EDP Sciences
Free access
Issue
A&A
Volume 383, Number 2, February IV 2002
Page(s) 558 - 567
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011746


A&A 383, 558-567 (2002)
DOI: 10.1051/0004-6361:20011746

The spectroscopic binaries 21 Her and $\gamma$ Gem

H. Lehmann1, S. M. Andrievsky2, 3, I. Egorova3, 4, G. Hildebrandt5, S. A. Korotin3, 4, K. P. Panov6, G. Scholz5 and D. Schönberner5

1  Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, 07778 Tautenburg, Germany
2  Instituto Astronômico e Geofísico, Universidade de São Paulo, Av. Miguel Stefano, 4200, São Paulo SP, Brazil
    sergei@andromeda.iagusp.usp.br
3  Department of Astronomy, Odessa State University, Shevchenko Park, 65014 Odessa, Ukraine
    scan@deneb.odessa.ua
4  Isaac Newton Institute of Chile, Odessa Branch, Ukraine
5  Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    ghildebrandt@aip.de, gscholz@aip.de, deschoenberner@aip.de
6  Institute of Astronomy, Bulgarian Academy of Sciences, Sofia, Bulgaria
    e-mail: kpanov@astro.bas.bg

(Received 21 May 2001 / Accepted 6 December 2001 )

Abstract
In the framework of a search campaign for short-term oscillations of early-type stars we analysed recently obtained spectroscopic and photometric observations of the early A-type spectroscopic binaries 21 Her and $\gamma$ Gem. From the radial velocities of 21 Her we derived an improved orbital period and a distinctly smaller eccentricity in comparison with the values known up to now. Moreover, fairly convincing evidence exists for an increase of the orbital period with time. In addition to the orbital motion we find further periods in the orbital residuals. The longest period of 57 $\fd$7 is most likely due to a third body which has the mass of a brown dwarf, whereas the period of 1 $\fd$48 could be related to the half rotational period of the star. For the spectral types we deduced A1 III for the primary and M for the secondary. Two further periods of 0 $\fd$21 and 0 $\fd$22 give hint to the existence of short-term pulsations in 21 Her. Their period difference is of the order of the expected rotational period so that one possible explanation could be rotational splitting of nonradial pulsation modes. Because of the very strong aliasing of the data this finding has to be confirmed by observations having a more suitable time sampling, however. The analysis of photometric series and the Hipparcos photometry give no certain evidence for periodic light variations. For $\gamma$ Gem, besides the orbital RV variation, no variations with amplitudes larger than about 100 m s -1 could be detected. The orbital elements of $\gamma$ Gem are only slightly changed compared to the previously known orbital solution by including our new radial velocities, but their accuracy is improved. For some chemical elements we determined their abundances, NLTE values of C, O, and Na as well as LTE values of Mg, Sc, Fe, Cr, and Ti. We find the abundances to be rather close to the solar values, only carbon shows a little underabundance.


Key words: stars: binaries: spectroscopic -- stars: individual: 21 Her , Gem -- stars: oscillations -- stars: abundances

Offprint request: H. Lehmann, lehm@tls-tautenburg.de

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© ESO 2002