62 research outputs found
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
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 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
WIYN OPEN CLUSTER STUDY. LXXII. A uvbyCaHβ CCD ANALYSIS OF THE METAL-DEFICIENT OPEN CLUSTER NGC 2506
Precision uvbyCaHβ photometry of the metal-deficient, old open cluster NGC 2506 is presented. The survey covers an area of and extends to for b − y and Hβ and to for c 1 and hk. For V brighter than 16.0, photometric scatter among the indices leads to the recovery of six known variables within the cluster core and five new variables in the outer of the survey field. Proper motions, radial velocities, and precise multicolor indices are used to isolate a highly probable sample of cluster members from the very rich color–magnitude diagram. From 257 highly probable members at the cluster turnoff, we derive a reddening estimate of (), where the errors refer to the internal standard errors of the mean. [Fe/H] is derived from the A/F dwarf members using both m 1 and hk, leading to [Fe/H] = −0.296 ± 0.011 (sem) and −0.317 ± 0.004 (sem), respectively. The weighted average, heavily dominated by hk, is [Fe/H] = −0.316 ± 0.033. Based on red giant members, we place an upper limit of ±0.010 on the variation in the reddening across the face of the cluster. We also identify two dozen potential red giant cluster members outside the cluster core. Victoria–Regina isochrones on the Strömgren system produce an excellent match to the cluster for an apparent modulus of and an age of 1.85 ± 0.05 Gyr
WIYN OPEN CLUSTER STUDY. LXXII. A uvbyCaHβ CCD ANALYSIS OF THE METAL-DEFICIENT OPEN CLUSTER NGC 2506
Precision uvbyCaHβ photometry of the metal-deficient, old open cluster NGC 2506 is presented. The survey covers an area of and extends to for b − y and Hβ and to for c 1 and hk. For V brighter than 16.0, photometric scatter among the indices leads to the recovery of six known variables within the cluster core and five new variables in the outer of the survey field. Proper motions, radial velocities, and precise multicolor indices are used to isolate a highly probable sample of cluster members from the very rich color–magnitude diagram. From 257 highly probable members at the cluster turnoff, we derive a reddening estimate of (), where the errors refer to the internal standard errors of the mean. [Fe/H] is derived from the A/F dwarf members using both m 1 and hk, leading to [Fe/H] = −0.296 ± 0.011 (sem) and −0.317 ± 0.004 (sem), respectively. The weighted average, heavily dominated by hk, is [Fe/H] = −0.316 ± 0.033. Based on red giant members, we place an upper limit of ±0.010 on the variation in the reddening across the face of the cluster. We also identify two dozen potential red giant cluster members outside the cluster core. Victoria–Regina isochrones on the Strömgren system produce an excellent match to the cluster for an apparent modulus of and an age of 1.85 ± 0.05 Gyr
A UvbyCa Hβ ANALYSIS OF THE OLD OPEN CLUSTER, NGC 6819
This is the published version. Copyright © 2014. The American Astronomical Society. All rights reserved.NGC 6819 is a richly populated, older open cluster situated within the Kepler field. A CCD survey of the cluster on the uvbyCaHβ system, coupled with proper-motion membership, has been used to isolate 382 highly probable, single-star unevolved main-sequence members over a 20' 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 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 ΔE(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
A uvbyCaHbeta CCD Analysis of the Open Cluster Standard, M67,and its Relation to NGC 752
Precision CCD uvbyCaHbeta photometry is presented of the old cluster, M67,
covering one square degree with typical internal precision at the 0.005-0.020
mag level to V~17. The photometry is calibrated using standards over a wide
range in luminosity and temperature from NGC 752 and zeroed to the standard
system via published photoelectric observations. Relative to NGC 752,
differential offsets in reddening and metallicity are derived using astrometric
members, supplemented by radial-velocity information. From single-star members,
offsets in the sense (M67 - NGC 752) are Delta E(b-y) = -0.005 +/-0.001 (sem)
mag from 327 F/G dwarfs and Delta [Fe/H] = 0.062 +/- 0.006 (sem) dex from the
combined m1 and hk indices of 249 F dwarfs, leading to E(b-y) = 0.021 +/- 0.004
(sem), and [Fe/H] = +0.030 +/- 0.016 (sem) for M67, assuming [Fe/H]{Hyades} =
+0.12. With probable binaries eliminated using c1,(b-y) indices, 83 members
with relative parallax errors < 0.02 generate (m-M)_0 = 8.220 +/- 0.005 (sem)
for NGC 752 and an isochronal age of 1.45 +/- 0.05 Gyr. Using the same parallax
restriction for 312 stars, M67 has (m-M) = 9.77 +/- 0.02 (sem), leading to an
age tied solely to the luminosity of the subgiant branch of 3.70 +/- 0.03 Gyr.
The turnoff color spread implies +/- 0.1 Gyr, but the turnoff morphology
defines a younger age/higher mass for the stars, consistent with recent binary
analysis and broad-band photometry indicating possible missing physics in the
isochrones. Anomalous stars positioned blueward of the turnoff are discussed.Comment: 36 pages, 8 figures. Accepted for Astronomical Journa
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