4 V649 Cas

V649 Cas (HR 8854; HD 219634) has been known as a spectroscopic binary since the work of Young (1939). It was discovered as an eclipsing binary by Jerzykiewicz & Sterken (1982) and independently by Gulliver et al. (1982). Radial velocities were published by Gulliver et al. (1985). BV light curves were obtained and solved by Martin et al. (1990). A solution of the light curve was also published by van Hamme (1992), who suggested that quite a large amount of third light - 40% - should be present in this system. Also the resulting masses were too low for the given spectral type. Note that there are several cases where originally erroneous masses were deduced due to the presence of third body lines: e.g., LY Aur (Popper 1982), SZ Cam (Mayer et al. 1994; Lorenz et al. 1998; Harries et al. 1997) and V1182 Aql (Lorenz et al. 1997, and a paper in preparation). Measurements by Jerzykiewicz were published in graphical form (Jerzykiewicz 1993); only a few measurements were taken during primary eclipse, so that a light curve solution and determination of the period are hardly possible.

Figure 5: Spectrum of V649 Cas at phase 0.360 (4670 to 4880 Å) and 0.404 (4880 to 5036 Å); the exposures are separated by a vertical bar.

 

 

Table 4: Times of minima of V649 Cas.

HJD-2400000	m.e. (d)	Epoch	O-C (d)	Source
47099.3861		0	+0.0209	1
48500.6277		586	0.0000	2
52171.170	0.005	2121	0.0000	3
1 Martin et al. (1990).
2 HIPPARCOS.
3 Hvar, this paper.

There are some doubts concerning the period of this binary. Gulliver et al. (1985) give $2\fd391253 \pm 0\fd000002$. This value is based on a series of radial velocity measurements covering 3300 days, so its actual accuracy is about one order of magnitude worse. Using the BV data published by Martin et al. (1990), van Hamme (1992) found "a phase shift of $0\fp9988$''; we got a similar value. Choosing an epoch near the middle of the time interval covered by the Martin et al. measurements, the zero epoch time given in Table 4 can be calculated. According to the HIPPARCOS catalogue, another time of minimum is HJD 2448500.5980. With the van Hamme ephemeris, such a value gives a rather large $\rm {O{-}C} = -0\fd0433$. However, if the Kwee-van Woerden method (Kwee & van Woerden 1956) is applied to the HIPPARCOS photometric measurements, a somewhat different time results (Table 4).

On request by the present authors the star was observed at Hvar Observatory during the second half of 2001. From several nights a normal minimum was calculated. For the phasing of our spectroscopic measurements, the second and third minimum times as given in Table 4 were used to yield an ephemeris valid during the time of our spectroscopic observations:

$${\rm Prim.~Min.} = {\rm HJD}~~2448500.6277 + 2\fd391233 \cdot E.$$

Due to line blending effects the time of conjunction calculated from old radial velocities appears unreliable. The rather large O-C in the first line of Table 4 indicates that the period of V649 Cas is probably variable.

Figure 6: Profiles of the He  I 4922 line in V649 Cas at phases 0.244 and 0.754 (shifted in flux by +0.15). The Gaussian fits for the three components are shown. Points represent the observed spectrum, dashed line the third component, thick line the resulting profile.

Figure 7: Profiles of the H$\alpha$ line in V649 Cas at phase 0.308 a), V382 Cyg at phase 0.285 ( b); shifted in flux by +0.10), and V431 Pup at phase 0.827 ( c); shifted in flux by +0.40).

An example of a V649 Cas spectrum is presented in Fig. 5. Profiles of the line He  I 4922 in spectra taken near quadratures (see Fig. 6) actually show the expected asymmetry of the primary line, which is conceivably caused by the third component. The lines were fitted with Gaussians. The third line position had to be kept fixed, and a position at $\lambda = 4921.1$Å appeared as an acceptable compromise between positions suggested by spectra taken at opposite quadratures. The FWHM of the line was assumed as 2.4 Å and its depth as 0.08 of the continuum. Note that a similar process had to be applied also in the case of V1182 Aql (Lorenz et al. 1997, and a paper in preparation). The results are given in Table 2. The third light contribution appears not so large as expected by van Hamme, nevertheless it is important. The velocities given by Gulliver et al. were of course strongly affected by the third light, namely K₁ was found too small. And it is apparent from Fig. 1 of the Gulliver et al. (1985) paper, that K₂ could be determined only with very low accuracy, since the peak of the cross-correlation function was hardly visible; so our K₂ differs, too. The newly determined masses are in better agreement with binaries of similar spectral type. The He  II 4686 line is quite strong (its equivalent width equals $\approx$0.25 Å), so the spectral type of the primary component cannot be later than B 0. Though the masses were revised, there is still a discrepancy with too low masses. This problem is known for other binaries as well. In case of V649 Cas it is however necessary to wait for a more reliable spectrum disentangling to confirm this deviation from theory.

The profile of the H$\alpha$ line is shown in Fig. 7.

Figure 8: The light curve of V649 Cas by Martin et al. (1990) in V (crosses) and B (open circles; see the differential magnitude scale at right) and by HIPPARCOS (filled circles, magnitude scale at left).

The HIPPARCOS satellite measured the brightness of the star on 120 occasions, and the corresponding light curve is presented in Fig. 8 together with measurements by Martin et al. (1990). The scatter in the HIPPARCOS data is considerable, so that a more precise solution of the light curve cannot be expected. For the purpose of obtaining masses of the components, we assumed an inclination of $69\fdg3$ as given by van Hamme for the solution including third light (however note that this solution was obtained for an erroneous mass ratio q = 0.351).

Figure 9: Radial velocity curve of V649 Cas; filled circles - He  I 4922; open circles - H$\alpha$, cross - He  II 4686; the dashed line represents the solution with $V_1\gamma = V_2\gamma$.

Results of our line fitting are given in Table 1. When the primary and secondary velocities are solved independently, the systemic velocities differ. Giving the secondary data half weight, the mean systemic velocity is -11.3 km s^-1, and the respective solution keeping $V\gamma$ fixed at this value differs only slightly from the individual solutions with different $V\gamma$ values for both components. A better coverage of the radial velocity curve is needed to disentangle the three spectra more reliably. The curve is shown in Fig. 9. It is encouraging that the H$\alpha$ and He  II 4686 line measurements lie close to the curve (which is mainly defined by the He  I 4922 line). We assume that the He  II 4686 line is produced only by the primary component, since the secondary as well as the tertiary components should have lower temperature. The velocity of the third line is -35 km s^-1, a value clearly different from the systemic velocity. If the system is considered as gravitationally bound, then this difference should change with time. Since the difference is large, the change should be observable within a few years. It would be certainly very interesting to confirm and monitor such radial velocity changes caused by a suspected third body.