Red giants in open clusters (original) (raw)
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Membership, binarity and metallicity of red giants in the southern open cluster NGC 2354
Astronomy and Astrophysics Supplement Series, 1999
We present new Coravel radial-velocity observations and photoelectric photometry in the UBV , DDO and Washington systems for a sample of red giant candidates in the field of the intermediate-age open cluster NGC 2354. Photometric membership probabilities show very good agreement with those obtained from Coravel radial velocities. The analysis of the photometric and kinematical data allow us to confirm cluster membership for 9 red giants, one of them being a spectroscopic binary, while 4 confirmed spectroscopic binaries appear to be probable members. We have also discovered 4 spectroscopic binaries not belonging to the cluster. A mean radial velocity of (33.40 ± 0.27) km s −1 and a mean reddening E(B−V) = 0.13 ± 0.03 were derived for the cluster giants. NGC 2354 has a mean ultraviolet excess < δ(U − B) > = −0.03 ± 0.01, relative to the field K giants, and a mean new cyanogen anomaly ∆CN = −0.035 ± 0.007, both implying [Fe/H] ≈ −0.3. The moderately metal-poor character of NGC 2354 is confirmed using five different metal abundance indicators of the Washington system. The cluster giant branch is formed by a well defined clump of 7 stars and 4 stars with high membership probabilities seem to define an ascending giant branch. The whole red giant locus cannot be reproduced by any theoretical track.
Photometric observations and Coravel radial velocities of red giant candidates in open clusters
BAAA, 2008
We present high precision multicolour photoelectric photometry together with Coravel radial-velocity data for a sample of red giant (RG) candidates in nine Galactic open clusters. Nearly 82% of the analized stars are found to be cluster RGs from photometric criteria, in excellent agreement with the Coravel membership probabilities. E(B − V) colour excesses, effective temperatures and metallicities are derived for the cluster giants. A new spectroscopic binary was discovered among the RGs of NGC 2482. Resumen. Presentamos fotometría fotoeléctrica multicolor de alta precisión y velocidades radiales (VRs) Coravel para una muestra de candidatas a gigantes rojas (GRs) en nueve cúmulos abiertos galácticos. Encontramos que aproximadamente el 82 % de las estrellas analizadas son GRs de los cúmulos, de acuerdo a criterios fotométricos, en excelente acuerdo con los resultados inferidos a partir de las VRs Coravel. Se determinan excesos de color E(B − V), temperaturas efectivas y metalicidades para las gigantes de los cúmulos. Una nueva binaria espectroscópica fue descubierta entre las GRs de NGC 2482.
Photometric membership and metallicities of red giant candidates in selected open clusters
Astronomische Nachrichten, 2008
Lodén 807)-stars: abundances-techniques: photometry-techniques: radial velocities As part of a long-term project to determine abundances and astrophysical properties of evolved red stars in open clusters, we present high-precision DDO photoelectric observations for a sample of 33 red giant candidates projected in the fields of nine Galactic open clusters. These data are supplemented with UBV photoelectric photometry of 24 of these stars as well as with CORAVEL radial-velocity observations for 13 red giant candidates in four of the clusters. We also present Washington photoelectric photometry of a small sample of red giant candidates of the open cluster Ruprecht 97. The likelihood of cluster membership for each star photometrically observed and for 23 additional red giant candidates with UBV and DDO data available in the literature, is evaluated by using two independent photometric criteria. Nearly 82% of the analysed stars are found to have a high probability of being cluster giants. Photometric membership probabilities show very good agreement with those obtained from CORAVEL radial velocities. While E(B − V) colour excesses were determined from combined B − V and DDO colours, calibrations of the DDO system were used to derive MK spectral types, effective temperatures and metallicities.The derived DDO metallicities range between values typical of moderately metal-poor clusters ([Fe/H] =-0.19) to moderately metal-rich ([Fe/H] = 0.25) ones.
Spectroscopy of red giants in the open clusters NGC 1545 and Tr2
Monthly Notices of the Royal Astronomical Society, 2011
The results of local thermodynamic equilibrium (LTE) abundance analysis are presented for four red giants in the fields of open clusters NGC 1545 and Tr2. The membership of HD 27276, HD 27292 and HD 16068 was confirmed and the age of NGC 1545 was updated: log age(yr) = 7.95. A mild deficiency of iron-group elements was found on average for NGC 1545 and Tr2, [Fe/H] = −0.13 and −0.07 with an uncertainty of about 0.08 dex. Oxygen and carbon are moderately depleted in all cluster giants, [O/Fe] = −0.23 ± 0.05 dex and [C/Fe] = −0.56 ± 0.07 dex on average. Nitrogen is enhanced, [N/Fe] values in range +0.4 to +1.2 dex, and lithium depleted. Sodium is slightly enhanced, [Na/Fe] = +0.26 ± 0.04 dex on average. Modified abundances are in agreement with the chemical evolution of intermediate-mass stars on the red giant branch. Some occasional radial velocity variations are probably due to convection in the atmospheres of luminous giants.
Multicolour photometry and Coravel observations of stars in the southern open cluster IC?2488
Astronomy and Astrophysics, 2003
We present new UBV photoelectric observations of 119 stars in the field of the southern open cluster IC 2488, supplemented by DDO and Washington photometry and Coravel radial velocities for a sample of red giant candidates. Nearly 50% of the stars sampled-including three red giants and one blue straggler-are found to be probable cluster members. Photometric membership probabilities of the red giant candidates show good agreement with those obtained from Coravel data. A mean radial velocity of (−2.63 ± 0.06) km s −1 is derived for the cluster giants. The reddening across the cluster is found to be uniform, the mean value being E(B − V) = 0.24 ± 0.04. IC 2488, located at a distance of (1250 ± 120) pc from the Sun and 96 pc below the Galactic plane, is most probably not related to the planetary nebula ESO 166-PN21. A metal abundance [Fe/H] = 0.10 ± 0.06 relative to the Sun is determined from DDO data of the red giant members, in good agreement with the [Fe/H] values derived from five independent Washington abundance indices. An age of 180 Myr is determined from the fitting of isochrones computed with convective overshooting for Z = 0.019. The isochrone for log t = 8.25 reproduces remarkably well not only the morphology of the upper main sequence but also the observed red giant pattern.
Asteroseismic inferences on red giants in open clusters NGC 6791, NGC 6819, and NGC 6811 usingKepler
Astronomy & Astrophysics, 2011
Context. Four open clusters are present in the Kepler field of view and timeseries of nearly a year in length are now available. These timeseries allow us to derive asteroseismic global oscillation parameters of red-giant stars in the three open clusters NGC 6791, NGC 6819 and NGC 6811. From these parameters and effective temperatures, we derive masses, radii and luminosities for the clusters as well as field red giants. Aims. We study the influence of evolution and metallicity on the observed red-giant populations. Methods. The global oscillation parameters are derived using different published methods and the effective temperatures are derived from 2MASS colours. The observational results are compared with BaSTI evolution models. Results. We find that the mass has significant influence on the asteroseismic quantities Δν vs. ν max relation, while the influence of metallicity is negligible, under the assumption that the metallicity does not affect the excitation/damping of the oscillations. The positions of the stars in the H-R diagram depend on both mass and metallicity. Furthermore, the stellar masses derived for the field stars are bracketed by those of the cluster stars. Conclusions. Both the mass and metallicity contribute to the observed difference in locations in the H-R diagram of the old metal-rich cluster NGC 6791 and the middle-aged solar-metallicity cluster NGC 6819. For the young cluster NGC 6811, the explanation of the position of the stars in the H-R diagram challenges the assumption of solar metallicity, and this open cluster might have significantly lower metallicity [Fe/H] in the range −0.3 to −0.7 dex. Also, nearly all the observed field stars seem to be older than NGC 6811 and younger than NGC 6791.
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
High-resolution abundance analysis of red giants in the globular cluster NGC 6522
Astronomy & Astrophysics, 2014
Context. The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are very metal-poor and in a few individual stars of the oldest known Milky Way globular cluster NGC 6522 have been interpreted as evidence of early enrichment by massive fastrotating stars (spinstars). Because NGC 6522 is a bulge globular cluster, the suggestion was that not only the very-metal poor halo stars, but also bulge stars at [Fe/H] ∼−1 could be used as probes of the stellar nucleosynthesis signatures from the earlier generations of massive stars, but at much higher metallicity. For the bulge the suggestions were based on early spectra available for stars in NGC 6522, with a medium resolution of R ∼ 22 000 and a moderate signal-to-noise ratio. Aims. The main purpose of this study is to re-analyse the NGC 6522 stars reported previously by using new high-resolution (R ∼ 45 000) and high signal-to-noise spectra (S /N > 100). We aim at re-deriving their stellar parameters and elemental ratios, in particular the abundances of the neutron-capture s-process-dominated elements such as Sr, Y, Zr, La, and Ba, and of the r-element Eu. Methods. High-resolution spectra of four giants belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration. The spectroscopic parameters were derived based on the excitation and ionization equilibrium of Fe i and Fe ii. Results. Our analysis confirms a metallicity [Fe/H] = −0.95 ± 0.15 for NGC 6522 and the overabundance of the studied stars in Eu (with +0.2 < [Eu/Fe] < +0.4) and alpha-elements O and Mg. The neutron-capture s-element-dominated Sr, Y, Zr, Ba, and La now show less pronounced variations from star to star. Enhancements are in the range 0.0 < [Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] < +0.35, and 0.05 < [Ba/Fe] < +0.55. Conclusions. The very high overabundances of [Y/Fe] previously reported for the four studied stars is not confirmed with the new high-quality spectra. The moderate enhancement in [Sr/Fe] previously reported for one of the re-studied stars is confirmed, but the strong enhancements of this ratio for the other two stars are not confirmed. Despite the moderate enhancements found for the neutroncapture s-element-dominated species, none of the four stars studied here show positive values for all [Sr/Ba], [Y/Ba] and [Zr/Ba] ratios. The re-studied stars are now compatible not only with the interpretation that the s-process enhancements in these very old stars are due to spinstars, but also with alternative models such as mass transfer from s-process-rich AGB stars. Note, however, that when our results are interpreted in the context of more extended datasets from the literature, the spinstar scenario still seems to be favoured.
Astronomy and Astrophysics Supplement Series, 1996
We present multiple-epoch radial-velocity observations for 104 stars in a 10 × 10 field of the intermediate-age open cluster IC 4651 to V ∼ 14.5. Only 13 stars (13%) of the full sample are field stars. From the 44 single member stars we find a mean radial velocity of −30.76 ± 0.20 km s −1 , and the 12 single red-giant members yield a true radial-velocity dispersion of 0.74 km s −1. Of the 19 giant members, 7 (37%) are spectroscopic binaries with periods up to 5000 days, while 35 (52%) of the 67 main-sequence and turnoff members are binaries with periods less than ∼1000 days. Combined with our deep, accurate CCD Strömgren photometry in a ∼21 × 21 field of IC 4651 (Meibom 2000), these data substantially improve the definition of the cluster locus in the colour-magnitude diagram and the spatial structure of the cluster, although the photometry shows that IC 4651 contains at least twice as many stars on the upper main sequence as was believed when the radialvelocity survey was initiated. The single cluster members define a very tight sequence in the CMD, and two sets of isochrones from stellar models with convective overshooting (d/Hp = 0.2) have been fit to it. Our best estimate for the age of IC 4651 is 1.7 ± 0.15 Gyr, assuming [Fe/H] = 0.12 (Hyades) and E (b−y) = 0.071. Including the ∼650 stars newly discovered from the photometry, we estimate the present total mass of IC 4651 to be ∼630 M , excluding any undetected stellar remnants. The corresponding tidal cutoff radius is ∼22. IC 4651 shows evidence of moderate mass segregation: Most of the turn-off stars and nearly all the red giants are located at radii smaller than ∼7 , while the lower main-sequence stars are less centrally concentrated. The spatial distributions of cluster and field stars indicate that additional cluster stars are probably still to be found outside the fields studied so far. Comparison of the present mass function of IC 4651 with plausible initial mass functions indicates that the cluster initially contained at least ∼8300 stars with a total mass of ∼5300 M. Thus, of the original cluster stars only ∼7%, containing ∼12% of the initial mass, remain today. Of the initial cluster mass, ∼35% has been lost due to evolution of the most massive stars into white dwarfs or other remnants while the remaining ∼53%, comprising ∼93% of the original low-mass stars, appear to have migrated out of the observed field or been lost from the cluster altogether. IC 4651 is currently 1 kpc closer to the Galactic center than its "sister" cluster NGC 3680 (Nordström et al. 1997), but their Galactic orbital parameters indicate that the mean orbital radius of IC 4651 is in fact larger by 0.7 kpc, providing a plausible reason why it is much less advanced in its dynamical evolution than the coeval cluster NGC 3680.