2 Observations and data reduction

2.1 Spectroscopy

The high-resolution spectroscopic observations were performed using three instruments at the European Southern Observatory (ESO), La Silla, Chile: the Cassegrain Echelle Spectrograph (CASPEC) attached to the ESO 3.6 m telescope, the Fiber-fed Extended Range Optical Spectrograph (FEROS) attached to the ESO 1.5 m telescope and CORALIE at the Euler 1.2 m Swiss telescope.

The first set of observations was obtained using CASPEC on February 1999. The CASPEC data reduction was performed using the Echelle reduction package available within the Munich Image Data Analysis System (MIDAS, version November 1997), plus some specially devised procedures making use of the algorithms prescribed by Verschueren & Hensberge (1990) for background subtraction and optimal order extraction. The nominal resolving power of these spectra, as measured from several isolated lines of the thorium-argon comparison spectrum, is $\lambda / \Delta\lambda \approx$ 22000. Unfortunately, CASPEC was very close to be decommissioned during our observing period and many technical problems related to the positioning of the cross-disperser prevented us from performing a reliable wavelength calibration of the spectra to derive radial velocities. Therefore, additional spectra for all the stars were obtained with FEROS in May 1999 and January 2001.

Despite of the problems with the cross-disperser, the CASPEC spectra allowed us to detect the Lithium resonance line at 6708 Å, wherever present, and to reveal a new double-lined spectroscopic binary (SB2). Systematic observations of this SB2 were immediately started with CORALIE, and since April 1999 also using FEROS (see Sect. 4.2).

The reduction of the FEROS data was performed using the specific FEROS data-reduction software (DRS) implemented in the ESO-MIDAS environment (from MIDAS version 98NOV on). The basic reduction consisted of the following steps: i) definition of the echelle orders on flat-field frames; ii) background subtraction; iii) extraction of the echelle orders; iv) flat-fielding of the extracted spectra (to remove pixel-to-pixel variations as well as correct for the blaze function); v) wavelength calibration using ThAr exposures; vi) rebinning to wavelength scale; vii) merging of the orders. For details on the instrument and on the data reduction procedures we refer to the FEROS User's Manual (Francois 1999, Vers. 1.1) and The FEROS Cookbook (Pompei & Francois 2000, Vers. 2.2), respectively.

The nominal resolving power of the FEROS spectra, as measured from several isolated lines of the thorium-argon comparison spectrum, is $\lambda / \Delta\lambda \approx$ 48000. For the CORALIE spectra with a resolution of 47000 all observations were taken with one fiber centered on the target star and the other fiber illuminated by the background sky. The reduction is performed by an on-line reduction procedure: after reading the CCD, the spectrum is extracted, calibrated in wavelength and flat-fielded. The on-line reduction system also performs the cross-correlation of the stellar spectrum with a numerical mask (Queloz 1995) for the determination of radial and rotational velocities.

In Table 1 a summary of the spectroscopic observations is presented. The number of observations of Cru-3 reported in this table refer only to the indicated periods. The sample of observations of Cru-3 are reported in Sect. 4.2, in Table 4.

   

Table 1: Summary of spectroscopic observations.

Star	CASPEC	FEROS	No.
Cru-1	5-Feb.-99	17-May-99; 09-Jan.-01	3
Cru-2E	5-Feb.-99	09-Jan.-01	2
Cru-2W	-	17-May-99	1
Cru-3 $^{\dagger}$	5-Feb.-99	-	1
Cru-4	6-Feb.-99	19-May-99; 04-Jan.-01	3
Cru-5	6-Feb.-99	20-May-99	2
Cru-6	6-Feb.-99	09-Jan.-01	2

$^{\dagger}$ See Table 4 for FEROS and CORALIE observation dates.

In Fig. 1, CASPEC and FEROS spectra of the sample stars in the range from H$\alpha$ to the Lithium $\lambda$6707 Å  absorption line are shown.

Figure 1: High-resolution CASPEC and FEROS spectra of the stars in our sample in the H$\alpha$ - Lithium range. Note the double spectral lines of the star Cru-3. The spectra of the stars Cru-2E, 3, 4, 5 and 6 are from CASPEC, while those of Cru-1 and Cru-2W are from FEROS. The spectral type (SpT), radial velocity (RV), and projected rotational velocity (vsini) are indicated for each star. For Cru-3 the systemic radial velocity is reported (see Sect. 4.2 for details). The ":'' in the values of Cru-1 indicate variability (see Sect. 4.1.)

As one can see from Fig. 1, with the exception of Cru-5, all the other stars show the Li I $\lambda$6708 absorption line. FL97 give an upper limit of 0.2 Å  for the Li equivalent width of Cru-5, but their spectrum for this star has a low S/N ratio. Unfortunately, FL97 do not specify which of the stars in the X-ray error boxes reported by PF96 they have observed. Therefore, unless we have observed a different object (which we consider rather unlikely because the spectral type we assign to this star is consistent with the one reported by FL97), we do not find any Li I absorption in Cru-5. As already reported by FL97, the stars Cru-1, Cru-3, Cru-4 and Cru-6 have H$\alpha$ in emission, while the couple Cru-2E and Cru-2W and the star Cru-5 show H$\alpha$ in absorption.

2.2 Near infrared imaging

Near infrared J, H, K, imaging of the six Crux stars was performed at ESO, La Silla, with the ESO 3.6 m telescope and the ADaptive Optics Near Infrared System (ADONIS) using the SharpII camera on January 11, 2001. A pixel scale of 0.05 arcsec/pixel was selected, and we used an internal chopping method for sky subtraction. A detector integration time (DIT) of 5 s for all sample stars and all three filters was chosen, except for Cru-3 (DIT = 2 s). Ten independent frames have been acquired in each filter for all objects except for Cru-1, for which we obtained 20 images per filter. The adaptive optics loop was closed on the science target itself for all the objects. The airmass during the observations varied between 1.19 and 1.26. Standard processing techniques within the ECLIPSE software was applied for flat-fielding, dark correction, bad pixel removal, and shift and add. From the IR standard stars AS16-2 and AS16-4 (Hunt et al. 1998) we determined the photometric zero points and the mean photometric errors. Table 2 summarizes the results for the six stars.

   

Table 2: Near-infrared photometry, radial and projected rotational velocities of the Crux stars.

Star	J	J-H	H-K	RV	vsini
				[km s-1]	[km s-1]
Cru-1	10.28	0.69	0.22	+16.0:	10.0:
Cru-2E	9.04	0.35	0.09	+2.5	7.0
Cru-2W	9.05	0.47	0.12	+5.5	7.5
Cru-3 $^{\dagger}$	8.23	0.68	0.15	+10.6	15.0
Cru-4	10.06	0.70	0.20	+12.0	13.2
Cru-5	10.56	0.70	0.19	-18.0	<2.0
Cru-6	10.04	0.73	0.15	+12.0	15.0

$^{\dagger}$ The systemic RV is given. See also Table 4 for FEROS and CORALIE measurements.
    The ":'' means variable radial and rotational velocity

Atmospheric extinction coefficients in the three bands were determined using the observations of the comparison star in the field of the PMS eclipsing binary RXJ 0529.4+0041, observed during several hours on the same night and spanning an air-mass from 1.0 to 1.8 (see ESO-press release 22/01 ; Covino et al., in preparation).

The mean zero points are 22.34 $\pm$ 0.01, 22.06 $\pm$ 0.02 and 21.51 $\pm$ 0.03 in the J, H and K bands respectively. The mean photometric errors are $\sigma_J$ = 0.04, $\sigma_H$ = 0.04 and $\sigma_K$ = 0.06. The zero points are in very good agreement with those reported by the ESO 3.6 m telescope team. More details on the data reduction will be reported in Covino et al. (in preparation). The star Cru-1 resulted to be a close visual pair with a separation of 0.25 arcsec (see Sect. 4.1), while the star Cru-2, previously known to be a visual binary, has a separation of 3.25 arcsec. The latter is thus sufficiently well separated to allow aperture photometry of the individual components.

When comparing the IR colours of the Crux stars with those of normal field stars and IRAS sources in star forming regions, it is found that the Crux stars lack IR excesses (cf. Fig. 2): while the stars Cru-2E and 2W have near-IR colours consistent with those of normal field stars, the other Crux stars fall in an intermediate region between the IRAS sources and the normal field stars, although they tend to follow the line of normal colours for field dwarfs, indicating the lack of near-IR flux excesses. The different position of Cru-2E and 2W in the J-H versus H-K diagram compared to the other Crux stars is mainly due to the earlier spectral type of Cru-2E and 2W. Since the components of the binary Cru-3 are practically equal, the IR colours are the same for both components. On the other hand, it was possible to resolve the visual binary Cru-1 only in the K band therefore, we could not determine the colours of the individual components.