In order to produce
synthetic photometry of stars in the MEGACAM filters, we have used 2
spectral libraries: Basel 3.1 and NextGen.
The Basel 3.1 library is a semi-empirical library, based on
previous Basel 2.2
). As we show below, the Basel library gives
good colour estimates at
> 4000 K but
does not give realistic colours for cool dwarfs. Hence we have used
complementary NextGen models from PHOENIX stellar atmosphere programs
(Hauschildt et al., in preparation and
< 4000 K. As they use a direct opacity sampling including over
500 million lines they give a more realistic description of the M dwarf
population. Conversely, the NextGen library seems to give systematic
from data colours in
-g' and for
substellar metallicities. Hence, we combine both libraries to get good
colour estimates at all temperatures and metallicities.
diagram of stars in the CFHTLS W3 field (see below) compared to
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
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,
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
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
> 4000 K. Note the striking difference in the atmosphere
models in g'-r' at low temperatures.
late type dwarfs the most
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
< 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
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
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
to r' ~ 25
mag, and the Very Wide is dedicated to the ecliptic and covers 1300
to r'~23. We refer for
a detailed description to the corresponding web pages (CFHTLS).
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
deg2 of the W3 field located at l = 98.85 and b = 58.4.
in i' is 20.5 imposed by the star/galaxy
- The colour-colour diagram: Figure 3 shows the
colour-colour diagrams of the CFHTLS (bottom panel) and the
model predictions (top panel) for the W3 field. We indicate the three
different galactic components by different colours : thin disc
(red), thick disc (green), spheroid (magenta).
Note the excellent overall agreement between observed and
predicted colours for the three populations.
Fig 3: g'-r' vs. r'-i' diagram. The bottom
panel shows the data of the W3 field. The top panel shows the
synthetic colours predicted by the Besançon model. The red
points are thin disc stars, the green points are for the thick disc and
magenta for the spheroid.
The blue part of the observed colour-colour
diagrams is populated by a population which has a
larger spread than the predicted stellar locus. We suspect these
objects to be compact faint galaxies misclassified as stars from the
morphological criterium used here.
Additional proper motion studies and/or spectroscopy would help to make
a cleaner separation between stars and galaxies.
- The colour-magnitude diagram: Figure 4 shows
colour-magnitude (CMD) diagrams of a subset of the W3 field. As for the
colour-colour diagram, the model predictions reproduce well the
observations.The overpopulation of blue objects in the observations is
due to compact
galaxies and quasars, as already seen in the colour-colour diagram. We
indicate the location of white dwarfs (open circles)
which are characterized by their blue r'-i'
4: Colour-magnitude diagram of a subset of data in the W3 field.
left panel shows the
W3 data and the right panel the synthetic colour-magnitude diagram
derived from the Besançon model. The symbols are the same as in
Fig.3. The open circles denote the location
of the white dwarf populations.
- The colour histogram: Figure 5 shows the
colour-distribution of the W3 field. Note the good agreement between
the numbers expected from the Galaxy model (dashed line) and the
observation (black solid line) for the three galactic components,
although some colour shift at high temperature and/or low metallicity
(blue side of the histogram) might be present.
Fig. 5: Histograms of the g'-r' colours.
The black solid line is the distribution in colours of the stars in a
field of 4 sqaure degrees, a subset of
the W3 field of CFHTLS. The dashed black line the colour distribution
predicted by the Besançon model. The red line is the thin disc
population, the green line the thick disc and in magenta
the spheroid population.
obs-besancon.fr, annie at obs-besancon.fr