A&A Supplement series, Vol 122, February 1997, 301-319
Received February 12; accepted June 7, 1996
C. Chiosi
, A. Vallenari
, and A. Bressan
Send offprint request: C. Chiosi
Department of Astronomy, University of Padua,
Vicolo Osservatorio 5, 35122 Padua, Italy
Astronomical Observatory, Vicolo Osservatorio 5,
35122, Padua, Italy
We present grids of magnitudes and colours for the WFPC2 of the refurbished HST using pass-bands from the far UV to the near infrared, i.e. the filters F170W, F218W, F255W, F300W, F336W, F439W, F450W, F555W, F606W, F702W, F814W, and F850LP. In addition to this, we consider also the filters F150W+F130LP, F175W, F220W and F342W of the old HST in the FOC mode in which a great deal of observations have been taken.
The theoretical magnitudes and colours are
calculated convolving spectral energy distributions
with different metallicity (Z), effective temperature (
),
and gravity with
the pass-band transmission, response of the camera, and
telescope assembly. In order to enable an easy usage of the present results
we provide tabulations of Bolometric Corrections
as function of gravity, 
, and chemical composition
for each pass-band under consideration.
As already pointed out by Yi et al. (1995), the UV
filters suffer from
the visible/red-leak problem, which destroys the monotonic relation
between
colours and 
and causes serious difficulties to analyze
the data.
With the aid of the above technique we calculate isochrones in the
observational colour - magnitude diagram (CMD) and integrated
magnitudes and colours of single stellar populations (SSP).
The ages span the range from 18 Gyr to 0.01 Gyr. For the sake of brevity,
results are
described here in some detail limited to the solar
chemical composition [Y=0.28 Z=0.020].
In addition to this, we present synthetic CMDs for a composite sample of stars
containing both young and old objects as they would appear in various
pass-bands.
Finally, we address the question whether the variation of the UV
colours of
elliptical galaxies as a function of the red-shift shows signatures from which
one can infer the age and the type of source emitting the UV flux.
The goal is achieved
looking at the red-shift dependence of colours obtained from
the pass-bands F170W, F210W, and F555W at varying the Hubble constant
and deceleration parameter 
defining the model of the Universe.
It is found that those colours run
differently according to underlying source, i.e either P-AGB or P-AGB plus
H-HB and
AGB-manqué stars (see below for details) and the cosmological parameters.
If the UV flux is mainly generated by
P-AGB plus H-HB and AGB-manqué stars a marked discontinuity in suitable
UV colours is expected at the red-shift corresponding to the age of
about 5.6 Gyr.
keywords: techniques: photometry -- stars: HR diagram -- galaxies: ellipticals -- galaxies: evolution -- ultraviolet: galaxies