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.
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.