A UBVI and uvbyCaHbeta Analysis of the Intermediate-Age Open Cluster, NGC 5822

NGC 5822 is a richly populated, moderately nearby, intermediate-age open cluster covering an area larger than the full moon on the sky. A CCD survey of the cluster on the UBVI and uvbyCaHbeta systems shows that the cluster is superposed upon a heavily reddened field of background stars with E(B-V) > 0.35 mag, while the cluster has small and uniform reddening at E(b-y) = 0.075 +/- 0.008 mag or E(B-V) = 0.103 +/- 0.011 mag, based upon 48 and 61 probable A and F dwarf single-star members, respectively. The errors quoted include both internal photometric precision and external photometric uncertainties. The metallicity derived from 61 probable single F-star members is [Fe/H] = -0.058 +/- 0.027 (sem) from m_1 and 0.010 +/- 0.020 (sem) from hk, for a weighted average of [Fe/H] = -0.019 +/- 0.023, where the errors refer to the internal errors from the photometry alone. With reddening and metallicity fixed, the cluster age and apparent distance modulus are obtained through a comparison to appropriate isochrones in both VI and BV, producing 0.9 +/- 0.1 Gyr and 9.85 +/- 0.15, respectively. The giant branch remains dominated by two distinct clumps of stars, though the brighter clump seems a better match to the core-He-burning phase while the fainter clump straddles the first-ascent red giant branch. Four potential new clump members have been identified, equally split between the two groups. Reanalysis of the UBV two-color data extending well down the main sequence shows it to be optimally matched by reddening near E(B-V) = 0.10 rather than the older value of 0.15, leading to [Fe/H] between -0.16 and 0.00 from the ultraviolet excess of the unevolved dwarfs. The impact of the lower reddening and younger age of the cluster on previous analyses of the cluster is discussed.Comment: 20 figures and 5 tables (portions of data tables 3 and 5 only

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.Comment: 41 pages, 9 figure files, LaTex. Appendix section and tables (tex or postscript) available at http://kubarb.phsx.ukans.edu/ ~twarog/ Submitted to Astronomical Journal July 199

A uvbyCaHbeta Analysis of the Old Open Cluster, NGC 6819

NGC 6819 is a richly populated, older open cluster situated within the Kepler field. A CCD survey of the cluster on the uvbyCaHbeta system, coupled with proper-motion membership, has been used to isolate 382 highly probable, single-star unevolved main-sequence members over a 20-arcminute field centered on the cluster. From 278 F dwarfs with high precision photometry in all indices, a mean reddening of E(b-y) = 0.117 +/- 0.005 or E(B-V) = 0.160 +/- 0.007 is derived, where the standard errors of the mean include both internal errors and the photometric zero-point uncertainty. With the reddening fixed, the metallicity derived from the same 278 stars is [Fe/H] = -0.116 +/- 0.101 from m_1 and -0.055 +/- 0.033 from hk, for a weighted average of [Fe/H] = -0.06 +/- 0.04, where the quoted standard errors of the mean values include the internal errors from the photometric scatter plus the uncertainty in the photometric zero points. If metallicity is derived using individual reddening values for each star to account for potential reddening variation across the face of the cluster, the analogous result is unchanged. The cluster members at the turnoff of the color-magnitude diagram are used to test and confirm the recently discovered variation in reddening across the face of the cluster, with a probable range in the variation of Delta[E(B-V)] = 0.045 +/-0.015. With the slightly higher reddening and lower [Fe/H] compared to commonly adopted values, isochrone fitting leads to an age of 2.3 +/- 0.2 Gyr for an apparent modulus of (m-M) = 12.40 +/-0.12.Comment: WIYN Open Cluster Study LXI; accepted to Astronomical Journal. 11 figures, 2 table

The Unevolved Main Sequence of Nearby Field Stars and the Open Cluster Distance Scale

The slope and zero-point of the unevolved main sequence as a function of metallicity are investigated using a homogeneous catalog of nearby field stars with absolute magnitudes defined with revised Hipparcos parallaxes, Tycho-2 photometry, and precise metallicities from high-dispersion spectroscopy. (B-V)-temperature relations are derived from 1746 stars between [Fe/H] = -0.5 and +0.6 and 372 stars within 0.05 dex of solar abundance; for T_e = 5770 K, the solar color is B-V= 0.652 +/- 0.002 (s.e.m.). From over 500 cool dwarfs between [Fe/H] = -0.5 and +0.5, Delta(B-V)/Delta[Fe/H] at fixed M_V = 0.213 +/- 0.005, with a very weak dependence upon the adopted main sequence slope with B-V at a given [Fe/H]. At Hyades metallicity this translates into Delta M_V/Delta[Fe/H] at fixed B-V = 0.98 +/- 0.02, midway between the range of values empirically derived from smaller and/or less homogeneous samples and model isochrones. From field stars of similar metallicity, the Hyades ([Fe/H] = +0.13) with no reddening has (m-M)_0 = 3.33 +/- 0.02 and M67, with E(B-V) = 0.041, A_V = 3.1E(B-V), and [Fe/H] = 0.00, has (m-M)_0 = 9.71 +/- 0.02 (s.e.m), where the errors quoted refer to internal errors alone. At the extreme end of the age and metallicity scale, with E(B-V) = 0.125 +/- 0.025 and [Fe/H] = +0.39 +/- 0.06, comparison of the fiducial relation for NGC 6791 to 19 field stars with (B-V) above 0.90 and [Fe/H] = +0.25 or higher, adjusted to the metallicity of NGC 6791, leads to (m-M)_0 = 13.07 +/- 0.09, internal and systematic errors included.Comment: 32 pages, 8 eps figures, latex; accepted for PAS

A uvbyCaHbeta CCD Analysis of the Open Cluster Standard, NGC 752

Precision uvbyCaHbeta photometry of the nearby old open cluster, NGC 752, is presented. The mosaic of CCD fields covers an area ~42' on a side with internal precision at the 0.005 to 0.010 mag level for the majority of stars down to V~15. The CCD photometry is tied to the standard system using an extensive set of published photoelectric observations adopted as secondary standards within the cluster. Multicolor indices are used to eliminate as nonmembers a large fraction of the low probability proper-motion members near the faint end of the main sequence, while identifying 24 potential dwarf members between V=15.0 and 16.5, eight of which have been noted before from Vilnius photometry. From 68 highly probable F dwarf members, we derive a reddening estimate of E(b-y)= 0.025 +/- 0.003 (E(B-V) = 0.034 +/- 0.004), where the error includes the internal photometric uncertainty and the systematic error arising from the choice of the standard (b-y), Hbeta relation. With reddening fixed, [Fe/H] is derived from the F dwarf members using both m_1 and hk, leading to [Fe/H] = -0.071 +/-0.014 (sem) and -0.017 +/- 0.008 (sem), respectively. Taking the internal precision and possible systematics in the standard relations into account, [Fe/H] for NGC 752 becomes -0.03 +/-0.02. With the reddening and metallicity defined, we use the Victoria-Regina isochrones on the Stromgren system and find an excellent match for (m-M) = 8.30 +/- 0.05 and an age of 1.45 +/- 0.05 Gyr at the appropriate metallicity.Comment: 37 pages, 11 figures, 4 tables. Accepted to Astronomical Journa

WIYN Open Cluster Study. XXXIX. Abundances in NGC 6253 from HYDRA Spectroscopy of the Li 6708 A 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 eliminates two of the dwarfs as members. The mean cluster radial velocity from probable single-star members is -29.4 +/- 1.3 km/sec (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 less than half the age of NGC 6791, NGC 6253 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.Comment: 5 Tables, 9 figures, 45 page

On a new theoretical calibration of the Stroemgren hk metallicity index: NGC6522 as a first test case

We present a new theoretical calibration of the Stroemgren metallicity index hk by using alpha-enhanced evolutionary models transformed into the observational plane by using atmosphere models with the same chemical mixture. We apply the new Metallicity--Index--Color (MIC) relations to a sample of 85 field red giants (RGs) and find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex with a dispersion of sigma = 0.19 dex. The outcome is the same if we apply the MIC relations to a sample of eight RGs in the bulge globular cluster NGC6522, but the standard deviation ranges from 0.26 (hk, v-y) to 0.49 (hk, u-y). The difference is mainly caused by a difference in photometric accuracy. The new MIC relations based on the (Ca-y) color provide metallicities systematically more metal-rich than the spectroscopic ones. We found that the Ca-band is affected by Ca abundance and possibly by chromospheric activity.Comment: Accepted for publication on The Astrophysical Journal Letter