Magnetic structure in cool stars. X. The CA II H+K flux density and rotation rate for main-sequence stars.

A sample of 55 F3 to M4 main-sequence stars with well determined rotation periods and Ca II H + K line-core flux densities is analysed. Single stars and members of synchronized binaries follow the same tight, color-dependent, linear relations between the Ca II H + K line-core flux density F_{Ca II} and the logarithm of the rotation period. Emission-line stars follow the same relations. From early K-type stars onwards the response of F_{Ca II} to rotation rate decreases rapidly towards later spectral type. Hence, the sample refutes the suggestion by Catalano and Marilli that the relation between the Ca II line-core luminosity and rotation period is independent of color. The larger sample of stars for which v sin i is known, confirms the results based on rotation periods; in addition, for stars with rotation periods less than about one day F_{Ca II} drops below the value predicted by the relations defined by other main-sequence stars. Somewhat evolved stars follow approximately the same relation as unevolved stars, provided that the Ca II flux density is compared with the rotation period and not with the rotation velocity. The Ca II H + K line-core flux density depends on rotation rate for stars with B-V down to 0.38, which suggests that the magnetic activity due to dynamo action extends down to spectral type F3.