Oxygen, Sodium, Magnesium and Aluminium as tracers of the Galactic Bulge Formation

By A. Lecureur, V. Hill, M. Zoccali, B. Barbuy, A. Gómez, D. Minniti, Sergio Ortolani, A. Renzini. 2006

173 citations recorded by the source corpus.

Aims. This paper investigates the peculiar behaviour of the light even (alpha-elements) and odd atomic number elements in red giants in the galactic bulge, both in terms of the chemical evolution of the bulge, and in terms of possible deep-mixing mechanisms in these evolved stars. Methods. Abundances of the four light elements O, Na, Mg, and Al are measured in 13 core He-burning giant stars (red clump stars) and 40 red giant branch stars in four 25′ fields spanning the bulge from -3 to -12° galactic latitude. Special care was taken in the abundance analysis, performing a differential analysis with respect to the metal-rich solar-neighbourhood giant μ Leo, which best resembles our bulge sample stars. This approach minimises systematic effects that can arise in the analysis of cool metal-rich stars due to continuum definition issues and blending by molecular lines (CN) and, cancels out possible model atmosphere deficiencies. Results. We show that the resulting abundance patterns point towards a chemical enrichment dominated by massive stars at all metallicities. Oxygen, magnesium, and aluminium ratios with respect to iron are overabundant with respect to both galactic disks (thin and thick) for [Fe/H] > -0.5. A formation timescale for the galactic bulge shorter than for both the thin and thick disks is therefore inferred. To isolate the massive-star contribution to the abundances of O, Mg, Al, and Na, we use Mg as a proxy for metallicity (instead of Fe), and further show that: (i) the bulge stars [O/Mg] ratio follows and extends the decreasing trend of [O/Mg] found in the galactic disks to higher metallicities. This is a challenge for predictions of O and Mg yields in massive stars, which so far predicted no metallicity dependence in this ratio; (ii) the [Na/Mg] ratio trend with increasing [Mg/H] is found to increase in three distinct sequences in the thin disk, the thick disk, and the bulge. The bulge trend is well represented by the predicted metallicity-dependent yields of massive stars, whereas the galactic disks have Na/Mg ratios that are too high at low metallicities, pointing to an additional source of Na from AGB stars; (iii) contrary to the case of the [Na/Mg] ratio, there appears to be no systematic difference in the [Al/Mg] ratio between bulge and disk stars, and the theoretical yields by massive stars agree with the observed ratios, leaving no space for AGB contribution to Al.

Subfields: Astronomy and Astrophysics, Instrumentation

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In Astronomy and Astrophysics

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Citations in the corpus

• Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars (2008, background): “In contrast to previous works on the topic (Zoccali et al. 2006; Fulbright et al. 2007; Lecureur et al. 2007), we find that the two populations are indistinguishable in their abundance patterns for the elements considered (C, N and O) up to [Fe/H] = −0.2, i.e. to the metallicity range where the…”
• The metal content of bulge field stars from FLAMES-GIRAFFE spectra - I. Stellar parameters and iron abundances (2008, methodology): “We note that the stars observed with UVES were 58 in total (c.f. Table 1 in Lecureur et al. 2007): 13 were Baade’s Window clump, and another 13 were giants, 11 more giants were Fig.”
• Nucleosynthesis in Stars and the Chemical Enrichment of Galaxies (2013, result): “These model predictions are in good agreement with observations (Fulbright, McWilliam & Rich 2007; Lecureur et al. 2007; Bensby et al. 2009; for a comparison, see Kobayashi & Nakasato 2011) except for the trend associated with [O/Mg].”
• Gaia FGK benchmark stars: Effective temperatures and surface gravities (2015, methodology): “The effective temperature adopted for the reference star was obtained by the infrared flux method, and the log g value was estimated from various independent methods (Lecureur et al. 2007).”
• Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS II. The complete high resolution extinction map and implications for Galactic bulge studies (2012, background): “…detail by analyzing the stellar ages and chemical abundances (McWilliam & Rich 1994; Zoccali et al. 2003; Fulbright et al. 2007; Rich et al. 2007; Lecureur et al. 2007; Zoccali et al. 2008; Clarkson et al. 2008; Brown et al. 2010; Bensby et al. 2010, 2011; Johnson et al. 2011; Gonzalez et al.…”
• Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants (2011, background): “…2010, and references therein) and that the formation of the bulge was fast as evidenced by the enhanced alpha elements in bulge stars (McWilliam & Rich 1994; Rich & Origlia 2005; Zoccali et al. 2006; Fulbright et al. 2006, 2007; Lecureur et al. 2007; Meléndez et al. 2008; Ryde et al. 2009, 2010).”
• Stellar Proper Motions in the Galactic Bulge from Deep Hubble Space Telescope ACS WFC Photometry (2008, background): “…rapid enrichment; Type II supernovae (SNe) produce the -elements and result from short-lived, high-mass stars, while iron enrichment requires the rather slower buildup of a significant number of Type I SNe (McWilliam&Rich1994; Zoccali et al. 2006; Lecureur et al. 2007; Fulbright et al. 2007).”
• Spectral models for solar-scaled and α-enhanced stellar populations (2007, result): “Besides, the recent work by Lecureur et al. (2007) found a significant overabundance of Na/Fe in bulge stars of our Galaxy, which supports the same findings for ellipticals (given that it is reasonable to assume that our bulge and elliptical galaxies share a similar star formation history).”
• Insights on the Milky Way bulge formation from the correlations between kinematics and metallicity (2010, background): “On the other hand, from medium and high-resolution spectroscopic data enhancements of α elements have been observed (McWilliam & Rich 1994; Zoccali et al. 2006; Fulbright et al. 2007; Lecureur et al. 2007) suggesting a short formation time-scale.”
• RECONCILING THE STELLAR AND NEBULAR SPECTRA OF HIGH-REDSHIFT GALAXIES (2016, background): “15; the squares are Galactic bulge stars: micro-lensed dwarfs from Bensby et al. (2013) (light green), and K-giants from Lecureur et al. (2007) (dark green).”
• THE THICK DISKS OF SPIRAL GALAXIES AS RELICS FROM GAS-RICH, TURBULENT, CLUMPY DISKS AT HIGH REDSHIFT (2009, background): “Bulges and thick disks have enhanced α elements compared to Fe (e.g., Lecureur et al. 2007; Zoccali et al. 2007), suggesting a star formation timescale shorter than 1 Gyr.”
• The metallicity distribution of bulge clump giants in Baade’s window (2011, methodology): “…of stars (10 to 20) in Baade’s window (McWilliam & Rich 1994; Fulbright et al. 2006, 2007), and more recently by our group (Zoccali et al. 2006; Lecureur et al. 2007) in a larger sample of 50 stars in four windows of the Galactic bulge, showed enhanced [α/Fe], compatible with a fast chemical…”
• Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca, and Ti (2010, methodology): “This is in contrast to some previous works on the topic (Zoccali et al. 2006; Fulbright et al. 2007; Lecureur et al. 2007), which argue that [O/Fe] in the bulge is higher than in the thick disk based on a comparison to disk dwarf stars.”
• Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies (2008, background): “Early-type galaxies tend to have ‘‘classical’’ bulges that are red, old, and high in /Fe ratio (indicative of rapid formation), and an n > 2:5 Sersic profile (Zoccali et al. 2006; Ballero et al. 2007; McWilliam et al. 2008; Lecureur et al. 2007).”
• TWO RED CLUMPS AND THE X-SHAPED MILKY WAY BULGE (2010, background): “…rapid formation timescale for the Galactic bulge (e.g., Ballero et al. 2007), as implied by the enhanced [Mg/Fe] ratios (e.g., Fulbright et al. 2007; Lecureur et al. 2007; McWilliam & Rich 1994); although, both the timescale and the Mg enhancements have associated uncertainties which may bring…”
• Chemodynamics of the Milky Way - I. The first year of APOGEE data (2013, result): “These preliminary results, while in agreement with earlier studies by, e.g., Zoccali et al. (2006), Fulbright et al. (2007) and Lecureur et al. (2007), are somewhat different from the more recent homogeneous abundance analyses of Meléndez et al. (2008), Alves-Brito et al. (2010)20 and Gonzalez et…”
• ARGOS - III. Stellar populations in the Galactic bulge of the Milky Way (2012, background): “Several authors have argued that the bulge is anα-enhanced metal-rich population (Zoccali et al. 2008; Fulbright, McWilliam, & Rich 2007; Lecureur et al. 2007; Meléndez et al. 2008).”