A&A 366, 965-971 (2001)
DOI: 10.1051/0004-6361:20000355

RX J1603.8-3938 -a surprising pre-main sequence spectroscopic binary

E. W. Guenther¹, G. Torres², N. Batalha^{3, 4}, V. Joergens⁵, R. Neuhäuser^{5, 6}, J. Vijapurkar⁷ and R. Mundt<sup>8

1</sup>  Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, Sternwarte 5, 07778 Tautenburg, Germany
²  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³  currently at the Planetary Systems Branch NASA Ames Research Center Mailstop 245-3 Moffett Field, CA 94035-1000, USA
⁴  Departamento de Astrofísica, Observatório Nacional, Rua General José Cristino 77, São Cristovão, Rio de Janeiro, RJ 20921, Brazil
⁵  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85741 Garching, Germany
⁶  University of Hawaii, Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
⁷  Homi Bhabha Centre for Science Education (Tata Inst. of Fundamental Research), V. N. Purav Marg Mankhurd, Mumbai 400088, India
⁸  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

(Received 21 August 2000 / Accepted 27 November 2000)

Abstract
We have determined the orbit of the double-lined spectroscopic binary RX J1603.8-3938. The binary consists of two weak-line T Tauri stars, both of which have a spectral type between K3 and K4. The period of the circular orbit is days and the mass ratio . To our knowledge RX J1603.8-3938 is thus the pre-main sequence binary with the longest period that has a circular orbit. Despite the fact that the masses and spectral types of the two components are almost identical, the photospheric lines are much stronger in one component than in the other. In the wavelength region between 5500 and 7800 Åwe find that the ratio of the equivalent widths of the primary to the secondary is . This ratio is constant in time, and is the same for all photospheric lines. Since the components are weak-line T Tauri stars, the effect cannot be explained by any kind of veiling. We are led to the conclusion that the secondary is mag fainter than the primary. It thus turns out that evolutionary tracks of pre-main sequence single stars are unable to explain the position of this system in the HR diagram.

Key words: stars: binaries: spectroscopic -- stars: formation -- stars: late-type -- stars: pre-main sequence

Offprint request: E. W. Guenther

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