14 HD 216489 = HR 8703 = IM Peg

14.1 Brief history

Harper (1920, 1935) found HR 8703 to be a single-lined spectroscopic binary with an orbital period of 24.649 days. From 1990 to 1992 Oláh et al. (1998) obtained 19 new radial velocities at NSO and KPNO and computed a new orbit with an improved period of 24.64876 days. Herbst (1973) found HR 8703 to have strong CaII H and K emission lines and discovered the light variability of the star. From three seasons of photometry Eaton et al. (1983) found a mean photometric period of 24.39 days. Recently, Strassmeier et al. (1997) obtained a long-term average of 24.494 days from 18 years of V-band photometry.

14.2 Orbital elements

The Vienna group collected 44 velocities at NSO during a single 1996-97 observing run. Independent orbital solutions for those NSO velocities and for Harper's (1920, 1935) 20 DAO velocities were computed with the SB1 program. From a comparison of those solutions Harper's velocities were given weights of 0.2, and -3.0 kms^-1 was added to each velocity. The velocities of Oláh et al. (1998) from NSO and KPNO, corrected to the Scarfe et al. (1990) velocity system, are poorly distributed in orbital phase. Thus, those velocities were assigned zero weight and included in a combined solution of Harper's velocities and the Vienna group's NSO velocities. As a result, we have given the Oláh et al. (1998) NSO velocities unit weight and added -3.0 kms^-1 to those velocities. The three KPNO velocities of Oláh et al. (1998) were also given unit weight, but no velocity correction was applied.

We have assumed that the velocity offsets determined for some of the above data sets result from asymmetric line profiles, as is the case in the most active systems. However, as we have shown for LN Peg and HD 106225, systematic velocity residuals are sometimes due to the presence of a third star. Although 83 velocities of HR 8703 have been obtained, they are not well suited to search for evidence of a long-period orbit since the velocities are extensively clumped around only four epochs. Additional velocities obtained over several years will be needed to confirm our assumption.

An all-data solution with six velocities given zero weight because of their large residuals was computed with SB1. The eccentricity of that solution was quite small, $0.013\pm 0.008$, and so a circular-orbit solution with SB1C was obtained for comparison. According to the precepts of Lucy & Sweeney (1971), the orbit is at the borderline of the 5% level of significance. We have adopted the circular-orbit solution, whose elements appear in Table 5. Since the eccentricity is zero, the time of periastron passage has been replaced by the time of maximum velocity. Our radial velocities and velocity residuals to the computed orbit are given in Table A15 of the Appendix. Also listed are the velocities of Harper (1920, 1935) and Oláh et al. (1998). Figure 12 compares the computed orbit with the observed velocities, where zero phase is the time of maximum positive velocity. The standard error of an observation of unit weight is 0.9 kms^-1, typical of our results for a very active star.

 

Figure 12: Radial-velocity curve of HR 8703 = IM Peg. Filled circles are from NSO, open circles from Oláh et al. (1998), and pluses from Harper (1920, 1935)

Acknowledgements

We are grateful to the Austrian Science Foundation (FWF) for support through grant FWF-S7302-AST. Special thanks go to Trudy Tilleman at NSO for taking some of the NSO spectra. We thank David Barlow, who generously provided us with copies of his circular-orbit programs. This research has been supported in part by NASA grants NCC5-228 as well as NSF grant HRD-9706268 to Tennessee State University.