Red Giants In NGC Research Articles

New red giants in NGC 6791 and NGC 6819 using Kepler superstamps

Context. Stars that are members of stellar clusters are assumed to be formed at the same time and place from material with the same initial chemical composition. These additional constraints on the ensemble of cluster stars make these stars suitable as benchmarks. Aims. We aimed (1) to identify previously unknown red giants in the open clusters NGC 6791 and NGC 6819, (2) to extract their asteroseismic parameters, and (3) to determine their cluster membership. Methods. We followed a dedicated method based on difference imaging to extract the light curves of potential red giants in NGC 6791 and NGC 6819 from Kepler superstamp data. We extracted the asteroseismic parameters of the stars that showed solar-like oscillations. We performed an asteroseismic membership study to identify which of these stars are likely to be cluster members. Results. We found 149 red giant stars within the Kepler superstamps, 93 of which are likely cluster members. We were able to find 29 red giants that are not primary targets of Kepler, and therefore, their light curves had not been released previously. Five of these previously unknown red giants have a cluster membership probability greater than 95%.

Convective-core Overshoot and Suppression of Oscillations: Constraints from Red Giants in NGC 6811

Abstract Using data from the NASA spacecraft Kepler, we study solar-like oscillations in red giant stars in the open cluster NGC 6811. We determine oscillation frequencies, frequency separations, period spacings of mixed modes, and mode visibilities for eight cluster giants. The oscillation parameters show that these stars are helium-core-burning red giants. The eight stars form two groups with very different oscillation power spectra; the four stars with the lowest Δν values display rich sets of mixed l = 1 modes, while this is not the case for the four stars with higher Δν. For the four stars with lowest Δν, we determine the asymptotic period spacing of the mixed modes, ΔP, which together with the masses we derive for all eight stars suggest that they belong to the so-called secondary clump. Based on the global oscillation parameters, we present initial theoretical stellar modeling that indicates that we can constrain convective-core overshoot on the main sequence and in the helium-burning phase for these ∼2 M ⊙ stars. Finally, our results indicate less mode suppression than predicted by recent theories for magnetic suppression of certain oscillation modes in red giants.

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.

The Abundances ofα- elements in the open cluster NGC 6791 from APOGEE data

The open cluster NGC 6791 is among the oldest, most massive open clusters in the Galaxy. High-resolution H-band spectra from the SDSS III / Apache Point Observatory Galactic Evolution Experiment (APOGEE) of 11 red giants in NGC 6791 are analyzed for their chemical abundances of 8 elements: C, N, O, Mg, Si, Ca, Ti and Fe. From the infrared spectra, this cluster is confirmed to be among the most metal-rich clusters in the Galaxy ( Fe / H ] >= 0.34 ± 0.06), and is found to have roughly solar values of the α-element abundances.

Aluminum abundances of multiple stellar generations in the globular cluster NGC 1851

We study the distribution of aluminum abundances among red giants in the peculiar globular cluster NGC 1851. Aluminum abundances were derived from the strong doublet Al I 8772-8773 A measured on intermediate resolution FLAMES spectra of 50 cluster stars acquired under the Gaia-ESO public survey. We coupled these abundances with previously derived abundance of O, Na, Mg to fully characterize the interplay of the NeNa and MgAl cycles of H-burning at high temperature in the early stellar generation in NGC 1851. The stars in our sample show well defined correlations between Al,Na and Si; Al is anticorrelated with O and Mg. The average value of the [Al/Fe] ratio steadily increases going from the first generation stars to the second generation populations with intermediate and extremely modified composition. We confirm on a larger database the results recently obtained by us (Carretta et al. 2011a): the pattern of abundances of proton-capture elements implies a moderate production of Al in NGC 1851. We find evidence of a statistically significant positive correlation between Al and Ba abundances in the more metal-rich component of red giants in NGC 1851.

CN Abundance Inhomogeneities among Bright Red Giants in the Globular Clusters NGC 288 and NGC 362

ABSTRACTElement inhomogeneities are studied in the second-parameter pair of globular clusters NGC 288 and NGC 362. Spectra obtained with the CTIO 1.5 m telescope of relatively high luminosity red giants (MV < -1.0) in each cluster have been used to measure a λ3883 CN band index denoted S(3839), and two indices centered on CH absorption in the λ4300 G band. Giants in both clusters exhibit a comparable spread in CN band strength, with only a small difference in mean G band absorption between CN-strong and CN-weak giants. By contrast, the S(3839) index anticorrelates with the [O/Fe] abundance measured by Shetrone & Keane. Part of this anticorrelation can be explained by the effect of reduced oxygen on the equilibria of CO and CN molecule formation in red giant atmospheres. However, the difference in S(3839) between CN-weak and CN-strong giants is sufficiently large that a factor of ∼10 enhancement in nitrogen in CN-strong stars also appears to be necessary. Additionally, the 3883 Å CN band strength correlates with [Na/Fe] abundance. These CN-O-Na relationships among the red giants of NGC 362 and NGC 288 are also a characteristic of other globular clusters of the Milky Way. At a given S(3839) and [Na/Fe] abundance, the CN-strong red giants of NGC 288 have a higher oxygen abundance than red giants in NGC 362. As such, NGC 288 and NGC 362 do not appear to follow a correlation between minimum [O/Fe] abundance and the blueward extent of the horizontal branch in the color-magnitude diagram.

Photometric and Coravel observations of red giant candidates in three open clusters: membership, binarity, reddening and metallicity

Aims. We present new CORAVEL radial-velocity observations and photoelectric photometry in the UBV and DDO systems for a sample of potential members of the red-giant branches of NGC 6192, NGC 6208 and NGC 6268, three open clusters projected close to the Galactic center direction. We also examine the properties of a sample of 42 inner disk open clusters projected towards almost the same direction as the three clusters. Methods. Cluster members and red field giants were discriminated by using the CORAVEL radial-velocity data and by applying two photometric criteria. Interstellar reddening and metal content of the clusters were derived from combined BV and DDO data. Results. Cluster membership for five red giants in NGC 6192, three in NGC 6208 and three in NGC 6268 has been confirmed by the analysis of the photometric and kinematic data. Photometric membership probabilities show very good agreement with those obtained from CORAVEL radial velocities. Three new spectroscopic binaries were discovered among the red giants of NGC 6192 and NGC 6208. Mean radial velocities and E(B − V) colour excesses were derived. Conclusions. The overall metallicities were found to be nearly solar for NGC 6208 and above solar for NGC 6192 and NGC 6268. Most of the clusters located closer than 2 kpc from the Sun in the considered direction are slightly more reddened than the absorption resulting from the Baade’s window absorption law.

Star-to-Star Na and O Abundance Variations along the Red Giant Branch in NGC 2808

We report for the first time Na and O abundances from high-resolution, high S/N echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multi-object spectrograph at the ESO VLT. In these stars, spanning about 3 mag from the red giant branch (RGB) tip, large variations are detected in the abundances of oxygen and sodium, anticorrelated with each other; this is a well known evidence of proton-capture reactions at high temperatures in the ON and NeNa cycles. One star appears super O-poor; if the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O depletion levels as large as those of M 13. This result confirms our previous findings based on lower resolution spectra (Carretta et al. 2003) of a large star-to-star scatter in proton capture elements at all positions along the RGB in NGC 2808, with no significant evolutionary contribution. Finally, the average metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from 19 stars.

Zeroing the Stellar Isochrone Scale: The Red Giant Clump Luminosity at Intermediate Metallicity

The color-magnitude diagrams of the open clusters NGC 2420 and NGC 2506 have been investigated as intermediate links between the solar neighborhood and the Magellanic Clouds. Two sets of theoretical isochrones that include convective overshoot are zeroed to the Sun at solar abundance and to the unevolved main-sequence dwarfs of the Hipparcos Catalogue at [Fe/H] = -0.4, requiring a differential of 0.4 mag between the unevolved main sequences at a given color. Adopting E(B-V) = 0.04 and [Fe/H] = -0.39 for NGC 2506 and E(B-V) = 0.04 and [Fe/H] = -0.29 for NGC 2420, the respective apparent moduli are (m-M) = 12.70 and 12.15, while the ages of both clusters are approximately 1.9 ± 0.2 Gyr or 2.2 ± 0.2 Gyr, depending on the choice of isochrones. From the composite giant branch of the two clusters, the mean clump magnitudes in V and I are found to be +0.47 and -0.48 (-0.17, +0.14), respectively. Applying a metallicity correction to the MI values, the cluster sample of Udalski leads to (m-M)0 = 18.42 (+0.17, -0.15) and 18.91 (+0.18, -0.16) for the LMC and SMC, respectively. A caveat to this discussion and potentially to the claim that clusters of the same abundance and age are identical is the observation that the V-I colors of the red giants in NGC 2506 are significantly redder at a given B-V than the giants in clusters of comparable age and/or metallicity. The distance scale above has been derived using the general cluster relation between B-V and V-I. If the CCD photometry in NGC 2506 is correctly tied to the standard system, MI for the clump will decrease and the distance moduli should increase by 0.1 mag.

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

FORMATION OF THE GALAXY: CLUES FROM GLOBULAR CLUSTER AGES AND ABUNDANCES

Chromospheres of two red giants in NGC 6752

Two red giant stars, A31 and A59, in the globular cluster NGC 6752 exhibit Mg II (2800 A) emission with surface fluxes comparable to those observed among metal-deficient halo field giants, and among low-activity Population I giants. Optical echelle spectra of these cluster giants reveal emission in the core of the Ca II K (3933.7 A) line, and in the wing of the H-alpha (6562.8 A) profile. Asymmetries exist both in the emission profiles and the line cores. These observations demonstrate unequivocally the existence of chromospheres among old halo population giants, and the presence of mass outflow in their atmospheres. Maintenance of a relatively constant level of chromospheric activity on the red giant branch contrasts with the decay of magnetic dynamo activity exhibited by dwarf stars and younger giants. A purely hydrodynamic phenomenon may be responsible for heating the outer atmospheres of these stars, enhancing chromospheric emission, thus extending the atmospheres and facilitating mass loss.

The cyanogen distribution of the giants in NGC 2808

view Abstract Citations (25) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The cyanogen distribution of the giants in NGC 2808. Norris, J. ; Smith, G. H. Abstract The authors report panoramic photometry of the strength of the λ3883 cyanogen band in the spectra of red giants in NGC 2808, in the magnitude range -0.5 < MV < 1.5. There is a range in band strength at all magnitudes surveyed, but the CN distribution shows little or no evidence for the type of bimodality predicted by Norris on the hypothesis that there is a connection between horizontal-branch morphology and the form of the CN distribution on the red giant branch. Publication: The Astrophysical Journal Pub Date: December 1983 DOI: 10.1086/161517 Bibcode: 1983ApJ...275..120N Keywords: Cosmochemistry; Cyanogen; Globular Clusters; Horizontal Branch Stars; Red Giant Stars; Abundance; Astronomical Photometry; Astrophysics full text sources ADS | data products SIMBAD (5)

A DDO astrophysical study of NGC 752

DDO intermediate-band photometry has provided physical parameters for 11 red giants in NGC 752. We have found a metallicity (Fe/H)=0.0 +- 0.1 with respect to the Sun, in good agreement with previous determinations, a reddening E (B-V) =0.02 +- 0.01, and a true distance of 380 +- 40 pc. From a comparison with Patenaude's (Ph. D. thesis, Obs. Geneve (1977)) theoretical isochrones, the age is (1.1 +- 0.1) x 10/sup 9/ yr. The mean value of the masses of the observed red giants was found to be 1.5 +- 0.2 (sigma/sub m/), which is consistent with the expected theoretical value. But the errors in the individual physical parameters which arise from photoelectric and calibration uncertainties exclude the possibility of studying mass loss in the giant branch as well as the He content of the cluster.

Physical Properties of the Red Giants in NGC 2420

The effective temperatures, surface gravities, and masses of 11 red giant members of the old open cluster NGC 2420 have been derived from existing intermediate-band photometry on the David Dunlap Observatory system. The mean mass of the stars was found to be 0·7 ℳ⊙. The data permit the helium abundance of the cluster to be estimated and result in the value Y = 0·30.

Red giants in NGC 6752

Red giants in NGC 6752

The Colour-Magnitude Diagram of the Globular Cluster NGC 7099

UBV photometry of the globular cluster NGC 7099 is reported. The ultraviolet excess of δ(U–B) = 0·33 mag shows that NGC 7099 is an extremely metal-poor cluster. The colour–magnitude diagram shows a strong blue population of the horizontal branch, with very few RR Lyrae variables and stars on the red side of the instability region. The giant branch shows very little spread in colour at a given magnitude. The metallicity parameter ∆V = 3·0 mag is consistent with the low metal abundance. A reddening of E(B–V) = 0·06 mag is derived from the two-colour diagram for field stars. The relative paucity of red giants in NGC 7099 probably indicates a high helium abundance.

Envelope Opacities and the Cepheid Loops at 5 ????_{sun}

view Abstract Citations (31) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Envelope Opacities and the Cepheid Loops at 5 ????_{sun} Robertson, J. W. Abstract Theoretical evolutionary tracks for the stage of core helium burning have previously been unable to explain the detailed distribution in the H-R diagram of the red giants in NGC 1866, a cluster in the Large Magellanic Cloud. Models calculated with improved envelope opacities develop extremely deep convective at the tip of the red-giant branch. The convective envelope modifies the hydrogen distribution in the region near the hydrogen-burning shell, which leads to core-helium-burning evolution in better agreement with the observations. Publication: The Astrophysical Journal Pub Date: May 1972 DOI: 10.1086/151449 Bibcode: 1972ApJ...173..631R full text sources ADS |

Core Helium Burning Evolution at 5M⊙

Observations of the red giants in NGC 1866 in the Large Magellanic Cloud, made by Arp and Thackeray provide a strict test for core helium burning models of about 5M⊙. The main observational features to be reproduced theoretically are the luminosity of the red giants, the range in effective temperature that they occupy and the ratio of the number of giants near the blue end of this range to the number at the bottom of the red giant branch. Meyer-Hofmeister attempted a theoretical fit to these observations using the initial (X, Y, Z) composition (0.602, 0.354, 0.044) and assuming the stars to have formed uniformly over a period of 1.5 × 107 years about a mean age of 5.75 × 107 years. Although she was able to fit the luminosity and effective temperature spread of the red giants, her theory predicted far too many stars at the bottom of the red giant branch relative to the number of bluer giants.