Chemical abundances of 11 bulge stars from high-resolution, near-IR spectra

Abstract

Context. It is debated whether the Milky Way bulge has characteristics more similar to those of a classical bulge than those of a pseudobulge. Detailed abundance studies of bulge stars are important when investigating the origin, history, and classification of the bulge. These studies provide constraints on the star-formation history, initial mass function, and differences between stellar populations. Not many similar studies have been completed because of the large distance and high variable visual extinction along the line-of-sight towards the bulge. Therefore, near-IR investigations can provide superior results. Aims. To investigate the origin of the bulge and study its chemical abundances determined from near-IR spectra for bulge giants that have already been investigated with optical spectra. The optical spectra also provide the stellar parameters that are very important to the present study. In particular, the important CNO elements are determined more accurately in the near-IR. Oxygen and other α elements are important for investigating the star-formation history. The C and N abundances are important for determining the evolutionary stage of the giants and the origin of C in the bulge. Methods. High-resolution, near-infrared spectra in the H band were recorded using the CRIRES spectrometer mounted on the Very Large Telescope. The CNO abundances are determined from the numerous molecular lines in the wavelength range observed. Abundances of the α elements Si, S, and Ti are also determined from the near-IR spectra. Results. The abundance ratios [O/Fe], [Si/Fe], and [S/Fe] are enhanced to metallicities of at least [Fe/H] = −0.3, after which they decline. This suggests that the Milky Way bulge experienced a rapid and early burst of star formation similar to that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems to be present. This similarity suggests that the bulge could have had a pseudobulge origin. The C and N abundances suggest that our giants are first-ascent red-giants or clump stars, and that the measured oxygen abundances are those with which the stars were born. Our [C/Fe] trend does not show any increase with [Fe/H], which is expected if W-R stars contributed substantially to the C abundances. No "cosmic scatter" can be traced around our observed abundance trends: the measured scatter is expected, given the observational uncertainties.

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

“…The presently revised oxygen abundances in bulge stars are in very good agreement with those by Cunha & Smith (2006) and Ryde et al (2010), who used H-band OH lines to derive the oxygen abundances, taking into account the molecular dissociative equilibrium, and simultaneously deriving the carbon and nitrogen abundances. It appears to us that this is the reason for the agreement, and the reliability of these results.…”

“…For these reasons, it appears that the present results, together with those by Ryde et al (2010) and Cunha & Smith (2006) are, in principle, the correct ones.…”

“…Their Si and Ca abundances also show some spread, probably due to their use of low resolution (R ∼ 18 000) spectra. On the other hand, their work provides important confirmation of similar mean [O/Fe] Cunha & Smith (2006) and Ryde et al (2010), which we selected as being the most reliable results, to the predictions of the chemodynamical models for a classical bulge. Figures 3a and b, exhibits the results of the model with specific star-formation rates of ν SF = 3 Gyr −1 , with and without hypernovae.…”

“…Recently, Dobrovolkas et al (2015) have shown that 3D calculations are needed to bring oxygen abundances from the IR OH lines into agreement with the forbidden atomic lines, in particular for metal-poor giants. These lines were analysed by Cunha & Smith (2006), Meléndez et al (2008), Ryde et al (2010), Rich & Origlia (2005) and Rich et al (2012), and they are presently being extensively observed in bulge stars in the APOGEE survey (Majewski et al 2016); (iv) the ultraviolet (UV) OH lines (A 2 Σ−X 2 Π electronic transition) are the only lines measurable in very metal-poor dwarfs. These lines were never observed in bulge stars due to the high extinction at these wavelengths.…”

“…In Table 2 we report the stellar parameters and oxygen abundances by Ryde et al (2010), where some stars of the present sample were observed in the infrared. There is good agreement between the results from the two analyses for the same stars.…”