152 research outputs found
Clump Distance to the Magellanic Clouds and Anomalous Colors in the Galactic Bulge
I demonstrate that the two unexpected results in the local Universe: 1)
anomalous intrinsic (V-I)_0 colors of the clump giants and RR Lyrae stars in
the Galactic center, and 2) very short distances to the Magellanic Clouds (LMC,
SMC) as inferred from clump giants, are connected with each other. The (V-I)_0
anomaly is partially resolved by using the photometry from the phase-II of the
Optical Gravitational Lensing Experiment (OGLE) rather than phase-I. The need
for V- or I-magnitude-based change in the bulge (V-I)_0 is one option to
explain the remaining color discrepancy. Such change may originate in a
coefficient of selective extinction A_V/E(V-I) smaller than typically assumed.
Application of the (V-I)_0 correction (independent of its source) doubles the
slope of the absolute magnitude - metallicity relation for clump giants, so
that M_I(RC) = -0.23 + 0.19[Fe/H]. Consequently, the estimates of the clump
distances to the LMC and SMC are affected. Udalski's (1998c) distance modulus
of mu_{LMC} = 18.18 +/- 0.06 increases to mu_{LMC} = 18.27 +/- 0.07. The
distance modulus to the SMC increases by 0.12 to mu_{SMC} = 18.77 +/- 0.08. I
argue that a more comprehensive assessment of the metallicity effect on M_I(RC)
is needed.Comment: accepted by ApJ Letters, brief review of the short distance scale
dropped, discussion of the absolute magnitude - metallicity relation for
clump giants shortened and made more qualitative, results basically unchange
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 Absolute Magnitude of RRc Variables From Statistical Parallax
We present the first definitive measurement of the absolute magnitude of RR
Lyrae c-type variable stars (RRc) determined purely from statistical parallax.
We use a sample of 247 RRc selected from the All Sky Automated Survey (ASAS)
for which high-quality light curves, photometry and proper motions are
available. We obtain high-resolution echelle spectra for these objects to
determine radial velocities and abundances as part of the Carnegie RR Lyrae
Survey (CARRS). We find that M_(V,RRc) = 0.52 +/- 0.11 at a mean metallicity of
[Fe/H] = -1.59. This is to be compared with previous estimates for RRab stars
(M_(V,RRab) = 0.75 +/- 0.13 and the only direct measurement of an RRc absolute
magnitude (RZ Cephei, M_(V, RRc) = 0.27 +/- 0.17). We find the bulk velocity of
the halo to be (W_pi, W_theta, W_z) = (10.9,34.9,7.2) km/s in the radial,
rotational and vertical directions with dispersions (sigma_(W_pi),
sigma_(W_theta), sigma_(W_z)) = (154.7, 103.6, 93.8) km/s. For the disk, we
find (W_pi, W_theta, W_z) = (8.5, 213.2, -22.1) km/s with dispersions
(sigma_(W_pi), sigma_(W_theta), sigma_(W_z)) = (63.5, 49.6, 51.3) km/s.
Finally, we suggest that UCAC2 proper motion errors may be overestimated by
about 25%Comment: Submitted to ApJ. 11 pages including 6 figure
Microlensing Maps for the Milky Way Galaxy
At any instant, there are roughly 1000 ongoing microlensing events to sources
brighter than 20th magnitude in the Milky Way Galaxy. Large-scale maps of the
microlensing optical depth and the mean timescale are constructed for a number
of models of the Galactic bar and disk, incorporating the effects of streaming
and spiral structure. Freudenreich's model can reproduce the high optical
depths towards the Bulge. It is also in good agreement with the data towards
the spiral arms (except for the gamma Norma field). Spiral structure tends to
increase the optical depth by about 20% and the mean timescale by about 100%.
Different bar morphologies give characteristically different shaped contours,
especially at low Galactic latitudes (|b| < 2 degrees). These could be traced
out with a K band microlensing survey, consuming about 100 minutes per night on
a telescope like VISTA.Comment: 15 pages, 5 figures, ApJ (Letters) in pres
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
The MACHO Project HST Follow-Up: The Large Magellanic Cloud Microlensing Source Stars
We present Hubble Space Telescope (HST) WFPC2 photometry of 13 microlensed
source stars from the 5.7 year Large Magellanic Cloud (LMC) survey conducted by
the MACHO Project. The microlensing source stars are identified by deriving
accurate centroids in the ground-based MACHO images using difference image
analysis (DIA) and then transforming the DIA coordinates to the HST frame. None
of these sources is coincident with a background galaxy, which rules out the
possibility that the MACHO LMC microlensing sample is contaminated with
misidentified supernovae or AGN in galaxies behind the LMC. This supports the
conclusion that the MACHO LMC microlensing sample has only a small amount of
contamination due to non-microlensing forms of variability. We compare the
WFPC2 source star magnitudes with the lensed flux predictions derived from
microlensing fits to the light curve data. In most cases the source star
brightness is accurately predicted. Finally, we develop a statistic which
constrains the location of the Large Magellanic Cloud (LMC) microlensing source
stars with respect to the distributions of stars and dust in the LMC and
compare this to the predictions of various models of LMC microlensing. This
test excludes at > 90% confidence level models where more than 80% of the
source stars lie behind the LMC. Exotic models that attempt to explain the
excess LMC microlensing optical depth seen by MACHO with a population of
background sources are disfavored or excluded by this test. Models in which
most of the lenses reside in a halo or spheroid distribution associated with
either the Milky Way or the LMC are consistent which these data, but LMC halo
or spheroid models are favored by the combined MACHO and EROS microlensing
results.Comment: 28 pages with 10 included PDF figures, submitted to Ap
Can CCM law properly represent all extinction curves?
We present the analysis of a large sample of lines of sight with extinction
curves covering wavelength range from near-infrared (NIR) to ultraviolet (UV).
We derive total to selective extinction ratios based on the Cardelli, Clayton
and Mathis (1989, CCM) law, which is typically used to fit the extinction data
both for diffuse and dense interstellar medium. We conclude that the CCM law is
able to fit most of the extinction curves in our sample. We divide the
remaining lines of sight with peculiar extinction into two groups according to
two main behaviors: a) the optical/IR or/and UV wavelength region cannot be
reproduced by the CCM formula; b) the optical/NIR and UV extinction data are
best fit by the CCM law with different values of R_V. We present examples of
such curves. The study of both types of peculiar cases can help us to learn
about the physical processes that affect dust in the interstellar medium, e.g.,
formation of mantles on the surface of grains, evaporation, growing or
shattering.Comment: 6 pages, 4 figures, in "Light, dust and chemical evolution", Journal
of Physics: Conference Serie
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