars (mostly type M6 or later) notcontained in the third edition of the variable star catalog or itssupplements are listed. Many identifications with the Caltech Two-MicronSurvey are given. The great majority of the stars are likely to begiants. For an assumed visual absolute magnituge of -0.9, the distanceof the stars from the Galactic plane is well represented by anexponential (barometric) distribution with scale height 1800 pc; or 900pc for an assumed absolute magnitude of zero. The sample may well berelated, therefore, to the recently discussed 'thick disk' component ofgalactic structure. Carbon stars appear to be at least twice asnumerous, relative to late M stars, at large Galactic Z as at small Z.Although difficult to quantify, it appears unlikely that large numbersof these late M stars are undiscovered variables of appreciableamplitude.
|Catalog of Indidual Radial Velocities, 0h-12h, Measured by Astronomers of the Mount Wilson Observatory|
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1970ApJS...19..387A&db_key=AST
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