In this paper we have reexamined the issue of the determination of the circularization period for the PMS stars and discussed its relevance to constrain tidal circularization scenarios. In particular, we have presented orbital and stellar parameters for the spectroscopic binary system RX J1603.9-3938. Based on its lithium content, age estimation and barycentric velocity we conclude that both components of this system are bona-fide PMS stars.
Next, we updated the
diagram for the low-mass PMS stars.
Special attention was accorded to the PMS system RX J1603.9-3938 with
and circular orbit because it sets the circularization period for the PMS
binary population. This new circularization period supports Zahn & Bouchet's (1989)
result that tidal circularization during the pre-main sequence will circularize orbits with
period up to 7.0-8.5 days.
However, the apparently circular orbit of a second
system with an orbital period of
days (RX J1301.0-7654a) poses an ambiguity in the sense that
it is not clear whether or not its circular orbit is a result of the PMS tidal circularization.
More observations of PMS spectroscopic binaries are needed to further test whether
the PMS tidal circularization scenario proposed by Zahn & Bouchet (1989) needs
to be revised to account for other
physical phenomena in addition to tidal-torque interactions due to eddy viscosity.
The time dependence of the circularization period for different
binary populations indicates a qualitative change at about the age of
the Hyades or Praesepe clusters. All younger populations appear to
have
around 7 or 8 days, independent of age. This suggests that
the orbital evolution in these young populations is a result of tidal
circularization during the PMS phase. For older binary populations, on the other hand, the
increase in the circularization period with age is evident.
Our main conclusions are that: i) PMS tidal circularization does occur and it seems to be responsible for the circularization period observed in the young clusters up to the age of Praesepe. More data are needed to constrain the efficiency of the tidal interaction during the PMS phase and to better understand the role of other phenomena which may cause angular momentum transfer such as disk interaction, magnetic fields, and accretion; and ii) orbital circularization is still occurring during the MS phase, contrary to Zahn & Bouchet's (1989) idea that nearly all tidal circularization occurs during the PMS phase. Both support Mathieu et al.'s (1992) suggestion that a hybrid scenario (PMS plus MS tidal circularization) could explain the observed circularization periods as a function of age. However, which mechanism is supposed to account for the continuous circularization on the MS is still an open question. Tassoul's hydrodynamical mechanism seems to be an alternative (Tassoul 1995 and references therein), although it has been the subject of intense debate on whether or not it can operate (Rieutord 1992; Rieutord & Zahn 1997; Tassoul & Tassoul 1997). Further efforts on that front would be important to improve our understanding, in particular, of the physical processes that transport angular momentum inside the stars.
Acknowledgements
CHFM acknowledges grants from CNPQ Proc. 200614/96-7 (NV). EC and JMA acknowledge financial support by the Italian Ministero dell'Università e della Ricerca Scientifica e Tecnologica. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. We thank the anonymous referee for his comments which helped to improve the paper.
Copyright ESO 2001