We examine the distribution of the O/Fe abundance ratio in stars across the Galactic disk using H-band spectra from the Apache Point Galactic Evolution Experiment (APOGEE). We minimize systematic ...errors by considering groups of stars with similar atmospheric parameters. The APOGEE measurements in the Sloan Digital Sky Survey data release 12 reveal that the square root of the star-to-star cosmic variance in the oxygen-to-iron ratio at a given metallicity is about 0.03–0.04 dex in both the thin and thick disk. This is about twice as high as the spread found for solar twins in the immediate solar neighborhood and the difference is probably associated to the wider range of galactocentric distances spanned by APOGEE stars. We quantify the uncertainties by examining the spread among stars with the same parameters in clusters; these errors are a function of effective temperature and metallicity, ranging between 0.005 dex at 4000 K and solar metallicity, to about 0.03 dex at 4500 K and Fe/H ≃ −0.6. We argue that measuring the spread in O/Fe and other abundance ratios provides strong constraints for models of Galactic chemical evolution.
ABSTRACT
We identified a sample of 58 candidate stars with metallicity Fe/H ≲ −0.8 that likely belong to the old bulge spheroid stellar population, and analyse their Na and Al abundances from Apache ...Point Observatory Galactic Evolution Experiment (APOGEE) spectra. In a previous work, we inspected APOGEE-Stellar Parameter and Chemical Abundance Pipeline abundances of C, N, O, Mg, Al, Ca, Si, and Ce in this sample. Regarding Na lines, one of them appears very strong in about 20 per cent of the sample stars, but it is not confirmed by other Na lines, and can be explained by sky lines, which affect the reduced spectra of stars in a certain radial velocity range. The Na abundances for 15 more reliable cases were taken into account. Al lines in the H band instead appear to be very reliable. Na and Al exhibit a spread in abundances, whereas no spread in N abundances is found, and we found no correlation between them, indicating that these stars could not be identified as second-generation stars that originated in globular clusters. We carry out the study of the behaviour of Na and Al in our sample of bulge stars and literature data by comparing them with chemodynamical evolution model suitable for the Galactic bulge. The Na abundances show a large spread, and the chemodynamical models follow the main data, whereas for aluminum instead, the models reproduce very satisfactorily the nearly secondary-element behaviour of aluminum in the metallicity range below Fe/H ≲ −1.0. For the lower-metallicity end (Fe/H < −2.5), hypernovae are assumed to be the main contributor to yields.
We present a new library of integrated spectra of 40 Galactic globular clusters, obtained with the Blanco 4 m telescope and the R-C spectrograph at the Cerro Tololo Inter-American Observatory. The ...spectra cover the range 63350-6430 AA with 63.1 AA (FWHM) resolution. The spectroscopic observations and data reduction were designed to integrate the full projected area within the cluster core radii in order to properly sample the light from stars in all relevant evolutionary stages. The S/N values of the flux-calibrated spectra range from 50 to 240 AA super(-1) at 4000 AA and from 125 to 500 AA super(-1) at 5000 AA. The selected targets span a wide range of cluster parameters, including metallicity, horizontal-branch morphology, Galactic coordinates, Galactocentric distance, and concentration. The total sample is thus fairly representative of the entire Galactic globular cluster population and should be valuable for comparison with similar integrated spectra of unresolved stellar populations in remote systems. For most of the library clusters, our spectra can be coupled with deep color-magnitude diagrams and reliable metal abundances from the literature to enable the calibration of stellar population synthesis models. In this paper we present a detailed account of the observations and data reduction. The spectral library is publicly available in electronic format from the National Optical Astronomical Observatory Web site.
ABSTRACT
The central part of the Galaxy hosts a multitude of stellar populations, including the spheroidal bulge stars, stars moved to the bulge through secular evolution of the bar, inner halo, ...inner thick disc, inner thin disc, as well as debris from past accretion events. We identified a sample of 58 candidate stars belonging to the stellar population of the spheroidal bulge, and analyse their abundances. The present calculations of Mg, Ca, and Si lines are in agreement with the ASPCAP abundances, whereas abundances of C, N, O, and Ce are re-examined. We find normal α-element enhancements in oxygen, similar to magnesium, Si, and Ca abundances, which are typical of other bulge stars surveyed in the optical in Baade’s Window. The enhancement of O/Fe in these stars suggests that they do not belong to accreted debris. No spread in N abundances is found, and none of the sample stars is N-rich, indicating that these stars are not second generation stars originated in globular clusters. Ce instead is enhanced in the sample stars, which points to an s-process origin such as due to enrichment from early generations of massive fast rotating stars, the so-called spinstars.
Metallicity gradients provide strong constraints for understanding the chemical evolution of the Galaxy. We report on radial abundance gradients of Fe, Ni, Ca, Si, and Mg obtained from a sample of ...304 red‐giant members of 29 disk open clusters, mostly concentrated at galactocentric distances between ∼8–15 kpc, but including two open clusters in the outer disk. The observations are from the APOGEE survey. The chemical abundances were derived automatically by the ASPCAP pipeline and these are part of the SDSS III Data Release 12. The gradients, obtained from least squares fits to the data, are relatively flat, with slopes ranging from –0.026 to –0.033 dex kpc–1 for the α ‐elements O/H, Ca/H, Si/H, and Mg/H, and –0.035 dex kpc–1 and –0.040 dex kpc–1 for Fe/H and Ni/H, respectively. Our results are not at odds with the possibility that metallicity (Fe/H) gradients are steeper in the inner disk (RGC ∼ 7–12 kpc) and flatter towards the outer disk. The open cluster sample studied spans a significant range in age. When breaking the sample into age bins, there is some indication that the younger open cluster population in our sample (log age <8.7) has a flatter metallicity gradient when compared with the gradients obtained from older open clusters.
We present an elemental abundance analysis of high-resolution spectra for five giant stars spatially located within the innermost regions of the bulge globular cluster NGC 6522 and derive Fe, Mg, Al, ...C, N, O, Si, and Ce abundances based on H-band spectra taken with the multi-object APOGEE-north spectrograph from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Of the five cluster candidates, two previously unremarked stars are confirmed to have second-generation (SG) abundance patterns, with the basic pattern of depletion in C and Mg simultaneous with enrichment in N and Al as seen in other SG globular cluster populations at similar metallicity. In agreement with the most recent optical studies, the NGC 6522 stars analyzed exhibit (when available) only mild overabundances of the s-process element Ce, contradicting the idea that NGC 6522 stars are formed from gas enriched by spinstars and indicating that other stellar sources such as massive AGB stars could be the primary polluters of intra-cluster medium. The peculiar abundance signatures of SG stars have been observed in our data, confirming the presence of multiple generations of stars in NGC 6522.
We report the peculiar chemical abundance patterns of 11 atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical ...giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low Mg abundances (Mg/Fe < 0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich (Fe/H −1.0) sample stars, which is at odds with actual observations of SG stars in Galactic GCs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy, a fundamental step forward to understanding the Galactic formation and evolution.
We present a combination of high-resolution Hubble Space Telescope and wide-held ground-based and Galaxy Evolution Explorer data of the Galactic globular cluster M10 (NGC 6254). By using this large ...data set, we determined the center of gravity of the cluster and we built its density profile from star counts over its entire radial extension. We find that the density profile is well reproduced by a single-mass King model with structural parameters c = 1.41 and rc = 41". We also studied the blue straggler star (BSS) population and its radial distribution. We count a total number of 120 BSS within the tidal radius. Their radial distribution is bimodal: highly peaked in the cluster center, decreasing at intermediate distances, and rising again outward. We discuss these results in the context of the dynamical clock scheme presented by Ferraro et al. and of recent results about the radial distribution of binary systems in this cluster.
Context. The Mount Wilson Ca ii index log\hbox{$(R'_{\rm HK})$}(RHK′) is the accepted standard metric of calibration for the chromospheric activity versus age relation for FGK stars. Recent results ...claim its inability to discern activity levels, and thus ages, for stars older than ~2 Gyr, which would severely hamper its application to date disk stars older than the Sun. Aims. We present a new activity-age calibration of the Mt. Wilson index that explicitly takes mass and Fe/H biases into account; these biases are implicit in samples of stars selected to have precise ages, which have so far not been appreciated. Methods. We show that these selection biases tend to blur the activity-age relation for large age ranges. We calibrate the Mt. Wilson index for a sample of field FGK stars with precise ages, covering a wide range of mass and Fe/H , augmented with data from the Pleiades, Hyades, M 67 clusters, and the Ursa Major moving group. Results. We further test the calibration with extensive new Gemini/GMOS log ()R'HK) data of the old, solar Fe/H clusters, M 67 and NGC 188. The observed NGC 188 activity level is clearly lower than M 67. We correctly recover the isochronal age of both clusters and establish the viability of deriving usable chromospheric ages for solar-type stars up to at least ~6 Gyr, where average errors are ~0.14 dex provided that we explicitly account for the mass and Fe/H dimensions. We test our calibration against asteroseismological ages, finding excellent correlation (ρ = + 0.89). We show that our calibration improves the chromospheric age determination for a wide range of ages, masses, and metallicities in comparison to previous age-activity relations.
Although the spectrum of a prototypical early-type galaxy is assumed to lack emission lines, a substantial fraction (likely as high as 30%) of nearby red sequence galaxy spectra contain emission ...lines with line ratios characteristic of LINERs. We use spectra of similar to 6000 galaxies from the SDSS in a narrow redshift slice (0.06 < z < 0.08) to compare the stellar populations of red sequence galaxies with and without UNER-like emission. The spectra are binned by internal velocity dispersion ( sigma ) and by emission properties to produce high-S/N stacked spectra. The recent stellar population models of R. Schiavon make it possible to measure ages, Fe/H, and individual elemental abundance ratios Mg/Fe, C/Fe, N/Fe, and Ca/Fe for each of the stacked spectra. We find that red sequence galaxies with strong LINER-like emission are systematically 2-3.5 Gyr (10%-40%) younger than their emission-free counterparts at the same sigma . This suggests a connection between the mechanism powering the emission (whether AGN, post-AGB stars, shocks, or cooling flows) and more recent star formation in the galaxy. We find that mean stellar age and Fe/H increase with sigma for all galaxies. Elemental abundance Mg/Fe increases modestly with sigma in agreement with previous results, and C/Fe and N/Fe increase more strongly with sigma than does Mg/Fe. Ca/Fe appears to be roughly solar for all galaxies. At fixed sigma galaxies with fainter r-band luminosities have lower Fe/H and older ages but similar abundance ratios compared to brighter galaxies.
![Cosmic variance in [O/Fe] i...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-6361/small "Cosmic variance in [O/Fe] in the Galactic disk")
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