Computer Science
Scientific paper
Jul 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984iue..prop.1895s&link_type=abstract
IUE Proposal ID #LGGTS
Computer Science
Scientific paper
There have been extensive studies of chromospheric activity cycles and the rotational modulation of chromospheric emission lines of solar-type dwarf stars at visible wavelengths and with IUE. These investigations provide valuable information on the size, spatial distribution (a real filling factor), longevity, and surface brightness of stellar active regions. In turn, these expressions of magnetic activity, in stars of different mass and effective temperature, place constraints on dynamo models of stellar activity. There have been many fewer studies of cyclic variations in the chromospheric spectra of cool giant stars, which would show how a lower surface gravity alters the time-dependence and characteristic spatial scalelengths of stellar dynamos. We propose to study the changes in UV emission lines formed in the upper chromospheres and transition regions of two, rapidly-rotating, G0III yellow giants, 31 Comae and psi3 Piscium, over an interval of 12 days, which corresponds to two rotation cycles. We will search for evidence of rotational modulation of the line fluxes as chromospheric structures (plages, active regions) appear on one limb of the star, move across the visible disk, and then pass out of view over the opposite limb of the star. The detection of cyclic changes in the UV emission lines of these luminous stars will allow us to draw a comparison with the significant rotational effects previously observed in young, active dwarf stars. A further comparison will be made with the active, rapidlyrotating subgiant components of RS CVn-type binaries. To our knowledge, 31 Com and psi3 Psc are both single stars in their first crossing of the H-R diagram, which maintain their rapid rotation because neither star has yet experienced substantial magnetic braking by a coronal wind like the Sun's. The dominant topology of surface magnetic fields on these stars is therefore likely to be in the form of closed loops, which confine the hot plasma detected at UV wavelengths. From the modulation of their UV emission, it will be possible to determine whether these loops are distributed non-uniformly over the surface of the stars.
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