2 The data

Our 90 high-resolution spectra (R=32000) of $\psi ^2$ Ori were obtained on La Palma, with the Isaac Newton Telescope and the fiber-fed ESA-Musicos echelle spectrograph, over a 6 night period (18-24 November 1997) which was mainly free of bad weather. The spectra have a wavelength coverage of about 3800-6800 Å, sampled by 41 echelle orders. The reddest orders are heavily affected by fringing. Most of the spectra were exposed 10 min or less. We discarded 8 spectra from the analysis because of poor count rates.

The spectra were reduced using standard packages in IRAF. The CCD overscan region was used to determine the bias level. One-dimensional spectra were extracted for the individual orders. It was found that pixel-to-pixel variations could not be removed properly using our abundant number of flatfield frames: unflatfielded spectra proved to have better S/N than flatfielded ones. A two-dimensional wavelength solution was obtained from 59 manually selected ThAr calibration lines. Spectra were shifted to, and acquisition times were transformed to, the heliocentric frame. The normalization of the orders was achieved in a way similar to that described by Telting & Schrijvers (1998) in order to minimise variable continuum misplacement in the absorption lines: for each night a spectrum with high S/N was used as a template and was divided into the other spectra of that night. The orders in the resulting quotient spectra were normalized by fitting a high-order cubic spline with typically 10 to 15 spline segments. The templates were normalized with a low-order cubic spline fit to the continuum regions. Then the normalized quotient spectra were transformed back by multiplying with their corresponding normalized template spectrum. The reduced spectra have a typical S/N ratio of between 250 and 450.

In Fig. 1 one can clearly see the orbital variability in the spectra, and the line-profile variations of the primary. We did not detect any significant variations in the line profiles of the secondary.

Figure 2: Orbital solution for the template giving the best $\chi ^2$value (see Table 1). The measured radial velocities of the primary are affected by the profile of the secondary in night 4 of our observations.

 

 

Table 1: Orbital solution. Listed are the results for the two templates, and the mean of these two solutions. The last column lists the number of points used in the fit.

Solution for the first template
K1	144.12	0.22
Eccentricity e	0.0521	0.0011
Period P	2.5335	0.0013
Periastron time	2450773.967	0.012
Periastron angle $\omega$	176.1	1.6
System velocity v0	24.77	0.17
K2	232.57	0.49
$\chi ^2$	574.90		129
rms	5.34		129
rms primary	5.74		82
rms secondary	4.56		47
Solution for the second template
K1	145.08	0.19
Eccentricity e	0.0536	0.0015
Period P	2.5244	0.0010
Periastron time	2450773.907	0.010
Periastron angle $\omega$	167.1	1.5
System velocity v0	14.01	0.15
K2	240.86	0.63
$\chi ^2$	716.18		129
rms	8.43		129
rms primary	4.37		82
rms secondary	12.72		47
Mean values and error in mean
K1 [kms-1]	144.6	0.5
Eccentricity e	0.053	0.001
Period P [days]	2.529	0.005
Periastron time [HJD]	2450773.94	0.03
Periastron angle $\omega$ [ $\hbox{$^\circ$ }$]	172	5
System velocity v0 [kms-1]	19	5
K2 [kms-1]	237	4
$A_1\sin i$ [106km]	5.02	0.02
$A_2\sin i$ [106km]	8.22	0.14
$m_1\sin^3 i$ [$M_\odot$]	9.02	0.34
$m_2\sin^3 i$ [$M_\odot$]	5.50	0.12