Abundances in bulge stars from high-resolution, near-IR spectra

It is debated whether the Milky Way bulge has the characteristics of a classical bulge sooner than those of a pseudobulge. Detailed abundance studies of bulge stars is a key to investigate the origin, history, and classification of the bulge. The aim is to add to the discussion on the origin of the bulge and to study detailed abundances determined from near-IR spectra for bulge giants already investigated with optical spectra, the latter also providing the stellar parameters which are very significant for the results of the present study. Especially, the important CNO elements are better determined in the near-IR. High-resolution, near-infrared spectra in the H band are recorded using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances can all be determined from the numerous molecular lines in the wavelength range observed. Abundances of the alpha elements are also determined from the near-IR spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up 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 star-formation history like that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems present. Such a similarity could suggest that the bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase with [Fe/H] which could have been expected if W-R stars have contributed substantially to the C abundances. No "cosmic scatter" can be traced around our observed abundance trends; the scatter found is expected, given the observational uncertainties.

The carbon, nitrogen, and oxygen abundances and trends in the bulge are discussed in the context of our recent analysis of these elements in an on-going project based on near-IR spectra (Rydeet al. 2009). We obtained these using the CRIRES spectrometer on the VLT. The formation and evolution of the Milky Way bulge can be constrained by studying elemental abundances of bulge stars. Due to the large and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred.

Bengt Gustafsson did pioneering work on the determination of the C, N and O abundances of stars in the early 1980s. Here, we present some preliminary results from an ongoing near-infrared (IR) study of these elements in bulge stars. The formation and evolution of the Milky Way bulge can be constrained by studying these types of elemental abundances. Due to the large and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred.

We obtained high-resolution near-IR spectra of 45 AGB stars located in the Galactic bulge. The aim of the project is to determine key elemental abundances in these stars to help constrain the formation history of the bulge. A further aim is to link the photospheric abundances to the dust species found in the winds of the stars. Here we present a progress report of the analysis of the spectra.

Based on the medium-high resolution (R ∼ 20,000), modest signal-to-noise ratio (S/N ≳ 70) FLAMES-GIRAFFE spectra, we investigated the copper abundances of 129 red giant branch stars in the Galactic bulge with [Fe/H] from −1.14 to 0.46 dex. The copper abundances are derived from both local thermodynamic equilibrium (LTE) and nonlocal thermodynamic equilibrium (NLTE) with the spectral synthesis method. We find that the NLTE effects for Cu i lines show a clear dependence on metallicity, and they gradually increase with decreasing [Fe/H] for our sample stars. Our results indicate that the NLTE effects of copper are important not only for metal-poor stars but also for supersolar metal-rich ones and the LTE results underestimate the Cu abundances. We note that the [Cu/Fe] trend of the bulge stars is similar to that of the Galactic disk stars spanning the metallicity range of −1.14 < [Fe/H] < 0.0 dex, and the [Cu/Fe] ratios increase with increasing metallicity when [Fe/H] is from ∼−1.2 to ∼−0.5 dex, favoring a secondary (metallicity-dependent) production of Cu.

We present abundances of C, N, O, F, Na, and Fe in six giant stars of the tidally disrupted globular cluster NGC 6712. The abundances were derived by comparing synthetic spectra with high-resolution infrared spectra obtained with the Phoenix spectrograph on the Gemini South telescope. We find large star-to-star abundance variations of the elements C, N, O, F, and Na. NGC 6712 and M4 are the only globular clusters in which F has been measured in more than two stars, and both clusters reveal F abundance variations whose amplitude is comparable to or exceeds that of O, a pattern which may be produced in -->M 5 -->M☉ AGB stars. Within the limited samples, the F abundance in globular clusters is lower than in field and bulge stars at the same metallicity. NGC 6712 and Pal 5 are tidally disrupted globular clusters whose red giant members exhibit O and Na abundance variations not seen in comparable metallicity field stars. Therefore, globular clusters such as NGC 6712 and Pal 5 cannot contribute many field stars and/or field stars do not form in environments with chemical enrichment histories like those of NGC 6712 and Pal 5. Although our sample size is small, from the amplitude of the O and Na abundance variations we infer a large initial cluster mass and tentatively confirm that NGC 6712 was once one of the most massive globular clusters in our Galaxy.

Context. Until now, heavy interstellar extinction has meant that only a few studies of chemical abundances have been possible in the inner Galactic bulge. However, it is crucial to learn more about this structure in order to better understand the formation and evolution of the centre of the Galaxy and galaxies in general. Aims. In this paper, we aim to derive high-precision α-element abundances using CRIRES high-resolution IR spectra of 72 cool M giants of the inner Galactic bulge. Methods. Silicon, magnesium, and calcium abundances were determined by fitting a synthetic spectrum for each star. We also incorporated recent theoretical data into our spectroscopic analysis (i.e. updated K-band line list, better broadening parameter estimation, non-local thermodynamic equilibrium (NLTE) corrections). We compare these inner bulge α abundance trends with those of solar neighbourhood stars observed with IGRINS using the same line list and analysis technique; we also compare our sample to APOGEE DR17 abundances for inner bulge stars. We investigate bulge membership using spectro-photometric distances and orbital simulations. We construct a chemical-evolution model that fits our metallicity distribution function (MDF) and our α-element trends. Results. Among our 72 stars, we find four that are not bulge members. [Si/Fe] and [Mg/Fe] versus [Fe/H] trends show a typical thick disc α-element behaviour, except that we do not see any plateau at supersolar metallicities as seen in other works. The NLTE analysis lowers [Mg/Fe] typically by ∼0.1 dex, resulting in a noticeably lower trend of [Mg/Fe] versus [Fe/H]. The derived [Ca/Fe] versus [Fe/H] trend has a larger scatter than those for Si and Mg, but is in excellent agreement with local thin and thick disc trends. With our updated analysis, we constructed one of the most detailed studies of the α abundance trends of cool M giants in the inner Galactic bulge. We modelled these abundances by adopting a two-infall chemical-evolution model with two distinct gas-infall episodes with timescales of 0.4 Gyr and 2 Gyr, respectively. Conclusions. Based on a very meticulous spectral analysis, we have constructed detailed and precise chemical abundances of Mg, Si, and Ca for cool M giants. The present study can be used as a benchmark for future spectroscopic surveys.

view Abstract Citations (24) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Spiral Galaxies. V. The Galactic Bulge Molla, Mercedes ; Ferrini, Federico Abstract We apply the multipopulation nonlinear model developed by Ferrini and coworkers to the evolution of the bulge of the Galaxy. We investigate the hypothesis of a bulge formed from the accumulation of gas by accretion from the halo in the center of the Galaxy. We extend the properties of phase interactions, determined in the solar neighborhood, to the bulge. The models show that bulge star formation in the bulge peaked during the early stages of the Galactic evolution and that the consequent element enrichment is in agreement with the observed metal abundances in bulge stars. The evolution of Galactic bulges and elliptical galaxies cannot be considered equivalent. Publication: The Astrophysical Journal Pub Date: December 1995 DOI: 10.1086/176524 Bibcode: 1995ApJ...454..726M Keywords: GALAXIES: EVOLUTION; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: SPIRAL; GALAXIES: STRUCTURE full text sources ADS | Related Materials (6) Part 1: 1992ApJ...387..138F Part 2: 1994ApJ...421..491P Part 3: 1994ApJ...427..745F Part 4: 1995ApJ...444..207P Part 6: 1996ApJ...466..668M Part 7: 1997ApJ...475..519M

We explore the elemental abundance features of metal-rich disk stars, highlighting the comparisons made with those of the recently revealed Galactic bulge stars. A similarity between two of the comparisons leads to a new theoretical picture of the bulge-disk connection in the Galaxy, where a supermassive black hole resides at the center. We postulate that a metal-rich outflow, triggered by feedback from a black hole, was generated and quenched the star formation, which had lasted several billion years in the bulge. The expelled gas cooled down in the Galactic halo without escaping from the gravitational potential of the Galaxy. The gas gradually started to accrete to the disk around five billion years ago, corresponding to the time of sun's birth, and replaced a low-metallicity halo gas that had been accreting over nearly ten billion years. The metal-rich infalling gas, whose elemental abundance reflects that of metal-rich bulge stars, mixed with the interstellar gas already present in the disk. Stars formed from the mixture compose the metal-rich stellar disk. This scheme is incorporated into models for chemical evolution of the disk. The resultant elemental features are compatible with the observed abundance trends of metal-rich disk stars, including the upturning feature exhibited in some [X/Fe] ratios, whose interpretation was theoretically puzzling. Furthermore, the predicted abundance distribution function of disk stars covers all observational facts to be considered: (i) the deficiency of metal-poor stars, (ii) the location of peak, and (iii) the extended metal-rich tail up to [Fe/H] ~ +0.4.

The stellar population of the Milky Way bulge is thoroughly studied, with a plethora of measurements from virtually the full suite of instruments available to astronomers. It is thus perhaps surprising that alongside well-established results lies some substantial uncertainty in its star-formation history. Cosmological models predict the bulge to host the Galaxy's oldest stars for [Fe/H] ≲ −1, and this is demonstrated by RR Lyrae stars and globular cluster observations. There is consensus that bulge stars with [Fe/H] ≲ 0 are older than t ≈ 10 Gyr. However, at super-solar metallicity, there is a substantial unresolved discrepancy. Data from spectroscopic measurements of the main-sequence turnoff and subgiant branch, the abundances of asymptotic giant branch stars, the period distribution of Mira variables, the chemistry and central-star masses of planetary nebulae, all suggest a substantial intermediate-age population (t ≈ 3 Gyr). This is in conflict with predictions from cosmologically motivated chemical evolution models and photometric studies of the main-sequence turnoff region, which both suggest virtually no stars younger than t ≈ 8 Gyr. A possible resolution to this conflict is enhanced helium-enrichment, as this would shift nearly all of the age estimates in the direction of decreasing discrepancy.

Carbon, nitrogen, and oxygen produced in the early universe come from a variety of possible astrophysical sites. Among these are early supernovae, winds of massive, rapidly-rotating, mega metal-poor stars, and intermediate mass AGB stars. Large-scale surveys such as the HK Survey of Beers and colleagues and the Hamburg/ESO Survey of Christlieb and colleagues have allowed for the identification of numerous metal-poor stars in the Galactic halo. Follow-up observations of these metal-poor stars is necessary to determine CNO abundances. Techniques have been developed such that [C/Fe], [N/Fe], and [O/Fe] can be estimated with considerable accuracy using medium-resolution observations alone. We present estimates of these species for a number of metal-poor stars based on analysis of near-UV, optical, and near-IR spectra. The data come from several different instruments on southern-hemisphere telescopes, including the Goodman HTS and OSIRIS on SOAR, GMOS-S on Gemini, and XSHOOTER on VLT. In this way, we present some of the first metal-poor stars with a full complement of CNO abundances based solely on the analysis of medium-resolution spectra.

view Abstract Citations (68) References (41) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galactic Bulge M Giants. IV. 0.5--2.5 Micron Spectrophotometry and Abundances for Stars in Baade's Window Terndrup, D. M. ; Frogel, Jay A. ; Whitford, A. E. Abstract Spectrophotometric observations of bulge and local M giants from 0.45 to 2.5 microns at a resolution of about 1000 are presented. From an analysis of strong atomic lines of Na I and Ca I in the K band, a mean metallicity of the M giants in Baade's Window is derived. It is demonstrated that J-K is a good temperature indicator for both the field and bulge nonvariable M giants, and that the relationship between the two quantities is the same for both types of stars. In addition, there is no difference in the surface gravity between bulge and field giants of the same J-K color (i.e., temperature). A major difference in the overall spectral energy distributions of bulge and local M giants is that the classical H-band bump attributed to the opacity minimum of the H(-) ion near 1.6 microns is considerably reduced in many of the bulge stars. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170475 Bibcode: 1991ApJ...378..742T Keywords: Abundance; Galactic Bulge; Giant Stars; Late Stars; M Stars; Opacity; Stellar Spectrophotometry; Absorption Spectra; Carbon Monoxide; Color-Magnitude Diagram; Infrared Photometry; Infrared Spectroscopy; Line Spectra; Water; Astrophysics; OPACITIES; SPECTROPHOTOMETRY; STARS: ABUNDANCES; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (37) Related Materials (3) Part 1: 1988AJ.....95.1400B Part 2: 1990ApJ...353..494F Part 3: 1990ApJ...357..453T

A cold, high-velocity (HV, ∼200 km s−1) peak was first reported in several Galactic bulge fields based on the Apache Point Observatory Galaxy Evolution Experiment (APOGEE) commissioning observations. Both the existence and the nature of the HV peak are still under debate. Here we revisit this feature with the latest APOGEE DR13 data. We find that most of the low-latitude bulge fields display a skewed Gaussian distribution with an HV shoulder. However, only 3 out of 53 fields show distinct HV peaks around 200 km s−1. The velocity distribution can be well described by Gauss–Hermite polynomials, except for the three fields showing clear HV peaks. We find that the correlation between the skewness parameter (h 3) and the mean velocity ( ), instead of a distinctive HV peak, is a strong indicator of the bar. It was recently suggested that the HV peak is composed of preferentially young stars. We choose three fields showing clear HV peaks to test this hypothesis using the metallicity, [α/M], and [C/N] as age proxies. We find that both young and old stars show HV features. The similarity between the chemical abundances of stars in the HV peaks and the main component indicates that they are not systematically different in terms of chemical abundance or age. In contrast, there are clear differences in chemical space between stars in the Sagittarius dwarf and the bulge stars. The strong HV peaks off-plane are still to be explained properly and could be different in nature.

Determining elemental abundances of bulge stars can, via chemical evolution modeling, help to understand the formation and evolution of the bulge. Recently there have been claims both for and against the bulge having a different [$\alpha$/Fe] vs. [Fe/H]-trend as compared to the local thick disk possibly meaning a faster, or at least different, formation time scale of the bulge as compared to the local thick disk. We aim to determine the abundances of oxygen, magnesium, calcium, and titanium in a sample of 46 bulge K-giants, 35 of which have been analyzed for oxygen and magnesium in previous works, and compare them to homogeneously determined elemental abundances of a local disk sample of 291 K-giants. We use spectral synthesis to determine both the stellar parameters as well as the elemental abundances of the bulge stars analyzed here. The method is exactly the same as was used for analyzing the comparison sample of 291 local K-giants in Paper I of this series. Compared to the previous analysis of the 35 stars in our sample, we find lower [Mg/Fe] for [Fe/H]>-0.5, and therefore contradict the conclusion about a declining [O/Mg] for increasing [Fe/H]. We instead see a constant [O/Mg] over all the observed [Fe/H] in the bulge. Furthermore, we find no evidence for a different behavior of the alpha-iron trends in the bulge as compared to the local thick disk from our two samples.

We report on the detailed abundances of giants in the Galactic bulge, measured with the HIRES echelle spectrograph on the 10-m Keck telescope. We also review other work on the bulge field population and globular clusters using Keck/HIRES. Our new spectra have 3 times the resolution and higher S/N than previous spectra obtained with 4m telescopes. We are able to derive log g from Fe II lines and excitation temperature from Fe I lines, and do not rely on photometric estimates for these parameters. We confirm that the iron abundance range extends from -1.6 to +0.55 dex. The improved resolution and S/N of the Keck spectra give (Fe/H) typically 0.1 to 0.2 dex higher than previous studies, for bulge stars more metal rich than the Sun. Alpha elements are enhanced even for stars at the Solar metallicity. We confirm our earlier abundance analysis of bulge giants and find that Mg and Ti are enhanced relative to Ca and Si even up to (Fe/H) equals +0.55. We also report the first reliable estimates the bulge oxygen abundance. Our element ratios confirm that bulge giants have a clearly identifiable chemical signature, and suggest a rapid formation timescale for the bulge.