The Wolf-Rayet + O binary WR 140 in Cygnus

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1 The Wolf-Rayet + O binary WR 140 in Cygnus Fachgruppe SPEKTROSKOPIE 1. The system The archetype of colliding-wind binary (CWB) systems is the 7.9-year period WR+O binary system WR140 (HD193793) where the stellar separation varies between ~ 2 AU at periastron to ~ 30 AU at apastron. The binary is frequently considered a textbook example of the colliding-wind phenomenon. The binary consists of a carbon-sequence Wolf-Rayet (WC7) star orbiting a more massive and luminous O4-5 main-sequence companion. The system s high eccentricity and rather favourable inclination help to probe different regions of the Wolf-Rayet wind and, at the same time, the profound change of conditions in the wind-wind collision zone at the times of periastron passage. This change is mainly reflected in rapid formation of dust clouds and can be detected as gigantic IR outbursts occurring on a strictly periodic (once per orbit) timescale. The status of the system as a strong non thermal, variable radio source, as well as an extremely bright (for a WR star) X-ray source makes WR140 an ideal laboratory for studying the properties of hot stellar winds. The almost perfectly phase-dependent behaviour of the system in radio and X-rays is remarkable. Many times over the past decade, WR 140 has been a target for long-term multiwavelength campaigns with subsequent state-of-the-art modelling of the colliding-wind phenomenon. In we intensified our large optical campaign in an attempt to follow the system with shorter time steps through the 2001 periastron passage. To our surprise, the clockwork behaviour of WR 140 changed dramatically during its periastron passage in (from Marchenko et al. 2003, The Astrophysical Journal, 596, 1295) Fig. 1: Orbit of WR 140. The line-of-sight is indicated by a straight line. 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 behaviour of WR 140 during periastron passage 2001 and represents a reasonable working and preparation text. It is important to note that they used relatively small size telescopes for their campaign. This was possible because the system had been observed for some months and they investigated nightly variations within the spectrum. That means WR 140 is a worthy target for amateur spectroscopists even with its relative 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. 2. 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 spectral ranges which are marked in the lower part of Figure 2. Depending on the number of such intervals visible, one should fit a spline or even a straight line (for just two intervals) for rectification.

2 The effect of this can be seen in figure 4. First, from the lines of CIII 5696 and HeI 5876 average spectra have been computed (top). By subtracting individual spectra from these averages, 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. Fig. 2: Spectrum of WR 140. 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 Sodium 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 1 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 periastron passage. Before passage the material moves towards the observer and afterwards it moves away from the observer. Figure 4: In wavelength moving excess emission of CIII and HeI during periastron passage. Figure 3: The wind-wind interaction produces a shock cone. Figure 5: Normalized excess flux at closest approach. 2

3 3. The parameters Type: WR+O binary Visual Magnitude = 7.07 mag Periastron passage at = HJD ± 3.7 (1985) Period P = ± 1.3 Inclination i = 50 +/- 15 Half opening angle of the shock cone q = 40 +/- 15 Eccentricity e = / Coordinates RA (2000) 20h 20' 28'', DEC (2000) ' 16'' 4. Frequently Asked Questions (FAQ) When should I observe WR 140? As often as possible. It is always a good idea to start training far in advance. First, during "easy" periods when the system is high in the sky, then during the January period Remember that it is a morning and evening object around periastron passage in January 12, 2009! And finally, we should know what we can expect in S/N for which exposure time. Last but not least we are interested in residuals during the passage. What are residuals? Global residuals: If we observe the system in a quiet period we find relatively stable lines. This is due to the large separation of the components. To find shock cone effects during periastron passage we have to separate those effects from the stable lines. This can be done by computing an average spectrum over the whole observation period and then subtracting each individual one from the mean. It is also possible to obtain a mean or minimum spectrum in advance and subtracting individual spectra from this minimum or mean. As a result we get the line features which are only present during periastron passage and which represent two crashing shock cones. Nightly residuals: The same technique can be applied for short term events in the lines, like clumps (turbulences) in the wind. Then we should obtain a minimum or a nightly mean and subtracting individual night spectra. This is a difficult task for small telescopes due to low S/N from short exposures. The spectrum should be centred in what wavelength? What bandwidth is advised? Due to its nature as a WR+O binary we majorly see emission lines. They are all of great interest and deliver worthy insights in the physics of the system. So a clear continuum is not easy to define. It is more important to choose a sufficient wavelength interval instead of defining a central interval to define a maximum number of (pseudo) continuum wavelength as it is done e.g. in the paper of Marchenko et al. Before deciding in more detail it is strongly recommended to read available literature. A number of papers can be found in this campaign web page. What calibration lamp must be used? Any lamp with sufficient emission lines for calibration plus a catalogue in the respectively observed wavelength region is fine. No real continuum? How can I estimate the Signal-to-Noise-Ratio then? By using two spectra one can estimate S/N by dividing one spectrum by the other. The lines will then be eliminated. If the two spectra have the same exposure time the S/N of the quotient is 2 of that of the single spectrum. The S/N of the sum spectrum is then better by a factor of two than the quotient. What S/N is sufficient? The more the better. But S/N should be 100 at least. I do not reach S/N > 100. What should I do? Just increase your exposure time. Nightly means are sufficient for the periastron event over some weeks. How long should I observe the star during periastron passage? Three months are ok. No line events were observed earlier or later by earlier campaigns. So, start around beginning of December and end around end of February. Why can I see a profile in the interstellar Na lines if I apply very high resolution? 3

4 Interstellar material along the line-of-sight has different velocities with respect to the observer and hence the interstellar absorption lines are broadened. 5. First shots Here are some trials plus respective errors in data reduction to help beginners trouble shoot their results and to understand how to proceed. The continuum does not match well in the blue wavelength region. All exposure: C14 (D=355 mm, f=3550mm), Slit spectrograph (Littrow), Grating 2400 l/mm, Slit width 50µm. Berthold Stober / Glan-Münchweiler 6. Some results Berthold Stober / Glan-Münchweiler The continuum has wrongly been fitted to the line peak of the CIII line at 5750 Å. Berthold Stober / Glan-Münchweiler The continuum is correct and the wavelength calibration has been performed by using the two interstellar Sodium lines around 5900 Å. The calibration is not in agreement with Marchenko et al. C14, Lhires with 2400 g/mm grating, Nova 1602E in 2x2 binning (0,27 A/Pix), 300s. Calibration with Neon lamp. Data reduction with IRIS and VSpec. FWHM of slit image 0,84A at 50 um slit width. Lothar Schanne / Voelklingen 4

5 C14, Lhires with 2400 g/mm grating, Nova 1602E in 2x2 binning (0,27 A/Pix), 300s. Calibration with Neon lamp. Data reduction with IRIS and VSpec. FWHM of slit image 0,84A at 50 um slit width. Lothar Schanne / Voelklingen 7. Literature A number of respective articles can be found at 8. Finding chart Mustermann, T. & Musterfrau, A., 1988, VdS- Journal, 11, 34 Referenz, 2., 2003, Irgendwo-Journal, 222,12 5