Chromospheric Activity in Old Stars: Halo Subdwarfs' Variability

Details Chromospheric Activity in Old Stars: Halo Subdwarfs' Variability Chromospheric Activity in Old Stars: Halo Subdwarfs' Variability

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7402p&link_type=abstract

HST Proposal ID #7402

Computer Science

Hst Proposal Id #7402 Cool Stars

Scientific paper

All normal stars cool enough to have convection zones share a common minimum level of chromospheric activity which decreases with advancing spectral type; most show higher emission levels which vary with time. The ubiquity of a minimum ``basal'' level suggests that this component of chromospheric activity is produced by some form of time- and metallicity-independent mechanism, such as acoustic waves arising from convection, while activity above this level most probably originates with strong magnetic fields generated by a rotationally-driven dynamo. Such a picture implies that even very old cool stars should show a minimum level of such activity, and that ones with rotationally-generated magnetic fields should show a higher level. Our Cycle 5 GHRS snapshot program has confirmed the first prediction: all nine of the old, low-metallicity, Population II field subdwarfs that were observed do indeed show chromospheric Mg II emission. In this STIS snapshot proposal we are requesting Mg II and Lyman-alpha observations for many of the same stars, to see whether the observed emission nonetheless arises from strong magnetic fields. One test is variability. Wherever possible, each target should be observed twice, to maximize the detection of variability and to minimize the difficulties inherent in intercomparing spectra from two different instruments.

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