1,006 research outputs found
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Rotational Spectra of Sulfur-Carbon Chains. I. The Radicals C and 4S, C5S, C6S, C7S, C8S, C9S
V. D. Gordon (currently with UT Austin), M. C. McCarthy, A. J. Apponi, and P. Thaddeus are with the
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; and Division of Engineering and Applied Sciences,
Harvard University, 29 Oxford Street, Cambridge, MA 02138The linear carbon-chain radicals C6S, C7S, C8S, and C9S have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy, and measurements of the previously studied
chains C4S and C5S have been extended using the same technique. The electronic ground state, as predicted, is found to be a triplet for those with an even number of carbon atoms and a singlet for those
with an odd number. The microwave spectra of all six chains have been fully characterized, and spectroscopic
constants, including fine-structure constants where applicable, have been determined to high precision.
Transitions from the J=N and J=N-1 fine-structure levels of C4S have been detected for the first time, and the lengths of the individual bonds of C5S have been determined from its isotopic species.
The carbon-sulfur chains here are all highly polar, calculated to possess dipole moments in the 4-7 D range; all are plausible candidates for astronomical detection. The spectroscopic constants determined
from this work allow transitions up to 40 GHz to be calculated with an uncertainty of less than 1 km
s -1 for each chain.
Subject headings : ISM: molecules -- line : identiĂcation -- molecular data -- molecular processes --
radio lines : ISMChemistr
Global structure and kinematics of stellar haloes in cosmological hydrodynamic simulations
We use the GalaxiesâIntergalactic Medium Interaction Calculation (GIMIC) suite of cosmological hydrodynamical simulations to study the global structure and kinematics of stellar spheroids of Milky Way mass disc galaxies. Font et al. have recently demonstrated that these simulations are able to successfully reproduce the satellite luminosity functions and the metallicity and surface brightness profiles of the spheroids of the Milky Way and M31. A key to the success of the simulations is a significant contribution to the spheroid from stars that formed in situ. While the outer halo is dominated by accreted stars, stars formed in the main progenitor of the galaxy dominate at rⲠ30âkpc. In the present study, we show that this component was primarily formed in a protodisc at high redshift and was subsequently liberated from the disc by dynamical heating associated with mass accretion. As a consequence of its origin, the in situ component of the spheroid has different kinematics (namely net prograde rotation with respect to the disc) than that of the spheroid component built from the disruption of satellites. In addition, the in situ component has a flattened distribution, which is due in part to its rotation. We make comparisons with measurements of the shape and kinematics of local galaxies, including the Milky Way and M31, and stacked observations of more distant galaxies. We find that the simulated disc galaxies have spheroids of the correct shape (oblate with a median axial ratio of âź0.6 at radii of â˛30âkpc, but note there is significant system-to-system scatter in this quantity) and that the kinematics show evidence for two components (due to in situ versus accreted), as observed. Our findings therefore add considerable weight to the importance of dissipative processes in the formation of stellar haloes and to the notion of a âdual stellar haloâ
Recommended from our members
Structures of the linear silicon carbides SiC 4 and SiC 6 : Isotopic substitution and Ab Initio theory
V. D. Gordon,a) E. S. Nathan, A. J. Apponi, M. C. McCarthy, and P. Thaddeus are with
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138 and
Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge,
Massachusetts 02138
-- P. Botschwina is with the
Institut fu¨r Physikalische Chemie der Universita¨t Go¨ttingen, Tammannstr, 6. D-37077 Go¨ttingen, GermanyThe structures of two linear silicon carbides, SiC4 and SiC6, have been determined by a combination
of isotopic substitution and large-scale coupled-cluster ab initio calculations, following detection of
all of the singly substituted isotopic species in a supersonic molecular beam with a Fourier transform
microwave spectrometer. Rotational constants obtained by least-squares fitting transition
frequencies were used to derive experimental structures; except for those nearest the center of mass,
individual bond lengths for both chains have an error of less than 0.008 Ă
. Accurate equilibrium
structures were derived by converting the experimental rotational constants to equilibrium constants
using the vibrationârotation coupling constants from coupled-cluster calculations, including
connected triple substitutions. Equilibrium dipole moments and harmonic vibrational frequencies
were also calculated for both chains. On the basis of the calculated vibrationârotation and l-type
doubling constants, weak rotational satellites from a low-lying vibrational state of SiC4 were
assigned to v6 , a bending mode calculated to lie about 205 cm -1 above the ground state. A
recommended ab initio equilibrium structure for SiC8 has also been established. Š 2000 American
Institute of Physics. @S0021-9606~00!01537-3#Chemistr
Survival of Staphylococcus aureus ST398 in the human nose after artificial inoculation.
There is evidence that MRSA ST398 of animal origin is only capable of temporarily occupying the human nose, and it is therefore, often considered a poor human colonizer.We inoculated 16 healthy human volunteers with a mixture of the human MSSA strain 1036 (ST931, CC8) and the bovine MSSA strain 5062 (ST398, CC398), 7 weeks after a treatment with mupirocin and chlorhexidine-containing soap. Bacterial survival was studied by follow-up cultures over 21 days. The human strain 1036 was eliminated faster (median 14 days; range 2-21 days) than the bovine strain 5062 (median 21 days; range 7-21 days) but this difference was not significant (pâ=â0.065). The bacterial loads were significantly higher for the bovine strain on day 7 and day 21. 4/14 volunteers (28.6%) showed elimination of both strains within 21 days. Of the 10 remaining volunteers, 5 showed no differences in bacterial counts between both strains, and in the other 5 the ST398 strain far outnumbered the human S. aureus strain. Within the 21 days of follow-up, neither human strain 1036 nor bovine strain 5062 appeared to acquire or lose any mobile genetic elements. In conclusion, S. aureus ST398 strain 5062 is capable of adequately competing for a niche with a human strain and survives in the human nose for at least 21 days
Incoherent dynamics of vibrating single-molecule transistors
We study the tunneling conductance of nano-scale quantum ``shuttles'' in
connection with a recent experiment (H. Park et al., Nature, 407, 57 (2000)) in
which a vibrating C^60 molecule was apparently functioning as the island of a
single electron transistor (SET). While our calculation starts from the same
model of previous work (D. Boese and H. Schoeller, Europhys. Lett. 54,
66(2001)) we obtain quantitatively different dynamics. Calculated I-V curves
exhibit most features present in experimental data with a physically reasonable
parameter set, and point to a strong dependence of the oscillator's potential
on the electrostatics of the island region. We propose that in a regime where
the electric field due to the bias voltage itself affects island position, a
"catastrophic" negative differential conductance (NDC) may be realized. This
effect is directly attributable to the magnitude of overlap of final and
initial quantum oscillator states, and as such represents experimental control
over quantum transitions of the oscillator via the macroscopically controllable
bias voltage.Comment: 6 pages, LaTex, 6 figure
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
Compensation between meridional flow components of the Atlantic MOC at 26°N
From ten years of observations of the Atlantic meridional overturning circulation (MOC) at 26°âŻN (2004â2014), we revisit the question of flow compensation between components of the circulation. Contrasting with early results from the observations, transport variations of the Florida Current (FC) and upper mid-ocean (UMO) transports (top 1000âŻm east of the Bahamas) are now found to compensate on sub-annual timescales. The observed compensation between the FC and UMO transports is associated with horizontal circulation and means that this part of the correlated variability does not project onto the MOC. A deep baroclinic response to wind-forcing (Ekman transport) is also found in the lower North Atlantic Deep Water (LNADW; 3000â5000âŻm) transport. In contrast, co-variability between Ekman and the LNADW transports does contribute to overturning. On longer timescales, the southward UMO transport has continued to strengthen, resulting in a continued decline of the MOC. Most of this interannual variability of the MOC can be traced to changes in isopycnal displacements on the western boundary, within the top 1000âŻm and below 2000âŻm. Substantial trends are observed in isopycnal displacements in the deep ocean, underscoring the importance of deep boundary measurements to capture the variability of the Atlantic MOC
Agro-materials : a bibliographic review
Facing the problems of plastic recycling and fossil resources exhaustion, the use of biomass to conceive new materials appears like a reasonable solution. Two axes of research are nowadays developed : on the one hand the synthesis of biodegradable plastics, whichever the methods may be, on the other hand the utilization of raw biopolymers, which is the object of this paper. From this perspective, the âplasticâ properties of natural polymers, the caracteristics of the different classes of polymers, the use of charge in vegetable matrix and the possible means of improving the durability of these agro-materials are reviewed
Compton scattering beyond the impulse approximation
We treat the non-relativistic Compton scattering process in which an incoming
photon scatters from an N-electron many-body state to yield an outgoing photon
and a recoil electron, without invoking the commonly used frameworks of either
the impulse approximation (IA) or the independent particle model (IPM). An
expression for the associated triple differential scattering cross section is
obtained in terms of Dyson orbitals, which give the overlap amplitudes between
the N-electron initial state and the (N-1) electron singly ionized quantum
states of the target. We show how in the high energy transfer regime, one can
recover from our general formalism the standard IA based formula for the cross
section which involves the ground state electron momentum density (EMD) of the
initial state. Our formalism will permit the analysis and interpretation of
electronic transitions in correlated electron systems via inelastic x-ray
scattering (IXS) spectroscopy beyond the constraints of the IA and the IPM.Comment: 7 pages, 1 figur
Black American Maternal Prenatal Choline, Offspring Gestational Age at Birth, and Developmental Predisposition to Mental Illness
Black Americans have increased risk for schizophrenia and other mental illnesses with prenatal origins. Prenatal choline promotes infant brain development and behavioral outcomes, but choline has not been specifically assessed in Black Americans. Pregnant women (N = 183, N = 25 Black Americans) enrolled in a study of prenatal stressors and interactions with prenatal choline. Black American women had lower 16-week gestation plasma choline than Whites. Lower choline was not related to obesity, income, or metabolic genotypes. Pregnant women in rural Uganda have higher choline levels than Black American women. Black Americans' lower choline was associated with higher hair cortisol, indicative of higher stress. Lower maternal choline was associated with offsprings' lower gestational age at birth and with decreased auditory P50 inhibition, a marker of inhibitory neuron development. Behavioral development was assessed on the Infant Behavior Questionnaire-R-SF (IBQ-R) at 3 months. Lower Black American maternal gestational choline was associated with lower infant IBQ-R Orienting/Regulation, indicating decreased attention and relation to caregivers. Additional evidence for developmental effects of choline in Black Americans comes from a randomized clinical trial of gestational phosphatidylcholine supplementation versus placebo that included 15 Black Americans. Phosphatidylcholine increased gestational age at birth and newborn P50 inhibition and decreased Social Withdrawn and Attention problems at 40 months of age in Black Americans' offspring compared to placebo. Inhibitory and behavioral deficits associated with lower prenatal choline in offspring of Black American women indicate potential developmental predispositions to later mental illnesses that might be ameliorated by prenatal choline or phosphatidylcholine supplementation
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