9 HD 106225 = HU Vir

9.1 Brief history

HD 106225 was transformed from an obscure eighth magnitude star into a stellar source of significant notoriety after Bidelman (1981) noted that this late-type star has strong CaII H and K emission. Following its discovery as a chromospherically active star, Fekel et al. (1984) obtained spectroscopic and photometric observations of it. They determined a preliminary spectroscopic period of 10.389 days and preferred a photometric period of 10.6 days with a V amplitude of 0$.\!\!^{\rm m}$25. Recently, Strassmeier et al. (1997) found a long-term average photometric period of 10.418 days from 14 years of V-band photometry, which is closer to the orbital period.

Fekel et al. (1986) found that the H$\alpha$ line of HD 106225 was slightly in emission above the continuum, while Strassmeier et al. (1990) obtained a spectrum showing H$\epsilon$ emission in addition to strong CaII H and K emission. Such H$\alpha$ and H$\epsilon$ emission is seen in only the most active RS CVn binaries. Strassmeier (1994b) found very significant absorption-line variations and used numerous spectroscopic and photometric observations of HD 106225 to produce two 1991 Doppler images of its surface-temperature distribution. He also obtained improved orbital elements for the binary.

9.2 Orbital elements

From 1981 through 1998, 178 observations were obtained of HD 106225. Of that total, 66 were made by FCF at McDonald Observatory and KPNO and 112 by the Vienna group at NSO and KPNO. Some of those velocities have been previously published by Fekel et al. (1984) and Strassmeier (1994b). The values used in this paper are slightly different due to the assumption of different standard-star velocities.

Determination of the orbital elements is complicated by the star's strong chromospheric activity, resulting in significant line-profile variability, and the discovery that the system is triple.

Assuming the orbital elements of Strassmeier (1994b) as starting values, a set of orbital elements for the FCF velocities was determined with SB1. The velocity residuals of that solution showed systematic variations indicating the presence of a third component. A long period of about 6.3 years was identified from a period search done on the residuals. Preliminary long-period elements for the residuals were computed with BISP. Then a simultaneous solution of the short- and long-period orbits was obtained with GLS.

Although the velocities of the Vienna group are more numerous, the seven sets of spectra were obtained primarily for spot modelling, and so the velocities are not well distributed in phase in the long-period orbit. To determine possible velocity offsets, the velocities obtained by the Vienna group were included with zero weight in a GLS solution of the FCF velocities. A mean residual was computed for each of the Vienna group's seven sets of velocities. As a result, velocity offsets were applied to three of the data sets, -1.3 kms^-1 for the April 1991 NSO data, +1.0 kms^-1 for the March 1994 KPNO velocities, and +2.0 kms^-1 for the February 1995 KPNO velocities. No velocity shift was applied to the three most recent velocity sets. Velocity residuals for the April 1991 KPNO velocities ranged from +6 to -10 kms^-1, and so, velocities in that data set were not included in the final solution.

Elements for the short- and long-period orbits, computed from a GLS solution that included 154 velocities, are given in Table 5. The short-period eccentricity is quite small $0.0093\pm 0.0033$, but has been retained because in triple systems the third star may cause a non-zero eccentricity (Mazeh & Shaham 1979; Söderhjelm 1984) in the short-period orbit.

Although only one component shows both long- and short-period orbital motion, for the purposes of the tables and figures we identify the long-period orbit as component A and the submotion or short-period orbit as Aa. The short-period velocities and residuals to the computed fit are listed in Table A9 in the Appendix, while those for the long-period are in Table A10. For the tables and figures each measured velocity has been separated into a short- and long-period component, which consists of the short- or long-period calculated velocity plus one-half of the computed residual. Thus, the sum of those separated short- and long-period velocities for each date of observation results in the actually measured velocity. The computed short-period velocity curve compared with our velocities is shown in the upper panel of Fig. 7, while the long-period velocity curve is shown in the lower panel of Fig. 7. The standard error of an observation of unit weight is 1.3 kms^-1, primarily the result of the broad and asymmetric lines of this star. A time of conjunction in the short-period orbit with the primary behind the secondary is HJD $2\,450\,195.909$.

 

Figure 7: Radial-velocity curves of HD 106225 = HU Vir. The upper panel shows the short-period orbit of Aa, the lower panel, the long-period orbit of A. Open symbols are NSO McMath-Pierce data, filled symbols are from KPNO, pluses are from McDonald Observatory