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recent observations on the origin of the uv upturn phenomenon of elliptical galaxies showed that hot hb and post - hb stars are the dominant uv sources in these systems ( ferguson et al . 1991 ; oconnell et al . 1992 ; king et al . 1992 ; bertola et al . 1995 ; brown et al 1997 ) . as demonstrated in our recent investigation ( yi et al . 1999 ) , it is still not clear , however , whether the dominant uv sources are metal poor (  metal - poor hb model " ; lee 1994 ; park & lee 1997 ) or metal rich (  metal - rich hb model " ; bressan et al . 1994 ; dorman et al 1995 ; yi et al 1998 ) . the  metal - poor hb model " suggests that the dominant uv sources are very old , hot metal - poor hb stars and their post - hb progeny , although metal - rich pagb stars also contribute some uv flux . in this picture , nearby giant ellipticals are about 3 gyr older than the milky way galaxy , which would imply non - zero cosmological constant from time scale test . on the other hand , the  metal - rich hb model " suggests that the dominant uv sources are super metal - rich hot hb stars that experienced enhanced mass - loss and helium enrichment . if so , nearby giant ellipticals are similar in age to the milky way galaxy , and there is no conflict with zero @xmath2 cosmology from time scale test . clearly , it is of considerable importance to understand the origin of the uv upturn . considering this situation , we have recently investigated the evolution of uv flux with look - back time to see if two models predict substantially different evolutionary patterns . we found that this would indeed provide some strong observational test on the origin of the uv upturn phenomenon ( yi et al . the major uncertainty in these model calculations is the mass of pagb stars , since it is only poorly constrained from direct observations . in this paper , we report our progress in investigating the effect of pagb mass in the modeling of uv evolution of elliptical galaxies . our model calculations show that hot hb and post - hb stars contribute most of the uv light in nearby giant ellipticals ( fig . 1 ) . as lookback time increses , however , pagb stars are getting more dominat uv sources since the mean masses of helium burning stars are too high to be hot enough hb stars at younger ages . ( at very large look - back times , hot main sequence stars eventually become the major uv source . ) since the contribution from pagb stars overwhelms the uv spectrum at higher redshifts , the model sed is strongly affected by the current uncertainty of the pagb mass as look - back time increases . we present , in figure 2 , the 1500 - v color evolution of giant ellipticals predicted by both the  metal - poor " and  metal - rich " hb models under two assumptions regarding the pagb mass . it is clear from figure 2 that the 1500 - v is not strongly affected by the uncertainty of the pagb mass for the nearby giant ellipticals since the pagb contribution to the total uv flux is less than  10% . pagb treatment becomes significant , however , for the ellipticals at @xmath3 . fortunately , both  metal - poor " and  metal - rich " models predict more or less the same 1500 - v colors at @xmath0 , and thus far - uv observations at these redshifts ( cf . brown et al . 1998 ) would provide crucial constraint on the pagb mass . our models in figure 2 suggest that the observations at lower redshifts ( @xmath1 ) would still provide strong constraints on two models on the origin of the uv upturn . future observations of ellipticals from the _ stis / hst _ and planned _ galex _ ( martin et al 1998 ) space uv facility will provide crucial database required for more concrete calibration of our uv dating techniques for old stellar systems . bertola , f. , et al . 1995 , , 438 , 680 bressan , a. , chiosi , c. , & fagotto , f. 1994 , , 94 , 63 brown , t. m. , ferguson , h. c. , davidsen , a. f. , & dorman , b. 1997 , , 482 , 685 brown , t. m. , ferguson , h. c. , deharveng , j .- m . , & jedrzejewski , r. 1998 , , 508 , l139 ferguson , h. c. , et al .  1991 , , 382 , l69 king , i. r. , et al . 1992 , , 397 , l35 lee , y .- w . 1994 , , 430 , l113 martin , c. , et al . 1998 , , 29 , 1309 oconnell , r. w. , et al .  1992 . , 395 , l45 park , j .- h . , & lee , y .- w . 1997 , , 476 , 28 yi , s. , demarque , p. , & oemler , a. 1998 , , 492 , 480 yi , s. , lee , y .- w . , woo , j .- h . , park , j .- h . , demarque , p. , & oemler , a. 1999 , , 513 , 128
the uv upturn phenomenon observed in elliptical galaxies is attractive for its potential value as an age indicator of old stellar systems . 
 we present our most recent population models for the uv evolution of elliptical galaxies . 
 we confirm that the dominant uv sources are either metal - poor or metal - rich hot horizontal - branch ( hb ) stars in local giant ellipticals , but we also note that the contribution from post - asymptotic - giant - branch ( pagb ) stars overwhelms the uv spectrum at higher redshifts ( look - back times ) . 
 the model uv spectral energy distribution ( sed ) is therefore strongly affected by the current uncertainty of the mean mass of pagb stars at higher redshifts . 
 fortunately , our models suggest that the far - uv observations at @xmath0 could produce strong constraint on the pagb mass , while observations at lower redshifts ( @xmath1 ) would still provide constraints on the models on the origin of the uv upturn . 
 future observations of ellipticals from the _ stis / hst _ and planned _ galex _ space uv facility will provide crucial database required for more concrete calibration of our uv dating techniques for old stellar systems .