3 Results

A rectified echelle spectrum of WR46 by Hamann et al. (1995b) is shown in Paper I (their Fig.14). We note that according to Smith et al. (1996) the N IV 3480 emission line is only faintly present, while a spectrum by Massey & Conti (1983) does show this line on the edge of their spectrum with a intensity of 0.2 in continuum units. Crowther et al. (1995, hereafter CSH) showed the spectrum to be similar to other weak-lined early-type WN stars, e.g. WR128 (WN4(h)) and WR152 (WN3(h)) (spectral types of Smith et al. 1996). The differences are that WR46 shows no sign of any hydrogen, the N V (resp. He II) lines are stronger (resp. weaker) and the wings are somewhat broader as a result of a faster wind. CSH modeled the observed triangular line profiles assuming a spherical, low-density WR stellar wind. We note that the spectrum of WR3 (HD9974, WN3) is very similar to that of WR46 (Marchenko priv. comm.), except for the O VI 3811/34 emission lines, which are absent in WR3, but very prominent in WR46. The discussion of this feature is deferred to Sect. 5.2.

To illustrate the variable behaviour of the spectral lines we present a gallery of grey-scale figures of various lines observed in different years. These figures show either the same line on different nights (Fig.1), different lines of different elements during a single night (Fig.2), the same element and ionization (Fig.3) during subsequent nights, or both (Fig.4). We discuss the characteristic behaviour of WR46 in the N V 4944 emission line. This line shows both an obvious radial-velocity curve (e.g., the second night in 1998 (Fig.4), or the fourth night in 1995 (top panel of Fig.3)), and, a stand-still with, or without, a change of flux (e.g., second night in 1995, Fig.4).

3.1 Continuum-corrected equivalent width ( ${EW}_{\rm cc}$)

We introduce a new variable, the so-called continuum-corrected equivalent width ${EW}_{\rm cc}$, which measures the line flux relative to the continuum of a specific spectrum, which was chosen to have truly simultaneous photometry available (V₀ with $\Delta t <3$ min) near the mean brightness. First, the different lines with adjacent continuum are extracted and rectified using a first- or second-order polynomial. Then, we measure the standard equivalent width relative to the continuum of the spectrum itself. Since the emission lines contribute only <10% to the broad-band photometry, the EW of all the lines can be transformed to ${EW}_{\rm cc}$, according to:

$$EW_{\rm cc}(t) = 10^{V(t) - V_0} * EW(t),$$ (1)

where V is in $\log I$. The photometry is simply linearly interpolated to the times of observation of the spectra V(t). The V-band is used, as it measures the continuum around He II 5411 accurately (see Paper I, Fig.14) and the color changes are only very small and do not affect the following analysis. In effect, ${EW}_{\rm cc}$ reflects purely the changes in the line flux. By this approach the problem of absolute calibration is avoided, while the variability is unraveled from the continuum variations. Moreover, since the specific spectrum is chosen to be near the mean value of an observing run, the values can still be compared to other observing runs as "standard'' EW.

 

 

Table 2: The ratio of the maximum over the minimum flux within different pass-bands during the days of simultaneous observations (corrected for the varying emission-line contribution) and the ratios of the maximum over minimum ${EW}_{\rm cc}$ (i.e., corrected for continuum) of the emission lines.

emission	1989	1990	1991	1995a	1998
W	1.03		1.047
U			1.047
$U_{\rm B/J}$
L	(1.0)		1.048
B	1.04		1.060
V	1.05		1.069
O VI 5290				1.22	1.37
O VI 3811/34			1.54	1.39
N V 4944	1.36	1.43	1.47	1.51	1.29
N V 4604/20	1.66	1.71	1.38	1.37
He II 4859	1.78	1.77	1.38	2.25	1.17a
He II 5412	1.33	1.53	1.26	1.20	1.27
He II 6560	1.26	1.45	1.17	1.23
He II 4686	1.25	1.33	1.16	1.21

^a Coverage during two nights only.

Figure 3: The variability of the N V lines in four nights in 1995 (time HJD-2449800 running upwards). The 1st (bottom) and 3rd panel show the N V 4520 emission line, and the 2nd and 4th (top) panel show the N V 4944 emission line. They illustrate the radial-velocity variability and the abrupt changes from night to night. Although these lines originate from different principal quantum levels (n = 9-7 and 7-6, respectively), they trace nearly the same atmospheric region according to Wolf-Rayet Standard model analyses by one of us (PAC).

Figures 5-9 present simultaneous measurements of the light curve (top row) and the ${EW}_{\rm cc}$of several lines (middle part) and the radial velocity (bottom part) separated by thick lines. It is obvious that the line fluxes vary on the same time scale as the light curves and that their behaviour is comparable, i.e., their maxima and minima coincide roughly. Such behaviour is already apparent from the EW values, but evidently enhanced when corrected for the continuum contribution. The temporal behaviour is analysed in Sect. 4.

Figure 4: Three nights (time measured upwards) of observing N V 4944 (left) and He II 5411 lines (right) in 1998 illustrate the variability from night-to-night and the differences in radial velocity behaviour between the N V and He II lines.

Figure 5: Top row: Walraven V-photometry versus HJD-2440000 obtained simultaneously with spectroscopy (lower panels); rows 2 to 5: equivalent width corrected for continuum changes (cc; see Sect.3.1) in Angström; rows 6 to 10: radial velocity in kms-1 measured as bisector at the specified height above continuum as percentage of the maximum flux (b50, b85, b60) during three nights in 1989 as indicated. The different emission lines (top down) are shown with increasing distance of its formation region from the star. Note that for some events the variability shows, among other changes, a clear time-delay increasing when going outwards.

Figure 6: As in Fig.5, but for the 1990 data.

Figure 7: As in Fig.5, but for the 1991 data.

Figure 8: As in Fig.5, but for the 1995 data. Note that the nights are not presented as a time sequence.

We determine the ratios of the maximum over the minimum flux-level of the continuum and the main emission lines for each observing run and the results are listed in Table2. Because of the low number of cycles (2-3) observed per season, the determinations can only serve as a relative measure, since all lines (and continuum) are observed simultaneously. The table is ordered from the inner to the outer layers of the atmosphere and shows as trend that the amplitude increases when going outwards, out to the formation region of either the spectral line He II 4859, or, in 1991, O VI 3811/34. Further out, the ratio of the line fluxes decreases, presumably, as the effect of the variable source wears out due to a larger distance. In Paper III, this change of amplitude dependent on the height in the atmosphere is interpreted as a change of the distortion of the (line- and continuum-) emission-forming layers. The measurements of these ratios are translated into a graphical representation in Fig.1 of Paper III.

3.2 Bi-sectorial radial velocity, K-amplitudes

We measure the bisector (i.e., the line center for each line) at three different intensities above the continuum, namely at 20, 50 and 85% of the maximum line intensity. A similar method was applied to WR6 by Robert et al.(1992). We check the stability of the wavelength calibration by measuring also the interstellar and atmospheric lines. A systematic offset was notable only a few times, which we subsequently corrected. The square root of the variance of these measurements (after correction) may be considered as error estimates for each data set: 30-50 kms^-1 in 1989-91 and 3-5 kms^-1 in 1995 and 1998.

Figure 9: As in Fig.5, but for the 1998 data. Time is measured as HJD-2450000.

Figure 10: Continuum corrected equivalent width ( ${EW}_{\rm cc}$; see Sect. 3.1) in Angström of N V 4604/20 (and He II 5411 for 1998, bottom row) folded with the 1989 and 1991 periods ($P_{\rm x}$, $P_{\rm sw}$, $P_{\rm dw}$). The preferred curves are presented in the thick-lined boxes. Note that the vertical scale is adjusted to the observed range of ${EW}_{\rm cc}$ values. The different symbols indicate the sequence of the nights: open symbols the first; grey symbols the second; solid symbols the third night.

Figure 11: Bisector at 50%-level of N V 4944 in kms-1 for all seasons folded with the various periods. From left to right we applied phasing with $P_{\rm x}$, $P^{89}_{\rm dw}$, $P^{91}_{\rm dw}$ (Paper I), P93/94 (Niemela et al. 1995), and P99 (Marchenko et al. 2000; hereafter referred to as MAB). For the different fill styles see Fig.10, and for 1995 dark-grey indicates the fourth night.

The determinations are presented in the lower half parts of the Figs.5-9. As already mentioned from the grey-scale graphs, the radial velocity behaves peculiarly. During one night the radial velocity shows a large-amplitude curve (e.g., 14 March 1989, 17 February 1991), while during another night it shows a stand-still (13 March 1989, 10 February 1998). Although the radial-velocity curve is not stable from night to night, showing a variable amplitude, it is clearly controlled by a time scale similar to the double-wave period of 0.2727/0.2825 d. We observe that in the case of a large-amplitude radial-velocity variation, the object is usually brightest, showing the largest line fluxes when the lines (formed in the inner wind region) are either at maximum velocity or minimum velocity. This relative timing can also be recognized in the 1999 data described by MAB. However, a counter-example may be the night of 1 March 1990 (Fig.6), where N V 4604/20 shows the line flux to be minimal during extreme radial velocity.

We conclude that the radial-velocity amplitude for most emission lines is in the range K = 50-100 kms^-1. We assume that the lines formed closer to the WR star give a better indication than the N V 4604/20 line complex, which results in an outstandingly high amplitude of 250-300 kms^-1. Such a large amplitude for this line-complex has also been observed by Niemela et al. (1995). Discussion of this phenomenon is deferred to Sect. 5.2.