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Astron. Astrophys. 338, 581-591 (1998)

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1. Introduction

In the last decade much effort has been spent on the studies of Miras (e.g. van der Veen 1988, Feast et al. 1989, Whitelock et al. 1991, Blommaert 1992, and Glass et al. 1995), OH/IR stars (e.g. Herman 1988, te Lintel-Hekkert 1990, Lindquist et al. 1992, van Langevelde 1992, Blommaert et al. 1994, and Habing 1996), and carbon stars (e.g. Willems 1987, Chan & Kwok 1988, Willems 1988a,b, Willems & de Jong 1988, de Jong 1989, Stephenson 1989, Zuckerman & Maddalena 1989, Azzopardi et al. 1991, Tyson & Rich 1991, Westerlund et al. 1991, Groenewegen et al. 1992, Groenewegen 1993, Marigo et al. 1996b, Ng 1997b & 1998, and Marigo 1998). Systematic investigations of semiregular variables (SRVs) were carried out by Kerschbaum & Hron (1992, 1994; hereafter respectively referred to as KH92 & KH94) and Jura & Kleinmann (1992) in recent years. From the temperatures, luminosities, mass loss rates and number densities KH92 & KH94 distinguished a `blue', `red', and a `Mira-like' group among their SRVs. Their result suggests an evolutionary sequence, where the `blue' SRVs evolve towards the thermally pulsing AGB, change into `red' SRVs, and then continue to evolve to the Mira phase.

AGB stars are ideal probes in studies of the galactic bulge, because of their high luminosities. Various photometric (Frogel & Whitford 1987, Terndrup 1988, Geisler & Friel 1992), spectroscopic (Rich 1990, McWilliam & Rich 1994, Sadler et al. 1996) and star counts (Ng et al. 1996a) studies from Baade's Window (hereafter referred to as BW; [FORMULA], [FORMULA]), show a large spread in metallicity ranging from -1.5 [FORMULA] Fe/H [FORMULA] 1.0. The bulge offers a good opportunity to study the evolutionary aspects of AGB-stars in relation with their metallicity, which is one of the major evolutionary parameters.

PG3 (field #3 of the Palomar-Groningen Variable Star Survey; [FORMULA], [FORMULA]; see Sect. 2.1 for details) is well searched for variable stars (Plaut 1971, Wesselink 1987; the latter will hereafter be referred to as Wess87). Blommaert (1992; hereafter referred to as Bl92) studied the properties of the PG3 Miras with nIR (near-infrared) and IRAS photometry. The period-colour relation for the PG3 Miras was found to be shifted significantly from the Large Magellanic Cloud (LMC) relation (Feast et al. 1989). With the theoretical relation from Wood et al. (1991) it was concluded that this must be due to metallicity, which could be about 1.6 times larger for the PG3 Miras (Bl92). Feast (1996) obtained similar results for the Miras in the galactic bulge versus those in the LMC. Comparable indications were found by Whitelock (1996) from the local Miras.

In this study a comparison is made between the pulsation and near-infrared properties of the PG3 SRVs and Miras and the field SRVs and Miras. In Sect. 2 a discussion is given about the SRV data: the observations and the comparison samples. We proceed in Sect. 3 with a description of the results of this analysis, continue with the discussion in Sect. 4 and summarize it in Sect. 5.

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© European Southern Observatory (ESO) 1998

Online publication: September 14, 1998