On the Variability of the Visual Binary WR 86?
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A&A 384, 1012–1022 (2002) Astronomy DOI: 10.1051/0004-6361:20020085 & c ESO 2002 Astrophysics On the variability of the visual binary WR 86? WC7 with a β-Cephei companion S. J. Paardekooper1,P.M.Veen1,A.M.vanGenderen1, and K. A. van der Hucht2 1 Leiden Observatory, Postbus 9513, 2300 RA Leiden, The Netherlands 2 SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Received 30 August 2001 / Accepted 15 January 2002 Abstract. We discuss the variability of WR 86 (WC7 (+B0 III)), a known visual binary, confirmed by Hubble Space Telescope observations (Niemela et al. 1998). Photometric observations show a fairly smooth light curve with a time scale of ∼3.5 h and a light amplitude of ∼0m. 01. Because of the spectral classification of the visual companion (B0 III) it might well be a β-Cephei star. From observations taken in 1989, 1990 and 1995 we derive two photometric frequencies, 6.914 cd−1 and 7.236 cd−1, where only the latter is given in the recent VIIth catalogue of galactic Wolf-Rayet stars (van der Hucht 2001). Our spectroscopy reveals a slightly variable WR star and almost certainly a pulsating companion. Comparison with stellar models suggests that WR 86 consists of a 20 M β-Cephei star in combination with a WR star of initial mass 40 M ,atZ =0.04. We derive a distance to WR 86 of 2.1 0.8kpc. Key words. stars: binaries: general – stars: Wolf-Rayet – stars: individual: WR 86 1. Introduction 2. Observations and reductions The Wolf-Rayet (WR) star WR 86 (HD 156327) is a well The 1989 and 1990 photometric observations were known visual binary of 000. 2 separation, consisting of a taken with the 90-cm Dutch telescope at the ESO, WC7 type WR star together with a B0 III companion La Silla, Chile, equipped with the VBLUW photome- (L´epine et al. 2001). The variability of WR 86 with a time ter of Walraven during eighteen nights in July 1989 and scale of ∼3 h was discovered by Monderen et al. (1988) and nine nights in May and June 1990. An overview of the subsequently attributed to a possible β-Cephei companion Walraven photometric system is given by Lub & Pel to the WR star by van Genderen et al. (1991). However, (1977). Integration times were of the order of 2 min for the the tentative objection against this supposition was due to variable and 0.5 min for the comparison star HD 158528, the absence of a progressively increasing light amplitude withanapertureof1600. 5. This comparison star has been up into the ultraviolet near 3300 A.˚ A possible explana- used before in a study of Cepheids (Moffet & Barnes 1984), tion is that the WR star substantially contributes to the and we found no variations. No check stars were observed. UV flux, surpressing the UV light amplitude appreciably. Both components of WR 86 were together in the aper- We present photometric monitoring during nine nights ture. Table 1 lists the average photometric parameters in 1990, from which we derive new periods, one night in in the VBLUW system (in log intensity scale) as well 1989 and one in 1995, which were used together with si- as the equivalent parameters of the UBV system V and multaneous spectroscopy to see whether the photometric B − V (with subscript J, in magnitude scale). The latter variations have any spectroscopic counterpart. were transformed from V and V − B by formulae of Pel We will describe the observations in Sect. 2, after which (1986). Note that these formulae apply only to absorption- we discuss the photometry in Sect. 3, and the spectroscopy line stars, and that for an emission-line object such as a in Sect. 4. We draw our conclusions in Sect. 5. WR star systematic differences can be introduced in the Send offprint requests to: A. M. van Genderen, order of a few tenths of a magnitude. The standard devi- e-mail: [email protected] ations (σ) represent the stellar variability as well as the ? Tables 3–6 and 8–13 are only available in electronic form instrumental scatter. These photometric averages are in at the CDS via anonymous ftp to good agreement with those of van Genderen et al. (1991), cdsarc.u-strasbg.fr (130.79.128.5) or via although their results are based only on two (short) nights http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/1012 of observations. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020085 S. J. Paardekooper et al.: On the variability of the visual binary WR 86 1013 Table 1. Photometric parameters of WR 86 in the VBLUW system (in log intensity scale) and the the transformed V and B − V parameters (UBV system) with subscript J (in magnitudes). Observer #nights VV− BB− UU− WB− LVJ (B − V )J epoch #points σV σV −B σB−U σU−W σB−L Spijkstra (data set 1989) 18 −0.965 0.342 0.210 0.147 0.153 9.27 0.770 07/07/89 – 31/07/89 110 0.01 0.004 0.005 0.005 0.005 Hollander (data set 1990-I) 3 −0.964 0.341 0.213 0.148 0.155 9.27 0.788 30/05/90 – 01/06/90 79 0.006 0.002 0.004 0.005 0.004 van Ojik (data set (1990-II) 6 −0.965 0.340 0.215 0.148 0.157 9.27 0.766 12/06/90 – 17/06/90 197 0.005 0.002 0.003 0.005 0.004 Veen (data set 1995) 1 9.21 06/04/95 21 Table 2. Photometric parameters of the comparison star HD 158528 in the VBLUW system (in log intensity scale) and the the transformed V and B − V parameters (UBV system) with subscript J (in magnitudes). VV− BB− UU− WB− LVJ (B − V )J σV σV −B σB−U σU−W σB−L −0.599 0.115 0.378 0.156 0.197 8.38 0.273 0.002 0.002 0.004 0.01 0.003 In Table 2 the average photometric parameters of the comparison star are listed. HD 158528 was found non- variable up to the instrumental errors. These parameters are representative for all observation nights. As can be Fig. 1. Light curve for part of the data set 1989. The vertical seen from the standard deviations, the variations in dif- scale is in units of log intensity, bright is up. On the right, ferential magnitudes are due to WR 86. an approximate magnitude scale is included. A 2σ error bar is The 1995 photometric data have been taken, using the shown on the left. ESO 50-cm telescope with, a single channel photometer. 1 × Sixteen data points have been recorded in about 2 2 hours, with a larger CCD detector (2048 2048 pixels). Exposure again relative to the comparison star HD 158528, using the timeswere20minforthefirstnight,and10minforthe V (ESO#99) filter. second night. The five spectra in the shorter wavelength Spectroscopy has been done by Greidanus in 1989, us- range show many absorption lines from the companion B- ing the Boller and Chivens spectrograph with the ESO type star, some of which were identified by Niemela et al. 1.5-m telescope. The spectra have been recorded on a (1998). 1024 × 640 pixel CCD and reduced with MIDAS (version Again, the stability of the wavelength calibration was nov99). Part of the reduction was done by van Dongen checked using atmospheric lines. All spectra taken in 1995 (1993). The exposure time was 2 min during the first night, are very stable in wavelength calibration up to 0.04 A.˚ and 30 s the second and third nights. The resolution was 3.65 A/pixel,˚ from λ = 4300 Ato˚ λ = 7300 A.˚ The stabil- 3. Light and color curves ity of the wavelength calibration has been checked using atmospheric lines, and it turned out that only the first The data set 1989 appeared to be scattered in time, and night could be used for radial velocity studies. The other the time series were short. This data set was therefore two nights showed significant scatter in the atmospheric not suitable for any period analysis. On the other hand, lines, and these have only been used for equivalent width for two nights out of eighteen, simultaneous spectroscopy (EW) determinations. was available, so we have used only these two nights in In 1995 again spectra have been taken during two combination with the spectroscopy. The differential light nights by one of us (P. M. V.), one night from λ = curve is shown in Fig. 1, and the data values can be found 3785 Ato˚ λ = 4785 A˚ (five spectra), and one night from in Table 3. We can see a rising branch on the left, and a λ = 4715 Ato˚ λ = 6735 A˚ (sixteen spectra), resulting in descending branch on the right. The amplitude amounts aresolutionof0.49 and 1.03 A/pixel,˚ respectively. The to ∼0.02 in log intensity scale. In Sect. 4 we will relate same telescope configuration was used as in 1989, but these two characteristics to the spectral behaviour. 1014 S. J. Paardekooper et al.: On the variability of the visual binary WR 86 Fig. 2. Light and color curves for the data set 1990-I. The vertical scale is in units of log intensity, bright and blue are up. Note the scale differences on the left.