THE TRANSIENT ACCRETION DISK IN THE ALGOL-TYPE BINARY U-SAGITTAE

U Sagittae (U Sge = HD 181182) belongs to the group of short-period Algol-type binaries which have orbital periods P < 5-6 days. One hundred and forty Ha spectra obtained at KPNO show evidence of weak emission components in the wings of the line profile which are rarely strong enough to extend above the continuum flux level. However, Balmer line emission with a strength of similar to 2% of the continuum flux was seen above the continuum at one phase near mid-primary eclipse; the first such detection in over 40 years. The observed emission components were enhanced by subtracting the photospheric Ha absorption profiles of the stars from the observed line profiles. The resulting Ha difference profiles of U Sge revealed the presence of double-peaked emission features at most phases outside of primary eclipse, and excess absorption at phases throughout primary eclipse. The strength of these absorption/emission features was typically within similar to 16% of the quadrature continuum flux. The double-peaked emission features indicate the presence of a transient accretion disk in U Sge. The radius of the trailing side of the disk was determined to be 1.68 +/- 0.10R(A) with electron density N-e greater than or equal to 8 x 10(10) cm(-3), and mass M less than or equal to 8 x 10(-12) M(circle dot) . These measurements were made during an epoch of enhanced mass transfer. The gas distribution in the disk was found to be highly variable in just a few orbital cycles.