432 research outputs found
Textural variations in Neogene pelagic carbonate ooze at DSDP Site 593, southern Tasman Sea, and their paleoceanographic implications
Changes in Neogene sediment texture in pelagic carbonate-rich oozes on the Challenger Plateau, southern Tasman Sea, are used to infer changes in depositional paleocurrent velocities. The most obvious record of textural change is in the mud:sand ratio. Increases in the sand content are inferred to indicate a general up-core trend towards increasing winnowing of sediments resulting from increasing flow velocity of Southern Component Intermediate Water (SCIW), the forerunner of Antarctic Intermediate Water. In particular, the intervals c. 19-14.5 Ma, c. 9.5-8 Ma, and after 5 Ma are suggested to be times of increased SCIW velocity and strong sediment winnowing. Within the mud fraction, the fine silt to coarse clay sizes from 15.6 to 2 ”m make the greatest contribution to the sediments and are composed of nannofossil plates. During extreme winnowing events it is the fine silt to very coarse clay material (13-3 ”m) within this range that is preferentially removed, suggesting the 10 ”m cohesive silt boundary reported for siliciclastic sediments does not apply to calcitic skeletal grains. The winnowed sediment comprises coccolithophore placoliths and spheres, represented by a mode at 4-7 ”m.
Further support for seafloor winnowing is gained from the presence in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma where the sand content increases to c. 20% of the bulk sample. Associated with the condensed section is a 6 m thick orange unit representing sediments subjected to particularly oxygen-rich, late early to early middle Miocene SCIW. Together these are inferred to indicate increased SCIW velocity resulting in winnowed sediment associated with faster arrival of oxygen-rich surface water subducted to form SCIW. Glacial development of Antarctica has been recorded from many deep-sea sites, with extreme glacials providing the mechanism to increase watermass flow. Miocene glacial zones Mi1b-Mi6 are identified in an associated oxygen isotope record from Hole 593, and correspond with times of particularly invigorated paleocirculation, bottom winnowing, and sediment textural changes
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
Searching for periodicities in the MACHO light curve of LMC X-2
Using the exceptional long-term monitoring capabilities of the MACHO project,
we present here the optical history of LMC X-2 for a continuous 6-yr period.
These data were used to investigate the previously claimed periodicities for
this source of 8.15 h and 12.54 d : we find upper amplitude limits of 0.10 mag
and 0.09 mag, respectively.Comment: 4 pages, 4 figures. Minor changes, including title. MNRAS, in pres
The MACHO Project Hubble Space Telescope Follow-Up: Preliminary Results on the Location of the Large Magellanic Cloud Microlensing Source Stars
We attempt to determine whether the MACHO microlensing source stars are drawn
from the average population of the LMC or from a population behind the LMC by
examining the HST color-magnitude diagram (CMD) of microlensing source stars.
We present WFPC2 HST photometry of eight MACHO microlensing source stars and
the surrounding fields in the LMC. 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. We consider in detail a model for the background population of
source stars based on that presented by Zhao, Graff & Guhathakurta. In this
model, the source stars have an additional reddening = 0.13 mag and a
slightly larger distance modulus ~ 0.3 mag than the average LMC
population. We also investigate a series of source star models, varying the
relative fraction of source stars drawn from the average and background
populations and the displacement of the background population from the LMC. Due
to the small number of analyzed events the distribution of probabilities of
different models is rather flat. A shallow maximum occurs at a fraction s_LMC ~
0.8 of the source stars in the LMC. This is consistent with the interpretation
that a significant fraction of observed microlensing events are due to lenses
in the Milky Way halo, but does not definitively exclude other models.Comment: revised version, results slightly changed, accepted by Ap
MACHO 96-LMC-2: Lensing of a Binary Source in the LMC and Constraints on the Lensing Object
We present photometry and analysis of the microlensing alert MACHO 96-LMC-2.
The ~3% photometry provided by the Global Microlensing Alert Network follow--up
effort reveals a periodic modulation in the lightcurve. We attribute this to
binarity of the lensed source. Microlensing fits to a rotating binary source
magnified by a single lens converge on two minima, separated by delta chi^2 ~
1. The most significant fit X1 predicts a primary which contributes ~100% of
the light, a dark secondary, and an orbital period (T) of 9.2 days. The second
fit X2 yields a binary source with two stars of roughly equal mass and
luminosity, and T = 21.2 days.
The lensed object appears to lie on the upper LMC main sequence. We estimate
the mass of the primary component of the binary system, M ~2 M_sun. For the
preferred model X1, we explore the range of dark companions by assuming 0.1
M_sun and 1.4 M_sun objects in models X1a and X1b, respectively. We find lens
velocities projected to the LMC in these models of v^hat_X1a = 18.3 +/- 3.1
km/s and v^hat_X1b = 188 +/- 32 k/ms. In both these cases, a likelihood
analysis suggests an LMC lens is preferred over a Galactic halo lens, although
only marginally so in model X1b. We also find v^hat_X2 = 39.6 +/- 6.1 k/ms,
where the likelihood for the lens location is strongly dominated by the LMC
disk. In all cases, the lens mass is consistent with that of an M-dwarf. The
LMC self-lensing rate contributed by 96-LMC-2 is consistent with model
self-lensing rates. (Abridged)Comment: 23 pages, including 3 tables and 6 figures; Accepted for publication
in The Astrophysical Journa
The MACHO project: Microlensing Optical Depth towards the Galactic Bulge from Difference Image Analysis
We present the microlensing optical depth towards the Galactic bulge based on
the detection of 99 events found in our Difference Image Analysis (DIA) survey.
This analysis encompasses three years of data, covering ~ 17 million stars in ~
4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique
improves the quality of photometry in crowded fields, and allows us to detect
more microlensing events with faint source stars. We find this method increases
the number of detection events by 85% compared with the standard analysis
technique. DIA light curves of the events are presented and the microlensing
fit parameters are given. The total microlensing optical depth is estimated to
be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at
l=2.68 and b=-3.35. For the bulge component we find
tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution
from disk sources. These optical depths are in good agreement with the past
determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994)
groups, and are higher than predicted by contemporary Galactic models. We show
that our observed event timescale distribution is consistent with the
distribution expected from normal mass stars, if we adopt the stellar mass
function of Scalo (1986) as our lens mass function. However, we note that as
there is still disagreement about the exact form of the stellar mass function,
there is uncertainty in this conclusion. Based on our event timescale
distribution we find no evidence for the existence of a large population of
brown dwarfs in the direction of the Galactic bulge.Comment: Updated references and corrected optical depth values. tau_tot=
[2.91(+0.47/-0.45) -> 2.43^(+0.39/-0.38)] x 10^(-6) tau_bul =
[3.88(+0.63/-0.60) -> 3.23^(+0.52/-0.50)] x 10^(-6
The MACHO Project: Microlensing Detection Efficiency
The MACHO project is a search for dark matter in the form of massive compact
halo objects (MACHOs). The project has photometrically monitored tens of
millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud
(SMC), and Galactic bulge in search of rare gravitational microlensing events
caused by these otherwise invisible objects. In 5.7 years of observations
toward the LMC some 13-17 microlensing events have been observed by the MACHO
survey, allowing powerful statements to be made about the nature of the dark
population in the halo of our Galaxy. A critical component of these statements
is an accurate determination of the survey's detection efficiency. The
detection efficiency is a complicated function of temporal sampling, stellar
crowding (the luminosity function), image quality, photometry, time-series
analysis, and criteria used to select the microlensing candidates. Such a
complex interdependence is most naturally solved using a Monte Carlo approach.
Here we describe the details of the Monte Carlo used to calculate the
efficiency presented in the MACHO 5.7-year LMC results. Here we correct several
shortcomings of past determinations, including (1) adding fainter source stars
(2.5 magnitudes below our faintest detected "stars"), (2) an up-to-date
luminosity function for the LMC, (3) better sampling of real images in both
stellar density and observing conditions, (4) an improved scheme for adding
artificial microlensing onto a random sample of real lightcurves, and many
other improvements. [Abridged]Comment: 32 pages, Latex with 16 postscript figures, submitted to ApJ
The MACHO Project LMC Variable Star Inventory. IX. Frequency Analysis of the First Overtone RR Lyrae Stars and the Indication for Nonradial Pulsations
More than 1300 variables classified provisionally as first overtone RR Lyrae
pulsators in the MACHO variable star database of the Large Magellanic Cloud
(LMC) have been subjected to standard frequency analysis. Based on the remnant
power in the prewhitened spectra, we found 70% of the total population to be
monoperiodic. The remaining 30% (411 stars) are classified as one of 9 types
according to their frequency spectra. Several types of RR Lyrae pulsational
behavior are clearly identified here for the first time. Together with the
earlier discovered double-mode (fundamental & first overtone) variables this
study increased the number of the known double-mode stars in the LMC to 181.
During the total 6.5yr time span of the data, 10% of the stars show strong
period changes. We also discovered two additional types of multifrequency
pulsators with low occurrence rates of 2% for each. In the first type there
remains one closely spaced component after prewhitening by the main pulsation
frequency. In the second type the number of remnant components is two, they are
also closely spaced, and, in addition, they are symmetric in their frequency
spacing relative to the central component. This latter type of variables is
associated with their relatives among the fundamental pulsators, known as
Blazhko variables. Their high frequency (~20%) among the fundamental mode
variables versus the low occurrence rate of their first overtone counterparts
makes it more difficult to explain Blazhko phenomenon by any theory depending
mainly on the role of aspect angle or magnetic field. Current theoretical
models invoke nonradial pulsation components in these stars.Comment: 20 pages, 21 figures (bitmapped), 7 tables, to appear in Ap.
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