Physics
Scientific paper
Jul 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995hst..prop.5869p&link_type=abstract
HST Proposal ID #5869
Physics
Hst Proposal Id #5869 Cool Stars
Scientific paper
All normal stars cool enough to have convection zones share a common level of chromospheric activity which decreases with advancing spectral type. Studies of indicators of low- temperature chromospheric activity in solar-type stars and K giants of near-solar metallicity, along with theoretical estimates of acoustic-wave heating, suggest strongly that this basal level of chromospheric activity is generated by acoustic waves; activity above this level is generated in conjunction with magnetic fields. Such a picture implies that even very old stars in this region of the HR diagram should show a minimum level of such activity. We propose to test this scenario with snapshot observations of Mg II in metal-poor solar-temperature field stars of the halo population, whose metallicities are 1/10 to 1/300 that of the Sun. No Ca II emission reversals have been detected in such stars to date. Because the magnesium abundance is more than a factor of ten higher than the calcium abundance, the Mg II lines may have large enough optical depth for the production of emission. More than a dozen targets are available which are bright enough for snapshot observations. This work may provide the first detections of chromospheric activity in such stars. If so, it will lend support to the idea of a basal level of chromospheric activity.
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