Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Observational discovery of the AGB-bump

in the color-magnitude diagram of nearby galaxies

Carme Gallart1 and Giampaolo Bertelli2,3

1Carnegie Observatories, 813 Santa Barbara St. Pasadena CA 91101 USA
2Department of Astronomy, Vicolo dell'Osservatorio 5, I-35122- Padova, Italy
3National Council of Research, CNR-GNA,Rome, Italy

Received 02nd April 1998
Abstract. The AGB-bump, predicted by stellar evolutionary models, has been recently observed in the densely populated color-magnitude diagrams (CMDs) of a number of Local Group galaxies. In this paper, we discuss the theoretical predictions for the AGB-bump and the RGB-bump in the case of two typical composite populations, and compare them with the observations of M31 and the LMC.

1. Introduction

Currently, huge photometric databases are being created for the Magellanic Clouds (e.g. Alcock et al. 1997; Zaritsky et al. 1997), and for Local Group galaxies in the HST archive. These data routinely show well-known structures in the CMD such as the horizontal-branch (HB) and red-clump (RC), which are key to interpreting the stellar populations. Moreover, other structures which have not been previously recognized, are also apparent in the most populated of these new CMDs. Gallart (1998) showed that one of these newly identified structures, the AGB-bump, appears as a consequence of stellar evolution at the beginning of the AGB phase. In this paper, we will use synthetic CMDs obtained from the Padova stellar evolutionary models (Bertelli et al. 1994) to further discuss the expected characteristics of the AGB-bump in composite stellar populations, and we will compare them with observations of the LMC and M31. A discussion of the AGB-bump in globular clusters can be found in Castellani et al. (1991). For a discussion of the physical mechanism producing the AGB-bump, see Castellani et al. (1991), Chiosi et al. (1992) and Gallart (1998).

2. An old population with large metallicity dispersion

In Fig. 1a we show a synthetic CMD obtained assuming a constant star formation rate (SFR) from 10 to 8 Gyr ago, and Z increasing linearly as a function of time from Z = 0.0001 to Z = 0.01. The AGB-bump is parallel to and about 1 magnitude brighter than the red HB. The color of the AGB-bump becomes redder as the metallicity increases. The RGB-bump has a similar dependence on Z, but its MI also changes with Z (see Fusi-Pecci et al. 1990). At a given age, it is brighter as Z decreases (it reaches the magnitude of the HB at Z ≅ 0.001).

The nearby dE galaxies and the M31 halo have CMD's similar to the one in Fig. 1a. Figure 1b displays a CMD of a halo field in M31 (Holland et al. 1996). The AGB-bump is indicated. The RGB-bump is not observed, possibly because it gets mixed with the red HB in its bright, low metallicity side, and is too close to the limit of the photometry in its faint, high metallicity portion.

[Click here to see Fig. 1!]

3. A population with constant SFR and small metallicity dispersion

In Fig. 2a we show a synthetic CMD computed assuming a constant SFR from 3 Gyr ago to the present time, and a linear increase in Z as a function of time from Z = 0.001 to 0.006. The AGB-bump appears about 1 magnitude brighter than the RC. At this metallicity, the RGB-bump has a magnitude similar to that of the RC.

The star formation history of dIrr galaxies generally appears to have the characteristics represented in Fig. 2a. In Fig. 2b, a CMD of a large field in the LMC is shown (Beaulieu & Sackett 1998). The AGB-bump is observed as an increase in the density of stars in the RGB. The fact that the RGB-bump has a magnitude similar to that of the RC, together with photometric errors makes it undetectable. In both Figs. 2a and 2b, the faint blue-loops, produced by stars a few hundred million years old, are also indicated.

[Click here to see Fig. 2!]
Acknowledgments. We thank S. Holland and P. Sackett for making available to us the M31 and LMC data respectively, and to A. Aparicio, C. Chiosi, W. Freedman, A. Renzini and P. Sackett for very useful discussions.

References

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First version: 15thAugust,1998
Last update: 29thSeptember,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)