4 The magnetic CP stars

The list of magnetic CP (MCP) stars comes from the period compilation of Catalano & Renson (1997 and earlier references therein) supplemented by other stars of interest. Figure 5 shows the standard error vs. mean magnitude plot with the left panel showing the full distribution and the right panel showing the plot with the same ranges as Figs. 1 and 3. The MCP stars are simply more variable than the non-magnetic CP stars. But their standard error vs. amplitude plot (Fig. 6) is the same in the part in common.

  

Figure 5: The standard error vs. mean Hipparcos magnitude diagram for the MCP stars. The left panel shows the full range of values while the right panel is sized comparable to Figs. 1 and 3. Comparison of these diagrams shows that one can construct a common lower bound as a function of magnitude for all three kinds of star

  

Figure 6: The amplitude vs. standard error diagram for MCP stars. The left panel shows the full range of values while the right panel is sized comparable to Figs. 2 and 4. Comparison of these diagrams shows that the same correlation probably applies to HgMn, Am, and MCP stars

To be able to detect the variability of such stars by differential photometry, one needs to use comparison and check stars which are more constant. Adelman (1998) showed that finding appropriate stars with amplitudes of 0.02 mag and standard errors of 0.0008 mag or less was usually possible. Thus most MCP stars should be found to be variable. Some apparently constant MCP stars will be found to be variable when observed over a time period greater than the approximately three years of Hipparcos observations. The Hipparcos data confirm that some MCP stars such as HD 204411 (= HR 8216) have been essentially constant stars during the working lifetime of the Hipparcos satellite.

Table 3 should be very useful for observers of MCP stars who want to be sure ahead of time that their observing programs will yield good results. Many of the brighter stars with amplitudes greater than 0.02 mag already have good periods, but this is not generally true of fainter stars with such variability. At a given apparent magnitude it is somewhat easier to define the shapes of the light curves as the amplitude of variability increases. Stars with large amplitude variability probably have relatively large magnetic fields. If their periods are sufficiently short (of order a day), they should be candidates to look for precessional effects (Shore & Adelman 1976).

 

Table 4: Noise model lower bounds to the standard error