Cluster Red Giants Research Articles

On the APOGEE DR14 Sodium Spread in the Galactic Open Cluster NGC 188

Since several years ago, the search for multiple populations (MPs) in Galactic open clusters (OCs) has become a field of increasing interest, particularly to the light of the general knowledge that MPs are observed in relatively massive clusters (>= 5 10^4 Mo). I report here measurements of stellar parameters and abundances of 17 different chemical species, including light and iron-peak elements of the old open cluster NGC188, from the APOGEE DR14 database. I selected 15 bonafide cluster red giants that reveal an spread in [Na/Fe] of +-0.16 dex, caused by two different groups of measurements whose mean [Na/Fe] values differ by ~ 0.30 dex. A hint for a subtle anti-correlation with Al and TiII, which also show relative large [X/Fe] dispersions, is also seen in the data. However, [Na/Fe] abundances have been found to be typically affected by uncertainties of 0.16 dex. Furthermore, I directly compared the available spectra for stars with similar atmospheric parameters and it would seem that they look similar. Therefore, I warn users of large spectroscopic surveys to be extra careful when finding peculiar abundance results. Analysis pipelines that run in an unsupervised fashion may produce bad results which may not be noticed before publication.

The Orbit of the New Milky Way Globular Cluster FSR1716 = VVV-GC05∗

Abstract We use deep, multi-epoch near-IR images of the VISTA Variables in the Vía Láctea (VVV) Survey to measure proper motions (PMs) of stars in the Milky Way globular cluster (GC) FSR1716 = VVV-GC05. The color-magnitude diagram of this object, made by using PM-selected members, shows an extended horizontal branch, nine confirmed RR Lyrae (RRL) members in the instability strip, and possibly several hotter stars extending to the blue. Based on the fundamental-mode (ab-type) RRL stars that move coherently with the cluster, we confirmed that FSR1716 is an Oosterhoff I GC with a mean period = 0.574 days. Intriguingly, we detect tidal extensions to both sides of this cluster in the spatial distribution of PM-selected member stars. Also, one of the confirmed RRabs is located ∼11 arcmin in projection from the cluster center, suggesting that FSR1716 may be losing stars due to the gravitational interaction with the Galaxy. We also measure radial velocities (RVs) for five cluster red giants selected using the PMs. The combination of RVs and PMs allow us to compute for the first time the orbit of this GC, using an updated Galactic potential. The orbit results to be confined within kpc, and has eccentricity 0.4 < e < 0.6, with perigalactic distance , and apogalactic distance . We conclude that, in agreement with its relatively low metallicity ([Fe/H] = −1.4 dex), this is an inner-halo GC plunging into the disk of the Galaxy. As such, this is a unique object with which to test the dynamical processes that contribute to the disruption of Galactic GCs.

On the Chemical Abundances of Miras in Clusters: V1 in the Metal-rich Globular NGC 5927*

Abstract We present the first spectroscopic abundance determination of iron, α-elements (Si, Ca, and Ti), and sodium for the Mira variable V1 in the metal-rich globular cluster NGC 5927. We use high-resolution (R ∼ 28,000), high signal-to-noise ratio (∼200) spectra collected with WINERED, a near-infrared (NIR) spectrograph covering simultaneously the wavelength range 0.91–1.35 μm. The effective temperature and the surface gravity at the pulsation phase of the spectroscopic observation were estimated using both optical (V) and NIR time-series photometric data. We found that the Mira is metal-rich ([Fe/H] = −0.55 ± 0.15) and moderately α-enhanced ([α/Fe] = 0.15 ± 0.01, σ = 0.2). These values agree quite well with the mean cluster abundances based on high-resolution optical spectra of several cluster red giants available in the literature ([Fe/H] = − 0.47 ± 0.06, [α/Fe] = + 0.24 ± 0.05). We also found a Na abundance of +0.35 ± 0.20 that is higher than the mean cluster abundance based on optical spectra (+0.18 ± 0.13). However, the lack of similar spectra for cluster red giants and that of corrections for departures from local thermodynamical equilibrium prevents us from establishing whether the difference is intrinsic or connected with multiple populations. These findings indicate a strong similarity between optical and NIR metallicity scales in spite of the difference in the experimental equipment, data analysis, and in the adopted spectroscopic diagnostics.

A phenomenological modification of thermohaline mixing in globular cluster red giants

Thermohaline mixing is a favoured mechanism for the so-called "extra mixing" on the red giant branch of low-mass stars. The mixing is triggered by the molecular weight inversion created above the hydrogen shell during first dredge-up when helium-3 burns via 3He(3He,2p)4He. The standard 1D diffusive mixing scheme cannot simultaneously match carbon and lithium abundances to NGC 6397 red giants. We investigate two modifications to the standard scheme: (1) an advective two stream mixing algorithm, and (2) modifications to the standard 1D thermohaline mixing formalism. We cannot simultaneously match carbon and lithium abundances using our two stream mixing approach. However we develop a modified diffusive scheme with an explicit temperature dependence that can simultaneously fit carbon and lithium abundances to NGC 6397 stars. Our modified diffusive scheme induces mixing that is faster than the standard theory predicts in the hotter part of the thermohaline region and mixing that is slower in the cooler part. Our results infer that the extra mixing mechanism needs further investigation and more observations are required, particularly for stars in different clusters spanning a range in metallicity.

Photometric metallicity map of the Large Magellanic Cloud

We have estimated a metallicity map of the Large Magellanic Cloud (LMC) using the Magellanic Cloud Photometric Survey (MCPS) and Optical Gravitational Lensing Experiment (OGLE III) photometric data. This is a first of its kind map of metallicity up to a radius of 4 - 5 degrees, derived using photometric data and calibrated using spectroscopic data of Red Giant Branch (RGB) stars. We identify the RGB in the V, (V$-$I) colour magnitude diagrams of small subregions of varying sizes in both data sets. We use the slope of the RGB as an indicator of the average metallicity of a subregion, and calibrate the RGB slope to metallicity using spectroscopic data for field and cluster red giants in selected subregions. The average metallicity of the LMC is found to be [Fe/H] = $-$0.37 dex ($\sigma$[Fe/H] = 0.12) from MCPS data, and [Fe/H] = $-$0.39 dex ($\sigma$[Fe/H] = 0.10) from OGLE III data. The bar is found be the most metal-rich region of the LMC. Both the data sets suggest a shallow radial metallicity gradient up to a radius of 4 kpc ($-$0.049$\pm$0.002 dex kpc$^{-1}$ to $-$0.066$\pm$0.006 dex kpc$^{-1}$). Subregions in which the mean metallicity differs from the surrounding areas do not appear to correlate with previously known features; spectroscopic studies are required in order to assess their physical significance.

SODIUM AND OXYGEN ABUNDANCES IN THE OPEN CLUSTER NGC 6791 FROM APOGEE H-BAND SPECTROSCOPY

The open cluster NGC 6791 is among the oldest, most massive and metal-rich open clusters in the Galaxy. High-resolution $H$-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) of 11 red giants in NGC 6791 are analyzed for their chemical abundances of iron, oxygen, and sodium. The abundances of these three elements are found to be homogeneous (with abundance dispersions at the level of $\sim$ 0.05 - 0.07 dex) in these cluster red giants, which span much of the red-giant branch (T$_{\rm eff}$ $\sim$ 3500K - 4600K), and include two red-clump giants. From the infrared spectra, this cluster is confirmed to be among the most metal-rich clusters in the Galaxy ($<$[Fe/H]$>$ = 0.34 $\pm$ 0.06), and is found to have a roughly solar value of [O/Fe] and slightly enhanced [Na/Fe]. Non-LTE calculations for the studied Na I lines in the APOGEE spectral region ($\lambda$16373.86\AA\ and $\lambda$16388.85\AA) indicate only small departures from LTE ($\leq$ 0.04 dex) for the parameter range and metallicity of the studied stars. The previously reported double population of cluster members with different Na abundances is not found among the studied sample.

NGC 5824: a luminous outer halo globular cluster with an intrinsic abundance spread

We present a detailed study of the strengths of the calcium triplet absorption lines in the spectra of a large sample of red giant members of the luminous outer Galactic halo globular cluster NGC 5824. The spectra were obtained with the FORS2 and GMOS-S multi-object spectrographs at the VLT and the Gemini-S telescope, respectively. By comparing the line strengths of the NGC 5824 stars with those for red giants in clusters with well established abundances, we conclude that there is an intrinsic abundance dispersion in NGC 5824 characterized by an inter-quartile range in [Fe/H] of 0.10 dex and a total range of ~0.3 dex. As for omega-Cen and M22, the abundance distribution shows a steep rise on the metal-poor side and a shallower decline on the metal-rich side. There is also some indication that the distribution is not unimodal with perhaps 3 distinct abundance groupings present. NGC 5824 has a further unusual characteristic: the outer surface density profile shows no signs of a tidal cutoff. Instead the profile has a power-law distribution with cluster stars detected to a radius exceeding 400 pc. We postulate that NGC 5824 may be the remnant nuclear star cluster of a now disrupted dwarf galaxy accreted during the formation of the Galaxy's halo. We further speculate that the presence of an intrinsic [Fe/H] spread is the characteristic that distinguishes former nuclear star clusters from other globular clusters.

DETECTION OF SOLAR-LIKE OSCILLATIONS FROM KEPLER PHOTOMETRY OF THE OPEN CLUSTER NGC 6819

Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -- one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation and the frequency of maximum oscillation power. We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

High-resolution optical spectroscopy of a newly discovered post-AGB star with a surprising metallicity in the globular cluster M79

An abundance analysis based on a high-resolution spectrum is presented for a newly discovered post-AGB star in the globular cluster M79. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues. Abundances relative to iron appear to be the same for the post-AGB star and the red giants for the 15 common elements. It is suggested that the explanation for the lower abundances of the post-AGB star may be that its atmospheric structure differs from that of a classical atmosphere; the temperature gradient may be flatter than predicted by a classical atmosphere.

FABRY-PÉROT ABSORPTION LINE SPECTROSCOPY OF THE GALACTIC BAR. II. STELLAR METALLICITIES

We measure the Ca II 8542 line strength in 3360 stars along three lines-of-sight in the Galactic bar: (l,b) ~ (+/-5.0,-3.5) and Baade's Window, using Fabry-Perot (FP) absorption line spectroscopy. This is the first attempt to show that reliable absorption line strengths can be measured using FP spectroscopy. The Ca II 8542 line is a good indicator of metallicity and its calibration to [Fe/H] is determined for globular cluster red giants in previous investigations. We derive such a calibration for the bulge giants and use it to infer metallicities for our full red clump sample (2488 stars) at all three lines-of-sight. We present the stellar metallicity distributions along the major axis of the bar. We find the mean [Fe/H] = -0.09 +/- 0.04 dex in Baade's Window, and find the distribution in this field to agree well with previous works. We find gradients in the mean metallicity and its dispersion w.r.t Baade's WIndow of -0.45 and -0.20 dex respectively at l = +5.5, and of -0.10 dex and -0.20 dex at l = -5.0. We detect a signature of a possible tidal stream at l = +5.5, in both our velocity and metallicity distributions. Its radial velocity indicates that it is not associated with the Sagittarius stream. We also measure the metallicity of a bulge globular cluster NGC 6522 in our Baade's Window field to be -0.90 +/- 0.10 dex, in agreement with recent measurements of Zoccali et al. (2008). This agreement demonstrates the reliability of our metallicity measurements.

The non-peculiar velocity dispersion profile of the stellar system ω Centauri

We present the results of a survey of radial velocities over a wide region extending from r~10 arcmin out to r~80 arcmin (~1.5 tidal radii) within the massive star cluster omega Centauri. The survey was performed with FLAMES@VLT, to study the velocity dispersion profile in the outer regions of this stellar system. We derived accurate radial velocities for a sample of 2557 newly observed stars, identifying 318 bona-fide cluster red giants. Merging our data with those provided by Pancino et al. (2007), we assembled a final homogeneous sample of 946 cluster members that allowed us to trace the velocity dispersion profile from the center out to r~32 arcmin. The velocity dispersion appears to decrease monotonically over this range, from a central value of sigma_{v}~17.2 Km/s down to a minimum value of sigma_{v}~5.2 Km/s. The observed surface brightness profile, rotation curve, velocity dispersion profile and ellipticity profile are simultaneously well reproduced by a simple dynamical model in which mass follows light, within the classical Newtonian theory of gravitation. The comparison with an N-body model of the evolution of a system mimicking omega Cen during the last 10 orbits into the Galactic potential suggests that (a) the rotation of stars lying in the inner ~20 arcmin of the clusters is not due to the effects of the tidal field of the Milky Way, as hypothized by other authors, and (b) the overall observational scenario is still compatible with the possibility that the outer regions of the cluster are subject to some tidal stirring.

DEEP MIXING AND METALLICITY: CARBON DEPLETION IN GLOBULAR CLUSTER GIANTS

We present the results of an observational study of the efficiency of deep mixing in globular cluster red giants as a function of stellar metallicity. We determine [C/Fe] abundances based on low-resolution spectra taken with the Kast spectrograph on the 3m Shane telescope at Lick Observatory. Spectra centered on the 4300 Angstrom CH absorption band were taken for 42 bright red giants in 11 Galactic globular clusters ranging in metallicity from M92 ([Fe/H]=-2.29) to NGC 6712 ([Fe/H]=-1.01). Carbon abundances were derived by comparing values of the CH bandstrength index S2(CH) measured from the data with values measured from a large grid of SSG synthetic spectra. Present-day abundances are combined with theoretical calculations of the time since the onset of mixing, which is also a function of stellar metallicity, to calculate the carbon depletion rate across our metallicity range. We find that the carbon depletion rate is twice as high at a metallicity of [Fe/H]=-2.3 than at [Fe/H]=-1.3, which is a result qualitatively predicted by some theoretical explanations of the deep mixing process.

Obscured clusters. II. GLIMPSE-C02 – A new metal rich globular cluster in the Milky Way

The estimated total number of Milky Way globulars is 160+-20. The question of whether there are any more undiscovered globular clusters in the Milky Way is particularly relevant with advances in near and mid-IR instrumentation. This investigation is a part of a long-term project to search the inner Milky Way for hidden star clusters and to study them in detail. GLIMPSE-C02 (G02) is one of these objects, situated near the Galactic plane (l=14.129deg, b=-0.644deg). Our analysis is based on SOFI/NTT JHKs imaging and low resolution (R~1400) spectroscopy of three bright cluster red giants in the K atmospheric window. We derived the metal abundance by analysis of these spectra and from the slope of the RGB. The cluster is deeply embedded in dust and undergoes a mean reddening of Av~24.8+-3 mag. The distance to the object is D=4.6+-0.7kpc. The metal abundance of G02 is [Fe/H](H96)=-0.33+-0.14 and [Fe/H](CG)=-0.16+-0.12 using different scales. The best fit to the radial surface brightness profile with a single-mass King's model yields a core radius rc=0.70 arcmin (0.9pc), tidal radius rt=15 arcmin (20pc), and central oncentration c=1.33. We demonstrate that G02 is new Milky Way globular cluster, among the most metal rich globular clusters in the Galaxy. The object is physically located at the inner edge of the thin disk and the transition region with the bulge, and also falls in the zone of the missing globulars toward the central region of the Milky Way.

An Improved Bandstrength Index for the CH G Band of Globular Cluster Giants

ABSTRACTSpectral indices are useful tools for quantifying the strengths of features in moderate-resolution spectra and relating them to intrinsic stellar parameters. This paper focuses on the 4300 Å CH G-band, a classic example of a feature interpreted through use of spectral indices. G-band index definitions, as applied to globular clusters of different metallicity, abound in the literature, and transformations between the various systems, or comparisons between different authors’ work, are difficult and not always useful. We present a method for formulating an optimized G-band index, using a large grid of synthetic spectra. To make our new index a reliable measure of carbon abundance, we minimize its dependence on [N/Fe] and simultaneously maximize its sensitivity to [C/Fe]. We present a definition for the new index S2(CH), along with estimates of the errors inherent in using it for [C/Fe] determination, and conclude that it is valid for use with spectra of bright globular cluster red giants over a large range in , [C/Fe], and [N/Fe].

Red giants in open clusters

Context. Radial velocities have proved to be an efficient method for membership determination if there are at least 2 or 3 red giants in a cluster. They are necessary for galactic studies, but are still missing for many open clusters.

CCD BVI photometry and Coravel observations of stars in the open cluster NGC 2489

Abstract We present CCD BVI photometry for the southern open cluster NGC 2489 and its surrounding field. The sample consists of 2182 stars measured in an area of 13.6 × 13.6 arcmin2, extending down to V∼ 21.5. These data are supplemented with CORAVEL radial-velocity observations for seven red giant candidates. A cluster angular radius of 6.7 ± 0.6 arcmin, equivalent to 3.5 ± 0.3 pc, is estimated from star counts carried out inside and outside the cluster region. The comparison of the cluster colour–magnitude diagrams with isochrones of the Padova group yields E(B−V) = 0.30 ± 0.05, E(V−I) = 0.40 ± 0.05 and V−MV= 12.20 ± 0.25 for log t= 8.70 (t= 500+130−100 Myr) and Z= 0.019. NGC 2489 is then located at 1.8 ± 0.3 kpc from the Sun and 25 pc below the Galactic plane. The analysis of the kinematical data allowed us to confirm cluster membership for six red giants, one of them being a spectroscopic binary. A mean radial velocity of 38.13 ± 0.33 km s−1 was derived for the cluster red giants. The properties of a sample of open clusters aligned along the line of sight of NGC 2489 are examined.

Spectroscopy of Six Red Giants in the Draco Dwarf Spheroidal Galaxy11The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M.

ABSTRACTKeck Observatory LRIS‐B (Low Resolution Imaging Spectrometer) spectra are reported for six red giant stars in the Draco dwarf spheroidal galaxy and several comparison giants in the globular cluster M13. Indexes that quantify the strengths of the Ca ii H and K lines, the λ3883 and λ4215 CN bands, and the λ4300 G band have been measured. These data confirm evidence of metallicity inhomogeneity within Draco obtained by previous authors. The four brightest giants in the sample have absolute magnitudes in the range − 2.6<MV < - 2.3. They have a range in B - V color, metallicities less than that of M13, and exhibit a dispersion in both calcium and CH indices. Our data indicate that a spread in [C/H] likely exists among the Draco tip giants and that some giants have higher [C/Fe] ratios than is typical of giants in the globular clusters M13 and M92. Several suggestions are made as to why some Draco stars may have higher [C/Fe] ratios than globular cluster red giants: deep mixing might be inhibited in these Draco stars, they may formerly have been mass‐transfer binaries that acquired carbon from a more massive companion, or the Draco dwarf galaxy may have experienced relatively slow chemical evolution over a period of several billion years, allowing carbon‐enhanced ejecta from intermediate‐mass asymptotic giant branch stars to enrich the interstellar medium while star formation was still occurring.

Abundances in Red Giant Stars of NGC 2808 and Correlations between Chemical Anomalies and Global Parameters in Globular Clusters

We present the abundance analysis of stars from the tip of the red giant branch (RGB) to below the RGB bump in the globular cluster NGC 2808 based on high-resolution echelle spectra. We derive abundances of Al, α-process elements (Si I, Ca I, Ti I, and Ti II), and Fe-group elements (Sc II, V I, Cr I, Cr II, Mn I, Co I, and Ni I). Apart from Mg being somewhat reduced, likely because it has been depleted at the expense of Al in the MgAl cycle, the other α-element ratios show the overabundance typical of halo stars of similar metallicity. Mn is underabundant, whereas Fe-group elements have typical abundance ratios near the solar value. We detect star-to-star differences in Al abundances from the RGB tip down to the faintest star below the RGB bump, correlated with Na abundances at all luminosities. The slope of the Na-Al correlation is similar to the one found in M13 by Sneden et al., but it is different from those in other globular clusters of similar metallicity. We find that the amount of chemical inhomogeneities along the Na-O and Mg-Al anticorrelations in globular cluster red giants is correlated with the present-day cluster mass and ellipticity. Moreover, we find for the first time a correlation between the spread in proton-capture elements and orbital parameters of clusters. The chemical anomalies are more extended in clusters having large-sized orbits and longer periods and those with larger inclination angles of the orbit with respect to the Galactic plane.

Stellar mass loss and the intracluster medium in Galactic globular clusters: a deep radio survey for H i and OH

We present the results of a survey, the deepest to date, for H i emission at 21 cm and OH emission at 18 cm (lines at 1612, 1665, 1667 and 1720 MHz) in the direction towards the Galactic globular clusters M15, M2, NGC6934, NGC7006 and Pal13. The aim is to measure the amount of hydrogen in the intra-cluster medium (ICM), and to find OH masers in the circumstellar envelopes of globular cluster red giants. We present a tentative detection of 0.3 M⊙ of neutral hydrogen in M15 and possible detections of neutral hydrogen in M2 and Pal13. We derive upper limits to the neutral hydrogen content of NGC6934 and NGC7006. No OH emission is detected. We also present deep H i data of the northern tip of the Magellanic Stream behind Pal13.

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

We summarize a selection process to identify red giants in the direction of the southern warp of the Galactic disk, employing VIC photometry and multiobject spectroscopy. We also present results from follow-up high-resolution, high signal-to-noise echelle spectroscopy of three field red giants, finding [Fe/H] values of about -0.5. The field stars, with galactocentric distances estimated at 10–15 kpc, support the conclusion of Yong and coworkers that the Galactic metallicity gradient disappears beyond RGC values of 10–12 kpc for the older stars and clusters of the outer disk. We summarize the detailed abundance patterns for 15 other elements for these stars and compare them with recently obtained results for old open cluster red giants in 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 α-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 gigayears. We also compare our results with those available for much younger objects. The limited results for the H II regions and B stars in the outer disk also suggest that the radial metallicity gradient in the outer disk is shallow or absent. The much more extensive results for Cepheids confirm these trends and that the change in slope of the metallicity gradient may occur at a larger galactocentric distance than for the older stars and clusters. However, the younger stars also show rising α-element enhancements with increasing RGC, at least beyond 12 kpc. These trends are consistent with the idea of a progressive growth in the size of the Galactic disk with time and episodic enrichment by Type II supernovae as part of the disk's growth.