Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca, and Ti

Abstract

Context. The formation and evolution of the Galactic bulge and its relationship with the other Galactic populations is still poorly understood. Aims. To establish the chemical differences and similarities between the bulge and other stellar populations, we performed an elemental abundance analysis of α-(O, Mg, Si, Ca, and Ti) and Z-odd (Na and Al) elements of red giant stars in the bulge as well as of local thin disk, thick disk and halo giants. Methods. We use high-resolution optical spectra of 25 bulge giants in Baade's window and 55 comparison giants (4 halo, 29 thin disk and 22 thick disk giants) in the solar neighborhood. All stars have similar stellar parameters but cover a broad range in metallicity (−1.5 < [Fe/H] < +0.5). A standard 1D local thermodynamic equilibrium analysis using both Kurucz and MARCS models yielded the abundances of O, Na, Mg, Al, Si, Ca, Ti and Fe. Our homogeneous and differential analysis of the Galactic stellar populations ensured that systematic errors were minimized. Results. We confirm the well-established differences for [α/Fe] at a given metallicity between the local thin and thick disks. For all the elements investigated, we find no chemical distinction between the bulge and the local thick disk, in agreement with our previous study of C, N and O but in contrast to other groups relying on literature values for nearby disk dwarf stars. For −1.5 < [Fe/H] < −0.3 exactly the same trend is followed by both the bulge and thick disk stars, with a star-to-star scatter of only 0.03 dex. Furthermore, both populations share the location of the knee in the [α/Fe] vs. [Fe/H] diagram. It still remains to be confirmed that the local thick disk extends to super-solar metallicities as is the case for the bulge. These are the most stringent constraints to date on the chemical similarity of these stellar populations. Conclusions. Our findings suggest that the bulge and local thick disk stars experienced similar formation timescales, star formation rates and initial mass functions, confirming thus the main outcomes of our previous homogeneous analysis of [O/Fe] from infrared spectra for nearly the same sample. The identical α-enhancements of thick disk and bulge stars may reflect a rapid chemical evolution taking place before the bulge and thick disk structures we see today were formed, or it may reflect Galactic orbital migration of inner disk/bulge stars resulting in stars in the solar neighborhood with thick-disk kinematics.

Tracing the evolution of the Galactic bulge with chemodynamical modelling of alpha-elements

“…3c compares the same model as in panel a with the data for the thick disk dwarfs from Bensby et al (2004) and thick disk giants from Alves-Brito et al (2010), together with the present oxygen abundances for 56 bulge giants. It should be noted that a thick disk is formed by a series of processes that are not included in the present Table 2.…”

“…In this case, the covariance analysis rejects the hypothesis that both subsamples are drawn from the same-population at a <0.01% significance level, but providing a downward average shift of 0.30 for the [O/Fe] values of thick disk data with respect to the bulge data. These results illustrate why this discussion is inconclusive: it is not clear if the thick disk would be more oxygen-rich than bulge stars as hinted by results from Alves-Brito et al (2010), or if the bulge stars would be more oxygen-rich than thick disk stars as can be seen from Bensby et al (2004Bensby et al ( , 2013 …”

“…Hydrodynamical three-dimensional (3D) model atmospheres show that this line is instead affected by granulation, increasing with decreasing metallicity and increasing temperatures, as shown by Nissen (2002), for example. Oxygen abundances in the Galactic bulge derived from these forbidden lines were presented by Zoccali et al (2006), Lecureur et al (2007), Fulbright et al (2006Fulbright et al ( , 2007, Alves-Brito et al (2010) and Johnson et al (2014). Bensby et al (2004) also used these lines to examine a sample of thick disk giants; (ii) the permitted OI λ 7771.96, 7774.18 and 7775.40 Å triplet lines measurable in dwarfs and subgiants (in supergiants, also the weaker triplet at λ 6156.03, 6156.80 and 6158.17 Å can be used); these lines are subject to strong NLTE effects.…”

“…In the lower panel, the present oxygen abundances for 56 bulge giants are plotted together with thick disc stars analysed by Alves-Brito et al (2010), Bensby et al (2004) and Reddy et al (2006).…”

“…Stellar parameters and CNO abundances from Ryde et al (2010), Zoccali et al (2006Zoccali et al ( , 2008, and Barbuy et al (2015). Alves-Brito et al (2010), and thick disk and bulge dwarfs by Bensby et al (2004Bensby et al ( , 2013. We preferred to use only the sample and results from Bensby et al (2004) (2004) show excellent agreement.…”