1,478 research outputs found
QSO 0130-4021: A third QSO showing a low Deuterium to Hydrogen Abundance Ratio
We have discovered a third quasar absorption system which is consistent with
a low deuterium to hydrogen abundance ratio, D/H = 3.4 times 10^-5. The z ~ 2.8
partial Lyman limit system towards QSO 0130-4021 provides the strongest
evidence to date against large D/H ratios because the H I absorption, which
consists of a single high column density component with unsaturated high order
Lyman series lines, is readily modeled -- a task which is more complex in other
D/H systems. We have obtained twenty-two hours of spectra from the HIRES
spectrograph on the W.M. Keck telescope, which allow a detailed description of
the Hydrogen. We see excess absorption on the blue wing of the H I Lyman alpha
line, near the expected position of Deuterium. However, we find that Deuterium
cannot explain all of the excess absorption, and hence there must be
contamination by additional absorption, probably H I. This extra H I can
account for most or all of the absorption at the D position, and hence D/H = 0
is allowed. We find an upper limit of D/H < 6.7 times 10^-5 in this system,
consistent with the value of D/H ~ 3.4 times 10^-5 deduced towards QSO
1009+2956 and QSO 1937-1009 by Burles and Tytler (1998a, 1998b). This
absorption system shows only weak metal line absorption, and we estimate [Si/H]
< -2.6 -- indicating that the D/H ratio of the system is likely primordial. All
four of the known high redshift absorption line systems simple enough to
provide useful limits on D are consistent with D/H = 3.4 +/- 0.25 times 10^-5.
Conversely, this QSO provides the third case which is inconsistent with much
larger values.Comment: 18 pages, 5 figures, submitted to Ap
Calculations of the Local Density of States for some Simple Systems
A recently proposed convolution technique for the calculation of local
density of states is described more thouroughly and new results of its
application are presented. For separable systems the exposed method allows to
construct the ldos for a higher dimensionality out of lower dimensional parts.
Some practical and theoretical aspects of this approach are also discussed.Comment: 5 pages, 3 figure
Intrinsic Properties of the <z>=2.7 Lyman Alpha Forest from Keck Spectra of QSO HS 1946+7658
We present the highest quality Lyman Alpha forest spectra published to date,
from the QSO HS 1946+7658. The distribution of H I column densities is a power
law of slope -1.5 from Log N = 12.1 - 14. This power law can extend to N = 0,
because lines weaker than Log N = 12.1 do not have a large H I optical depth.
Low column lines with Log N > 9 could account for all observed He II
absorption, but lines with Log N > 12 alone are unlikely to do so. The b
distribution between 20 and 60 km/sec is a Gaussian with a mean of 23 km/sec
(less than reported in past at this z), and a sigma b of 14 km/sec. We report
no evolution in the Lyman alpha forest (except the number of lines), because Lu
et al. (1997) found the same N and b distributions at = 3.7. We see lines
with 14 80 km/sec that cannot be accounted for by noise
or blending effects. We discover that the lower cutoff in the b distribution
varies with N, from b = 14 km/sec at Log N = 12.5 to b = 22 km/sec at Log N =
14.0, but otherwise b and N are not correlated. We see no Lyman Alpha line
clustering above 50 \kms, in disagreement with previous results from lower
signal to noise data, but we do see a 3 sigma clustering signal at 25 - 50
km/sec among lines with Log N > 13.6Comment: (Minor changes including new identifications for two weak lines) 46
pages including 16 Figures, Latex Table 1 Available at
http://nately.ucsd.edu/~david . To appear in Ap
Non-Gaussian Features of Transmitted Flux of QSO's Ly Absorption: Intermittent Exponent
We calculate the structure function and intermittent exponent of the 1.) Keck
data, which consists of 29 high resolution, high signal to noise ratio (S/N)
QSO Ly absorption spectra, and 2.)the Ly forest simulation
samples produced via the pseudo hydro scheme for the low density cold dark
matter (LCDM) model and warm dark matter (WDM) model with particle mass
and 1000 eV. These two measures detect not only
non-gaussianities, but also the type of non-gaussianty in the the field. We
find that, 1.) the structure functions of the simulation samples are
significantly larger than that of Keck data on scales less than about 100
h kpc, 2.) the intermittent exponent of the simulation samples is more
negative than that of Keck data on all redshifts considered, 3.) the
order-dependence of the structure functions of simulation samples are closer to
the intermittency of hierarchical clustering on all scales, while the Keck data
are closer to a lognormal field on small scales. These differences are
independent of noise and show that the intermittent evolution modeled by the
pseudo-hydro simulation is substantially different from observations, even
though they are in good agreement in terms of second and lower order
statistics. (Abridged)Comment: 17 pages, 13 figures. Accepted by Ap
QSOs and Absorption Line Systems Surrounding the Hubble Deep Field
We have imaged a 45x45 sq. arcmin. area centered on the Hubble Deep Field
(HDF) in UBVRI passbands, down to respective limiting magnitudes of
approximately 21.5, 22.5, 22.2, 22.2, and 21.2. The principal goals of the
survey are to identify QSOs and to map structure traced by luminous galaxies
and QSO absorption line systems in a wide volume containing the HDF. We have
selected QSO candidates from color space, and identified 4 QSOs and 2 narrow
emission-line galaxies (NELGs) which have not previously been discovered,
bringing the total number of known QSOs in the area to 19. The bright z=1.305
QSO only 12 arcmin. away from the HDF raises the northern HDF to nearly the
same status as the HDF-S, which was selected to be proximate to a bright QSO.
About half of the QSO candidates remain for spectroscopic verification.
Absorption line spectroscopy has been obtained for 3 bright QSOs in the field,
using the Keck 10m, ARC 3.5m, and MDM 2.4m telescopes. Five heavy-element
absorption line systems have been identified, 4 of which overlap the
well-explored redshift range covered by deep galaxy redshift surveys towards
the HDF. The two absorbers at z=0.5565 and z=0.5621 occur at the same redshift
as the second most populated redshift peak in the galaxy distribution, but each
is more than 7Mpc/h (comoving, Omega_M=1, Omega_L=0) away from the HDF line of
sight in the transverse dimension. This supports more indirect evidence that
the galaxy redshift peaks are contained within large sheet-like structures
which traverse the HDF, and may be precursors to large-scale ``pancake''
structures seen in the present-day galaxy distribution.Comment: 36 pages, including 9 figures and 8 tables. Accepted for publication
in the Astronomical Journa
The Deuterium to Hydrogen Abundance Ratio Towards a Fourth QSO: HS0105+1619
We report the measurement of the primordial D/H abundance ratio towards QSO
\object. The column density of the hydrogen in the Lyman limit
system is high, \lnhi \cmm, allowing for the deuterium to
be seen in 5 Lyman series transitions. The measured value of the D/H ratio
towards QSO \object is found to be D/H. The
metallicity of the system showing D/H is found to be solar,
indicating that the measured D/H is the primordial D/H within the measurement
errors. The gas which shows D/H is neutral, unlike previous D/H systems which
were more highly ionized. Thus, the determination of the D/H ratio becomes more
secure since we are measuring it in different astrophysical environments, but
the error is larger because we now see more dispersion between measurements.
Combined with prior measurements of D/H, the best D/H ratio is now D/H, which is 10% lower than the previous value. The new
values for the baryon to photon ratio, and baryonic matter density derived from
D/H are and \ob
respectively.Comment: Minor text and reference changes. To appear in the May 10, 2001 issue
of the Astrophysical Journa
Constraints on massive gravity theory from big bang nucleosynthesis
The massive gravity cosmology is studied in the scenario of big bang
nucleosynthesis. By making use of current bounds on the deviation from the
fractional mass, we derive the constraints on the free parameters of the
theory. The cosmological consequences of the model are also discussed in the
framework of the PAMELA experiment.Comment: 5 page
Retinopathy predicts progression of fasting plasma glucose: An Early Diabetes Intervention Program (EDIP) analysis
Background
Retinopathy is increasingly recognized in prediabetic populations, and may herald increased risk of metabolic worsening. The Early Diabetes Intervention Program (EDIP) evaluated worsening of glycemia in screen-detected Type 2 diabetes, following participants for up to 5 years. Here we have evaluated whether the presence of retinopathy at the time of detection of diabetes was associated with accelerated progression of glycemia.
Methods
We prospectively studied 194 participants from EDIP with available baseline retinal photographs. Retinopathy was determined at baseline using 7-field fundus photography and defined as an Early Treatment of Diabetic Retinopathy Study Scale grading score of â„ 20.
Results
At baseline, 12% of participants had classical retinal lesions indicating retinopathy. In univariate Cox proportional hazard analysis, the presence of retinopathy at baseline was associated with a doubled risk of progression of fasting plasma glucose (HR 2.02; 95% CI 1.05â3.89). The retinopathy effect was robust to individual adjustment for age and glucose, the most potent determinants of progression in EDIP.
Conclusion
Retinopathy was associated with increased risk of progression of fasting plasma glucose among adults with screen-detected, early diabetes. Early detection of retinopathy may help individualize more aggressive therapy to prevent progressive metabolic worsening in early diabetes
Molecular functionalization of graphite surfaces : Basal Plane versus Step Edge electrochemical activity
The chemical functionalization of carbon surfaces has myriad applications, from tailored sensors to electrocatalysts. Here, the adsorption and electrochemistry of anthraquinone-2,6-disulfonate (AQDS) is studied on highly oriented pyrolytic graphite (HOPG) as a model sp2 surface. A major focus is to elucidate whether adsorbed electroactive AQDS can be used as a marker of step edges, which have generally been regarded as the main electroactive sites on graphite electrode surfaces. First, the macroscopic electrochemistry of AQDS is studied on a range of surfaces differing in step edge density by more than 2 orders of magnitude, complemented with ex situ tapping mode atomic force microscopy (AFM) data. These measurements show that step edges have little effect on the extent of adsorbed electroactive AQDS. Second, a new fast scan cyclic voltammetry protocol carried out with scanning electrochemical cell microscopy (SECCM) enables the evolution of AQDS adsorption to be followed locally on a rapid time scale. Subsequent AFM imaging of the areas probed by SECCM allows a direct correlation of the electroactive adsorption coverage and the actual step edge density of the entire working area. The amount of adsorbed electroactive AQDS and the electron transfer kinetics are independent of the step edge coverage. Last, SECCM reactive patterning is carried out with complementary AFM measurements to probe the diffusional electroactivity of AQDS. There is essentially uniform and high activity across the basal surface of HOPG. This work provides new methodology to monitor adsorption processes at surfaces and shows unambiguously that there is no correlation between the step edge density of graphite surfaces and the observed coverage of electroactive AQDS. The electroactivity is dominated by the basal surface, and studies that have used AQDS as a marker of steps need to be revised
- âŠ