Branch Stars In NGC Research Articles

Observing multiple populations in globular clusters with the ESO archive: NGC 6388 reloaded

The metal-rich and old bulge globular cluster (GC) NGC 6388 is one of the most massive Galactic GCs (M ~ 106 M⊙). However, the spectroscopic properties of its multiple stellar populations rested only on 32 red giants (only 7 of which observed with UVES, the others with GIRAFFE), given the difficulties in observing a rather distant cluster, heavily contaminated by bulge and disc field stars. We bypassed the problem using the resources of the largest telescope facility ever: the European Southern Observatory (ESO) archive. By selecting member stars identified by other programmes, we derive atmospheric parameters and the full set of abundances for 15 species from high-resolution UVES spectra of another 17 red giant branch stars in NGC 6388. We confirm that no metallicity dispersion is detectable in this GC. About 30% of the stars show the primordial composition of first-generation stars, about 20% present an extremely modified second-generation composition, and 50% have an intermediate composition. The stars are clearly distributed in the Al–O and Na–O planes in three discrete groups. We find substantial hints that more than a single class of polluters is required to reproduce the composition of the intermediate component in NGC 6388. In the heavily polluted component the sum Mg+Al increases as Al increases. The sum Mg+Al+Si is constant, and is the fossil record of hot H-burning at temperatures higher than about 70 MK in the first-generation polluters that contributed to form multiple populations in this cluster.

High-resolution Spectroscopic Abundances of Red Giant Branch Stars in NGC 6584 and NGC 7099

Abstract We obtain high-resolution spectra of red giant branch stars in NGC 6584 and NGC 7099 to perform a detailed abundance analysis. We confirm cluster membership for these stars based on consistent radial velocities measured in this study and small pixel offsets between the observations of Sarajedini et al. and Piotto et al. We find mean metallicities of [Fe/H] = −1.53 ± 0.08 dex and [Fe/H] = −2.29 ± 0.07 dex for NGC 6584 and NGC 7099, respectively. We also find these clusters to be enhanced in their [α/Fe] ratios, consistent with what is expected for metal-poor globular clusters. Additionally, we find evidence of a statistically significant Na–O anti-correlation in both clusters. Finally, with the use of HST photometry, we compare the location of the enhanced and pristine populations in chromosome maps of the clusters to confirm previous photometric evidence of multiple stellar populations. Although we cannot confirm the nature of the polluter stars responsible for the abundance differences, our results can be used to constrain pollution models.

Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters(Corrigendum)

Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters<i>(Corrigendum)</i>

High-resolution Spectroscopic Abundances of Red Giant Branch Stars in NGC 6681* †

Abstract We obtain high-resolution spectra of nine red giant branch stars in NGC 6681 and perform the first detailed abundance analysis of stars in this cluster. We confirm cluster membership for these stars based on consistent radial velocities of 214.5 ± 3.7 km s−1 and find a mean [Fe/H] = −1.63 ± 0.07 dex and [α/Fe] = 0.42 ± 0.11 dex. Additionally, we confirm the existence of a Na–O anti-correlation in NGC 6681 and identify two populations of stars with unique abundance trends. With the use of HST photometry from Sarajedini et al. and Piotto et al. we are able to identify these two populations as discrete sequences in the cluster CMD. Although we cannot confirm the nature of the polluter stars responsible for the abundance differences in these populations, these results do help put constraints on possible polluter candidates.

The chemical composition of the low-mass Galactic globular cluster NGC 6362★

We present chemical abundances for 17 elements in a sample of 11 red giant branch stars in NGC 6362 from UVES spectra. NGC 6362 is one of the least massive globulars where multiple populations have been detected, yet its detailed chemical composition has not been investigated so far. NGC 6362 turns out to be a metal-intermediate ([Fe/H]=-1.07\pm0.01 dex) cluster, with its \alpha- and Fe-peak elements content compatible with that observed in clusters with similar metallicity. It also displays an enhancement in its s-process element abundances. Among the light elements involved in the multiple populations phenomenon, only [Na/Fe] shows star-to-star variations, while [Al/Fe] and [Mg/Fe] do not show any evidence for abundance spreads. A differential comparison with M4, a globular cluster with similar mass and metallicity, reveals that the two clusters share the same chemical composition. This finding suggests that NGC 6362 is indeed a regular cluster, formed from gas that has experienced the same chemical enrichment of other clusters with similar metallicity.

CHEMICAL ABUNDANCES IN NGC 5053: A VERY METAL-POOR AND DYNAMICALLY COMPLEX GLOBULAR CLUSTER

NGC 5053 provides a rich environment to test our understanding of the complex evolution of globular clusters (GCs). Recent studies have found that this cluster has interesting morphological features beyond the typical spherical distribution of GCs, suggesting that external tidal effects have played an important role in its evolution and current properties. Additionally, simulations have shown that NGC 5053 could be a likely candidate to belong to the Sagittarius dwarf galaxy (Sgr dSph) stream. Using the Wisconsin-Indiana-Yale-NOAO-Hydra multi-object spectrograph, we have collected high quality (signal-to-noise ratio $\sim$ 75-90), medium-resolution spectra for red giant branch stars in NGC 5053. Using these spectra we have measured the Fe, Ca, Ti, Ni, Ba, Na, and O abundances in the cluster. We measure an average cluster [Fe/H] abundance of -2.45 with a standard deviation of 0.04 dex, making NGC 5053 one of the most metal-poor GCs in the Milky Way (MW). The [Ca/Fe], [Ti/Fe], and [Ba/Fe] we measure are consistent with the abundances of MW halo stars at a similar metallicity, with alpha-enhanced ratios and slightly depleted [Ba/Fe]. The Na and O abundances show the Na-O anti-correlation found in most GCs. From our abundance analysis it appears that NGC 5053 is at least chemically similar to other GCs found in the MW. This does not, however, rule out NGC 5053 being associated with the Sgr dSph stream.

Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters

NGC288 is a globular cluster with a well-developed blue horizontal branch covering the u-jump that indicates the onset of diffusion. It is therefore well suited to study the effects of diffusion in blue horizontal branch (HB) stars. We compare observed abundances with predictions from stellar evolution models calculated with diffusion and from stratified atmospheric models. We verify the effect of using stratified model spectra to derive atmospheric parameters. In addition, we investigate the nature of the overluminous blue HB stars around the u-jump. We defined a new photometric index sz from uvby measurements that is gravity-sensitive between 8000K and 12000K. Using medium-resolution spectra and Stroemgren photometry, we determined atmospheric parameters (T_eff, log g) and abundances for the blue HB stars. We used both homogeneous and stratified model spectra for our spectroscopic analyses. The atmospheric parameters and masses of the hot HB stars in NGC288 show a behaviour seen also in other clusters for temperatures between 9000K and 14000K. Outside this temperature range, however, they instead follow the results found for such stars in omega Cen. The abundances derived from our observations are for most elements (except He and P) within the abundance range expected from evolutionary models that include the effects of atomic diffusion and assume a surface mixed mass of 10^-7 M0. The abundances predicted by stratified model atmospheres are generally significantly more extreme than observed, except for Mg. When effective temperatures, surface gravities, and masses are determined with stratified model spectra, the hotter stars agree better with canonical evolutionary predictions. Our results show definite promise towards solving the long-standing problem of surface gravity and mass discrepancies for hot HB stars, but much work is still needed to arrive at a self-consistent solution.

On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models★

Globular clusters display significant variations in their light-element content, pointing to the existence of a second stellar generation formed from the ejecta of an earlier generation. The nature of these internal polluters is still a matter of debate: the two most popular scenarios indicate intermediate-mass AGB stars and fast rotating massive stars. Abundances determination for some key elements can help distinguish between these competitor candidates. We present in this paper Y abundances for a sample of 103 red giant branch stars in NGC 6121. Within measurement errors, we find that the [Y/Fe] is constant in this cluster contrary to a recent suggestion. For a subsample of six stars we also find [Rb/Fe] to be constant, consistent with previous studies showing no variation in other s-process elements. We also present a new set of stellar yields for intermediate-mass AGB stellar models of 5 and 6 solar masses, including heavy element s-process abundances. The uncertainties on the mass-loss rate, the mixing-length parameter, and the nuclear reaction rates have a major impact on the stellar abundances. Within the IM-AGB pollution scenario, the constant abundance of heavy elements inside the cluster requires a marginal s-process efficiency in IM-AGB stars. Such a constrain could still be satisfied by the present models assuming a stronger mass-loss rate. The uncertainties mentioned above are limiting the predictive power of intermediate-mass AGB models. For these reasons, at the moment we are not able to clearly rule out their role as main polluters of the second population stars in globular clusters.

POTASSIUM IN GLOBULAR CLUSTER STARS: COMPARING NORMAL CLUSTERS TO THE PECULIAR CLUSTER NGC 2419

Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and omega Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds both for giants and less evolved stars. At present, NGC 2419 seems unique among GCs.

C and N abundances of main sequence and subgiant branch stars in NGC 1851

We present the first chemical analysis of stars on the double subgiant branch (SGB) of the globular cluster NGC 1851. We obtained 48 Magellan IMACS spectra of subgiants and fainter stars covering the spectral region between 3650-6750\AA, to derive C and N abundances from the spectral features at 4300\AA (G-band) and at ~ 3883\AA (CN). We added to our sample ~ 45 unvevolved stars previously observed with FORS2 at the VLT. These two datasets were homogeneously reduced and analyzed. We derived abundances of C and N for a total of 64 stars and found considerable star-to-star variations in both [C/H] and [N/H] at all luminosities extending to the red giant branch (RGB) base (V~18.9). These abundances appear to be strongly anticorrelated, as would be expected from the CN-cycle enrichment, but we did not detect any bimodality in the C or N content. We used HST and ground-based photometry to select two groups of faint- and bright-SGB stars from the visual and Str\"omgren color-magnitude diagrams. Significant variations in the carbon and nitrogen abundances are present among stars of each group, which indicates that each SGB hosts multiple subgenerations of stars. Bright- and faint-SGB stars differ in the total C+N content, where the fainter SGB have about 2.5 times the C+N content of the brighter ones. Coupling our results with literature photometric data and abundance determinations from high-resolution studies, we identify the fainter SGB with the red-RGB population, which also should be richer on average in Ba and other s-process elements, as well as in Na and N, when compared to brighter SGB and the blue-RGB population.

LOCAL GROUP DWARF ELLIPTICAL GALAXIES. II. STELLAR KINEMATICS TO LARGE RADII IN NGC 147 AND NGC 185

We present kinematic and metallicity profiles for the M31 dwarf elliptical (dE) satellite galaxies NGC 147 and NGC 185. The profiles represent the most extensive spectroscopic radial coverage for any dE galaxy, extending to a projected distance of eight half-light radii (8 r_eff = 14'). We achieve this coverage via Keck/DEIMOS multislit spectroscopic observations of 520 and 442 member red giant branch stars in NGC 147 and NGC 185, respectively. In contrast to previous studies, we find that both dEs have significant internal rotation. We measure a maximum rotational velocity of 17+/-2 km/s for NGC 147 and 15+/-5 km/s for NGC 185. The velocity dispersions decrease gently with radius with an average dispersion of 16+/-1 km/s for NGC 147 and 24+/-1 km/s for NGC 185. Both dEs have internal metallicity dispersions of 0.5 dex, but show no evidence for a radial metallicity gradient. We construct two-integral axisymmetric dynamical models and find that the observed kinematical profiles cannot be explained without modest amounts of non-baryonic dark matter. We measure central mass-to-light ratios of ML_V = 4.2+/-0.6 and ML_V = 4.6+/-0.6 for NGC 147 and NGC 185, respectively. Both dE galaxies are consistent with being primarily flattened by their rotational motions, although some anisotropic velocity dispersion is needed to fully explain their observed shapes. The velocity profiles of all three Local Group dEs (NGC 147, NGC 185 and NGC 205) suggest that rotation is more prevalent in the dE galaxy class than previously assumed, but is often manifest only at several times the effective radius. Since all dEs outside the Local Group have been probed to only inside the effective radius, this opens the door for formation mechanisms in which dEs are transformed or stripped versions of gas-rich rotating progenitor galaxies.

Is a binary fraction-age relation responsible for the lack of EHB binaries in globular clusters?

Context. The recently-discovered lack of close binaries, among extreme horizontal branch (EHB) stars in Galactic globular clusters, has thus far constituted a major puzzle, in view of the fact that blue subdwarf stars ‐ the field counterparts of cluster EHB stars ‐ are well-known to present a high binary fraction. Aims. In this Letter, we provide new results that confirm the lack of close EHB bina ries in globular clusters, and present a first scenario to explain the difference between field and cluster EHB stars. Methods. First, in order to confirm that the lack of EHB binaries in glob ular clusters is a statistically robust result, we undertoo k a new analysis of 145 horizontal branch stars in NGC 6752, out of which forty-one belong to the EHB. To search for radial-velocity variations as a function of time, we repeated high-resoluti on (R = 18 500) spectroscopy of all stars, four times during a single night of observations. Results. We detected a single, hot (25 000 K), radial-velocity variable star as a close-binary candidate. From these results, we e stimate an upper-limit for the close (period P≤ 5 day) binary fraction f among NGC 6752 EHB stars of 16% (95% confidence level), with the most probable value being f = 4%. Thus our results clearly confirm the lack of close binarie s among the hot HB stars in this cluster. Conclusions. We suggest that the confirmed discrepancy between the binary fractions for field and cluster EHB stars is the consequence of an f -age relation, with close binaries being more likely in the c ase of younger systems. We analyze theoretical and observational results available in the literature, which s upport this scenario. If so, an age difference between the EHB progenitors in the field and in clusters, the former being younger (on aver age) by up to several Gyr, would naturally account for the startling differences in binary fraction between the two populations.

Blue horizontal branch stars in NGC 6522 and Baade's Window

view Abstract Citations (28) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Blue Horizontal Branch Stars in NGC 6522 And Baade's Window Terndrup, Donald M. ; Walker, Alistair R. Abstract We present color-magnitude diagrams in BVI (Cousins) of the globular cluster NGC 6522 in Baade's Window. Our photometry, which extends to V approximately equals 19, shows that NGC 6522 has an extended blue horizontal branch (BHB), typical of clusters of moderate metallicity close to the galactic center. We derive (Fe/H) = -1.6 +/- 0.2, E(B-V)zero = 0.52 +/- 0.05 for the cluster from comparison to the giant branches of other globulars and from the level of the cluster's horizontal branch. We identify a region in the color-magnitude diagrams which is populated mainly by BHB stars, and show that most of these are spatially concentrated around the cluster and so are not members of the bulge; the spatial density of bulge BHB stars is 0.4 arcmin-2. If the bulge has the same (1750 A--V) color as the bulge of M31, then about 20% the light at 1750 A is from BHB stars. Publication: The Astronomical Journal Pub Date: May 1994 DOI: 10.1086/116986 Bibcode: 1994AJ....107.1786T Keywords: Abundance; Astronomical Photometry; Blue Stars; Color-Magnitude Diagram; Galactic Bulge; Globular Clusters; Horizontal Branch Stars; Metallicity; Spatial Distribution; Calibrating; Charge Coupled Devices; Data Reduction; High Resolution; Optical Filters; Spatial Resolution; Telescopes; Astronomy; GLOBULAR CLUSTERS: INDIVIDUAL: NGC 6522; STARS: HORIZONTAL-BRANCH full text sources ADS | data products SIMBAD (6)

Asymptotic giant branch stars in NGC 6822: Probes of star formation history

view Abstract Citations (32) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Asymptotic Giant Branch Stars in NGC 6822: Probes of Star Formation History Gallart, C. ; Aparicio, A. ; Chiosi, C. ; Bertelli, G. ; Vilchez, J. M. Abstract About 500 asymptotic giant branch (AGB) stars have been resolved and photometrically measured in the Local Group (LG) dwarf irregular galaxy NGC 6822. They draw an AGB well shaped, densely populated, and extending to very red colors. It is the first time that this kind of AGB has been clearly detected. Using recent stellar evolutionary models and isochrones, we discuss the stellar population of the galaxy, showing, in particular, how the structure of the AGB can be used as an observable to trace the intermediate-age and old star formation history of the galaxy. The powerful telescopes and detectors currently available offer the opportunity of detecting this structure in other nearby galaxies. This opens new paths for the study of the intermediate-age and old populations of the LG. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/187298 Bibcode: 1994ApJ...425L...9G Keywords: Asymptotic Giant Branch Stars; Cosmology; Dwarf Galaxies; Galactic Evolution; Irregular Galaxies; Late Stars; Local Group (Astronomy); Stellar Composition; Stellar Evolution; Color-Magnitude Diagram; Hertzsprung-Russell Diagram; Photometry; Stellar Color; Stellar Luminosity; Stellar Spectra; Astronomy; GALAXIES: EVOLUTION; GALAXIES: LOCAL GROUP; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 6822; GALAXIES: IRREGULAR; GALAXIES: STELLAR CONTENT; STARS: LATE-TYPE full text sources ADS | data products SIMBAD (4) NED (1)

The distribution and age of asymptotic giant branch stars in NGC 205

V and I CCD images are used to investigate the spatial distribution and age of bright asymptotic giant branch (AGB) stars in the central regions of the Local Group dwarf elliptical galaxy NGC 205. These data reveal a large intermediate-age component, which is uniformly distributed over the central regions of the galaxy. The redward extent of the bright AGB sequence on the (I, V-I) diagram suggests that the majority of these objects have metallicities which are roughly solar