ORFEUS Observations of the Contact Binary 44i Boo

Details ORFEUS Observations of the Contact Binary 44i Boo ORFEUS Observations of the Contact Binary 44i Boo

May 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998aas...192.5603b&link_type=abstract

American Astronomical Society, 192nd AAS Meeting, #56.03; Bulletin of the American Astronomical Society, Vol. 30, p.902

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Scientific paper

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 lambda 977; lambda 1176 indicates relatively high electron density, N_e = 2 x 10(10) cm(-3) at T_e ~ 6 x 10(4) K, with a pressure five orders of magnitude less than the coronal pressure at T_e = 6 x 10(6) K determined from EUVE (Brickhouse & Dupree 1998, Ap J, 502, in press). The shape of the emission measure distribution for the system from T_e ~ 3 x 10(4) to the limit temperature T_e ~ 2 x 10(7) K is determined from IUE, ORFEUS, EUVE, and ASCA, with ORFEUS providing the critical O VI lambda 1032 (T_e ~ 3 x 10(5) K), linking the upper transition region to the coolest observed EUV Fe ion (Fe IX at T_e ~ 6 x 10(5) K). The minimum occurs near 6 x 10(5) K, reminiscent of that of Capella. Emission lines shifted by the relative orbital velocity ( ~ 200 km/s) between the two stars would be separable at ORFEUS resolution at both phases; however, the line profiles of Si III lambda 1206, C III lambda 977, and O VI lambda 1032, sampling a decade of temperatures (from 3 x 10(4) to 3 x 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, & Rahunen 1989, A& 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.

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