1,880 research outputs found
Starcounts Redivivus. IV. Density Laws Through Photometric Parallaxes
In an effort to more precisely define the spatial distribution of Galactic
field stars, we present an analysis of the photometric parallaxes of 70,000
stars covering nearly 15 square degrees in seven Kapteyn Selected Areas. We
address the affects of Malmquist Bias, subgiant/giant contamination,
metallicity and binary stars upon the derived density laws. The affect of
binary stars is the most significant. We find that while the disk-like
populations of the Milky Way are easily constrained in a simultaneous analysis
of all seven fields, no good simultaneous solution for the halo is found. We
have applied halo density laws taken from other studies and find that the
Besancon flattened power law halo model (c/a=0.6, r^-2.75) produces the best
fit to our data. With this halo, the thick disk has a scale height of 750 pc
with an 8.5% normalization to the old disk. The old disk scale height is
280-300 pc. Corrected for a binary fraction of 50%, these scale heights are 940
pc and 350-375 pc, respectively. Even with this model, there are systematic
discrepancies between the observed and predicted density distributions. Our
model produces density overpredictions in the inner Galaxy and density
underpredictions in the outer Galaxy. A possible solution is modeling the
stellar halo as a two-component system in which the halo has a flattened inner
distribution and a roughly spherical, but substructured outer distribution.
Further reconciliation could be provided by a flared thick disk, a structure
consistent with a merger origin for that population. (Abridged)Comment: 66 pages, accepted to Astrophysical journal, some figures compresse
The Star Formation Epoch of the Most Massive Early-Type Galaxies
We present new Keck spectroscopy of early-type galaxies in three galaxy
clusters at z~0.5. We focus on the fundamental plane (FP) relation, and combine
the kinematics with structural parameters determined from HST images. The
galaxies obey clear FP relations, which are offset from the FP of the nearby
Coma cluster due to passive evolution of the stellar populations. The z~0.5
data are combined with published data for 11 additional clusters at
0.18<z<1.28, to determine the evolution of the mean M/L(B) ratio of cluster
galaxies with masses M>10^11 M_sun, as implied by the FP. We find
dlog(M/L(B))/dz = -0.555+-0.042, stronger evolution than was previously
inferred from smaller samples. The observed evolution depends on the
luminosity-weighted mean age of the stars in the galaxies, the initial mass
function (IMF), selection effects due to progenitor bias, and other parameters.
Assuming a normal IMF but allowing for various other sources of uncertainty we
find z* = 2.01+-0.20 for the luminosity-weighted mean star formation epoch. The
main uncertainty is the slope of the IMF in the range 1-2 Solar masses: we find
z* = 4.0 for a top-heavy IMF with slope x=0. The M/L(B) ratios of the cluster
galaxies are compared to those of recently published samples of field
early-type galaxies at 0.32<z<1.14. Assuming that progenitor bias and the IMF
do not depend on environment we find that the present-day age of stars in
massive field galaxies is 4.1 +- 2.0 % (~0.4 Gyr) less than that of stars in
massive cluster galaxies, consistent with most, but not all, previous studies
of local and distant early-type galaxies. This relatively small age difference
is surprising in the context of expectations from ``standard'' hierarchical
galaxy formation models. [ABRIDGED]Comment: Accepted for publication in ApJ. Minor corrections to match published
versio
W UMa-type Binary Stars in Globular Clusters
A sample of 86 contact binary systems in 14 globular clusters with available
color index data in (B-V) or in (V-I) has been analyzed. At least one third of
all systems are numerous foreground Galactic Disk projections over long lines
of sight to the clusters. Since the selection of the cluster members has been
based on the MV(logP, color) calibrations, the matter of a
metallicity-correction required particular attention with the result that such
a correction is apparently not needed at the present level of accuracy.
Analysis of the color-magnitude and period-color relations shows that globular
cluster members are under-luminous relative to the Galactic Disk contact
systems mainly because of the smaller sizes and, consequently, shorter orbital
periods; the color-index effect of the diminished blanketing is less important,
especially for (V-I). The variability amplitudes for the Blue Straggler systems
show a significantly different distribution from that for systems below the
Turn Off Point (TOP): The BS systems show only small amplitudes while the
distribution for the systems below the TOP is peculiar in containing only large
amplitude systems. This difference is linked to an observational selection
effect so that efforts at determining the frequency of occurrence of the
contact systems below the TOP have been judged to be premature; the frequency
among the BS stars could be at about 45+-10 BS stars per one contact BS binary.Comment: submitted for publication in Astronomical Journal; 8 figures, 3
tables (Table 1 in landscape
Comprehensive analysis of RGU photometry in the direction to M5
The RGU-photographic investigation of an intermediate latitude field in the
direction to the Galactic center is presented. 164 extra-galactic objects,
identified by comparison of Minnesota and Basel charts, are excluded from the
program. Also, a region with size 0.104 square-degrees, contaminated by cluster
(M5) stars and affected by background light of the bright star HD 136202 is
omitted. Contrary to previous investigations, a reddening of ,
corresponding to E(G-R)=0.07 mag is adopted. The separation of dwarfs and
evolved stars is carried out by an empirical method, already applied in some of
our works. A new calibration for the metallicity determination is used for
dwarfs, while the absolute magnitude determination for stars of all categories
is performed using the procedures given in the literature. There is good
agreement between the observed logarithmic space density histograms and the
galactic model gradients. Also, the local luminosity function agrees with
Gliese's (1969) and Hipparcos' (Jahreiss & Wielen 1997) luminosity functions,
for stars with mag. For giants, we obtained two different local
space densities from comparison with two Galactic models, i.e. ,
close to that of Gliese (1969), and . A metallicity gradient,
dex/kpc, is detected for dwarfs (only) with absolute
magnitudes , corresponding to a spectral type interval F5-K0.Comment: 17 pages, including 13 figures and 3 tables, accepted for publication
in PAS
Exploration of the BaSeL stellar library for 9 F-type stars COROT potential targets
The Basel Stellar Library (BaSeL models) is constituted of the merging of
various synthetic stellar spectra libraries, with the purpose of giving the
most comprehensive coverage of stellar parameters. It has been corrected for
systematic deviations detected in respect to UBVRIJHKLM photometry at solar
metallicity, and can then be considered as the state-of-the-art knowledge of
the broad band content of stellar spectra. In this paper, we consider a sample
of 9 F-type stars with detailed spectroscopic analysis to investigate the Basel
Stellar Library in two photometric systems simultaneously, Johnson (B-V, U-B)
and Stromgren (b-y, m_1, and c_1). The sample corresponds to potential targets
of the central seismology programme of the COROT space experiment, which have
been recently observed at OHP. The atmospheric parameters T_eff, [Fe/H], and
log g obtained from the BaSeL models are compared with spectroscopic
determinations as well as with results of other photometric calibrations. For a
careful interpretation of the BaSeL solutions, we computed confidence regions
around the best ^2-estimates and projected them on T_eff-[Fe/H],
T_eff-log g, and log g-[Fe/H] diagrams. (Abridged)Comment: 16 pages, LaTeX2e; version accepted for publication in the new A&A
Journal: minor changes + figures in black and white for better readabilit
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