4. Magnetic Cycles in Late-type Stars: Maunder minima?
Starting in the 1960's at the Mt. Wilson Observatory O.C. Wilson (sic) began a long-term study of magnetic cycles in cool stars using as his observational indicator the variable emission flux of the H and K resonance lines of ionized calcium whose appearance in emission is characteristic of stellar chromospheres. There is an excess of H and K emission in the faculae which surround sunspots, and epochs of sunspot maximum coincide with epochs of maximum H and K. If the Sun were a distant star, we could observe its 11-year cycle as a variation in integrated H+K flux with this period and with a peak-to-peak amplitude some 30% of the mean. Wilson set out to see whether stars of similar spectral type and luminosity class (i.e. similar surface temperature and mass) show comparable variations. This entailed a major project, the monitoring of the fluxes of a group of stars during decades. Wilson's project was taken up by his students and successors, notably by Vaughan and by Baliunas, with the result that there are now well sampled records of H+K intensity for over a hundred stars covering 30 years, plus samples of many hundreds more, including coeval starclusters, covering the past 15 years. A full summary of the project and its results is given in Baliunas et al. (1998). One of the manifold implications of these results is the possible detection of Maunder minima in Sun-like stars.
In very broad terms, for two similar stars the one which rotates faster will have a stronger magnetic field, and also stronger H+K emission. Again, in general terms, the older a star the slower it rotates. Thus H+K emission is an indicator of age, and has been calibrated against open stellar clusters, whose ages can be determined via collective photometry of their complete populations. One obvious problem here is that if a star shows H+K variability, with amplitude similar to that of the Sun, say, a single measurement which catches the star at a maximum or minimum will give a misleading age estimate; what is required is a mean over a stellar cycle, or over a long enough period to cover short-term variations. In a cluster these effects can be cancelled even at a single epoch by averaging over its population, but for a single star this is not possible. The situation is worsened if stars have Maunder minima, because a measurement of H+K during a Maunder minimum would give the false impression of very low activity, and very great age. Stars with low activity have indeed been found. A significant fraction, maybe 20% of the isolated solar-type stars of the Mt. Wilson survey have low, constant H+K levels. These might just be very old stars, but a similar situation is found in the stars of the open cluster M 67, which is just a little younger than the Sun.
This strongly suggests that solar-type stars do go through phases of low magnetic activity, and that these Maunder minima last some 20% of the time. Many doubts remain; only one star has possibly been ``caught'' in transition from low to ``normal'' activity, from a sample for which more than 5 such transitions could be expected during their period of observation. Possibly, the stars with low activity are always like this and are simply slow rotators. Up to now no observable correlation of activity with rotational period has been possible (the periods must be of order weeks or months, which requires extreme spectral resolution to measure). Without going further into detail, the whole question is open and is a subject of active investigation. In particular direct measurements of indices of total stellar luminosity are being taken together with the H+K indices, to see whether, and to what degree, stellar (and thus solar) total power is correlated with magnetic field strength. The solar work gave the first clue, and the stellar work gives a framework to quantify, to predict, and eventually to model theoretically, the behaviour of the Sun.
Originally Posted by aurora
