273 research outputs found
CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk
The relative abundances of atomic and molecular species in planet-forming
disks around young stars provide important constraints on photochemical disk
models and provide a baseline for calculating disk masses from measurements of
trace species. A knowledge of absolute abundances, those relative to molecular
hydrogen (H2), are challenging because of the weak rovibrational transition
ladder of H and the inability to spatially resolve different emission
components within the circumstellar environment. To address both of these
issues, we present new contemporaneous measurements of CO and H2 absorption
through the "warm molecular layer" of the protoplanetary disk around the
Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode
of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2
absorption in this region for the first time. An analysis of the emission and
absorption spectrum of RW Aur shows components from the accretion region near
the stellar photosphere, the molecular disk, and several outflow components.
The warm H2 and CO absorption lines are consistent with a disk origin. We model
the 1092-1117A spectrum of RW Aur to derive log10
N(H2)~=~19.90 at T(H2) ~=~440~+/-~39 K. The CO
~--~ bands observed from 1410-1520A are best fit by log10
N(CO)~=~16.1~ at T(CO) ~=~200 K.
Combining direct measurements of the HI, H2, and CO column densities, we find a
molecular fraction in the warm disk surface of ~>=~0.47 and derive a
molecular abundance ratio of CO/H2~=~1.6~x~10, both
consistent with canonical interstellar dense cloud values.Comment: ApJ - accepted. 13 pages, 8 figure
Time Reversal Invariance Violation in Neutron Deuteron Scattering
Time reversal invariance violating (TRIV) effects for low energy elastic
neutron deuteron scattering are calculated for meson exchange and EFT-type of
TRIV potentials in a Distorted Wave Born Approximation, using realistic
hadronic strong interaction wave functions, obtained by solving three-body
Faddeev equations in configuration space. The relation between TRIV and parity
violating observables are discussed
P- and T-violating Schiff moment of the Mercury nucleus
The Schiff moment of the Hg nucleus was calculated using finite range
P- and T-violating weak nucleon-nucleon interaction. Effects of the core
polarization were considered in the framework of RPA with effective residual
forces.Comment: 10 pages and 2 figures,to appear in Yad. Fi
D vs d: CP Violation in Beta Decay and Electric Dipole Moments
The T-odd correlation coefficient D in nuclear beta decay probes CP violation
in many theories beyond the Standard Model. We provide an analysis for how
large D can be in light of constraints from electric dipole moment (EDM)
searches. We argue that the neutron EDM d_n currently provides the strongest
constraint on D, which is 10 - 10^3 times stronger than current direct limits
on D (depending on the model). In particular, contributions to D in leptoquark
models (previously regarded as "EDM safe") are more constrained than previously
thought. Bounds on D can be weakened only by fine-tuned cancellations or if
theoretical uncertainties are larger than estimated in d_n. We also study
implications for D from mercury and deuteron EDMs.Comment: 17 pages, 6 figure
Spectropolarimetry of the Classical T Tauri Star TW Hydrae
We present high resolution (R ~ 60,000) circular spectropolarimetry of the
classical T Tauri star TW Hydrae. We analyze 12 photospheric absorption lines
and measure the net longitudinal magnetic field for 6 consecutive nights. While
no net polarization is detected the first five nights, a significant
photospheric field of Bz = 149 \pm 33 G is found on the sixth night. To rule
out spurious instrumental polarization, we apply the same analysis technique to
several non-magnetic telluric lines, detecting no significant polarization. We
further demonstrate the reality of this field detection by showing that the
splitting between right and left polarized components in these 12 photospheric
lines shows a linear trend with Lande g-factor times wavelength squared, as
predicted by the Zeeman effect. However, this longitudinal field detection is
still much lower than that which would result if a pure dipole magnetic
geometry is responsible for the mean magnetic field strength of 2.6 kG
previously reported for TW Hya. We also detect strong circular polarization in
the He I 5876 and the Ca II 8498 emission lines, indicating a strong field in
the line formation region of these features. The polarization of the Ca II line
is substantially weaker than that of the He I line, which we interpret as due
to a larger contribution to the Ca II line from chromospheric emission in which
the polarization signals cancel. However, the presence of polarization in the
Ca II line indicates that accretion shocks on Classical T Tauri stars do
produce narrow emission features in the infrared triplet lines of Calcium.Comment: One tar file. The paper has 22 pages, 5 figures. Accepted by AJ on
Sep 10, 200
Collective T- and P- Odd Electromagnetic Moments in Nuclei with Octupole Deformations
Parity and time invariance violating forces produce collective P- and T- odd
moments in nuclei with static octupole deformation. Collective Schiff moment,
electric octupole and dipole and also magnetic quadrupole appear due to the
mixing of rotational levels of opposite parity and can exceed single-particle
moments by more than a factor of 100. This enhancement is due to two factors,
the collective nature of the intrinsic moments and the small energy separation
between members of parity doublets. The above moments induce T- and P- odd
effects in atoms and molecules. Experiments with such systems may improve
substantially the limits on time reversal violation.Comment: 9 pages, Revte
Measuring the Magnetic Field on the Classical T Tauri Star TW Hydrae
We present infrared (IR) and optical echelle spectra of the Classical T Tauri
star TW Hydrae. Using the optical data, we perform detailed spectrum synthesis
to fit atomic and molecular absorption lines and determine key stellar
parameters: Teff = 4126 \pm 24 K, log g = 4.84 \pm 0.16, [M/H] = -0.10 \pm
0.12, vsini = 5.8 \pm 0.6 km/s. The IR spectrum is used to look for Zeeman
broadening of photospheric absorption lines. We fit four Zeeman sensitive Ti I
lines near 2.2 microns and find the average value of the magnetic field over
the entire surface is 2.61 \pm 0.23 kG. In addition, several nearby
magnetically insensitive CO lines show no excess broadening above that produced
by stellar rotation and instrumental broadening, reinforcing the magnetic
interpretation for the width of the Ti I lines. We carry out extensive tests to
quantify systematic errors in our analysis technique which may result from
inaccurate knowledge of the effective temperature or gravity, finding that
reasonable errors in these quantities produce a 10% uncertainty in the mean
field measurement.Comment: The tar file includes one Tex file and four .eps figures. The paper
is accepted and tentatively scheduled for the ApJ 1 December 2005, v634, 2
issue. ApJ manuscript submission # 6310
P,T-Violating Nuclear Matrix Elements in the One-Meson Exchange Approximation
Expressions for the P,T-violating NN potentials are derived for ,
and exchange. The nuclear matrix elements for and
exchange are shown to be greatly suppressed, so that, under the assumption of
comparable coupling constants, exchange would dominate by two orders of
magnitude. The ratio of P,T-violating to P-violating matrix elements is found
to remain approximately constant across the nuclear mass table, thus
establishing the proportionality between time-reversal-violation and
parity-violation matrix elements. The calculated values of this ratio suggest a
need to obtain an accuracy of order for the ratio of the
PT-violating to P-violating asymmetries in neutron transmission experiments in
order to improve on the present limits on the isovector pion coupling constant.Comment: 17 pages, LaTeX, no figure
Nuclear Octupole Correlations and the Enhancement of Atomic Time-Reversal Violation
We examine the time-reversal-violating nuclear ``Schiff moment'' that induces
electric dipole moments in atoms. After presenting a self-contained derivation
of the form of the Schiff operator, we show that the distribution of Schiff
strength, an important ingredient in the ground-state Schiff moment, is very
different from the electric-dipole-strength distribution, with the Schiff
moment receiving no strength from the giant dipole resonance in the
Goldhaber-Teller model. We then present shell-model calculations in light
nuclei that confirm the negligible role of the dipole resonance and show the
Schiff strength to be strongly correlated with low-lying octupole strength.
Next, we turn to heavy nuclei, examining recent arguments for the strong
enhancement of Schiff moments in octupole-deformed nuclei over that of 199Hg,
for example. We concur that there is a significant enhancement while pointing
to effects neglected in previous work (both in the octupole-deformed nuclides
and 199Hg) that may reduce it somewhat, and emphasizing the need for
microscopic calculations to resolve the issue. Finally, we show that static
octupole deformation is not essential for the development of collective Schiff
moments; nuclei with strong octupole vibrations have them as well, and some
could be exploited by experiment.Comment: 25 pages, 4 figures embedded in tex
Nab: Measurement Principles, Apparatus and Uncertainties
The Nab collaboration will perform a precise measurement of 'a', the
electron-neutrino correlation parameter, and 'b', the Fierz interference term
in neutron beta decay, in the Fundamental Neutron Physics Beamline at the SNS,
using a novel electric/magnetic field spectrometer and detector design. The
experiment is aiming at the 10^{-3} accuracy level in (Delta a)/a, and will
provide an independent measurement of lambda = G_A/G_V, the ratio of
axial-vector to vector coupling constants of the nucleon. Nab also plans to
perform the first ever measurement of 'b' in neutron decay, which will provide
an independent limit on the tensor weak coupling.Comment: 12 pages, 6 figures, 1 table, talk presented at the International
Workshop on Particle Physics with Slow Neutrons, Grenoble, 29-31 May 2008; to
appear in Nucl. Instrum. Meth. in Physics Research
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