Astron. Astrophys. 338, 435-441 (1998)

1. Introduction

The near-IR colours and medium-low resolution spectra (Azzopardi et al. 1991- hereafter referred to as ALRW91, Tyson & Rich 1991- hereafter referred to as TR91, Westerlund et al. 1991) obtained for the so-called `bulge' carbon stars, identified by Azzopardi et al. (1985, 1988), show similarities with the low- to medium bolometric luminosity SMC carbon stars. The main difference is that the galactic carbon stars are photometrically bluer and that they have spectroscopically stronger NaD-doublets.

The radial velocities together with the direction in which these stars are located suggest a Bulge membership. In addition, a high metallicity for the Bulge lead TR91 and Westerlund et al. (1991) to suggest that the stars should be old and possess a mass of about 0.8 M, while evolutionary calculations (Boothroyd et al. 1993, Groenewegen & de Jong 1993, Groenewegen et al. 1995, Marigo 1998, Marigo et al. 1996a b) demonstrate that the initial mass of carbon stars in general has to be at least 1.2 M ( Gyr) for both Z = 0.004 and Z = 0.008. Furthermore, the initial mass increases towards higher metallicity (Lattanzio 1989). The ALRW91 C-stars are a mystery (Lequeux 1990, TR91, Westerlund et al. 1991, Chiosi et al. 1992, Azzopardi 1994), because they are in bolometric luminosity about too faint to be regarded as genuine AGB (Asymptotic Giant Branch) stars, if located in the metal-rich Bulge.

The serendipitous identification of the Sagittarius dwarf galaxy (SDG) was made by Ibata et al. (1994, 1995). The difference of the distance modulus between the dwarf galaxy and the Galactic Centre at 8 kpc lead Ng & Schultheis (1997, hereafter referred to as NS97) to suggest that the ALRW91 C-stars could actually be located at the distance of the dwarf galaxy. Its presence was unknown at the time when the C-stars were identified and a different location could solve the standing question about the origin of the `bulge' carbon stars.

Ng (1997, 1998) analysed the possibility that the ALRW91 C-stars are related to the SDG. With this hypothesis there is no need of exotic stellar evolutionary scenarios to explain these stars. Ng demonstrated that the photometric sequence of the ALRW91 C-stars is not exceptional, but comparable with the sequence found for the SMC. The estimated metallicity and age were respectively Z 0.008 and 0.1 - 1 Gyr.

The organisation of the paper is that Sect. 2 begins with an overview about the age and metallicities of the various populations identified in the SDG. Sect. 3 deals with the improvement of the photometric metallicity and age estimates of the ALRW91 C-stars. In Sect. 4 additional constraints are obtained from the velocity dispersion. In Sect. 5 it is further argued that the present position of the C-stars does not violate any of the reliable observational constraints. The discussion continues in Sect. 6 with additional tests to verify independently the results summarized in Sect. 7.

© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998
helpdesk.link@springer.de