430 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
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
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. 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
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