Astron. Astrophys. 363, 1029-1039 (2000)

3. Results - spectroscopy

3.1. Optical spectral features

The optical spectrum of the object is characterized by emission lines of the Balmer series and neutral and ionized helium on a blue continuum (Fig. 1, Fig. 2). The HeII 4686 emission line is relatively weak but present as also pointed out by Thorstensen (1999). The OI 6345 emission line is possibly detected in the averaged spectrum. The emission at H, HeI 7065, OI 7772, HeII 8236, and Paschen lines is also seen in the snapshot taken with grating #13 (Fig. 2). The OI 7772 is in absorption, suggesting a nearly edge-on accretion disk and a high inclination angle of the system (Smith 1990). The spectra as a whole have the general features seen in those of dwarf novae during quiescence.

Fig. 1. HeI 5876 (left) and H (right) profiles from the averaged spectrum of V893 Sco. The flux is normalized by the continuum level.

Fig. 2. The spectrum of V893 Sco taken on 1998 April 17.

All the emission lines seen in the spectrum have double-peaked profiles, suggesting these lines are produced in the accretion disk. Relatively strong absorption cores below the continuum level appear for the neutral helium lines, while those of the HeII are relatively weak. The depth of the self-absorption has no dependence on spectroscopic phase. No absorption features from the secondary are seen.

3.2. Radial velocities and the orbital period

To search for the orbital period of the object, radial velocities were obtained for the H emission line using the convolved double-Gaussian method (Schneider & Young 1980; Shafter 1983; Horne et al. 1986) and various separations between 25 and 110  for the double-peaked convolution profile. A sinusoidal fit to the radial velocity curves for each separation was obtained using the expression

where and are the systemic velocity and the spectroscopic phase shift, respectively, and A and B are coefficients for the semi-amplitude . The diagnostic parameter was computed for each separation (Fig. 3; see e.g., Shafter et al. 1986), and has a minimum for separations around . This gives a 0.07610 d periodicity as the strongest signal in the radial velocity curve. The final periodogram is shown in Fig. 4. Thorstensen (1999)'s alias of 0.08220 d can also be seen. However, we regard the period as the orbital period, being consistent with the result of Thorstensen (1999) and the photometrically determined period by Bruch et al. (2000).

Fig. 3. The diagnostic diagram for H radial velocity measurements.

Fig. 4. The periodogram for the radial velocity of H.

The spectroscopic ephemeris of the strongest radial velocity signal (corresponding to the change of radial velocity from positive to negative relative to the systemic velocity) is

The radial motion of the white dwarf was investigated by following the velocity variations of the wings of H line using the same double-Gaussian method (). The profile of the emission line suggests that the radial velocity variation at 80-90  separation is a better tracer of the orbital motion of the white dwarf: given the orbital period, the diagnostic diagram (Fig. 3) shows a rapid increase in for separations larger than 90 . We adopt the orbital parameters associated with the separation before the noise starts to dominate, i.e., : ; ; and . The radial velocity curve sampled for the 85  separation is shown folded on the orbital period in Fig. 5.

Fig. 5. The best-fit velocity curve of the H wings (85  separation) folded with the orbital period. The data are averages of each 10 successive points. Note that the spectroscopic phase is based on the ephemeris given in Eq. (1) ( separation). The zero-crossing point is phase shifted from that based on Eq. (1). The phase is repeated twice for clarity.

We find a relatively larger radial velocity scatter around spectroscopic phase , which indicates a rotational disturbance. This feature is most clearly seen in a velocity curve sampled with the minimum of the separation: in Fig. 5, the lower S/N of the sampling at the wider separation weakens the feature.

3.3. Equivalent widths

The Balmer emission lines dominate the spectrum and are characterized by large equivalent widths. The values are typically about and for H and H, respectively, while those of the helium lines are relatively weaker with about for the HeI 5876 line, and about for the other neutral or ionized helium lines and the FeII 5169 line. Fig. 6 shows that the equivalent width has a weak dependence on the orbital period. On May 31, our last night of spectroscopy, an outburst started according to the VSNET database ³. Because of the outburst, the strengths of the all lines suddenly decreased: the equivalent widths of the H and H were respectively about and , and those for the other lines were around 1 or .

Fig. 6. The equivalent width of H folded by the orbital period and plotted for the individual nights (April 18-20 (top) and May 29-31 (bottom) for left to right, respectively).

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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