Received 14th March 1998
Abstract.
The
Deep Near Infrared Southern Sky
Survey (DeNIS) instrument (Epchtein et al. 1997) is surveying the
sourthern hemisphere using the
ESO 1 m
telescope in the
I,
J and
K photometric bands and
with limiting magnitudes of 18 mag in
I (0.8 µm),
16 mag in
J (1.25 µm) and 14 mag in
Ks (2.15 µm).
The survey is made strip by strip where each strip covers a region of
30° × 12'.
Large parts of both Magellanic Clouds have already been covered.
We expect to find of the order of 10
7 point sources.
A first inspection shows already considerable differences in AGB population
between the Large and the Small Cloud and with the bulge of our Milky Way.
From a comparison of our color-color diagram and the one in the Galaxy
(Schultheis et al. 1997) we conclude that there is a greater population
of M-type stars in the Galaxy while in the Magellanic Clouds C-type stars
seem to dominate.
We believe that with all the reduced strips, that will completely cover
the Clouds (about 90 for the LMC and 50 for the SMC), this behaviour will
strongly emerge.
Each object as part of a final catalogue will constitute the more complete
catalogue of point sources in the Magellanic Clouds at these frequencies.
1. Observations
La Silla Observatory (Chile) is one of
the most beautiful places in the world to observe the Magellanic Clouds.
The DeNIS observations started
in December 1995 and have been carried out every season following the same
survey mode.
Unfortunately due to bad weather conditions and some technical problems full
coverage has not yet been reached.
The Southern Sky is divided in three zones, upper, lower and intermediate,
each covering 30° of declination from δ = +2 to δ = -98.
Each zone is divided in strips of 12' wide and each strip is composed of
180 images of 12' × 12'.
Between strips and between images there is an overlap of 2'.
Three independent cameras constitute the DeNIS focal instrument that is
attached to the 1 m ESO telescope.
The cameras simultaneously observe each image for an integration time of
approximately 9 s.
The pixel size is 1" in I and 3" in J and
Ks.
We focus our interest on some strips to verify the image quality and the
characterization of the extracted sources.
2. Data reduction
Data reduction takes place at two different centres.
At the Paris Data Analysis Centre (PDAC): cleaning, flat-field, sky
substraction and PSF calculation is carried out.
At the Leiden Data Analysis Centre (LDAC) we perform point source extraction
and calibration as it concerns both astrometry and photometry.
The reduction is done strip by strip.
Overlapping regions are essential for a reliable identification.
Images, if they are sharply focused and have no interferences, present no
problems in the central part.
For example the variation of the PSF is larger close to the image border.
Therefore overlaps between frames and between strips allow for a double,
or even higher, coverage of the four image borders.
The SExtractor program (Bertin & Arnout 1996) extracts point sources
on single images.
During the extraction in case, for example, of bad or saturated pixels,
the object is properly flagged.
The resulting list of objects is organized in catalogues, one per color.
Every subsequent pipeline step adds information to these catalogues.
The astrometric solution gives an error of about 0.1 arcsec to each
object, except for complicated extractions.
The pipeline performs also the search for ghosts, glitches and saturated
objects not previously identified.
According to selection criteria the final catalogue is built.
The resulting photometric error is less then 0.05 mag for bright stars.
We limit our discussion to 0.1 mag.
3. CH stars in LMC and SMC
So far 20 strips mainly in the LMC have been reduced.
From color-color diagrams and color-magnitude diagrams we can clearly recognize
four "sequences":
Main Sequence stars, Red Giant stars, M-type and C-type AGB stars
(Loup et al. 1997) and many LPVs.
The strip mode observation is perhaps not the best technic to study objects
distribution in the Magellanic Clouds, yet we see a clear difference in the
number of objects between the center and the external part of the Clouds.
We extracted a list of CH stars from Hartwick & Cowley (1988) and
Cowley & Hartwick (1991).
We cross identified them using coordinates and finding charts where available.
Results are in very good agreement with previous determinations but our
accuracy is better in both astrometry and photometry.
The known CH stars in the LMC are quite bright compared to the limiting
magnitudes of our survey.
The interesting position they occupy in the color-color diagram (I-J
around 1 mag and J-Ks ≥ 1.2 mag) suggests us
that there may be a large number of CH stars in our data.
They should be binary stars: an evolved giant that has obtained a high carbon
abundance due to mass exchange with its companion (Feast & Whitelock 1992).
The corresponding position of this additional branch in the color-magnitude
diagram (Ks versus J-Ks) tells us that
they are fainter than previously known CH stars (Mbol around
-2.8 mag).
We obtained the bolometric magnitude assuming a distance modulus of 18.55,
neglecting the correction for reddening and applying the bolometric correction
according to Groenewegen 1997.
A comparison with the luminosity of carbon stars in MCs clusters induces us to
attribute Population II nature to these objects (Bessel et al. 1983).
We are confident in the study of the luminosity distribution and of the colors
distribution of these stars to strengthen our hypothesis:
is the new branch in the color-color diagram constituted by CH stars?
References
- Bertin E., Arnouts S., 1996, A&AS 117, 393
- Bessel M.S., Wood P.R., Lloyd Evans T., 1983, MNRAS 202, 59
- Cowley A.P., Hartwick F.D.A., 1991, ApJ 373, 80
- Epchtein N., Burton W.B., Borsenberger J., et al., 1997, ESOMe 87, 27
- Feast M.W., Whitelock P.A., 1992, MNRAS 259, 6
- Groenewegen M.A.T., 1997, in 'The impact of Large Scale Near-IR Surveys',
Garzon F., et al. (eds), Proc., p. 165
- Hartwick F.D.A., Cowley A.P., 1988, ApJ 334, 135
- Loup C., Omont A., Duc P.A., Fouqué P., Bertin E., Epchtein N.,
in the 3rd Euroconference on Near Infrared Surveys, Proc.,
in press
- Schultheis M., Simon G., Hron J., in the 3rd Euroconference
on Near Infrared Surveys, Proc., in press
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| First version: | 08th | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)