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Session 56 - Frontiers of Ultraviolet Astrophysics.
Display session, Wednesday, June 10
We report ORFEUS observations of the contact binary 44i Boo (HD 133640) at two orbital phases (0.91 and 0.40). The contact binaries provide the exception to the rule of increasing magnetic activity with rotation rate among cool stars, as the radiation reaches a constant maximum level for the fastest rotators. The ratio of emission lines C III \lambda977; \lambda1176 indicates relatively high electron density, N_e = 2 \times 10^10 cm^-3 at T_e \sim 6 \times 10^4 K, with a pressure five orders of magnitude less than the coronal pressure at T_e = 6 \times 10^6 K determined from EUVE (Brickhouse amp; Dupree 1998, Ap J, 502, in press). The shape of the emission measure distribution for the system from T_e \sim 3 \times 10^4 to the limit temperature T_e \sim 2 \times 10^7 K is determined from IUE, ORFEUS, EUVE, and ASCA, with ORFEUS providing the critical O VI \lambda1032 (T_e \sim 3 \times 10^5 K), linking the upper transition region to the coolest observed EUV Fe ion (Fe IX at T_e \sim 6 \times 10^5 K). The minimum occurs near 6 \times 10^5 K, reminiscent of that of Capella.
Emission lines shifted by the relative orbital velocity (\sim 200 km/s) between the two stars would be separable at ORFEUS resolution at both phases; however, the line profiles of Si III \lambda1206, C III \lambda977, and O VI \lambda1032, sampling a decade of temperatures (from 3 \times 10^4 to 3 \times 10^5 K), are not consistent with such a simple model. Moreover, these three line profiles show significant differences from each other. Since IUE light curves of lower transition region lines (C II, C IV; see Vilhu, Neff, amp; Rahunen 1989, Aamp; A, 208, 201) are consistent with the partial eclipses observed in the photosphere, these new line profile measurements present an intriguing clue to the puzzle of coronal heating and confinement.
Program listing for Wednesday