1,924 research outputs found
Band structure of helimagnons in MnSi resolved by inelastic neutron scattering
A magnetic helix realizes a one-dimensional magnetic crystal with a period
given by the pitch length . Its spin-wave excitations -- the
helimagnons -- experience Bragg scattering off this periodicity leading to gaps
in the spectrum that inhibit their propagation along the pitch direction. Using
high-resolution inelastic neutron scattering the resulting band structure of
helimagnons was resolved by preparing a single crystal of MnSi in a single
magnetic-helix domain. At least five helimagnon bands could be identified that
cover the crossover from flat bands at low energies with helimagnons basically
localized along the pitch direction to dispersing bands at higher energies. In
the low-energy limit, we find the helimagnon spectrum to be determined by a
universal, parameter-free theory. Taking into account corrections to this
low-energy theory, quantitative agreement is obtained in the entire energy
range studied with the help of a single fitting parameter.Comment: 5 pages, 3 figures; (v2) slight modifications, published versio
Versatile module for experiments with focussing neutron guides
We report the development of a versatile module that permits fast and
reliable use of focussing neutron guides under varying scattering angles. A
simple procedure for setting up the module and neutron guides is illustrated by
typical intensity patterns to highlight operational aspects as well as typical
parasitic artefacts. Combining a high-precision alignment table with separate
housings for the neutron guides on kinematic mounts, the change-over between
neutron guides with different focussing characteristics requires no
readjustments of the experimental set-up. Exploiting substantial gain factors,
we demonstrate the performance of this versatile neutron scattering module in a
study of the effects of uniaxial stress on the domain populations in the
transverse spin density wave phase of single crystal Cr
Cimetidine, C10H16N6S, formC: crystal structure and modelling of polytypes using suoperspace approach
An efficient method for modelling a polytypic family is presented with the example of cimetidine in the form C polymorph. The method exploits the (3 + 1)-dimensional superspace model, which is a powerful tool for the description, prediction and understanding of polytype modifications in small-molecule crystallography, as illustrated with this pharmaceutical example
HST/WFPC2 and VLT/ISAAC observations of PROPLYDS in the giant HII region NGC 3603
We report the discovery of three proplyd-like structures in the giant HII
region NGC 3603. The emission nebulae are clearly resolved in narrow-band and
broad-band HST/WFPC2 observations in the optical and broad-band VLT/ISAAC
observations in the near-infrared. All three nebulae are tadpole shaped, with
the bright ionization front at the head facing the central cluster and a
fainter ionization front around the tail pointing away from the cluster.
Typical sizes are 6,000 A.U. x 20,000 A.U. The nebulae share the overall
morphology of the proplyds (``PROto PLanetarY DiskS'') in Orion, but are 20 to
30 times larger in size. Additional faint filaments located between the nebulae
and the central ionizing cluster can be interpreted as bow shocks resulting
from the interaction of the fast winds from the high-mass stars in the cluster
with the evaporation flow from the proplyds. The striking similarity of the
tadpole shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests
that the physical structure of both types of objects might be the same. We
present 2D radiation hydrodynamical simulations of an externally illuminated
star-disk-envelope system, which was still in its main accretion phase when
first exposed to ionizing radiation from the central cluster. The simulations
reproduce the overall morphology of the proplyds in NGC 3603 very well, but
also indicate that mass-loss rates of up to 10^-5 Mo/yr are required in order
to explain the size of the proplyds. (abbreviated)Comment: 10 pages, 4 Postscript figures, uses emulateapj.sty and psfig.tex.
Astronomical Journal, in press (January 2000 issue
The Luminosity & Mass Function of the Trapezium Cluster: From B stars to the Deuterium Burning Limit
We use the results of a new, multi-epoch, multi-wavelength, near-infrared
census of the Trapezium Cluster in Orion to construct and to analyze the
structure of its infrared (K band) luminosity function. Specifically, we employ
an improved set of model luminosity functions to derive this cluster's
underlying Initial Mass Function (IMF) across the entire range of mass from OB
stars to sub-stellar objects down to near the deuterium burning limit. We
derive an IMF for the Trapezium Cluster that rises with decreasing mass, having
a Salpeter-like IMF slope until near ~0.6 M_sun where the IMF flattens and
forms a broad peak extending to the hydrogen burning limit, below which the IMF
declines into the sub-stellar regime. Independent of the details, we find that
sub-stellar objects account for no more than ~22% of the total number of likely
cluster members. Further, the sub-stellar Trapezium IMF breaks from a steady
power-law decline and forms a significant secondary peak at the lowest masses
(10-20 times the mass of Jupiter). This secondary peak may contain as many as
\~30% of the sub-stellar objects in the cluster. Below this sub-stellar IMF
peak, our KLF modeling requires a subsequent sharp decline toward the planetary
mass regime. Lastly, we investigate the robustness of pre-main sequence
luminosity evolution as predicted by current evolutionary models, and we
discuss possible origins for the IMF of brown dwarfs.Comment: 74 pages, 30 figures, AASTeX5.0. To be published in the 01 July 2002
ApJ. For color version of figure 1 and online data table see
http://www.astro.ufl.edu/~muench/PUB/publications.htm
USING FUNCTIONAL DATA ANALYSIS TO EVALUATE EFFECT OF SHADE ON BODY TEMPERATURE OF FEEDLOT HEIFERS DURING ENVIRONMENTAL HEAT STRESS
Heat stress can be a serious problem for cattle. Body temperature (Tb) is a good measure of an animalâs thermo-regulatory response to an environmental thermal challenge. Previous studies found that Tb increases in response to increasing ambient temperature in a controlled chamber. However, when animals are in an uncontrolled environment, Tb is subject to many uncontrolled environmental factors, such as sunshade, wind, and humidity, that increase variation in the data. Hence, functional data analysis (FDA) was applied to analyze the data with uncontrolled environmental factors as curves in the whole series of days in this study. Breed (Angus, MARCIII, MARC-I, Charolais) and availability of shade (access versus no access to sunshade) were included as treatment factors in the statistical model. This study illustrates the potential of FDA to retain all information in the curves. The specific objectives are to use FDA to smooth Tb with large noise, to detect treatment effects on Tb, and to assess the interactions between breed and availability of shade with functional regression coefficients. The results show that FDA can be used to detect significant treatment interactions that may otherwise remain undetected using regular linear or nonlinear models. Significant interactions were found, indicating that access to sun-shade influences the way animals respond to a thermal challenge. Overall, it was found that breeds of cattle with dark-hides were more affected by temperature changes and peak temperatures than breeds of cattle with light-hides. Angus cattle (black) had the highest body temperatures in both shade and no shade areas, while Charolais (white) had the lowest body temperatures in the no shade area. However, MARC III (dark red) experienced the largest temperature differential between shade and no shade. Therefore, breed and availability of shade interactions are important considerations when making predictions to aid in management decisions involving feedlot cattle
A First Estimate Of The X-Ray Binary Frequency As A Function Of Star Cluster Mass In A Single Galactic System
We use the previously-identified 15 infrared star-cluster counterparts to
X-ray point sources in the interacting galaxies NGC 4038/4039 (the Antennae) to
study the relationship between total cluster mass and X-ray binary number. This
significant population of X-Ray/IR associations allows us to perform, for the
first time, a statistical study of X-ray point sources and their environments.
We define a quantity, \eta, relating the fraction of X-ray sources per unit
mass as a function of cluster mass in the Antennae. We compute cluster mass by
fitting spectral evolutionary models to K_s luminosity. Considering that this
method depends on cluster age, we use four different age distributions to
explore the effects of cluster age on the value of \eta and find it varies by
less than a factor of four. We find a mean value of \eta for these different
distributions of \eta = 1.7 x 10^-8 M_\sun^-1 with \sigma_\eta = 1.2 x 10^-8
M_\sun^-1. Performing a \chi^2 test, we demonstrate \eta could exhibit a
positive slope, but that it depends on the assumed distribution in cluster
ages. While the estimated uncertainties in \eta are factors of a few, we
believe this is the first estimate made of this quantity to ``order of
magnitude'' accuracy. We also compare our findings to theoretical models of
open and globular cluster evolution, incorporating the X-ray binary fraction
per cluster.Comment: 20 pages, 6 figures, accepted by Ap
A Hubble View of Star Forming Regions in the Magellanic Clouds
The Magellanic Clouds (MCs) offer an outstanding variety of young stellar
associations, in which large samples of low-mass stars (with masses less than 1
solar mass) currently in the act of formation can be resolved and explored
sufficiently with the Hubble Space Telescope. These pre-main sequence (PMS)
stars provide a unique snapshot of the star formation process, as it is being
recorded for the last 20 Myr, and they give important information on the
low-mass Initial Mass Function (IMF) of their host environments. We present the
latest results from observations with the Advanced Camera for Surveys (ACS) of
such star-forming regions in the MCs, and discuss the importance of Hubble}for
a comprehensive collection of substantial information on the most recent
low-mass star formation and the low-mass IMF in the MCs.Comment: To appear in the proceedings of the 41st ESLAB Symposium: The Impact
of HST on European Astronomy, 4 pages, LaTeX ESA Publications style, 5
Figure
Mid Infrared Properties of Low Metallicity Blue Compact Dwarf Galaxies From Spitzer/IRS
We present a {\em Spitzer}-based mid-infrared study of a large sample of Blue
Compact Dwarf galaxies (BCD) using the Infrared Spectrograph (IRS), including
the first mid-IR spectrum of IZw18, the archetype for the BCD class and among
the most metal poor galaxies known. We show the spectra of Polycyclic Aromatic
Hydrocarbon (PAH) emission in low-metallicity environment. We find that the
equivalent widths (EW) of PAHs at 6.2, 7.7, 8.6 and 11.2 m are generally
weaker in BCDs than in typical starburst galaxies and that the fine structure
line ratio, [NeIII]/[NeII], has a weak anti-correlation with the PAH EW. A much
stronger anti-correlation is shown between the PAH EW and the product of the
[NeIII]/[NeII] ratio and the UV luminosity density divided by the metallicity.
We conclude that PAH EW in metal-poor high-excitation environments is
determined by a combination of PAH formation and destruction effects.Comment: 41 pages, 14 figure
- âŠ