Astron. Astrophys. 357, 871-880 (2000)

2. Data

2.1. Observations

The data discussed in this paper consist of two sets of photographic plates and films taken with the ESO 1m Schmidt telescope at La Silla Observatory (see Table 1), each of them covering an area of deg² on the sky.

Table 1. Observations

The first set of plates was part of the DUO project aimed at detecting microlensing events towards the Galactic Bulge (Alard & Guibert 1997, hereafter AG97). This field, centred on Galactic coordinates , has already been processed and presented in A96. The second set is new and includes 69 films centred on a field shifted towards the centre of density of the Sagittarius dwarf galaxy and slightly overlapping with the former . Throughout the remainder of this paper, we will call the first field DUO field while the new field will be referred to as the SAG field.

2.2. Data reduction

The plates were scanned at CAI/Paris Observatory with the high speed microdensitometer MAMA ¹ (Machine Automatique à Mesurer pour l'Astronomie), yielding images with a pixel size of 10 µm (.).

The photometric reduction has been performed with the software Extractor written by Alard. The process is as follows: first a reference catalogue is extracted from a plate of good quality (seeing). For all the other plates, a new extraction is performed (implying a new detection of each object) and the new catalogue associated to the reference catalogue. The light curves were built in this way plate by plate and stored in a database. For more details on the photometric reduction process see AG97. The final sample contains light curves for stars in the DUO field and stars in the SAG field.

2.3. Photometry

2.3.1. Calibration

The DUO field has been calibrated with a CCD sequence taken at the ESO/Danish 1.5 m telescope at La Silla. The photometric system for this field is (Blair & Gilmore 1982). The Emulsion/Filter combination was different for the SAG field and consisted of a Kodak Tech-Pan 4415 emulsion together with a BG12 Filter. The Tech-Pan 4415 emulsion is an extremely fine-grained, high resolution film with a sensitivity extending to 0.69 µm. For more informations about the 4415 emulsion, see Phillipps & Parker (1993) and references therein. We were not aware of any photometric relation published for the band used in SAG .

The resulting bandpasses for both fields are shown on Fig. 1. The calibration in SAG has been performed with a sequence provided by Sarajedini & Layden (1995), located north of the globular cluster M54 and consisting of 1638 stars calibrated in V and I bands. A polynomial fit to the instrumental magnitude of these stars yielded the photometric system , where stands for the magnitude in the color band used in SAG . The scatter about this relation is 0.17 mag.

Fig. 1. Approximate bandpasses used in SAG (solid line) and DUO (dotted line).

2.3.2. Correction for extinction

The reddening has been estimated separately for each field. For the DUO field we used the well known property that the color of RR Lyrae stars at minimum magnitude is approximately constant and depends only slightly on the period and the metallicity (Sturch, 1966). A reddening map has been estimated for this field by computing the mean colors of RRab in small regions of 1010´. The corrected magnitude is (Wesselink 1987). For the SAG field, where no color information was available, we used the extinction map of Schlegel et al. (1998, hereafter SFD) which provides reddening estimates with a precision of 16 for . However, the SAG field extends to where, according to SFD, the reddening map might become inaccurate. From the relation (Cardelli et al. 1989; hereafter CCM) we derive . This ratio in the overlap between DUO and SAG yields 1.32 0.24, in reasonable agreement with the theoretical expectation, showing that even at the western edge of the SAG field the SFD map provides a satisfactory estimation for the extinction. Assuming E(V-I)=1.55 E(B-V) from CCM and a normal extinction law =3.10 E(B-V) we obtain - 5.38 E(B-V) for the de-reddened magnitude in the SAG field.

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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