129,104 research outputs found
Weighing the galactic disc using the Jeans equation: lessons from simulations
Using three-dimensional stellar kinematic data from simulated galaxies, we examine the efficacy of a Jeans equation analysis in reconstructing the total disk surface density, including the dark matter, at the ‘Solar’ radius. Our simulation data set includes galaxies formed in a cosmological context using state-of-the-art high-resolution cosmological zoom simulations, and other idealized models. The cosmologically formed galaxies have been demonstrated to lie on many of the observed scaling relations for late-type spirals, and thus offer an interesting surrogate for real galaxies with the obvious advantage that all the kinematical data are known perfectly. We show that the vertical velocity dispersion is typically the dominant kinematic quantity in the analysis, and that the traditional method of using only the vertical force is reasonably effective at low heights above the disk plane. At higher heights the inclusion of the radial force becomes increasingly important. We also show that the method is sensitive to uncertainties in the measured disk parameters, particularly the scalelengths of the assumed double exponential density distribution, and the scalelength of the radial velocity dispersion. In addition, we show that disk structure and low number statistics can lead to significant errors in the calculated surface densities. Finally, we examine the implications of our results for previous studies of this sort, suggesting that more accurate measurements of the scalelengths may help reconcile conflicting estimates of the local dark matter density in the literature
Liquid Polymorphism and Density Anomaly in a Lattice Gas Model
We present a simple model for an associating liquid in which polymorphism and
density anomaly are connected. Our model combines a two dimensional lattice gas
with particles interacting through a soft core potential and orientational
degrees of freedom represented through thermal \char`\"{}ice
variables\char`\"{} . The competition between the directional attractive forces
and the soft core potential leads to a phase diagram in which two liquid phases
and a density anomaly are present. The coexistence line between the low density
liquid and the high density liquid has a positive slope contradicting the
surmise that the presence of a density anomaly implies that the high density
liquid is more entropic than the low density liquid
Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption
Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky
Way. It became the second-brightest star in the sky during its mid-19th century
"Great Eruption," but then faded from view (with only naked-eye estimates of
brightness). Its eruption is unique among known astronomical transients in that
it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3
kpc away, spatially resolved studies of the nebula have constrained the ejected
mass and velocity, indicating that in its 19th century eruption, Eta Car
ejected more than 10 M_solar in an event that had 10% of the energy of a
typical core-collapse supernova without destroying the star. Here we report the
discovery of light echoes of Eta Carinae which appear to be from the 1838-1858
Great Eruption. Spectra of these light echoes show only absorption lines, which
are blueshifted by -210 km/s, in good agreement with predicted expansion
speeds. The light-echo spectra correlate best with those of G2-G5 supergiant
spectra, which have effective temperatures of ~5000 K. In contrast to the class
of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption,
the effective temperature of its outburst is significantly cooler than allowed
by standard opaque wind models. This indicates that other physical mechanisms
like an energetic blast wave may have triggered and influenced the eruption.Comment: Accepted for publication by Nature; 4 pages, 4 figures, SI: 6 pages,
3 figures, 5 table
Freeze-Thaw Cycling as a Chemical Weathering Agent on a Cold and Icy Mars
Liquid water was abundant on early Mars, but whether the climate was warm and wet or cold and icy with punctuated periods of melting is still poorly understood. Modern climate models for Mars tend to predict a colder, icier early climate than previously imagined. In addition, ice and glaciation have been major geologic agents throughout the later Hesperian and Amazonian eras. One process that can act in such climates is repeated freezing and thawing of water on the surface and in the subsurface, and is significant because it can occur anywhere with an active layer and could have persisted for a time after liquid water was no longer stable on Mars surface. As freeze-thaw is the dominant mechanical weathering process in most glacial/periglacial terrains, it was likely a significant geomorphologic driver at local to regional scales during past climates, and would potentially have been most active when day-average surface temperatures exceeded 0 C for part of the year. Indeed, freeze-thaw involving liquid water in the Amazonian is evidenced by abundant geomorphic features including polygonal ground and solifluction lobes requiring seasonal thawing. In addition to physical modification, freezing can drive solutions towards supersaturation and force dissolved solutes out as precipitates. In Mars-like terrains, dissolved solutes are typically dominated by silica. In polar regions on Earth, freeze-thaw cycles have been shown to promote deposition of silica, and freeze-thaw experiments on synthetic solutions found stable amorphous silica that built up over multiple cycles. Freeze-thaw may therefore be an important but overlooked chemical weathering process on Mars. However, our ability to assess its impact on alteration of martian terrains is majorly limited by the current lack of understanding of the alteration phases produced (and formation rates) under controlled freeze-thaw weathering of Mars-relevant materials. To address this knowledge gap, we report results from (1) freeze-thaw weathering products found at a glacial Mars analog site at the Three Sisters, Oregon, and (2) new controlled freeze-thaw experiments on basaltic material
Destination images of non-visitors
This article provides much needed understanding of destination images held by non-visitors. Recognizing the characteristics of non-visitor images and their formation is important in order to understand images more widely. This qualitative study assesses images of London. The views of three hundred people in the Czech Republic who have never visited London were obtained via an innovative open-ended research instrument. The study showed that non-visitors imagine destinations through comparisons with their own experiences of places. Findings indicate that images can be very persistent and that the first images formed of a destination endure over time. Although the research is based on people with no direct experience of London, the research highlights that a range of secondary ‘experiences’ influence image formation
Integer quantum Hall effect on a six valley hydrogen-passivated silicon (111) surface
We report magneto-transport studies of a two-dimensional electron system
formed in an inversion layer at the interface between a hydrogen-passivated
Si(111) surface and vacuum. Measurements in the integer quantum Hall regime
demonstrate the expected sixfold valley degeneracy for these surfaces is
broken, resulting in an unequal occupation of the six valleys and anisotropy in
the resistance. We hypothesize the misorientation of Si surface breaks the
valley states into three unequally spaced pairs, but the observation of odd
filling factors, is difficult to reconcile with non-interacting electron
theory.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Theory of low transitions in CO discharge lasers
A self consistent theoretical model which couples the electron and heavy particle finite rate kinetics with the optical and fluid dynamic processes has been employed to identify the various parameters and explain the mechanism responsible for producing low lying transitions in slow flowing CO lasers. It is found that lasing on low lying transitions can be achieved at low temperatures for low pressures (or low flow rates) together with high partial pressures of the He and N2. The role of N2 has been identified as an additive responsible for reducing the electron temperature to a range where the transfer of electrical power to the lower vibrational modes of CO is optimum
Computer Mapping of Water Quality in Saginaw Bay with LANDSAT Digital Data
The author has identified the following significant results. LANDSAT digital data and ground truth measurements for Saginaw Bay (Lake Huron), Michigan, for 31 July 1975 were correlated by stepwise linear regression and the resulting equations used to estimate invisible water quality parameters in nonsampled areas. Chloride, conductivity, total Kjeldahl nitrogen, total phosphorus, and chlorophyll a were best correlated with the ratio of LANDSAT Band 4 to Band 5. Temperature and Secchi depth correlate best with Band 5
Redshift-Independent Distances to Type Ia Supernovae
We describe a procedure for accurately determining luminosity distances to
Type Ia supernovae (SNe Ia) without knowledge of redshift. This procedure,
which may be used as an extension of any of the various distance determination
methods currently in use, is based on marginalizing over redshift, removing the
requirement of knowing a priori. We demonstrate that the Hubble diagram
scatter of distances measured with this technique is approximately equal to
that of distances derived from conventional redshift-specific methods for a set
of 60 nearby SNe Ia. This indicates that accurate distances for cosmological
SNe Ia may be determined without the requirement of spectroscopic redshifts,
which are typically the limiting factor for the number of SNe that modern
surveys can collect. Removing this limitation would greatly increase the number
of SNe for which current and future SN surveys will be able to accurately
measure distance. The method may also be able to be used for high- SNe Ia to
determine cosmological density parameters without redshift information.Comment: 12 pages, 3 figures, accepted for publication in Astrophysical
Journal Letter
Experimental observations of dynamic critical phenomena in a lipid membrane
Near a critical point, the time scale of thermally-induced fluctuations
diverges in a manner determined by the dynamic universality class. Experiments
have verified predicted 3D dynamic critical exponents in many systems, but
similar experiments in 2D have been lacking for the case of conserved order
parameter. Here we analyze time-dependent correlation functions of a quasi-2D
lipid bilayer in water to show that its critical dynamics agree with a recently
predicted universality class. In particular, the effective dynamic exponent
crosses over from to as the correlation
length of fluctuations exceeds a hydrodynamic length set by the membrane and
bulk viscosities.Comment: 5 pages, 3 figures and 2 additional pages of supplemen
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