Bruce A. Twarog

Some Revised Observational Constraints on the Formation and Evolution of the Galactic Disk

A set of 76 open clusters with abundances based upon DDO photometry and/or moderate dispersion spectroscopy has been transformed to a common [Fe/H] scale and used to study the local structure and evolution of the galactic disk. The metallicity distribution of clusters with R_GC is best described by two distinct zones. Between R_GC = 6.5 and 10 kpc, the distribution has a mean [Fe/H] = 0.0 and a dispersion of 0.1 dex; there is only weak evidence for a shallow abundance gradient over this distance range. Beyond R_GC = 10 kpc, the metallicity distribution has a dispersion between 0.10 and 0.15 dex, but with a mean [Fe/H] = -0.3, implying a sharp discontinuity at R_GC = 10 kpc. After correcting for the discontinuity, no evidence is found for a gradient perpendicular to the plane. Adopting the clusters interior to 10 kpc as a representative sample of the galactic disk over the last 7 Gyr, the cluster metallicity range is found to be about half that of the field stars. When coupled with the discontinuity in the galactocentric gradient, the discrepancy in the metallicity distribution is interpreted as an indication of significant diffusion of field stars into the solar neighborhood from beyond 10 kpc. These results imply that the sun is NOT atypical of the stars formed in the solar circle 4.6 Gyr ago. It is suggested that the discontinuity reflects the edge of the initial galactic disk as defined by the disk globular cluster system and the so-called thick disk; the initial offset in [Fe/H] created by the differences in the chemical history on either side of the discontinuity has carried through to the current stage of galactic evolution. If correct, diffusion coupled with the absence of an abundance gradient could make the separation of field stars on the basis of galactocentric origin difficult.

A PHOTOMETRIC AND RADIAL-VELOCITY ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER NGC 752

Using all available proper-motion and radial-velocity data, including new radial-velocity observations obtained for this investigation, probable members of the open cluster NGC 752 have been identified. Photoelectric data on six systems have been transformed and collated to form an internally consistent sample on the BV system. Binaries have been identified using photometric and radial-velocity data, including a photographic survey for variability and the radial-velocity observations of this study. Analysis of the data leads to the following cluster parameters and their probable errors: E(B-V) = 0.035 ± 0.005 mag, [Fe/H] = -0.15 ± 0.05 dex, and (m - M) = 8.25 ± 0.10. The spread in color among stars in the color-magnitude diagram (cmd) along the main sequence from the turnoff to the unevolved main sequence is the consequence of a rich population of binaries. Due to its age and the comprehensive data available for the cluster, NGC 752 provides an ideal test of a variety of evolutionary phenomena. Comparison with theoretical isochrones normalized in an internally consistent manner leads to the conclusion that the morphology and distribution of stars in the cmd can best be matched using models that include convective overshoot, particularly those of Schaller et al. (1992). Despite their differences, the traditional and the overshoot isochrones both imply very similar ages, 1.9 ± 0.2 Gyr and 1.7 ± 0.1 Gyr, respectively, for the cluster. The Li abundances for the giants confirm that the giant branch is dominated by clump stars and first-ascent giants below the luminosity of the clump. The position and size of the Li dip among the main sequence stars, compared to the Hyades, is readily explained by stellar evolution with convective overshoot. It is predicted that among turnoff stars in the intermediate-age range Li will cease to be a unique function of age at a given color. Chromospheric flux is shown to be a well-defined function of color for single, unevolved stars, identical to that found for the Hyades, and the relation for NGC 752 falls within the Vaughan-Preston gap. However, the slope of the relation requires that increasing color implies increasing age for the bluer portion of hte weak-emission boundary. The combined effect of small samples, random errors, emission limits, a possible selection bias in favor of turnoff stars, and metallicity corrections is to render highly questionable any interpretation of time-variable star formation within the Galaxy based upon chromosopheric ages.

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.

LITHIUM IN THE INTERMEDIATE-AGE OPEN CLUSTER, NGC 3680

High-dispersion spectra centered on the Li 6708 A line have been obtained for 70 potential members of the intermediate-age open cluster NGC 3680, with an emphasis on stars in the turnoff region of the cluster color-magnitude diagram (CMD). A measurable Li abundance has been derived for 53 stars, 39 of which have radial velocities and proper motions consistent with cluster membership. After being transferred to common temperature and abundance scales, previous Li estimates have been combined to generate a sample of 49 members, 40 of which bracket the cluster Li-dip. Spectroscopic elemental analysis of eight giants and five turnoff stars produces [Fe/H] = –0.17 ± 0.07 (sd) and –0.07 ± 0.02 (sd), respectively. We also report measurements of Ca, Si, and Ni which are consistent with scaled-solar ratios within the errors. Adopting [Fe/H] = –0.08 (Section 3.6), Y 2 isochrone comparisons lead to an age of 1.75 ± 0.1 Gyr and an apparent modulus of (m – M) = 10.30 ± 0.15 for the cluster, placing the center of the Li-dip at 1.35 ± 0.03 M ☉. Among the giants, five of the nine cluster members are now known to have measurable Li with A(Li) near 1.0. A combined sample of dwarfs in the Hyades and Praesepe is used to delineate the Li-dip profile at 0.7 Gyr and [Fe/H] = +0.15, establishing its center at 1.42 ± 0.02 M ☉ and noting the possible existence of a secondary dip on its red boundary. When evolved to the typical age of the clusters NGC 752 (age = 1.45 Gyr, (m – M) = 8.4), IC 4651 (age = 1.5 Gyr, (m – M) = 10.4), and NGC 3680, the Hyades/Praesepe Li-dip profile reproduces the observed morphology of the combined Li-dip within the CMDs of the intermediate-age clusters while implying a metallicity dependence for the central mass of the Li-dip given by M/M ☉ = 1.38 ± 0.04 + 0.4 ± 0.2 [Fe/H]. The implications of the similarity of the Li-dichotomy among giants in NGC 752 and IC 4651 and the disagreement with the pattern among NGC 3680 giants are discussed.

CA II H and K filter photometry on the UVBY system. I - The standard system

A fifth filter (fwhm = 90 A) centered on Ca II H and K has been developed for use with the standard uvby system. The filter, called Ca, is designed primarily for applications to metal-poor dwarfs and red giants, regions where the uvby metallicity index, m(l), loses some sensitivity. An index, hk, is defined by replacing v in m(l) by Ca. The effects of interstellar extinction on the index are modeled and demonstrated to be modest and relatively insensitive to spectral type. Observations of V, (b-y), and hk for 163 primary standards are detailed and transformed to the standard V and (b-y) system. A qualitative analysis using only the primary standards indicates that hk is more sensitive than m(l) over the regions of interest by about a factor of 3. 58 refs.

vbyCaHβ CCD Photometry of Clusters. VIII. The Super-Metal-Rich, Old Open Cluster NGC 6791

CCD photometry on the intermediate-band vbyCaHβ system is presented for the metal-rich, old open cluster NGC 6791. Preliminary analysis led to [Fe/H] above +0.4 with an anomalously high reddening and an age below 5 Gyr. A revised calibration between (b - y)0 and [Fe/H] at a given temperature shows that the traditional color-metallicity relations underestimate the color of the turnoff stars at high metallicity. With the revised relation, the metallicity from hk and the reddening for NGC 6791 become [Fe/H] = +0.45 ± 0.04 and E(b - y) = 0.113 ± 0.012 or E(B - V) = 0.155 ± 0.016. Using the same technique, reanalysis of the photometry for NGC 6253 produces [Fe/H] = +0.58 ± 0.04 and E(b - y) = 0.120 ± 0.018 or E(B - V) = 0.160 ± 0.025. The errors quoted include both the internal and external errors. For NGC 6791, the metallicity from m1 is a factor of 2 below that from hk, a result that may be coupled to the consistently low metal abundance from DDO photometry of the cluster and the C-deficiency found from high-dispersion spectroscopy. E(B - V) is the same value predicted from Galactic reddening maps. With E(B - V) = 0.15 and [Fe/H] = +0.45, the available isochrones predict an age of 7.0 ± 1.0 Gyr and an apparent modulus of (m - M) = 13.60 ± 0.15, with the dominant source of the uncertainty arising from inconsistencies among the isochrones. The reanalysis of NGC 6253 with the revised lower reddening confirms that on both the hk and m1 metallicity scales, NGC 6253, while less than half the age of NGC 6791, remains at least as metal-rich as NGC 6791, if not richer.

CN AND CA ABUNDANCE VARIATIONS AMONG THE GIANTS IN M22

We have obtained uvbyCa data for over 300 giants and horizontal branch stars in three fields of M22. The spread in (b-y) for the giant and horizontal branches is consistent with a spread in foreground reddening Delta-E (B-V) ~0.08. Reanalysis of the data of Norris and Freeman (1983) indicates not only positive correlations between CH, CN and a unimodal distribution for Ca. Our photometric indices, m1, and hk, demonstrate a range in metallicity that persists to two magnitudes below the horizontal branch, and confirm the correlation between calcium abundance and CN/CH. We infer from comparisons to spectroscopic data that m1 is dominated by the CN and CH abundance and find no independent evidence of a range in [Fe/H]. The excessive ranges in m1 and hk also suggest the influence of a continuous opacity source, reminiscent of the Bond-Neff effect, that is correlated with CNO abundance. The relative contributions of internal mixing and primordial variations for M22s giants are discussed.

CCD uvbyCaHβ Photometry of Clusters. III. The Most Metal-rich Open Cluster, NGC 6253

CCD photometry on the intermediate-band uvbyCaHβ system is presented for the old open cluster NGC 6253. Despite a high level of field star contamination because of its location toward the Galactic center, combination of the data from the multiple color indices with the core cluster sample derived from radial star counts leads to the identification of a set of highly probable, single cluster members. Photometric analysis of a select sample of 71 turnoff stars produces a reddening value of E(b-y) = 0.190 ± 0.002 (s.e.m.) or E(B-V) = 0.260 ± 0.003 (s.e.m.). The metallicity indices, δm1 and δhk, both identify this cluster as the most metal-rich object studied on either system to date. Simple extrapolation of the available metallicity calibrations leads to [Fe/H] values ranging from +0.7 to +0.9. Metal-rich isochrones with overshoot imply an age between 2.5 and 3.5 Gyr, with an apparent distance modulus between (m - M) = 11.6 and 12.2, depending upon the isochrones used. The improvement in the fit using α-enhanced isochrones may indicate that the cluster [Fe/H] is closer to +0.4, but the photometric indices are distorted by an elemental distribution other than a scaled solar. The Galactocentric position of the cluster, in conjunction with data for other clusters and Cepheids, is consistent with the inner disk reaching and maintaining a metallicity well above solar since the early history of the disk, unlike the solar neighborhood.

WIYN OPEN CLUSTER STUDY. XXXIX. ABUNDANCES IN NGC 6253 FROM HYDRA SPECTROSCOPY OF THE Li 6708 Å REGION

High-dispersion spectra of 89 potential members of the old, super-metal-rich open cluster, NGC 6253, have been obtained with the HYDRA multi-object spectrograph. Based upon radial-velocity measurements alone, 47 stars at the turnoff of the cluster color–magnitude diagram (CMD) and 18 giants are identified as potential members. Five turnoff stars exhibit evidence of binarity while proper-motion data eliminate two of the dwarfs as members. The mean cluster radial velocity from probable single-star members is −29.4 ± 1.3 km s −1 (sd). A discussion of the current estimates for the cluster reddening, derived independently of potential issues with the BV cluster photometry, lead to an adopted reddening of E(B − V ) = 0.22 ± 0.04. From equivalent width analyses of 38 probable single-star members near the CMD turnoff, the weighted average abundances are found to be [Fe/H] = +0.43 ± 0.01, [Ni/H] = +0.53 ± 0.02, and [Si/H] = +0.43 0.03 0.04 , where the errors refer to the standard errors of the weighted mean. Weak evidence is found for a possible decline in metallicity with increasing luminosity among stars at the turnoff. We discuss the possibility that our turnoff stars have been affected by microscopic diffusion. For 15 probable single-star members among the giants, spectrum synthesis leads to abundances of +0.46 0.02 0.03 for [Fe/H]. While NGC 6253 is less than half the age of NGC 6791, it is at least as metal-rich and, within the uncertainties, exhibits the same general abundance pattern as that typified by super-metal-rich dwarfs of the galactic bulge.

A LITHIUM-RICH RED GIANT BELOW THE CLUMP IN THE KEPLER CLUSTER, NGC 6819

WIYN/HYDRA spectra in the Li 6708 A region have been obtained for 332 probable members of the old open cluster, NGC 6819. Preliminary analysis shows a pattern of Li depletion from the top of the turnoff to the base of the giant branch. Starting 1 mag below the level of the clump, all brighter giants have A(Li) below 1.0, with most having upper limits below 0.5. Star W007017, located below the first-ascent red giant bump is Li-rich with A(Li) = 2.3. As a highly probable single-star astrometric and radial-velocity cluster member, its discrepant asteroseismic membership could be a by-product of the processes that triggered Li enhancement. Its color-magnitude diagram location is consistent with only one proposed enhanced mixing process among first-ascent red giants.