2 The observational data

 

 

Table 1: Radial Velocity data for RX J1603.9-3938. Column 1: heliocentric julian date; Col. 2: orbital phase; Col. 3: radial velocity measurements for the primary star; Col. 4: error on the radial velocity (primary); Col. 5: residuals (primary); Col. 6: radial velocity measurements for the secondary star; Col. 7: error on the radial velocity (secondary); Col. 8: residuals (secondary). Columns 3 to 8 are given in km s^-1.

HJD	Phase	$Vr_{\rm a}$	$\sigma$	O-C	$Vr_{\rm b}$	$\sigma$	O-C
51209.8218	0.1788	19.54	0.02	0.03	-16.67	0.05	0.20
51210.8160	0.3103	-13.04	0.04	0.08	17.80	0.08	-0.20
51212.7694	0.5688	-35.19	0.04	-0.21	41.33	0.07	-0.01
51213.7936	0.7044	-9.56	0.02	0.02	14.43	0.05	0.23
51214.8325	0.8418	24.02	0.03	-0.07	-21.42	0.06	0.34
51215.8039	0.9704	41.87	0.04	0.01	-40.83	0.08	-0.01
51253.9018	0.0119	42.48	0.75	0.03	-41.48	0.75	-0.11
51257.8665	0.5366	-37.73	0.75	-0.08	44.50	0.75	0.31
51311.9213	0.6897	-13.06	0.04	0.05	17.87	0.08	-0.10
51320.8742	0.8745	30.45	0.03	-0.11	-28.85	0.05	-0.18
51684.7691	0.0291	41.91	0.02	0.03	-40.83	0.03	-0.07

2.1 The system RX J1603.9-3938

In Table 1 we present radial velocity observations for the double-lined spectroscopic binary RX J1603.9-3938 carried out with the spectrograph CORALIE attached to the Swiss Euler Telescope at ESO, La Silla. The radial velocity curve is shown in Fig. 1 and the respective orbital elements and stellar parameters for both components are given in Table 2. The system has a circular orbit and an orbital period of 7.56 days. Its barycentric velocity ( $\gamma=1.921$ km s^-1) is in agreement with the mean radial velocity ( $<V_{\rm rad}>\,=-0.03\pm1.2$ km s^-1) of the 9 Lupus bona-fide CTTS and with the mean radial velocity ( $<V_{\rm rad}>\,=1.29\pm0.87$ km s^-1) of the "on-cloud'' Lupus ROSAT WTTS both listed in Wichmann et al. (1999).

Determination of luminosities and spectral types were obtained for both components through a matching of the binary spectrum with that of a synthetic binary following the prescription of Lee et al. (1994). Details on the application of the matching technique and on the radial and rotational velocities determination are given in Covino et al. (2000, 2001a). Once individual luminosities have been derived, we can estimate the true Li I $\lambda$ 6708 Å equivalent width for each component (i.e. the equivalent width corrected from the veiling due to continuum of the companion). The values given in Table 2 show that the Li I content in both components of RX J1603.9-3938 are higher than those observed in stars of same spectral type at the age of Pleiades (${\sim}350$ mÅ for K2 and ${\sim}320$ mÅ for a K4). Finally, ages, masses and the radii for both components were determined from evolutionary models by D'Antona & Mazzitelli (1997). The choice of using the evolutionary models by D'Antona & Mazzitelli to derive ages and radii is completely arbitrary since there is no standard PMS evolutionary model. The reader should be aware that other models can yield different masses and ages (e.g., Baraffe et al. 1998; see e.g., Simon et al. 2000; Covino et al. 2001a for a comparison between different evolutionary tracks). However, we stress that the use of other sets of PMS tracks does not change the main conclusions of this paper. Together, the Li content of each component, the barycentric velocity of the system and its age strongly suggest that both components of RX J1603.9-3938 are in fact bona-fide T-Tauri stars.

Figure 1: Radial velocity curve of RX J1603.9-3938. Radial velocities observations for the primary and secondary components are drawn as a filled and open circles, respectively.

 

 

Table 2: Orbital and stellar parameters for RX J1603.9-3938.

	$e \equiv 0$	e free
$P_{\rm orb}$ (days)	$7.56679\pm 0.00015$	$7.55678\pm 0.00019$
$T_0-2\,400\,000^{1}$ (HJD)	$51004.4375\pm 0.0049$	$51003.5\pm 1.8$
e	0.	$0.0009\pm 0.0014$
$\gamma$ (km s-1)	$1.921\pm 0.034$	$1.925\pm 0.036$
$K_{\rm a}$ (km s-1)	$40.643\pm 0.060$	$40.624\pm 0.068$
$K_{\rm b}$ (km s-1)	$43.411\pm 0.080$	$43.395\pm 0.086$
$a_{\rm a}\sin i$ (109 m)	$4.2234\pm 0.0063$	$4.2214\pm 0.0071$
$a_{\rm b}\sin i$ (109 m)	$4.5110\pm 0.0082$	$4.5093\pm 0.0090$
$M_{\rm a}\sin^3 i$ ($M_{\odot}$)	$0.24069\pm 0.00091$	$0.24041\pm 0.00100$
$M_{\rm b}\sin^3 i$ ($M_{\odot}$)	$0.22535\pm 0.00082$	$0.22506\pm 0.00091$
$q=M_{\rm b}/M_{\rm a}$	$0.936\pm 0.003$	$0.936\pm 0.004$
$\sigma({\rm O{-}C})$ (km s-1)	0.143	0.145
Number of measur.	11	11
	Primary (a)	Secondary (b)
$V\sin i$ (km s-1)	6	5
$L (L_{\odot})$	0.72	0.53
$\log T_{\rm eff}~({\rm K})$	3.678	3.664
ST	K2	K4
$EW_{{\rm Li} \lambda 6708}$ (mÅ)	420	390
$R (R_{\odot})$	1.25	1.19
$M (M_{\odot})$	1.1	0.9
age ( Myr)	7	7

¹ For the eccentric case, T₀ gives the time of periastron passage while for the circular case, T₀ indicates the time of the radial velocity maximum of the primary star.

2.2 Data available for other PMS systems

The orbital period and eccentricity for the PMS binary systems known to the authors are listed in Table 3. When available in respective reference, the spectral type of the components is given. When only the effective temperature is available we use the calibration of de Jager & Nieuwenhuijzen (1987) to transform effective temperatures into spectral types. We also mark the systems in which at least one of the components is not within the mass range investigated in Zahn & Bouchet (1989). The nature of the spectroscopic binary system (i.e., single- or double-lined) is indicated as well. Comparing our Table 3 to Mathieu's (1994) Tables A1 and A2, we list 15 new entries. Also, some entries listed in Mathieu (1994) had their reference updated, but a few of them still remain unpublished. In Table 4 we list age and circularization period with their respective references for other low-mass binary populations for which the circularization period is determined.