1,909 research outputs found
Gas Seepage and Pockmark Formation From Subsurface Reservoirs:Insights From Table-Top Experiments
Pockmarks are morphological depressions commonly observed in ocean and lake floors. Pockmarks form by fluid (typically gas) seepage thorough a sealing sedimentary layer, deforming and breaching the layer. The seepage-induced sediment deformation mechanisms, and their links to the resulting pockmarks morphology, are not well understood. To bridge this gap, we conduct laboratory experiments in which gas seeps through a granular (sand) reservoir, overlaid by a (clay) seal, both submerged under water. We find that gas rises through the reservoir and accumulates at the seal base. Once sufficient gas over-pressure is achieved, gas deforms the seal, and finally escapes via either: (a) doming of the seal followed by dome breaching via fracturing; (b) brittle faulting, delineating a plug, which is lifted by the gas seeping through the bounding faults; or (c) plastic deformation by bubbles ascending through the seal. The preferred mechanism is found to depend on the seal thickness and stiffness: in stiff seals, a transition from doming and fracturing to brittle faulting occurs as the thickness increases, whereas bubble rise is preferred in the most compliant, thickest seals. Seepage can also occur by mixed modes, such as bubbles rising in faults. Repeated seepage events suspend the sediment at the surface and create pockmarks. We present a quantitative analysis that explains the tendency for the various modes of deformation observed experimentally. Finally, we connect simple theoretical arguments with field observations, highlighting similarities and differences that bound the applicability of laboratory experiments to natural pockmarks.</p
Comparing the Ancient Star Formation Histories of the Magellanic Clouds
We present preliminary results from a new HST archival program aimed at
tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs)
of the field populations of the SMC and LMC. We demonstrate the quality of the
archival data by constructing HST/WFPC2-based color-magnitude diagrams (CMDs;
M_{F555W} ~ +8) for 7 spatially diverse fields in the SMC and 8 fields in the
LMC. The HST-based CMDs are >2 magnitudes deeper than any from ground based
observations, and are particularly superior in high surface brightness regions,
e.g., the LMC bar, which contain a significant fraction of star formation and
are crowding limited from ground based observations. To minimize systematic
uncertainties, we derive the SFH of each field using an identical maximum
likelihood CMD fitting technique. We then compute an approximate mass weighted
average SFH for each galaxy. We find that both galaxies lack a dominant burst
of early star formation, which suggests either a suppression or an
under-fueling of early star formation. From 10-12 Gyr ago, the LMC experienced
a period of enhanced stellar mass growth relative to the SMC. Similar to some
previous studies, we find two notable peaks in the SFH of the SMC at ~4.5 and 9
Gyr ago, which could be due to repeated close passages with the LMC, implying
an interaction history that has persisted for at least 9 Gyr. We find little
evidence for strong periodic behavior in the lifetime SFHs of both MCs,
suggesting that repeated encounters with the Milky Way are unlikely. Beginning
~3.5 Gyr ago, both galaxies show increases in their SFHs, in agreement with
previous studies, and thereafter, track each other remarkably well. (abridged)Comment: 9 pages, 5 Figures, Accepted for Publication in MNRA
Loss of intranetwork and internetwork resting state functional connections with Alzheimer\u27s disease progression
Alzheimer\u27s disease (AD) is the most common cause of dementia. Much is known concerning AD pathophysiology but our understanding of the disease at the systems level remains incomplete. Previous AD research has used resting-state functional connectivity magnetic resonance imaging (rs-fcMRI) to assess the integrity of functional networks within the brain. Most studies have focused on the default-mode network (DMN), a primary locus of AD pathology. However, other brain regions are inevitably affected with disease progression. We studied rs-fcMRI in five functionally defined brain networks within a large cohort of human participants of either gender (n = 510) that ranged in AD severity from unaffected [clinical dementia rating (CDR) 0] to very mild (CDR 0.5) to mild (CDR 1). We observed loss of correlations within not only the DMN but other networks at CDR 0.5. Within the salience network (SAL), increases were seen between CDR 0 and CDR 0.5. However, at CDR 1, all networks, including SAL, exhibited reduced correlations. Specific networks were preferentially affected at certain CDR stages. In addition, cross-network relations were consistently lost with increasing AD severity. Our results demonstrate that AD is associated with widespread loss of both intranetwork and internetwork correlations. These results provide insight into AD pathophysiology and reinforce an integrative view of the brain\u27s functional organization
Mixed Dark Matter from Axino Distribution
We study the possibility of mixed dark matter obtained through the phase
space distribution of a single particle. An example is offered in the context
of SUSY models with a Peccei-Quinn symmetry. Axinos in the 100 keV range can
naturally have both thermal and non-thermal components. The latter one arises
from the lightest neutralino decays and derelativizes at z ~ 10^4.Comment: Figures added, references fixed. Version accepted for publication on
Phys. Rev. D. LaTeX. 9 pages, 3 figures, uses epsfig.st
The young age of the extremely metal-deficient blue compact dwarf galaxy SBS 1415+437
We use Multiple Mirror Telescope (MMT) spectrophotometry and Hubble Space
Telescope (HST) Faint Object Spectrograph (FOS) spectra and Wide Field and
Planetary Camera 2 (WFPC2) V and I images to study the properties and
evolutionary status of the nearby (D = 11.4 Mpc) extremely metal-deficient blue
compact dwarf (BCD) galaxy SBS 1415+437=CG 389. The oxygen abundance in the
galaxy is 12+log(O/H)=7.60+/-0.01 or Zsun/21. The helium mass fraction in SBS
1415+437 is Y=0.246+/-0.004 which agrees with the primordial helium abundance
determined by Izotov & Thuan using a much larger sample of BCDs. The
alpha-elements-to-oxygen abundance ratios (Ne/O, S/O, Ar/O) are in very good
agreement with the mean values for other metal-deficient BCDs and are
consistent with the scenario that these elements are made in massive stars. The
Fe/O abundance ratio is ~2 times smaller than the solar ratio. The Si/O ratio
is close to the solar value, implying that silicon is not significantly
depleted into dust grains. The values of the N/O and C/O ratios imply that
intermediate-mass stars have not had time to evolve in SBS 1415+437 and release
their nucleosynthesis products and that both N and C in the BCD have been made
by massive stars only. This sets an upper limit of ~100 Myr on the age of SBS
1415+437. The (V-I) color of the low-surface-brightness component of the galaxy
is blue (<0.4 mag) indicative of a very young underlying stellar population.
The (V-I) - I color-magnitude diagrams of the resolved stellar populations in
different regions of SBS 1415+437 suggest propagating star formation from the
NE side of the galaxy to the SW. All regions in SBS 1415+437 possess very blue
spectral energy distributions (SED). We find that the ages of the stellar
populations in SBS 1415+437 to range from a few Myr to 100 Myr.Comment: 25 pages, 12 PS and 5 JPG figures, to appear in Ap
Discovery of Extended Blue Horizontal Branches in Two Metal-Rich Globular Clusters
We have used WFPC2 to construct B, V color-magnitude diagrams of four
metal-rich globular clusters, NGC 104 (47 Tuc), NGC 5927, NGC 6388, and NGC
6441. All four clusters have well populated red horizontal branches (RHB), as
expected for their metallicity. However, NGC 6388 and 6441 also exhibit a
prominent blue HB (BHB) extension, including stars reaching as faint in V as
the turnoff luminosity. This discovery demonstrates directly for the first time
that a major population of hot HB stars can exist in old, metal-rich systems.
This may have important implications for the interpretation of the integrated
spectra of elliptical galaxies.
The cause of the phenomenon remains uncertain. We examine the possibility
that NGC 6388 and 6441 are older than the other clusters, but a simple
difference in age may not be sufficient to produce the observed distributions
along the HB. The high central densities in NGC 6388 and 6441 suggest that the
existence of the blue HB (BHB) tails might be caused by stellar interactions in
the dense cores of these clusters, which we calculate to have two of the
highest collision rates among globular clusters in the Galaxy. Tidal collisions
might act in various ways to enhance loss of envelope mass, and therefore
populate the blue side of the HB. However, the relative frequency of tidal
collisions does not seem large enough (compared to that of the clusters with
pure RHBs) to account for such a drastic difference in HB morphology. While a
combination of an age difference and dynamical interactions may help, prima
facie the lack of a radial gradient in the BHB/RHB star ratio seems to argue
against dynamical effects playing a role.Comment: LaTeX, includes one Postscript figure. To appear in ApJ
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