Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006spd....37.1201s&link_type=abstract
American Astronomical Society, SPD meeting #37, #12.01; Bulletin of the American Astronomical Society, Vol. 38, p.240
Astronomy and Astrophysics
Astronomy
3
Scientific paper
It has recently been suggested that there are very few dwarf stars in the equivalent of a solar Maunder-like magnetic minimum (MM), implying that the Sun is an unusual star for having such states. I investigate this issue by comparing Ca II HK fluxes and stellar properties(Teff, gravity, metallicity M/H) from two large analyses of exoplanet search spectra. The joint sample includes 731 stars, of which 570 are dwarfs (as defined by gravity).The apparent lack of MM dwarfs arises from two problems: the definition of what properties an MM star should have, and issues in the calibration of Ca II HK fluxes. There are indeed very few dwarf stars with normalized Ca II HK fluxes log R'HK < -5.1, the traditional MM level. However, this level was defined using a stellar sample which included subgiants. The new data show that the minimum R'HK in subgiants almost always less than that in dwarfs for all Teff and M/H. Thus, in a sense, MM dwarfs have been defined out of existence by requiring an R'HK level more appropriate for evolved stars. Furthermore, the minimum R'HK observed in dwarfs is a decreasing function of M/H, so that metal-poor minimum activity dwarfs have unusually high R'HK values (e.g., log R'HK -4.85 at log M/H -1.0, compared with log R'HK -4.95 at solar minimum). If MM dwarfs can be metal poor, these stars may also be candidates. The true scarcity of MM dwarfs, and by extension, the uniqueness of the solar case, therefore remains an open question.This work is supported by NASA Origins grant NNG04GL54G.
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