Astron. Astrophys. 336, L17-L20 (1998)

2. Radial velocities

To detect orbital motion in a highly unstable photosphere like the one of AC Her is by no means obvious since at some pulsation phases strong line asymmetries and even line-splitting is observed (e.g. Gillet et al. 1990). Our sample of radial velocities was obtained with the CORAVEL radial velocity spectrometers installed at the Swiss telecope at the Haute Provence Observatory in France and at the Danish 1.5 meter telescope at La Silla, Chile (Baranne et al., 1979). Since the spectrum is correlated with a hardware mask spectrum of Arcturus always the same lines contribute to the radial velocity determination which makes this sample very homogeneous. In total 144 radial velocities were obtained over a total time-span of 3329 days but 4 measurements were rejected because the line-splitting was such that no single cross-correlation function could be obtained.

Gillet et al. (1990) describe in detail one pulsation cycle observed with CORAVEL. In Fig. 4a of their work one can see that the peak-to-peak amplitude is as high as 32 . Thanks to the fact that AC Her is one of the stablest RV Tauri stars known we could correct the total velocity set for the pulsation amplitude by using the pulsation period determination of 75.47 days by Zsoldos (1993). The zero-point of the pulsation phase was optimized for the reference cycle and put to JD+7359. The first orbital solution was used to correct the reference pulsation cycle for orbital motion.

Fig. 1. The radial velocity measurements of AC Her (asterisk) folded on the 1194 days period, together with the best fit as discussed in the text. The radial velocities were corrected for the pulsation amplitude obtained in between JD+7359.4 and 7434.3. Our total dataset covers 2.8 cycles. The julian date is given from JD2440000 onwards.

The final residuals were found to be periodic with a final period of 1194 6 days which we interpret as the orbital period. The radial velocity curve together with the best fit is shown in Fig 1, and the obtained orbital elements are listed in Table 1. The standard deviation on the total O-C set is 2.61 and the total measured peak-to-peak amplitude of the residuals is 33 , which is similar to the pulsation amplitude. In total 2.8 orbital cycles are sampled. According to the Lucy & Sweeney (1971) criterion, the hypothesis that the orbit is circular can be rejected at a significance level of 0.2%. Since the rest frame velocity of the reference pulsation cycle was -45.0 , the -velocity of the system is -33 . We confirm the binary nature of AC Her which was originally proposed by Sanford (1931, 1955) who suggested a period of 1240 days with a similar amplitude.

Table 1. The orbital elements of AC Her. The velocity is the system velocity after correction of the pulsation amplitude using one well-sampled pulsation cycle.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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