Astron. Astrophys. 328, 211-218 (1997)

2. Data and method

2.1. Near-infrared photometry

The near-IR photometry of the Bulge carbon stars presented by ALRW91 was reduced to the standards from the homogenized ESO photometric system proposed by Koornneef et al (1983a; b). Bouchet et al. (1991) found in this system no systematic differences in the magnitudes from the standards observed at ESO in the period 1983 - 1989.

The near-IR photometry from WIC96 with the SAAO 1.9m telescope is appropriately transformed to the ESO system (Hron et al. 1997). Note, that in general many transformations from SAAO to the ESO near-IR system refer to the transformation from the system defined by Glass (1974) or Carter (1990) to an older ESO system defined by Engels et al. (1981) and Wamsteker (1981).

2.2. Extinction and distance

The extinction for the Bulge carbon stars is not homogeneous. For each field defined by ALRW91 a general correction is applied to all stars. Instead of A_V = 1 87 (Glass et al. 1995) an extinction of A_V = 1 71 is adopted for the Sgr I field. It is the average value from Walker & Mack (1986) and Terndrup et al. (1990). For Baade's Window field around NGC 6522 A_V = 1 50. This value was obtained by Ng et al. (1996) from the (V,V-I) CMD obtained by the OGLE (Optical Gravitational Lensing Experiment, see Paczyski et al. 1994 for details) and is in good agreement with average value E(B-V) = 0 49 determined for this field. For the extended clear region around and near NGC 6558 the same value as used by ALR88, E(B-V) = 0 41 or A_V = 1 27 from Zinn (1980), is adopted. No actual determinations of the extinction are found for the field intermediate to NGC 6522 and NGC 6558 and A_V = 1 38 is adopted. For the remaining Bulge fields () a relation (Blommaert 1992, Schultheis et al. 1997) based on the reddening map constructed by Wesselink (1987) from the colour excess of the RR Lyrae stars at minimum light, is used: , where b is the galactic latitude.
The value A_V = 0 48 obtained by Mateo et al. (1995) is adopted for the stars in the SDG observed by WIC96. The extinction in the near-IR passbands is determined under the assumption that A_J /A_V = 0.282, A_H /A_V = 0.175, and A_K /A_V = 0.112 (Rieke & Lebofsky 1985).

The distance determined with RR Lyrae stars for the SDG ranges from 22.0 - 27.3 kpc. Fig. 1 shows that these distances are actually correlated with the galactic latitude at which they were determined. Additional distance determinations (Sarajedini&Layden 1995 - hereafter referred to as SL95, Fahlman et al. 1996) obtained with other methods were added to this figure. An unweighted linear least-squares fit through those distances gives . Fig. 1 also shows a two section line drawn through these points.

Fig. 1. The distance of the Sagittarius dwarf galaxy determined for various galactic latitudes. The dashed line refers to an unweighted linear least-squares fit for the points, while the long-dashed line shows a two section fit drawn through the points

2.3. Colour-magnitude diagram

Fig. 2a shows the de-reddened CMD. A distance correction with the linear least-squares line was applied to all the stars to reduce the scatter due to differential distance effects. Fig. 2b shows the diagram with the distances corrected with the two section line. There is no significant difference between the Figs. 2a and 2b. Some stars are very bright. In SDG their M_bol would range from - 6 0 to - 7 0. They could be bright members of SDG or they could be located in the Bulge and have a lower luminosity. For a graphical purpose those stars are placed in Fig. 2c at a distance of 8.0 kpc. A sample of LPVs from Schultheis et al. (1997) are included in panel 2c. The dotted line in Figs. 2a-c is the lower LMC limit at which carbon stars are found, while the lower magnitude boundary corresponds with the SMC limit.

Fig. 2. Panel (a and b) The ALRW91 carbon stars placed at the distance of the Sagittarius dwarf galaxy (open circles; small open circles are used to indicate a larger uncertainty in the extinction adopted), together with the carbon stars from IGI95 (filled triangles). Open triangles and crosses are used for respectively the candidate carbon stars and giant branch stars of the dwarf galaxy observed by WIC96. The open square indicates the carbon stars S283 found by NS97 among the variables studied by Plaut (1971) and Wesselink (1987). The dotted horizontal line indicates the observational lower LMC limit at which carbon stars are found, while the lower magnitude limit corresponds with the SMC limit (Azzopardi 1994). The difference between panel (a) and (b) is the method used to assign a distance to each star, see Sect. 2.3. Panel (c) shows the stars which are possibly located in the Bulge. The open crosses indicate the giant branch stars observed by WIC96, the open square is a carbon star (L199, unpublished) found among the variables studied by Plaut (1971) and Wesselink (1987), and the dots show for comparison the location of the semiregular and Mira variables selected from respectively Schultheis et al. (1997) and Blommaert (1992). The isochrones (Bertelli et al. 1994; only for log  ) displayed in the three panels are as follows: a) 10 Gyr RGBs for Z = 0.0004, 0.004,0.008, and 0.020; b) 1 and 10 Gyr AGBs for Z = 0.001 (long-dashed line) and 0.1 and 1 Gyr AGBs for Z = 0.008 (dotted line); and c) 5 and 10 Gyr AGBs for Z = 0.004 (dotted and solid line) and Z = 0.02 (dot-dashed and dashed line). See Sect. 3.1 for additional details about the magnitude and the colour transformations. Note that the left vertical axes in each panel give the K-magnitude scale, while the right vertical axes indicate the Mbol -scale

Figs. 2a and b show that three of the four IGI95 carbon stars observed by WIC96 form an extension to the ALRW91 sequence of carbon stars. This clearly is a strong indication that the ALRW91 carbon stars belong to the SDG. The fourth carbon star (star C1 from WIC96) appears to be redder and located in the parallel sequence formed by the giant branch stars. It is not clear if this is due to a mis-identification in the photometry with another redder, nearby star. The finding chart provided from WIC96 does not rule out this possibility. The figure further shows that eight (#s 1 - 3, 5, 6, 12, 15, 21) out of the 26 stars from WIC96 are highly eligible carbon star candidates, because they are located on the combined ALRW91 & WIC96 carbon star sequence.

A significant number of the ALRW91 carbon stars are located below the LMC limit at which carbon stars are found. This limit might be related to a low metallicity of the stars, but in Sect. 4.1 it is argued that this is not the case. A similar trend is present among some of the carbon stars found in other dwarf spheroidals (see Fig. 2b from WIC96). However, Azzopardi et al. (1997) found carbon stars with M - 1 2 in the Fornax dwarf galaxy, assuming (m-M)₀ = 21 0. This is even fainter than the present limit M - 2 0 for the `bulge' carbon stars if located in the SDG and M - 1 4 for the SMC carbon stars (Azzopardi 1994; Westerlund et al. 1993, 1995).
Four LPVs, most likely belonging to the dwarf galaxy (NS97), are included in this figure. Note that they are the first Mira and the semiregular variables found belonging to the SDG. In fact, they are the first LPVs discovered in a dwarf spheroidal galaxy. Two LPVs are located inside the carbon star sequence, while the other two form at M - 7 5 a blue extension to the carbon sequence.

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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