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The elemental abundance pattern of twenty lambda Bootis candidate stars
Detailed elemental abundances were derived for twenty bona fide lambdaBootis as well as two MK standard stars. Other than LTE abundances forten elements (including C and O), NLTE values for Na were determined.The group of lambda Bootis stars consists of non-magnetic, Population I,late B to early F-type dwarfs with a typical abundance pattern (Fe-peakelements being underabundant whereas C, N, O and S being almost solarabundant). Since classification resolution spectroscopy in the opticaldomain is not capable of determining the abundance of the lightelements, a detailed abundance analysis is the ultimate test for themembership of an object to this group. Another important point is thedetection of apparent spectroscopic binary systems in which two solarabundance objects mimic one metal-weak star, as proposed as a workinghypothesis by Faraggiana & Bonifacio (\cite{farag99}). From twentyprogram stars we are able to confirm or establish the membership fornine objects (HD 23258, HD 36726, HD 40588, HD 74911, HD 84123, HD91130, HD 106223, HD 111604 and HD 290799). Five stars (HD 90821, HD98772, HD 103483, HD 108765 and HD 261904) can be definitely ruled outas being members of the lambda Bootis group whereas no unambiguousdecision can be drawn for another six stars (HD 66684, HD 105058, HD120500, HD 141851, HD 201184 and HD 294253). One very important resultis the apparent overabundances found for Na which cannot be explained byaccretion or mass-loss alone. Based on observations from theOsservatorio Astronomico di Padova-Asiago, McDonald Observatory, KittPeak National Observatory and Observatório do Pico dosDias-LNA/CNPq/MCT.
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| A spectroscopic survey for lambda Bootis stars. I. Strategy, techniques and first results
In recent years, the chemically peculiar (CP) stars of the upper mainsequence have become a fruitful field for the testing of astrophysicaltheories. Processes such as diffusion, convection and mass loss havebeen developed theoretically and introduced into models. The group oflambda Bootis stars, however, is remarkable among the chemicallypeculiar stars as they are nonmagnetic, Population I, A to F-type dwarfswhich show significant underabundances of metals (except for C, N, O andS). Unfortunately, the small number of confirmed members of the lambdaBootis class makes a sound statistical analysis of their propertiesimpossible. Thus, it is still difficult to decide between the twotheories - mass loss with diffusion and the accretion theory - whichhave been proposed to explain the origin of these stars. We thereforehave started a spectroscopic survey to find new lambda Bootis stars inthe field as well as in open clusters and associations. The presence oflambda Bootis stars in open clusters and associations would permit adetermination of the ages of these stars, and thus would yield animportant test for distinguishing between the two theories. In thispaper we describe the selection of candidates using photometriccriteria, the basic requirements, spectroscopic follow-up observationsand results from the first three observing runs. Special care was takento avoid misclassification of our programme stars (e.g. lambda Bootisstars are often confused with intermediate Population II, He-weak orhigh v sin i stars), using a refined MK system. The discovery of atleast six new lambda Bootis stars (including three in the Orion OB1association) shows the efficacy of our selection criteria. Based onobservations obtained at the Observatoire de Haute-Provence and the DarkSky Observatory.
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| Takýmyýldýz: | Yengec |
| Sað Açýklýk: | 08h47m10.16s |
| Yükselim: | +09°23'00.7" |
| Görünürdeki Parlaklýk: | 8.513 |
| Uzaklýk: | 200 parsek |
| özdevim Sað Açýklýk: | -12.6 |
| özdevim Yükselim: | -14.4 |
| B-T magnitude: | 8.675 |
| V-T magnitude: | 8.527 |
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