High-resolution spectroscopic study of red clump stars in the Galaxy: iron-group elements (original) (raw)

Elemental Abundance Ratios in Stars of the Outer Galactic Disk. II. Field Red Giants

Astronomical Journal - ASTRON J, 2005

We summarize a selection process to identify red giants in the direction of the southern warp of the Galactic disk, employing VI_C photometry and multi-object spectroscopy. We also present results from follow-up high-resolution, high-S/N echelle spectroscopy of three field red giants, finding [Fe/H] values of about -0.5. The field stars, with Galactocentric distances estimated at 10 to 15 kpc, support the conclusion of Yong, Carney, & de Almeida (2005) that the Galactic metallicity gradient disappears beyond R_GC values of 10 to 12 kpc for the older stars and clusters of the outer disk. The field and cluster stars at such large distances show very similar abundance patterns, and, in particular, all show enhancements of the "alpha" elements O, Mg, Si, Ca, and Ti and the r-process element Eu. These results suggest that Type II supernovae have been significant contributors to star formation in the outer disk relative to Type Ia supernovae within the past few Gyrs. We also com...

The Hipparcos HR diagram of nearby stars in the metallicity range -1.0 < [Fe/H] < 0.3

arXiv (Cornell University), 1999

The Hipparcos mission has provided very high quality parallaxes of a sample of a hundred nearby disk stars, of spectral types F to K. In parallel, bolometric fluxes, effective temperatures, and accurate Fe/H ratios of many of these stars became available through infrared photometry and detailed spectroscopic analyses. These new accurate data allow to build the Hertzsprung-Russell diagram of stars of the solar neighbourhood with the smallest error bars ever obtained. We analyse these observations by means of theoretical stellar models, computed with the most recent input physics. We first examine the positions of the objects versus standard theoretical isochrones, corresponding to their chemical composition and age. For these isochrones we have first assumed that the helium content was varying in locksteps with metallicity. The comparison becomes age-independent in the lower part of the HR diagram, where evolutionary effects are negligible. We show that for the unevolved stars, the agreement between real stars and models is fairly satisfactory for stars with metallicity within ± 0.3 dex of the solar metallicity, but that a conflict exists for stars with metallicity less than [Fe/H] = −0.5. This conflict cannot be resolved by decreasing the helium abundance: values of this abundance below the primordial abundance would be required. On the basis of recent works, we show that the addition of two processes not included in standard models can help solving the above discrepancy. These are (i) correcting the LTE iron abundances using a non-LTE approach and (ii) including microscopic diffusion of He and heavier elements in the stellar interior. The case of the binary star µ Cas is particularly useful to support this conclusion as its mass is also known from its orbit. After inclusion of the two

Metal Abundances of Red Clump Stars in Open Clusters. I. NGC 6819

The Astronomical Journal, 2001

We present an analysis of high dispersion spectra (R ∼ 40, 000) of three red clump stars in the old open cluster NGC 6819. The spectra were obtained with SARG, the high dispersion spectrograph of the Telescopio Nazionale Galileo. The spectra were analyzed using both equivalent widths measured with an automatic procedure, and comparisons with synthetic spectra. NGC 6819 is found to be slightly metal-rich ([Fe/H]=+0.09 ± 0.03, internal error); there are no previous high resolution studies to compare with. Most element-to-element abundance ratios are close to solar; we find a slight excess of Si, and a significant Na overabundance. Our spectra can also be used to derive the interstellar reddening towards the cluster, by comparing the observed colours with those expected from line excitation: we derive E(B − V ) = 0.14 ± 0.04, in agreement with the most recent estimate for this cluster.

OCCASO – II. Physical parameters and Fe abundances of red clump stars in 18 open clusters

Monthly Notices of the Royal Astronomical Society, 2017

Open clusters have long been used to study the chemodynamical evolution of the Galactic disc. This requires a homogeneously analysed sample covering a wide range of ages and distances. In this paper, we present the Open Clusters Chemical Abundances from Spanish Observatories (OCCASO) second data release. This comprises a sample of high-resolution (R > 65 000) and high signal-to-noise spectra of 115 red clump stars in 18 open clusters. We derive atmospheric parameters (T eff , log g, ξ), and [Fe/H] abundances using two analysis techniques: equivalent widths and spectral synthesis. A detailed comparison and a critical review of the results of the two methods are made. Both methods are carefully tested between them, with the Gaia FGK benchmark stars, and with an extensive sample of literature values. We perform a membership study using radial velocities and the resulting abundances. Finally, we compare our results with a chemodynamical model of the Milky Way thin disc concluding that the oldest open clusters are consistent with the models only when dynamical effects are taken into account.

Intrinsic iron spread and a new metallicity scale for globular clusters

Astronomy & Astrophysics, 2009

We have collected spectra of about 2000 red giant branch (RGB) stars in 19 Galactic globular clusters (GC) using FLAMES@VLT (about 100 stars with GIRAFFE and about 10 with UVES, respectively, in each GC). These observations provide an unprecedented, precise, and homogeneous data-set of Fe abundances in GCs. We use it to study the cosmic scatter of iron and find that, as far as Fe is concerned, most GCs can still be considered mono-metallic, since the upper limit to the scatter of iron is less than 0.05 dex, meaning that the degree of homogeneity is better than 12%. The scatter in Fe we find seems to have a dependence on luminosity, possibly due to the well-known inadequacies of stellar atmospheres for upper-RGB stars and/or to intrinsic variability. It also seems to be correlated with cluster properties, like the mass, indicating a larger scatter in more massive GCs which is likely a (small) true intrinsic scatter. The 19 GCs, covering the metallicity range of the bulk of Galactic GCs, define an accurate and updated metallicity scale. We provide transformation equations for a few existing scales. We also provide new values of [Fe/H], on our scale, for all GCs in the Harris catalogue.

A Large Sample Study of Red Giants in the Globular Cluster Omega Centauri (NGC 5139)

The Astrophysical Journal, 2009

We present abundances of several light, α, Fe-peak, and neutron-capture elements for 66 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri (ω Cen). Our observations lie in the range 12.0<V<13.5 and focus on the intermediate and metal-rich RGBs. Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution (R≈18,000) spectra obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. Combining these data with previous work, we find that there are at least four peaks in the metallicity distribution function at [Fe/H]=-1.75, -1.45, -1.05, and -0.75, which correspond to about 55%, 30%, 10%, and 5% of our sample, respectively. Additionally, the most metal-rich stars are the most centrally located. Na and Al are correlated despite exhibiting starto-star dispersions of more than a factor of 10, but the distribution of those elements appears to be metallicity dependent and are divided at [Fe/H]≈-1.2. About 40-50% of stars with [Fe/H]<-1.2 have Na and Al abundances consistent with production solely in Type II supernovae and match observations of disk and halo stars at comparable metallicity. The remaining metal-poor stars are enhanced in Na and Al compared to their disk and halo counterparts and are mostly consistent with predicted yields from >5 M ⊙ asymptotic giant branch (AGB) stars. At [Fe/H]>-1.2, more than 75% of the stars are Na/Al enhanced and may have formed almost exclusively from AGB ejecta. Most of these stars -2are enhanced in Na by at least 0.2 dex for a given Al abundance than would be expected based on "normal" globular cluster values. All stars in our sample are α-rich with [Ca/Fe] =+0.36 (σ=0.09) and [Ti/Fe] =+0.23 (σ=0.14). The Fepeak elements give solar-scaled abundances and similarly small dispersions with [Sc/Fe] =+0. 09 (σ=0.15) and [Ni/Fe] =-0.04 (σ=0.09). Europium does not vary extensively as a function of metallicity and has [Eu/Fe] =+0.19 (σ=0.23). However, [La/Fe] varies from about -0.4 to +2 and stars with [Fe/H] -1.5 have [La/Eu] values indicating domination by the s-process. A quarter of our sample have [La/Eu]≥+1

Homogeneous photometry and metal abundances for a large sample of Hipparcos metal-poor stars

Monthly Notices of the Royal Astronomical Society, 1999

Homogeneous photometric data (Johnson V , B − V , V − K, Cousins V − I and Strömgren b − y), radial velocities, and abundances of Fe, O, Mg, Si, Ca, Ti, Cr and Ni are presented for 99 stars with high precision parallaxes measured by the HIPPARCOS satellite. These data have been previously used to assist the derivation of accurate distances and ages of galactic globular clusters. Magnitudes and colours for the programme stars were obtained combining and standardizing carefully selected literature data available in the Simbad data-base and V and B − V values measured by the Hipparcos/Tycho mission. Comparison of colours for our targets suggests: (i) ground-based and Tycho B − V 's agree well for colours bluer than 0.75 mag, but have a lot of scatter for redder colours; (ii) the Hipparcos V − I colours have a very large scatter and a zero point offset of +0.02 mag compared to the literature values. The programme stars have metal abundances in the range −2.5 <[Fe/H]< 0.2. The spectroscopic observational data set consists of high dispersion (15 000 < R < 60 000), high S/N (> 200) spectra obtained at the Asiago and McDonald Observatories for 66 stars. The analysis is carried out following the same precepts used in previous papers of this series and includes corrections for departures from LTE in the formation of the O lines. The main results are: (i) the equilibrium of ionization of Fe is well satisfied in late F-early K-dwarfs; (ii) Oxygen and α−elements are overabundant by ∼ 0.3 dex.

Fe and Al Abundances for 180 Red Giants in the Globular Cluster Omega Centauri (NGC 5139)

The Astrophysical Journal, 2008

We present radial velocities, Fe, and Al abundances for 180 red giant branch (RGB) stars in the Galactic globular cluster Omega Centauri (ω Cen). The majority of our data lie in the range 11.0<V<13.5, which covers the RGB from about 1 mag. above the horizontal branch to the RGB tip. The selection procedures are biased towards preferentially observing the more metal-poor and luminous stars of ω Cen. Abundances were determined using equivalent width measurements and spectrum synthesis analyses of moderate resolution spectra (R≈13,000) obtained with the Blanco 4m telescope and Hydra multifiber spectrograph. Our results are in agreement with previous studies as we find at least four different metallicity populations with [Fe/H]=-1.75, -1.45, -1.05, and -0.75, with a full range of -2.20 [Fe/H] -0.70. [Al/Fe] ratios exhibit large star-to-star scatter for all populations, with the more than 1.0 dex range of [Al/Fe] decreasing for stars more metal-rich than [Fe/H]∼-1.4. The minimum [Al/Fe] abundance observed for all metallicity populations is [Al/Fe]∼+0.15. The maximum abundance of log ǫ(Al) is reached for stars with [Fe/H]∼-1.4 and does not increase further with stellar metallicity. We interpret these results as evidence for type II SNe providing the minimum [Al/Fe] ratio and a mass spectrum of intermediate mass asymptotic giant branch stars causing the majority of the [Al/Fe] scatter.

Detailed Iron-peak Element Abundances in Three Very Metal-poor Stars

The Astrophysical Journal, 2020

We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z=21-30) in three very metal-poor ([Fe/H]≈−3) stars: BD+03 o 740, BD−13 o 3442, and CD−33 o 1173. High-resolution ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300-3050Å were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for a number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with [Fe/H]−2. The abundances of these elements in BD+03 o 740, BD−13 o 3442, CD−33 o 1173, and HD 84937 (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of [Cr I/Fe]=+0.01) and ionized species (mean of [Cr II/Fe]=+0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of [Co I/H]=−2.53), while no such enhancement is seen for the ionized species (mean of [Co II/H]=−2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.

First Stars. III. A detailed elemental abundance study of four extremely metal-poor giant stars

Astronomy and Astrophysics, 2003

This paper reports detailed abundance analyses for four extremely metal-poor (XMP) giant stars with [Fe/H] < −3.8, based on high-resolution, high-S /N spectra from the ESO VLT (Kueyen/UVES) and LTE model atmosphere calculations. The derived [α/Fe] ratios in our sample exhibit a small dispersion, confirming previous findings in the literature, i.e. a constant overabundance of the α-elements with a very small (if any) dependence on [Fe/H]. In particular, the very small scatter we determine for [Si/Fe] suggests that this element shows a constant overabundance at very low metallicity, a conclusion which could not have been derived from the widely scattered [Si/Fe] values reported in the literature for less metal-poor stars. For the iron-peak elements, our precise abundances for the four XMP stars in our sample confirm the decreasing trend of Cr and Mn with decreasing [Fe/H], as well as the increasing trend for Co and the absence of any trend for Sc and Ni. In contrast to the significant spread of the ratios [Sr/Fe] and [Ba/Fe], we find [Sr/Ba] in our sample to be roughly solar, with a much lower dispersion than previously found for stars in the range −3.5 < [Fe/H] < −2.5.