203 research outputs found
The Zero Point of Extinction Toward Baade's Window
We measure the zero point of the Stanek (1996) extinction map by comparing
the observed (V-K) colors of 206 K giant stars with their intrinsic (V-K)_0
colors as derived from their H\beta indices. We find that the zero point of the
Stanek map should be changed by \Delta A_V = -0.10 +/- 0.06 mag, obtaining as a
bonus a three-fold reduction of the previous statistical error. The most direct
way to test for systematic errors in this determination would be to conduct a
parallel measurement based on the (V-K) colors of RR Lyraes (type ab).Comment: 10 pages, 1 figur
Anomaluos RR Lyrae (V-I)_0 colors in Baade's Window
We compare (V-I)_0-(V-K)_0 color-color and (V-I)_0-log P period-color
diagrams for Baade's Window and local RRab Lyrae stars. We find that for a
fixed log P the Baade's Window RR Lyrae stars are ~0.17 magnitudes redder in
(V-I)_0 than the local RR Lyrae stars. We also show that there is no such
effect observed in (V-K)_0. We argue that an extinction misestimate towards
Baade's Window is not a plausible explanation of the discrepancy. Unlike
Baade's Window RR Lyrae stars, the local ones follow a black-body color-color
relation and are well approximated by theoretical models. We test two
parameters, metallicity and surface gravity, and find that their effects are
too small to explain the (V-I)_0 discrepancy between the two groups of stars.
We do not provide any explanation for the anomalous (V-I)_0 behavior of the
Baade's Window RR Lyrae stars. We note that a similar effect for clump giant
stars has been recently reported by Paczynski and we caution that RR Lyrae
stars and clump giants, often used as standard candles, can be subject to the
same type of systematics.Comment: 10 pages, 7 figures, submitted to Ap
Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions
We sing the praises of the central limit theorem. Having previously removed
all other possible causes of significant systematic error in the statistical
parallax determination of RR Lyrae absolute magnitudes, we investigate
systematic errors from two final sources of input data: apparent magnitudes and
extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the
statistical error. However, these are of opposite sign and so roughly cancel.
The apparent magnitude system that we previously adopted from Layden et al. was
calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we
show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were
previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V
should rather be based on new COBE/IRAS dust-emission map of Schlegel,
Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find
M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or
M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from
716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen.
These are 2 and 3 sigma fainter than recent determinations of M_V from main
sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid
and Gratton et al. Since statistical parallax is being cleared of systematic
errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we
conclude that these brighter determinations may be in error. In the course of
three papers, we have corrected 6 systematic errors whose absolute values total
0.20 mag. Had these, contrary to the expectation of the central limit theorem,
all lined up one way, they could have resolved the conflict in favor of the
brighter determinations. In fact, the net change was only 0.06 mag.Comment: submitted to ApJ, 21 pages, 2 tables, 4 figure
The Large-Scale Extinction Map of the Galactic Bulge from the MACHO Project Photometry
We present a (V-R)-based reddening map of about 43 square degrees of the
Galactic bulge/bar. The map is constructed using template image photometry from
the MACHO microlensing survey, contains 9717 resolution elements, and is based
on (V-R)-color averages of the entire color-magnitude diagrams (CMDs) in 4 by 4
arc-minute tiles. The conversion from the observed color to the reddening
follows from an assumption that CMDs of all bulge fields would look similar in
the absence of extinction. Consequently, the difference in observed color
between various fields originates from varying contribution of the disk
extinction summed along different lines of sight. We check that our (V-R)
colors correlate very well with infrared and optical reddening maps. We show
that a dusty disk obeying a cosec|b| extinction law, E(V-R) = 0.0274 cosec|b|,
provides a good approximation to the extinction toward the MACHO bulge/bar
fields. The large-scale (V-R)-color and visual extinction map presented here is
publicly available in the electronic edition of the Journal and on the World
Wide Web.Comment: 24 pages, 3 tables, 7 figures (6 in color), version accepted to AJ,
added comparisons with Schlegel et al. (1998) and Dutra et al. (2003)
reddening map
Dark Matter Density in Disk Galaxies
I show that the predicted densities of the inner dark matter halos in LCDM
models of structure formation appear to be higher than estimates from real
galaxies and constraints from dynamical friction on bars. This inconsistency
would not be a problem for the LCDM model if physical processes that are
omitted in the collisionless collapse simulations were able to reduce the dark
matter density in the inner halos. I review the mechanisms proposed to achieve
the needed density reduction.Comment: 12 pages, 5 figures, to appear in "The Galaxy Disk in Cosmological
Context" eds Andersen, Bland-Hawthorn & Nordstro
An inner ring and the micro lensing toward the Bulge
All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
(). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.Comment: 9 pages, 7(15) figs, LaTeX, AJ (accepted
High precision microlensing maps of the Galactic bulge
We present detailed maps of the microlensing optical depth and event density
over an area of 195 sq. deg towards the Galactic bulge. The maps are computed
from synthetic stellar catalogues generated from the Besancon Galaxy Model,
which comprises four stellar populations and a three-dimensional extinction map
calibrated against the Two-Micron All-Sky Survey. The optical depth maps have a
resolution of 15 arcminutes, corresponding to the angular resolution of the
extinction map. We compute optical depth and event density maps for all
resolved sources above I=19, for unresolved (difference image) sources
magnified above this limit, and for bright standard candle sources in the
bulge. We show that the resulting optical depth contours are dominated by
extinction effects, exhibiting fine structure in stark contrast to previous
theoretical optical depth maps. Optical depth comparisons between Galactic
models and optical microlensing survey measurements cannot safely ignore
extinction or assume it to be smooth. We show how the event distribution for
hypothetical J and K-band microlensing surveys, using existing ground-based
facilities such as VISTA, UKIRT or CFHT, would be much less affected by
extinction, especially in the K band. The near infrared provides a substantial
sensitivity increase over current I-band surveys and a more faithful tracer of
the underlying stellar distribution, something which upcoming variability
surveys such as VVV will be able to exploit. Synthetic population models offer
a promising way forward to fully exploit large microlensing datasets for
Galactic structure studies.Comment: 8 pages, submitted to MNRA
Gravitational microlensing as a test of a finite-width disk model of the Galaxy
The aim of this work is to show, in the framework of a simple finite-width
disk model, that the amount of mass seen through gravitational microlensing
measurements in the region is consistent with the dynamical mass
ascertained from Galaxy rotation after subtracting gas contribution. Since
microlensing only detects compact objects, this result suggests that a
non-baryonic mass component may be negligible in this region.Comment: 10 pages, 12 figure
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