1 Introduction

The bright (V=2.3 mag) B0.3 IV star $\delta$ Scorpii (HD143275, HR5953) has been considered a typical B0-type object for many years. It was suspected of binarity nearly a century ago by Innes (1901), who claimed visual detection of the secondary during a lunar occultation. van Hoof et al. (1963) found its radial velocity (RV) to vary with an amplitude of $\sim$15 kms^-1 on a time scale of 20 days. Their data were obtained in February 1955 and showed a smooth, nearly sinusoidal, variation over the period of the observations. Based on 7 observations obtained in 5 different nights in 1974 and 1976, Levato et al. (1987) also detected RV variations with even a larger amplitude of $\sim$25 kms^-1 and suggested a period of 83 days. These facts might imply that $\delta$ Sco is a single-lined spectroscopic binary, and such RV variations are easily detectable with modern high-resolution spectrographs, similar to those we used for this study. Below we further discuss this problem.

Smith (1986) performed high-resolution spectroscopic observations of the Si III 4552-4574 Å lines and found short-term line profile variability. He attributed this phenomenon to nonradial pulsations and treated the star as single due to the absence of mass transfer.

Long-term speckle interferometric observations of $\delta$ Sco (Bedding 1993; Hartkopf et al. 1996) determined that it is a binary system with a highly-eccentric orbit. The orbital parameters found by these authors are significantly different even though they used almost the same data sets (see Table 1). Hartkopf et al. (1996) noticed that the positional residuals tend to decrease as the eccentricity approaches 1. Their orbital solution predicts very large radial velocity (RV) variations near periastron for both components, with the line peak separation reaching 150 kms^-1. The components' brightness ratio is estimated to be 1.5-2.0 mag based on an optical interferometric observation obtained at the Anglo-Australian Telescope on 1991 May 31/June 1 (Bedding 1993).

 

 

Table 1: Orbital solutions for $\delta$ Sco.

T0	a	e	P	$\omega$	$\Omega$	i	Ref.
	arcsec		years	degrees	degrees	degrees
1979.3	0.11	0.82	10.5	170	0	70	1
$1979.41\pm0.14$	$0.107\pm0.007$	$0.92\pm0.02$	$10.58\pm0.08$	$24\pm13$	$159.3\pm7.6$	$48.5\pm6.6$	2
$2000.693\pm0.008$	$0.107^{\rm a}$	$0.94\pm0.01$	$10.58^{\rm a}$	$-1\pm5$	175	$38\pm5$	3

T₀ is the periastron passage epoch, a is the orbital semi-major axis, e is the orbit eccentricity, P is the orbital period, $\omega$ is the periastron longitude,
$\Omega$ is the node line longitude, i is the orbital inclination angle. References: 1 - Bedding (1993), 2 - Hartkopf et al. (1996), 3 - this work.
$^{\rm a}$ - The parameter is taken from the Hartkopf et al. (1996) solution.

Analysing profiles of Si III and Si IV ultraviolet lines, Snow (1981) calculated a mass loss rate of ${\sim} 3 \times 10^{-11}~M_{\odot}$ yr^-1 from the star. This value is the lowest one among those obtained with the same technique for the stars of his sample, which mainly contained Be stars.

Coté & van Kerkwijk (1993), who were searching for unidentified Be stars, displayed an H$\alpha$ line profile of $\delta$ Sco obtained at ESO in 1990 (close to the predicted periastron passage), which showed a weak double-peaked emission component inside a broad photospheric absorption. Previous high-resolution spectroscopic data obtained by Heasley & Wolff (1983), apparently in 1981/2 at CFHT, and by Grigsby et al. (1992) in 1986 at KPNO showed no emission component in H$\alpha$.

The HIPPARCOS parallax of the star is $8.12\,\pm\,0.88$ milliarcsec (ESA 1997), which corresponds to a distance $D=123\pm15$ pc. Other important parameters are $v\sin i=148\pm8$ kms^-1 (Brown & Verschueren 1997), E_B-V=0.14 mag, and log $L_{\rm bol}/L_{\odot}=4.4\pm0.1$(with the contribution of the secondary subtracted). The star's effective temperature as determined by different methods resulted in slightly different values: $T_{\rm eff}= 29\,760$ K (Blackwell, Petford, & Shallis 1980, infrared flux method) and 27500 K (Heasley et al. 1982, model atmospheres). Grigsby et al. (1992) fitted profiles of several hydrogen and helium lines to non-LTE, line-blanketed model atmospheres and concluded that the fits for $T_{\rm eff}=27\,000$ K and 28000 K (log g=4.0 in both cases) are almost equally good for $\delta$ Sco. The infrared flux method is less accurate because it involves the bolometric stellar flux, which is poorly known for such a hot star due to the lack of far-UV observations (see Hummer et al. 1988).

In June 2000, S. Otero discovered a brightening of $\delta$ Sco by visual comparison with nearby 1st and 2nd magnitude stars. This information along with the results of first spectroscopic observations, showing the H$\alpha$line in emission, was reported by Fabregat et al. (2000) in late July. Since that time the star was monitored by visual and photoelectric photometry as well as by spectroscopy (our team) until it became inaccessible in November. The photometric data have been recently published by Otero et al. (2001). The results of our spectroscopic observations are presented in this paper.

Our observations are described in Sect. 2, characteristics of the detected spectral lines in Sect. 3, refinement of the orbital solution by means of the RV measurements in Sect. 4, a brief discussion of possible mechanisms responsible for the appearance of the Be phenomenon in $\delta$ Sco in Sect. 5. Detailed modelling of the observed event is beyond the scope of this paper and will be presented elsewhere.