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Astron. Astrophys. 361, L49-L52 (2000)

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4. Clues to the pre-main sequence nature

4.1. Spatial location and kinematics

First of all, the PMS nature of RXJ 0529.4+0041 has been assessed on the basis of its spatial location and kinematics, indicating that the system is a very likely member of the Orion OB1a association, and on the high lithium content observed in each of its individual components (Alcalá et al., 2000). RXJ 0529.4+0041 is seen projected close to the North-East side of the Orion B cloud, and its systemic radial velocity also strongly supports its association to the SFR. An estimate of the distance to the system is obtained by scaling the observed flux with the total luminosity calculated directly from the stellar radii and effective temperatures of the primary and secondary components, as obtained from the light-curve solution. This yields a distance of about 350 pc, which is in good agreement with the result based on HIPPARCOS parallaxes for subgroup 1a of the Orion OB1 association (Brown et al. 1998).

4.2. Lithium abundance

Equivalent widths of the Li 6708 Å, WLi, for the individual components of RXJ 0529.4+0041 have been measured by two-components gaussian fits on the residual spectrum obtained by subtracting a synthetic binary spectrum from the observed one, as illustrated in Fig. 2. This procedure offers the advantage to eliminate any possible contamination of the lithium line from blends with nearby iron lines (Lee et al. 1994). The synthetic spectrum has been built up using two stars of appropriate spectral type, HD 115404 (K2 V) and HD 201092 (K7 V), for the primary and secondary component respectively, acquired with the same instrumental set-up. Li 6708 Å equivalent widths have been measured from five different spectra, in which the binary components are seen best separated, and then corrected by the corresponding weighting factors derived from the synthetic spectrum fit, to account for the actual contribution of each star to the total continuum flux. The average, corrected Lithium equivalent widths are given in Table 1. The errors on the EW have been estimated determining the S/N ratio in two windows on the right and left-hand side of the Li line in the residual spectrum and taking into account the errors on the weighting factors. These translate to a Lithium abundance, [FORMULA], of 3.2 and 2.4 for the primary and secondary component, respectively, (in the usual scale in which [FORMULA]), by using the theoretical curves of growth based on NLTE atmospheric models by Pavlenko & Magazzù (1996) corresponding to [FORMULA]=4.5. The main sources of error on the derived [FORMULA] are the uncertainties on the effective temperatures and on the weighting factors used to correct the observed equivalent widths. These yield estimated mean errors of about 0.3 and 0.5 dex on the derived lithium abundance, for the primary and secondary component, respectively.

[FIGURE] Fig. 2a-c. Upper panel a : match of observed spectrum (crosses) and `synthetic' spectrum (solid line) by fit of the three components with combinations of K2 and K7 standard stars. Lower panels (b and c ): details of the residual line spectrum in the H[FORMULA] and the Li [FORMULA]6708 Å ranges. Dots represent two-gaussian fits to H[FORMULA] and Li I lines of A and B components.

4.3. HR diagram

We can now compare the fundamental stellar parameters derived from the orbital solution (reported in Table 1) with those inferred from currently available theoretical evolutionary models. As an example, Fig. 3 shows the position on the theoretical HR diagram of the binary components with respect to the evolutionary tracks and isochrones by Baraffe et al. (1998). The comparison with other widely used sets of tracks is shown in Covino et al. (2000). In all cases, the mass inferred from the theoretical tracks for the primary component is fairly consistent, within the errors, with the observational value. However, in the case of the secondary component, better agreement is found with the Baraffe et al.'s and Swenson's (1994) tracks, whereas the sets of tracks by D'Antona & Mazzitelli (1994) and Palla & Stahler (1999) apparently show less consistence with the stellar parameters we have deduced. New PMS evolutionary models by D'Antona, Ventura & Mazzitelli (2000), taking into account zero-order thermal modifications due to a dynamo-generated magnetic field, show the great sensitivity of the Teff location of the PMS tracks to the magnetic field strength and that non-magnetic models actually provide an upper limit to the Teff location of the PMS tracks.

[FIGURE] Fig. 3. HR diagram for the components of RXJ 0529.3+0041: comparison with the evolutionary tracks and isochrones by Baraffe et al. (1998). The black dot indicates the location of the visual companion adopting the distance of 350 pc.

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© European Southern Observatory (ESO) 2000

Online publication: October 10, 2000