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 AV = 1 87 (Glass et
al. 1995) an extinction of AV = 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 AV = 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 AV = 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 AV = 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 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.
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 Mbol 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.
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,
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998