Fig
2:
Stellar atmospheres:
Colour-colour
diagram of stars in the CFHTLS W3 field (see below) compared to
the
synthetic colour-colour
diagram from Basel 3.1
stellar libraries(left panel) and NextGen (right panel) for different
metallicities. The red lines are for
[Fe/H]=0.0, the green lines [Fe/H]=-1.0 and the blue lines
[Fe/H]=-2.0. We use
only
stars with a photometric error smaller than 0.01 mag in each filter for
a more accurate stellar locus. Approximate
temperatures are also indicated.
Figure 2 shows g'-r' versus r'-i' diagrams observed with Megacam,
superimposed
with the synthetic colours of dwarf stars using the Basel3.1 stellar
library for solar metallicity,
[Fe/H]=-1.0 and [Fe/H]=-2.0 (left panel) and the NextGen library
for [Fe/H]=0.0 and [Fe/H]=-1.0 (right panel). We
used here only stars which have a photometric error smaller than
0.01mag in each filter. The diagram
illustrates the sensitivity of the colours in the MEGACAM
system to metallicity and also the differences in
the stellar libraries. Especially at temperatures below 3500 K which
corresponds to K/M
dwarfs,
the Basel3.1 library does not give realistic colours. The r'-i' colour
is sensitive to
effective temperature while g'-r' is only sensitive to it at
Teff
> 4000 K. Note the striking difference in the atmosphere
models in g'-r' at low temperatures.
For
late type dwarfs the most
sensitive
metallicity indicator seems to be the g'-r'colour .
For
objects with
g'-r' > 1.0 one can distinguish, assuming a very accurate
photometry, between stars with solar metallicity and [Fe/H]=-1.0. The
comparison between both libraries and MEGACAM data lead us
to adopt Basel3.1 colours as a base and complementary NextGen
colours for
Teff
< 4000 K.
The
Besançon model with the
MEGACAM filter system
In the case of the MEGACAM photometric system, we have used the
ccd+filter definition of the passbands, and applied these bandpasses to
the spectral libraries. The model is otherwise the same as the standard
version (Robin et al, 2003).
Comparison
between
CFHTLS data and the Besançon model
simulations
The CFHT Legacy Survey project is a large
observationnal project with the CFH Telescope equiped with the new
primary focus Megaprime and the Megacam instrument. It consists of
three different
surveys. The Deep is a
survey of 4 patches (D1,D2,D3,D4) of 1 square degree in the
filters u*,g',r',i' and z', complete to r' ~ 28 mag; the Wide
survey covers 3 patchs (W1,W2,W3) of ~ 7 deg2, each
observed
to r' ~ 25
mag, and the Very Wide is dedicated to the ecliptic and covers 1300
deg2
to r'~23. We refer for
a detailed description to the corresponding web pages (CFHTLS).
All the
observations are reduced by the TERAPIX
pipeline at the Institut
d'Astrophysique de Paris.
As a test for the implementation of the Megacam filter set, we here
present a simulation
from the Besançon model in the MEGACAM filters to be compared
with preliminary data of a subset of
4
deg2 of the W3 field located at l = 98.85 and b = 58.4.
The
cut-off limit
in i'
is 20.5 imposed by the star/galaxy
separation.