Chromospheric emission luminosity versus rotation in late main sequence stars

From an extended sample of single main sequence stars later than F7, for which well determined rotation periods and Ca II H + K or Mg II h + k emission luminosities are available, the chromospheric emission rotation period correlation is reexamined. The linear relation between the logarithm of the emission luminosity and the rotation period found by Catalano and Marilli (1983), log L(HK) = -aP + b, is confirmed, but both the coefficients in the relation are found to be color-dependent. The slope, a, decreases from early to late spectral types and remains almost constant for spectral types later than G8, which implies a weak response of the chromospheric emission to the rotation rate in this spectral interval. The b term that represents the H - K limiting luminosity for a very rapidly rotating star (very active star), decreases steadily with increasing B - V. The influence of photospheric parameters and the ability of the chromosphere to respond to the magnetic activity in very late type stars is discussed.