2 20 Eri

Adelman & Boyce (1995) used 78 FCAPT uvby observations to improve the ephemeris of Renson & Manfroid (1981) for the mCP star 20 Eri (HR 1100, HD 22470) whose u, v, b, and y magnitudes vary in phase and found

HJD (light minimum) = 2443485.50 + 1.92893 E.

Small differences in the light curves including their shapes and their amplitudes suggested that 20 Eri might be showing a second period perhaps due to precession. Hence in the 1997-98 and 1998-99 observing seasons 44 and 86 new FCAPT observations, respectively, were obtained (Table 2). Plotting the new observations using the previously adopted ephemeris shows a phase shift. As the light curves are shifted toward smaller phase values, the period should be increased to 1.928890 $\pm$ 0.00005 days while retaining the zero phase value. To bring the previous u, v, b, and y FCAPT values into agreement with those reported in this paper for 1998-99, -0.02, 0.05, 0.033, and 0.022 mag, respectively, need to be added. These differential shifts most likely result from differential extinction errors exposed by the relocation of the telescope in 1996. They are worse for 20 Eri than most other stars due to its Southerly declination. No shifts were seen between 1997-98 and 1998-99 values for u and v, but the 1997-98 b and y required 0.015 and 0.012 mag, respectively, to added for agreement. To make the values of Renson & Manfroid (1981) also agree 0.89, 1.18, 1.213, and 1.25 mag must be added to the published u, v, b, and y values, respectively. Both sets of observations used HR 1128 as a comparison star, but they used $\tau$⁵ Eri instead of $\tau$⁶ Eri as the other constant star.

If a mCP star shows a second period due to precession of the rotational axis, then its various spots and background regions will transit across the visible surface with periodically varying distances from the center of the visible disk. This will result in changes in the amplitudes of all maxima and minima as well as in the phases of the rising and falling branches of the light curves. When the precessional period is known, one can remove the effects of the precessional period on the rotational period. Until then, attempts to find the best period by matching the primary maximum for 20 Eri, may emphasize the differences at other parts of the rotational cycle. The rotational period can be found when light curves from several consecutive years are the same as other light curves from a similar set of consecutive years. This has not happened yet for 20 Eri and may be the result of not obtaining a sufficient number of significantly well determined light curves in different observing seasons.

Figure 1: Differential FCAPT uvby photometry of 20 Eri plotted with the ephemeris HJD (light minimum) = 2443485.50 + 1.92889 E. The 1998-99 FCAPT values are shown as open squares, the 1997-98 FCAPT values as solid circles, Renson & Manfroid (1981)'s values as + signs, and the FCAPT 1993-94 values from Adelman & Boyce (1995) as solid triangles

Figure 1 shows the u, v, b, and y photometry of this paper for 1997-98 (as solid circles) and for 1998-99 (as open squares), of Renson & Manfroid (1981) (as + signs), and of Adelman & Boyce (1995) for 1993-94 (as open upward pointing triangles). Figure 2 shows these u values along with those of Adelman & Boyce (1995) for 1990-91 (as solid squares), for 1991-92 (as solid downward pointing triangles), for 1992-93 (as x's), and for 1994-95 (as open circles). These additional values were not included in Fig. 1 both as these data sets were not as complete and to simplify the graphs.

In Fig. 1, the u values for 1998-99 tend to be lower and scatter more at primary maximum than the other data. The 1997-98 values tend to show a less deep primary minimum than the Renson & Manfroid (1981) and Adelman & Boyce (1995) 1993-94 values than do the 1998-99 values. In Fig. 2, the 1990-91 and 1991-92 values show an upward displacement of order 0.01 magnitude near phase 0.7 while the 1992-93 and 1994-95 values are shifted downward here. These all indicate that the u light curve is changing and is consistent with what is expected for precession. The v values for 1997-98 and for 1998-99 differ mostly at primary minimum. The differences between sets of values is the least for b. For y it is the Renson & Manfroid (1981) values which show a deeper minimum that those of 1998-99 which in turn a deeper minimum than do the 1997-98 values.

Figure 2: Differential FCAPT u photometry of 20 Eri using the ephemeris HJD (light minimum) = 2443485.50 + 1.92889 E. The symbols are the same as those of Fig. 1 with the addition of FCAPT values from Adelman & Boyce (1995): solid squares are 1990-91 values, solid triangles with downward pointing apexes are 1991-92 values, x's are 1992-93 values, and open circles are 1994-95 values

These results indicating subtle, yet observable changes in the shapes of the light curve mean further observations of 20 Eri are warranted. It is desirable to obtain light curves with of order 50 to 100 values to well define them. If the pattern of light curve changes begins to repeat, then the period is determined.