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

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1. Introduction

1.1. Bulge carbon stars

For a long time carbon stars are searched in the direction of the Galactic Centre. From a low dispersion, near infrared grism survey a total of five carbon stars were found amid 2187 M-giants (Blanco et al. 1978, McCarthy et al. 1983 and Blanco & Terndrup 1989). The stars are mainly identified by the CN-bands at 7945, 8125, and 8320 Å. Azzopardi et al. (1985a 1985b;1986) demonstrated that additional, especially blue carbon stars can be found with the strong Swan C2 -bands (4737 and 5165 Å) in the spectral range 4350 - 5300 Å . Using this technique, Azzopardi et al. (1985b, 1988; the latter is hereafter referred to as ALR88) found 33 carbon stars in 8 different fields of the Galactic Bulge. Near-IR photometry and medium-low resolution spectra have been obtained for these carbon stars (Azzopardi et al. 1991 - hereafter referred to as ALRW91, Tyson & Rich 1991, Westerlund et al. 1991). These stars show similarities with the low- to medium bolometric luminosity SMC carbon stars, but the galactic carbon stars have stronger NaD-doublets. Various studies suggest that a wide metallicity range is present in the bulge (Whitford & Rich 1983, Rich 1988 & 1990, Geisler et al. 1992, McWilliam & Rich 1994, Ng 1994, Bertelli et al. 1995, Ng et al. 1995 & 1996, Sadler et al. 1996). According to ALR88 the carbon stars are expected to be metal-rich if they belong to the Bulge.

1.2. Sagittarius dwarf galaxy

The serendipitous identification of the Sagittarius dwarf galaxy (SDG) was made by Ibata et al. (1994,1995; hereafter respectively referred to as IGI94 and IGI95). It is the closest dwarf spheroidal and moves away from us at about 160 km/s. Accurate distance determinations from RR Lyrae stars belonging to this galaxy range from 22.0 - 27.3 kpc (Alcock et al. 1997; Alard 1996; Mateo et al. 1995ab, 1996; Ng & Schultheis 1997 - hereafter referred to as NS97). The photometric metallicity estimates made thus far depend heavily on the assumed age and the values for [Fe/H] range from -0.5 to -1.8. The mean age (10 - 12 Gyr) and metallicity ([Fe/H] = - 1.5) adopted is a trade-off, such that the age conveniently allows for the presence RR Lyrae and carbon stars (Ibata et al. 1997). IGI95 identified four carbon stars belonging to this galaxy, Whitelock et al. (1996, hereafter referred to as WIC96) performed a near-IR study of 26 candidates, and NS97 identified one more carbon star in the outer edge of the dwarf galaxy. The next section deals with how the Bulge carbon stars mentioned above are related to the (candidate) carbon stars from the dwarf galaxy.

1.3. A clue?

A supposedly high metallicity lead Tyson & Rich (1991) and Westerlund et al. (1991) to suggest that the Bulge carbon stars should be old and posses a mass of about 0.8 M [FORMULA], while evolutionary calculations (Boothroyd et al. 1993, Groenewegen & de Jong 1993, Groenewegen et al. 1995, Marigo et al. 1996 a b) demonstrate that the initial mass of carbon stars is at least 1.2
M [FORMULA] ([FORMULA]  Gyr) for Z = 0.008. Furthermore, the initial mass increases towards higher metallicity and decreases towards lower metallicity (Lattanzio 1989). The Bulge carbon stars are a mystery (Lequeux 1990, Tyson & Rich 1991, Westerlund et al. 1991, Chiosi et al. 1992, Azzopardi 1994), because they are in bolometric luminosity about 2 [FORMULA] 5 too faint to be regarded as genuine AGB (Asymptotic Giant Branch) stars, if located in the metal-rich Bulge.

NS97 compiled a catalogue of candidate RR lyrae and long period variables (LPVs) in the outer edge of the SDG. After de-reddening of the K-band magnitudes for the candidate LPVs, the period-K0 relation for the Mira variables from the Sgr I field in the Bulge from Glass et al. (1995) put these stars at 25.7 kpc. This distance is in good agreement with the distance obtained from the RR Lyrae stars found in the same field. One of the LPVs is a carbon star. The difference between the distance modulus of the dwarf galaxy and the Galactic Centre at 8 kpc is [FORMULA] 2 [FORMULA] 5. This lead to the suggestion that the Bulge carbon stars could actually be located in the dwarf galaxy, whose presence was unknown at the time the carbon stars were identified. This would solve the standing question about the origin of the `bulge' carbon stars.

The possibility that the `bulge' carbon stars could be member of the SDG is analysed. The organization of the paper is as follows. In Sect. 2 the formerly Bulge carbon from ALRW91 are placed in a CMD (colour-magnitude diagram) at the distance of the SDG. A comparison is made with the near-IR magnitudes and colours from actual and candidate carbon star members from the dwarf galaxy (WIC96, NS97). In Sect. 3 isochrones are placed in the CMD and it is demonstrated that the majority of the ALRW91 carbon stars are still fainter than the tip of the red giant branch. A discussion of the results is given in Sect. 4. Some stars are too faint, even if carbon stars are formed immediately after they enter the TP-AGB phase, and a possible binary evolution origin is suggested for some of the carbon stars fainter than the red giant branch tip. Arguments are given that the expected number of carbon stars related to the SDG is at least two times larger. The results are summarized in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998