Stellar rotation in lower main-sequence stars measured from time variations in H and K emission-line fluxes. I - Initial results

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Abstract

Fluxes at 1 Å bands at the centers of the H and K lines in 46 lower main sequence field stars, and in eight selected subgiants and giant stars, have been measured at nightly intervals in the course of a nearly continuous 14-week observing run. In 19 stars we have found clear evidence of rotational modulation, from which values of the rotational periods can be assigned by inspection. In nine others, periods have been found by an autocorrelation analysis of the flux records. The periods obtained imply rotation velocities that are in good accord with spectroscopically determined values of V sin i in the literature for 13 of the stars we have observed. Much of the short term scatter in H-K flux observed by Wilson appears to be caused by rotational modulation, although variations on other time scales are also present. As many as 80% of the chromospherically active (i.e., young) stars display prominent rotational modulation, and in some cases the phase of the modulation remained unchanged for the entire observing period, suggesting that markedly asymmetric and long-lived distributions of active regions are common in such stars. At a given (B - V) < 1.0, the strength of H-K emission is shown to vary as a function of rate of rotation, suggesting that rotation, rather than initial conditions or age per se, is the chief parameter influencing chromospheric output. From data on stellar activity cycles available at present, it is suggested that periodic cycles resembling the Sun's are almost exclusively found in stars with rotation periods in excess of about 20 days; and, except for this threshold effect, the cycle periods are uncorrelated with rotation rate.

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