Sergey Marchenko et al. published 2003 the investigation "The Unusual 2001 Periastron Passage in the ``Clockwork'' Colliding-Wind Binary WR 140" (see Literature). It describes the behavior of WR 140 during periastron passage 2001 and represents a reasonable working and preparation text.

It is important to note that they used realively small size telescopes for their campaign. This is possible because the system has been observed during some months and they investigated nightly variations within the spectrum. That means WR 140 is a worthy target for amateur spectroscopists even with its relativ faintness of 7mag. A nightly average spectrum is sufficient and delivers a good S/N. However, the observational conditions in January (periastron passage) have the disadvantage of a maximum of three hours exposure time.
Fig. 1: Spectrum of WR 140.

Data reduction

The spectrum of WR 140 is dominated by emission lines like all Wolf-Rayet stars. The photosphere is invisible due to a strong stellar wind and we can not HR-classify WR stars. As a consequence one can not define a clear continuum and a reliable rectification is difficult. Marchenko et al. defined spactral ranges which are marked in the lower part of Figure 1. Depending on the visible number of such intervals one should fit a spline or even a strait line (for just two intervals) for rectification.

WR 140 is a binary. For this reason we find a number of absorption lines from the O component within its spectrum. They are indicated as well as the interstellar absorptions. Those around 5900 Å are Natrium D1 and D2.

The wavelength calibration should be done by with an optical slit and a calibration lamp, if possible. Using interstellar lines for calibration is somewhat problematic. They are too rare to estimate the spectral dispersion.

Some results

The orbit has an advantageous inclination to the line-of-sight. In Figure 2 this line is defined by a straight line and one can see that the shock cone material, produced by wind-wind interaction of the two components (see Fig. 3), changes its direction from blue to red during periastronp passage. Before passage the material moves towards the observer and afterwards it moves away from the observer.
Figure 2: Orbit of WR 140. The line-of-sight is indicated by a straight line.
Figure 3: The wind-wind interaction produces a shock cone.

This situation is observed by using figure 4. First, from the lines of CIII 5696 and HeI 5876 average spectra have been computed (top). By subtracting individual spectra from these averafes residuals have been obtained. These residuals represent excess material which is produced within the shock cone. Instead of an average spectrum one can also use a minimum obtained far before periastron passage. In both lines one can observe that the excess appears shortly before periastron, moves from blue red and disappears after periastron. In addition one can see in figure 5 that the excess quickly increases and decreases during periastron.

Figure 4: In wavelength moving excess emission of CIII and HeI during periastron passage.
Figure 5: Normalized excess flux at closesed approach.