Lower Giant Branch Research Articles

Mixed-mode Ensemble Asteroseismology of Low-luminosity Kepler Red Giants

We present measurements of the dipole mode asymptotic period spacing (ΔΠ1), the coupling factor between the p- and g-modes (q), the g-mode phase offset (ϵ g ), and the mixed-mode frequency rotational splitting (δ ν rot) for 1074 low-luminosity red giants from the Kepler mission. Using oscillation mode frequencies extracted from each star, we apply Bayesian optimization to estimate ΔΠ1 from the power spectrum of the stretched-period spectrum and to perform the subsequent forward modeling of the mixed-mode frequencies. With our measurements, we show that the mode coupling factor q shows significant anticorrelation with both the stellar mass and metallicity, and can reveal highly metal-poor stars. We present the evolution of ϵ g up the lower giant branch up to before the luminosity bump, and find no significant trends in ϵ g or δ ν rot with the stellar mass and metallicity in our sample. Additionally, we identify six new red giants showing anomalous distortions in their g-mode pattern. Our data products, code, and results are provided in a public repository.

Models for metal-poor stars with different initial abundances of C, N, O, Mg, and Si. II. Application to the colour–magnitude diagrams of the globular clusters 47 Tuc, NGC 6362, M 5, M 3, M 55, and M92

ABSTRACTStellar models for −2.5 ≤ [Fe/H] ≤−0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M 5 M 3, M 55, and M 92. The bolometric corrections (BCs) used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other α elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour–magnitude diagrams (CMDs) to within ∼0.03 mag. In particular, they appear to match the spreads in the observed (MF336W − MF438W)0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] ≲ −0.5). Both the absolute (MF438W − MF606W)0 colours and the variations in these colours at a given MF606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] ≳ +0.5). Allowing for moderate He abundance variations ($\delta \, Y \sim 0.05$) improves the fits to the observations.

CN AND CH BAND STRENGTH VARIATIONS IN M71 GIANTS

CN and CH band strengths for fourteen bright giants in the globular cluster M71 have been measured from archival spectra obtained with the Multiple Mirror Telescope. Adding the collected. data from the literature we confirm a bimodality of CN distribution on the red giant branch and the honzontal branch, and CN-CH anti-correlations on the lower giant branch and horizontal branch. However a CN-CH anti-correlation on the upper red giant branch is not quite clear as those of other branches. The small number If statistics could not be excluded as a possible cause. To confirm this, a greater number of sample stars are needed. We also confirm that the ratio of CN-strong to CN-weak stars is quite different from that in 47 Tuc, although the anti-correlation between CN and CH bands, the bimodality of the CN distribution, and the spatial distribution of CN stars in M71 are found to be similar to those III 47 Tuc.

The Unusual Luminosity Function of the Globular Cluster M10

We present the I-band luminosity function of the differentially reddened globular cluster M10. We combine photometric analysis derived from wide-field (23' x 23') images that include the outer regions of the cluster and high-resolution images of the cluster core. After making corrections for incompleteness and field star contamination, we find that the relative numbers of stars on the lower giant branch and near the main-sequence turnoff are in good agreement with theoretical predictions. However, we detect significant (> 6 \sigma) excesses of red giant branch stars above and below the red giant branch bump using a new statistic (a population ratio) for testing relative evolutionary timescales of main-sequence and red giant stars. The statistic is insensitive to assumed cluster chemical composition, age, and main-sequence mass function. The excess number of red giants cannot be explained by reasonable systematic errors in our assumed cluster chemical composition, age, or main-sequence mass function. Moreover, M10 shows excesses when compared to the cluster M12, which has nearly identical metallicity, age, and color-magnitude diagram morphology. We discuss possible reasons for this anomaly, finding that the most likely cause is a mass function slope that shows significant variations as a function of mass.

S1040 in M67: A Post–Mass Transfer Binary with a Helium Core White Dwarf

We have obtained spectra of the yellow giant S1040 in the open cluster M67 using the Goddard High-Resolution Spectrograph (GHRS) and the Faint Object Spectrograph on the Hubble Space Telescope. S1040 is a single-lined spectroscopic binary with a 42.8 day period that occupies a red position in the M67 color-magnitude diagram (CMD), 0.2 mag blueward of the giant branch. A detection of S1040 at 1620 A with the Ultraviolet Imaging Telescope provided evidence that the secondary is a hot white dwarf and thus that the anomalous location of S1040 in the CMD is likely due to a prior episode of mass transfer. Our GHRS spectrum shows a broad Lyα absorption profile that confirms the white dwarf identification of the S1040 secondary. A model atmosphere fit to the GHRS spectrum yields Teff = 16,160 K, log g = 6.7, and a mass of about 0.22 M☉, for an assumed cluster distance of 820 pc and a reddening of E(B - V) = 0.02. The unusually low mass derived for the white dwarf implies that it must have a helium core and that a mass transfer episode must have begun while the progenitor was on the lower giant branch. We construct a plausible mass transfer history for S1040 in which it originated as a short-period (~2 days) binary and evolved through a blue straggler phase to reach its current state.

The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri

view Abstract Citations (164) References (80) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Abundance Spread Among Giants and Subgiants in the Globular Cluster Omega Centauri Suntzeff, Nicholas B. ; Kraft, Robert P. Abstract We present spectroscopic abundances and radial velocities for giant stars in the Galactic globular cluster ω Centauri based on the Ca II infrared triplet. Two samples of stars were observed: 234 stars at Mv∼1.25 on the lower giant branch at radial distances between 8' and 23', and 145 stars at Mv∼-1.3 at radial distances between 3' and 22'. We found 199 and 144 radial velocity members, respectively, in the two samples. These samples were corrected for evolutionary effects to provide an unbiased distribution of the underlying stellar metallicity. We find <vr>=234.7±1.3 σobs=11.3 km s-1 (bright sample), and (Vr=232.9±1.2, σobs=10.6 km s-1 (faint sample). The statistical errors of the dispersions are less than 1 km s-1. Previous metallicity studies found a non-Gaussian metallicity distribution containing a tail of metal-rich stars. We confirm these results except our unbiased cluster metallicity distributions are narrower. They contain the following key features: (1) No very metal-poor stars, (2) a sudden rise in the metal-poor distribution to a modal [Fe/H] value of - 1.70 consistent with an homogeneous metallicity unresolved at the 0.07 dex level, (3) a tail to higher metallicities with more stars than predicted by simple chemical evolution models, and (4) a weak correlation between metallicity and radius such that the most metal-rich stars are concentrated to the cluster core. The unresolved metal-weak tail implies that the gas out of which ω Cen formed was well-mixed up to the modal metallicity of the cluster. Therefore, ω Cen like other Galactic globular clusters, seems to have formed in a pre-enriched and homogenized (up to the modal metallicity) environment. The existence of a weak metallicity gradient supports the idea that co Cen self-enriched, with the enriched gas sinking to the cluster center due to gas dissipation processes. We also note, however, that the metal-rich stars are more massive than the bulk of the stars in the cluster, and could also have sunk to the center by dynamical mass segregation over the lifetime of the cluster. Publication: The Astronomical Journal Pub Date: May 1996 DOI: 10.1086/117930 arXiv: arXiv:astro-ph/9601013 Bibcode: 1996AJ....111.1913S Keywords: GLOBULAR CLUSTERS: INDIVIDUAL: OMEGA CENTAURI; STARS: ABUNDANCES; STARS: GIANTS; Astrophysics E-Print: Accepted for publication in the AJ. 40 pages, AASTeX V4.0 latex format (including figures), 13 ps figures, 8 separate AASTeX V4.0 latex tables full text sources arXiv | ADS | data products SIMBAD (514)

Pal 1: A Globular Cluster Anyway?

CCD photometry of the halo cluster Pall in Thuan-Gunn system is presented. The color - magnitude diagrams (Fig. 1) show a well defined red horizontal branch, lower giant branch and main-sequence down to about two magnitudes below the main-sequence turnoff. The giant branch is absent and the brightest stars are horizontal-branch stars. The horizontal branch is composed of red stars only. The age of the cluster, determined by comparison with the isochrones of Bell and Vanden Berg (1987) is consistent with an age between 12 – 14 Gyr. and a metallicity [Fe/H] = −0.79. The observed luminosity function shows a deficiency of stars from about 0.5 mag below turnoff up to g = 21 mag. A distance modulus of (m – M)g0 = 15.35 ± 0.1 magnitude has been derived. An estimate of the cluster structural parameters gives rc = 1.7 pc and c = 1.46. A mass estimate of 1.6 × 103M⊙ and mass-to-light ratio 1.77 has been obtained, using King's (1966) method.

CN and CH inhomogeneities among lower giant branch stars in M71

Spectra obtained with the 2.5-m Isaac Newton Telescope show that a bimodal distribution of both 3883- and 4215-A CN band strengths exists among M71 lower giant branch stars with magnitudes in the range +0.8 < M V < 2.4. The strength of the 4300 A CH band is found to be anticorrelated with the CN band strength. Enhanced CN bands are found among fewer than half of the evolved stars in M71, indicating that it has one of the lowest incidences of CN-enhanced giants of any globular cluster observed to date

The effect of helium diffusion on the ages of globular clusters

view Abstract Citations (69) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Effect of Helium Diffusion on the Ages of Globular Clusters Chaboyer, Brian ; Deliyannis, Constantine P. ; Demarque, Pierre ; Pinsonneault, M. H. ; Sarajedini, Ata Abstract Evolutionary tracks for halo stars were calculated, and isochrones which include the effects of microscopic diffusion of helium were constructed. The isochrones were fitted to a metal poor (M92) and a moderately metal rich (NGC 288) globular cluster using an updated version of the Revised Yale Isochrone color calibration. Ages of the two clusters were also determined using the difference between the turnoff magnitude and horizontal branch magnitude, and the difference in color between the main-sequence turnoff and lower giant branch. Considering all methods and constraints, diffusion is argued to reduce the derived ages of M92 and NGC 288 by 0.5-1 Gyr. The maximum age reduction that diffusion could cause is 3 Gyr. Age estimates including diffusion indicate that M92 is 16 +/- 2 Gyr old, and that M92 is about 3 Gyr older than NGC 288, assuming that the clusters have the same O/Fe of +0.4. Publication: The Astrophysical Journal Pub Date: April 1992 DOI: 10.1086/171160 Bibcode: 1992ApJ...388..372C Keywords: Gaseous Diffusion; Globular Clusters; Helium Atoms; Stellar Evolution; Stellar Interiors; Color-Magnitude Diagram; Halos; Milky Way Galaxy; Astrophysics; DIFFUSION; GALAXY: GLOBULAR CLUSTERS: GENERAL; STARS: EVOLUTION; STARS: INTERIORS full text sources ADS | data products SIMBAD (2)

The main-sequence color-magnitude diagram of M 92

view Abstract Citations (55) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Main-Sequence Color-Magnitude Diagram of M92 Heasley, J. N. ; Christian, C. A. Abstract A new color-magnitude (C-M) diagram for main-sequence stars in the globular cluster M 92 is presented. The observations upon which this diagram is based were obtained with an RCA CCD at the 3.6 m Canada-France-Hawaii telescope. These new data refine the position of the cluster ridge lines in the lower main sequence, the subgiant branch, and the lower giant branch. A comparison of the new C-M diagram with new theoretical isochrones shows that the shapes of the models are in good agreement with the observations, but a large shift in color is required to bring the theory and observations into agreement. Possible explanations for this discrepancy are discussed. The new M 92 observations and recent work on M13, M15, and 47 Tuc combined with the new isochromes show that the globular cluster distance scale developed by Sandage from the RR Lyrae period-luminosity-amplitude relation is consistent for the relative distances if all the clusters are the same age. Publication: The Astrophysical Journal Pub Date: August 1986 DOI: 10.1086/164459 Bibcode: 1986ApJ...307..738H Keywords: Color-Magnitude Diagram; Globular Clusters; Main Sequence Stars; Stellar Evolution; Charge Coupled Devices; Chronology; Stellar Models; Stellar Spectrophotometry; Astrophysics; CLUSTERS: GLOBULAR; STARS: EVOLUTION full text sources ADS | data products SIMBAD (10)

Photometry, proper motions, and membership in the globular cluster M71

view Abstract Citations (113) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometry, proper motions, and membership in the globular cluster M 71. Cudworth, K. M. Abstract New photographic photometry and proper motions have been obtained for over 350 stars with V less than about 16 in the region of the globular cluster M71 (NGC 6838). The photometry has been calibrated via new photoelectric observations which resolve the zero-point discrepancy of previous sequences. The proper motions have been used to derive cluster membership probabilities. The horizontal branch and lower giant branch very closely resemble those of 47 Tuc, but the upper part of the giant branch is slightly brighter and bluer in M71. Three probable UV-bright stars are likely members. Four variable stars near the tip of the giant branch are probable members, as are most of the stars observed spectroscopically by various authors. The internal velocity dispersion has been detected. The cluster space velocity is well determined and is characteristic of a disk-population object: about 50 km/s. Publication: The Astronomical Journal Pub Date: January 1985 DOI: 10.1086/113709 Bibcode: 1985AJ.....90...65C Keywords: Astrometry; Globular Clusters; Star Distribution; Stellar Motions; Stellar Spectrophotometry; Astronomical Photography; Electrophotometry; Giant Stars; Horizontal Branch Stars; Ultraviolet Astronomy; Variable Stars; Astronomy full text sources ADS | data products SIMBAD (197)

A search for cyanogen variations on the lower giant branch of M3

A search for cyanogen variations on the lower giant branch of M3