Relying on infrared surface brightness fluctuactions to trace AGB properties in a sample of elliptical galaxies in the Virgo and Fornax clusters , we assess the puzzling origin of the “ UV-upturn ” phenomenon , recently traced down to the presence of a hot horizontal branch ( HB ) stellar component .
We find that the UV-upturn actually signals a profound change in the c-m diagram of stellar populations in elliptical galaxies , involving both the hot stellar component and red-giant evolution .
First , we encounter that the strengthening of the UV rising branch is always seen to correspond to a shortening in AGB deployment ; this trend can be readily interpreted as an age effect , perhaps mildly modulated by metal abundance .

A comparison between galaxy \overline { K } magnitudes and population synthesis models confirms that , all the way , brightest stars in ellipticals are genuine AGB members , reaching the thermal-pulsing phase , and with the AGB tip exceeding the RGB tip by some 0.5-1.5 mag .
The inferred core mass of these stars is found to be \lesssim 0.57 M _ { \odot } among giant ellipticals .
Coupled with the recognized severe deficiency of planetary nebulae in these galaxies , this result strongly calls for an even more critical blocking effect due to a lengthy transition time needed by the post-AGB stellar core to become a hard UV emitter and eventually “ fire up ” the nebula .

The combined study of galaxy ( 1550 - V ) _ { o } color and integrated H \beta index points , as an explanation for the UV-upturn phenomenon , to a composite HB with a bimodal temperature distribution , i.e .
with both a red clump and an extremely blue component , in a relative proportion of roughly [ N ( RHB ) : N ( BHB ) ] \sim [ 80:20 ] .
As far as metallicity of the BHB stellar population is concerned , we find that [ Fe/H ] values of either \simeq - 0.7 dex or \gtrsim + 0.5 may provide the optimum ranges to feed the needed low-mass stars ( M _ { * } \ll 0.58 M _ { \odot } ) , that at some stage begin to join the standard red-clump stars .