X-Ray and Ultraviolet Spectroscopy of Intermediate-Mass, First Crossing Giants

Chandra, FUSE, and HST STIS jointly have obtained spectra of four intermediate-mass giants (≲3 M_☉) crossing the Hertzsprung gap for the first time, passing through a "rapid braking phase" analogous to, but much briefer than, the more gradual prolonged decay of magnetic activity experienced by single low-mass stars like the Sun. All four giants display hot, dense coronal plasmas (6-30 MK, ~10¹² cm^-3), with near solar abundances. The UV spectra show a remarkable congruence of shapes of the 0.06-0.2 MK lines of Si IV, C IV, and N V, which further can be decomposed into narrow and broad Doppler components. The profile isomorphism extends to the yellow dwarf α¹ Cen (G2 V), utilized as a solar surrogate. The broad component fraction of the total increases with L_X/L_bol and is suggestive of persistent "microflaring." Indeed, the most active of the targets, HR 9024 (G1 III), experienced a macroflare during the Chandra pointing, reaching a remarkable 100 MK. In prebraking phase 31 Com (G0 III) and solar proxy α¹ Cen emission levels in the 0.03-0.3 MK "transition zone" regime are very similar to those of the 1-10 MK corona, but in the three cooler giants, the hotter plasma is dominant. The high coronal densities of all four giants contrast to much lower values (~10¹⁰ cm^-3) at 0.15 MK from O IV line ratios, contrary to expectations for isobaric magnetic loops, but possibly analogous to the bimodal pressure behavior of certain solar impulsive flares.