The Chemical Composition of Two Supergiants in the Dwarf Irregular Galaxy WLM

Abstract

The chemical composition of two stars in WLM have been determined from high quality UVES data obtained at the VLT UT2 1 . The model atmospheres analysis shows that they have the same metallicity, [Fe/H] = −0.38 ±0.20 (±0.29) 2 . Reliable magnesium abundances are determined from several lines of two ionization states in both stars resulting in [Mg/Fe] = −0.24 ±0.16 (±0.28). This result suggests that the [α(Mg)/Fe] ratio in WLM may be suppressed relative to solar abundances (also supported by differential abundances relative to similar stars in NGC 6822 and the SMC). The absolute Mg abundance, [Mg/H] = −0.62 is high relative to what is expected from the nebulae though, where two independent spectroscopic analyses of the H II regions in WLM yield [O/H] = −0.89. Intriguingly, the oxygen abundance determined from the O I λ6158 feature in one WLM star is [O/H] = −0.21 ±0.10 (±0.05), corresponding to five times higher than the nebular oxygen abundance. This is the first time that a significant difference between stellar and nebular oxygen abundances has been found, and presently, there is no simple explanation for this difference. The two stars are massive supergiants with distances that clearly place them in WLM. They are young (≤10 Myr) and should have a similar composition to the ISM. Additionally, differential abundances suggest that the O/Fe ratio in the WLM star is consistent with similar stars in NGC6822 and the SMC, galaxies where the average stellar oxygen abundances are in excellent agreement with the nebular results. If the stellar abundances reflect the true composition of WLM, then this galaxy lies well above the metallicity-luminosity relationship for dwarf irregular galaxies. It also suggests that WLM is more chemically evolved than currently interpreted from its color-magnitude diagram. The similarities between the stars in WLM and NGC6822 suggest that these two galaxies may have had similar star formation histories.1 Based on observations collected at the European Southern Observatory, proposal number 65.N-0375 2 In this paper, we adopt the standard notation [X/H] = log(X/H) -log(X/H) ⊙ . Also, abundances shall be reported with two uncertainties: the first is the line-to-line scatter, and the second (in parentheses and italics) is an estimate of the systematic error due to uncertainties in the atmospheric parameters.

B-type supergiants in the SMC: Chemical compositions and comparison of static and unified models

“…These studies include those of Lennon et al (1997), Gies & Lambert (1992), Kudritzki et al (1999), McErlean et al (1999) and Repolust et al (2004) in our Galaxy, Fitzpatrick & Bohannan (1993), Puls et al (1996), Dufton et al (2000), Korn et al (2002), Trundle et al (2004Trundle et al ( , 2005 and Venn (1999) for the Magellanic System, Venn et al (2000a), Smartt et al (2001) and Trundle et al (2002) for M 31, Monteverde et al (1997Monteverde et al ( , 2000 and Urbaneja et al (2003) for M 33 and Kaufer et al (2004), Urbaneja et al (2003) and Venn et al (2001Venn et al ( , 2000b for other Local Group galaxies. Studies of the progenitor O-type stars (see Bouret et al 2003;Hillier et al 2003;Heap et al 2004) and B-type giants (see Korn et al 2002, Lennon et al 2003 have also provided relevant observations to aid our understanding of the evolution of the surface chemical composition of early-type stars.…”

The Metallicity of Galaxy Disks: Infall versus Outflow

“…For example, for infall to produce galaxies with log y eA ¼ À3:0, the galaxies initially must have had f gas < 0:025, which is typical of gas fractions in elliptical galaxies, not disks. Moreover, a galaxy with such a low initial gas mass fraction would have a very high stellar metallicity, which conflicts with the low stellar metallicities derived for dwarf galaxies using broadband colors and resolved stellar populations (e.g., MacArthur et al 2004;Skillman et al 2003;Bell & de Jong 2000;Holtzman et al 2000;van Zee et al 1997a); this would also lead to a large metallicity difference between the gas phase and the stars (although see Venn et al 2003 andLee et al 2005 for a possible example of such an offset in the dwarf galaxy WLM). Finally, the necessary accretion would have to be extremely large.…”

A close look at the Centaurus A group of galaxies

“…We can however assume that galaxies outside the Local Group follow similar trends to the Local Group ones. The existing studies for the Local Group tend to suggest that for younger populations and/or higher metallicities the [α/Fe] ratios are around solar values, while for very old populations and/or low metallicities the [α/Fe] ratios tend to be similarly high as in the Galactic halo (see, e.g., Venn et al 2001Venn et al , 2003Tautvaišienė et al 2007;Tolstoy et al 2009). If the [α/Fe] ratios at those early ages were higher than they are now, then this would affect the RGB stars in our CMDs.…”