4,319 research outputs found
Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7
Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a
spin-freezing transition and magnetic irreversibilities below a temperature Tf
similar to what is observed nonlinear magnetization measurements on Y2Mo2O7
that provide strong evidence that there is an underlying thermodynamic phase
transition at Tf, which is characterized by critical exponents \gamma \approx
2.8 and \beta \approx 0.8. These values are typical of those found in random
spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is
immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included).
Submitted to Phys. Rev. Letters
A closer look at nitrification in pelagic sediments
Nutrient profiles in Southwest Pacific interstitial solutions suggest that in environments of oxic pelagic sedimentation microbially mediated nitrification is recognizable as a two-step process. During the first step partially oxidized nitrogenous intermediaries accumulate in distinctive ammonia and nitrite maxima along with nitrate. During the second step nitrification continues and all intermediate species are fully oxidized to nitrate. Both steps occur within a zone that corresponds in thickness to the biologically active surface layer. Similarly, experimental nitrogen regeneration from decomposition of plankton in seawater (VON BRAND and RAKESTRAW, 1941: VON BRAND et al., 1942) suggests that each step corresponds to a distinct reaction in the microbially mediated transformation of N-org → NH3 → NO2 → NO3. The resolution of distinct reaction zones in pore water nutrient profiles possibly depends on the nature and mode of supply of the organic matter undergoing nitrification or reflects the spatial succession downcore of microbial populations capable of deamination, ammonium oxidation and nitrite oxidation, respectively. Finally, stoichiometric ratios of nutrients in the free water column - here demonstrated on published data from Saanich Inlet - reflect the same two steps of nitrification as delineated by the dissolved pore water species. Future pore water studies should include dissolved oxygen measurements as well as accurate ∑CO2, PO4 and nitrogenous species profiles, to verify and better quantify these separate steps in nitrification mechanism of oxic pelagic sediments
On the helium flash in low-mass Population III Red Giant stars
We investigate the evolution of initially metal-free, low-mass Red Giant
stars through the He core flash at the tip of the Red Giant Branch. The low
entropy barrier between the helium- and hydrogen-rich layers enables a
penetration of the helium flash driven convective zone into the inner tail of
the extinguishing H-burning shell. As a consequence, protons are mixed into
high-temperature regions triggering a H-burning runaway. The subsequent
dredge-up of matter processed by He and H burning enriches the stellar surface
with large amounts of helium, carbon and nitrogen. Extending previous results
by Hollowell et al. (1990) and Fujimoto et al. (2000), who claimed that the
H-burning runaway is an intrinsic property of extremely metal-poor low-mass
stars, we found that its occurrence depends on additional parameters like the
initial composition and the treatment of various physical processes.
We perform some comparisons between predicted surface chemical abundances and
observational measurements for extremely metal-deficient stars. As in previous
investigations, our results disclose that although the described scenario
provides a good qualitative agreement with observations, considerable
discrepancies still remain. They may be due to a more complex evolutionary path
of `real' stars, and/or some shortcomings in current evolutionary models.
In addition, we analyze the evolutionary properties after the He core flash,
during both the central and shell He-burning phases, allowing us to deduce some
interesting differences between models whose Red Giant Branch progenitor has
experienced the H-flash and canonical models. In particular, the Asymptotic
Giant Branch evolution of extremely metal-deficient stars and the occurrence of
thermal pulses are strongly affected by the previous RGB evolution.Comment: 7 figures, AASTeX, submitted to Ap
Antisymmetric-Tensor and Electromagnetic effects in an alpha'-non-perturbative Four-Dimensional String Cosmology
Starting from an exact (in the Regge slope alpha') functional method for a
bosonic stringy sigma-model, we investigate four-dimensional cosmological
string solutions in graviton, dilaton and antisymmetric tensor backgrounds,
compatible with world-sheet conformal invariance, and valid beyond perturbative
expansions in powers of alpha'. The antisymmetric tensor field, playing the
role of an axion in the four-dimensional target space time, leads to spatial
anisotropies of the emergent Robertson-Walker expanding Universe, and, upon
coupling the system to the electromagnetic field, it results in non-trivial
optical activity. Some estimates of the corresponding effects are made and
their relevance to current cosmology is briefly discussed
Contribution of White Dwarfs to Cluster Masses
I present a literature search through 31 July 1997 of white dwarfs (WDs) in
open and globular clusters. There are 36 single WDs and 5 WDs in binaries known
among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among
11 globular clusters. From these data I have calculated WD mass fractions for
four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one
globular cluster (NGC 6121). I develop a simple model of cluster evolution that
incorporates stellar evolution but not dynamical evolution to interpret the WD
mass fractions. I augment the results of my simple model with N-body
simulations incorporating stellar evolution (Terlevich 1987; de la Feunte
Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters
undergo moderate to strong kinematical evolution the WD mass fraction is
relatively insensitive to kinematical evolution. By comparing the cluster mass
functions to that of the Galactic disk, and incorporating plausibility
arguments for the mass function of the Galactic halo, I estimate the WD mass
fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old
(Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that
the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et
al. 1998), although the WD mass fraction is insensitive to age in this range. I
find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps
as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The
Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent
with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et
al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be
published in AJ, April, 199
A Far-Ultraviolet Spectroscopic Survey of Luminous Cool Stars
FUSE ultraviolet spectra of 8 giant and supergiant stars reveal that high
temperature (3 X 10^5 K) atmospheres are common in luminous cool stars and
extend across the color-magnitude diagram from Alpha Car (F0 II) to the cool
giant Alpha Tau (K5 III). Emission present in these spectra includes
chromospheric H-Lyman Beta, Fe II, C I, and transition region lines of C III, O
VI, Si III, Si IV. Emission lines of Fe XVIII and Fe XIX signaling temperatures
of ~10^7 K and coronal material are found in the most active stars, Beta Cet
and 31 Com. A short-term flux variation, perhaps a flare, was detected in Beta
Cet during our observation. Stellar surface fluxes of the emission of C III and
O VI are correlated and decrease rapidly towards the cooler stars, reminiscent
of the decay of magnetically-heated atmospheres. Profiles of the C III (977A)
lines suggest that mass outflow is underway at T~80,000 K, and the winds are
warm. Indications of outflow at higher temperatures (3 X 10^5K) are revealed by
O VI asymmetries and the line widths themselves. High temperature species are
absent in the M-supergiant Alpha Ori. Narrow fluorescent lines of Fe II appear
in the spectra of many giants and supergiants, apparently pumped by H Lyman
Alpha, and formed in extended atmospheres. Instrumental characteristics that
affect cool star spectra are discussed.Comment: Accept for publication in The Astrophysical Journal; 22 pages of
text, 23 figures and 8 table
Transition to Long Range Magnetic Order in the Highly Frustrated Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7
Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at K. The Curie-Weiss temperature, =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below 10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the ground state for Gd. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4Comment: Letter, 6 POSTSCRIPT figures included. (NOTE: Figure 5 is not
included --) To appear in Physical Review B. Contact:
[email protected]
Chandra X-ray Observations of the Quadruply Lensed Quasar RX J0911.4+0551
We present results from X-ray observations of the quadruply lensed quasar RX
J0911.4+0551 using data obtained with the Advanced CCD Imaging Spectrometer
(ACIS) on board the Chandra X-ray Observatory. The 29 ks observation detects a
total of ~404 X-ray photons (0.3 to 7.0 keV) from the four images of the lensed
quasar. Deconvolution of the aspect corrected data resolves all four lensed
images, with relative positions in good agreement with optical measurements.
When compared to contemporaneous optical data, one of the lensed images
(component A3) is dimmer by a factor of ~6 in X-rays with respect to the 2
brighter images (components A1 and A2). Spectral fitting for the combined
images shows significant intrinsic absorption in the soft (0.2 to 2.4 keV)
energy band, consistent with the mini-BAL nature of this quasar, while a
comparison with ROSAT PSPC observations from 1990 shows a drop of ~6.5 in the
total soft bandpass flux. The observations also detect ~157 X-ray photons
arising from extended emission of the nearby cluster (peaked ~42" SW of
RXJ0911.4+0551) responsible for the large external shear present in the system.
The Chandra observation reveals the cluster emission to be complex and
non-spherical, and yields a cluster temperature of kT = 2.3^{+1.8}_{-0.8} keV
and a 2.0 to 10 keV cluster luminosity within a 1 Mpc radius of L_X =
7.6_{-0.2}^{+0.6} x 10^{43} ergs/s (error bars denote 90% confidence limits).
Our mass estimate of the cluster within its virial radius is 2.3^{+1.8}_{-0.7}
x 10^{14} solar, and is a factor of 2 smaller than, although consistent with,
previous mass estimates based on the observed cluster velocity dispersion.Comment: 16 pages, 3 figures (figure 1 is color ps). Accepted by Ap
Spectroscopic Analysis of H I Absorption Line Systems in 40 HIRES Quasars
We list and analyze H I absorption lines at redshifts 2 < z < 4 with column
density (12 < log(N_HI) < 19) in 40 high-resolutional (FWHM = 8.0 km/s) quasar
spectra obtained with the Keck+HIRES. We de-blend and fit all H I lines within
1,000 km/s of 86 strong H I lines whose column densities are log(N_HI/[cm^-2])
> 15. Unlike most prior studies, we use not only Lya but also all visible
higher Lyman series lines to improve the fitting accuracy. This reveals
components near to higher column density systems that can not be seen in Lya.
We list the Voigt profile fits to the 1339 H I components that we found. We
examined physical properties of H I lines after separating them into several
sub-samples according to their velocity separation from the quasars, their
redshift, column density and the S/N ratio of the spectrum. We found two
interesting trends for lines with 12 < log(N_HI) < 15 which are within 200-1000
km/s of systems with log(N_HI) > 15. First, their column density distribution
becomes steeper, meaning relatively fewer high column density lines, at z <
2.9. Second, their column density distribution also becomes steeper and their
line width becomes broader by about 2-3 km/s when they are within 5,000 km/s of
their quasar.Comment: 32 pages, 14 figures, accepted for publication in the Astronomical
Journal. A complete version with all tables and figures is available at
http://www.astro.psu.edu/users/misawa/pub/Paper/40hires.ps.g
Field induced transitions in a kagome antiferromagnet
The thermal order by disorder effect in magnetic field is studied for a
classical Heisenberg antiferromagnet on the kagome lattice. Using analytical
arguments we predict a unique H-T phase diagram for this strongly frustrated
magnet: states with a coplanar and a uniaxial triatic order parameters
respectively at low and high magnetic fields and an incompressible collinear
spin-liquid state at a one-third of the saturation field. We also present the
Monte Carlo data which confirm existence of these phases.Comment: 4 pages, 2 figures, accepted versio
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