The 2003 Eclipse of 31 Cygni

The long-period (P = 10.36 yr) binary 31 Cygni (K4 Ib + B3 V) contains one of the brightest late-type supergiants in an eclipsing system. The hot companion, with a diameter about 3% of that of the supergiant, offers an unparalleled opportunity to probe the K star’s outer atmosphere and wind at high spatial resolution. Observations of the ultraviolet spectrum are especially advantageous because there the primary’s flux is negligible and the difficulties of disentangling composite spectra are avoided. Near eclipse, the sightline sweeps deep into the chromosphere of the supergiant and absorption features observed superimposed on the companion’s early-type continuum can be used to map the supergiant’s chromosphere in unprecedented detail. The most recent eclipse of 31 Cyg occurred in mid-2003, a fortuitous event given the length of the orbital period compared to the expected lifetime of the FUSE mission. Observations of the 2003 eclipse were made at several epochs by the Cycle 4 GI program D123 during ingress, totality and egress phases. We present a summary of the observations and a preliminary empirical model of the chromosphere of 31 Cygni derived from these FUSE observations.