467 research outputs found
VLT/UVES shows no cosmological variability of alpha
The cosmological variability of alpha is probed from individual observations
of pairs of FeII lines. This procedure allows a better control of the
systematics and avoids the influence of the spectral shifts due to ionization
inhomogeneities in the absorbers and/or non-zero offsets between different
exposures. Applied to the FeII lines of the metal absorption systems at zabs =
1.839 in Q1101--264 and at zabs = 1.15 in HE0515--4414 observed by means of
UVES at the ESO-VLT, it provides da/a = 0.4 (+/- 1.5 stat)x10^{-6}. The result
is shifted with respect to the Keck/HIRES mean da/a = -5.7(+/- 1.1
stat})x10^{-6} (Murphy et al. 2004) at a high confidence level (95%). Full
details of this work are given in Levshakov et al (2005)Comment: 3 pages, 1 postscript figur
UVES radial velocity accuracy from asteroid observations. Implications for the fine structure constant variability
High resolution observations of the asteroids Iris and Juno have been
performed by means of the UVES spectrograph at the ESO VLT to obtain the
effective accurac y of the spectrograph's radial velocity. The knowledge of
this quantity has impo rtant bearings on studies searching for a variability of
the fine structure cons tant carried on with this instrument. Asteroids provide
a precise radial velocit y reference at the level of 1 m/s which allows
instrumental calibration and the recognition of small instrumental drifts and
calibration systematics. In particu lar, radial velocity drifts due to non
uniform slit illumination and slit optica l misalignment in the two UVES
spectrograph arms can be investigated. The positi on of the solar spectrum
reflected by the asteroids are compared with the solar wavelength positions or
with that of asteroid observations at other epochs or wi th the twilight to
asses UVES instrumental accuracy . Radial velocities offsets in the range
10--50 m/s are generally observed likely due to a non uniform slit
illumination. However, no radial velocity patterns with wavelength are detected
and the two UVES arms provide consistent radial velocities. These results
suggest that the detected alpha variability by Levshakov et al. (2007) deduced
from a drift of -180 (+/- 85) m/s at z =1.84, between two sets of FeII lines
falling in the two UVES arms may be real or induced by other kinds of
systematics than those investigated here. The proposed technique allows real
time quality check of the spectrograph and should be followed for very accurate
measurements.Comment: Accepted A&
Implementation and modeling of a femtosecond laser-activated streak camera
8 June 2017) A laser-activated streak camera was built to measure the duration of femtosecond electron pulses. The streak velocity of the device is 1.89 mrad/ps, which corresponds to a sensitivity of 34.9 fs/pixels. The streak camera also measures changes in the relative time of arrival between the laser and electron pulses with a resolution of 70 fs RMS. A full circuit analysis of the structure is presented to describe the streaking field and the general behavior of the device. We have developed a general mathematical model to analyze the streaked images. The model provides an accurate method to extract the pulse duration based on the changes of the electron beam profile when the streaking field is applied
The UVES Large Program for testing fundamental physics - III. Constraints on the fine-structure constant from 3 telescopes
Large statistical samples of quasar spectra have previously indicated
possible cosmological variations in the fine-structure constant, . A
smaller sample of higher signal-to-noise ratio spectra, with dedicated
calibration, would allow a detailed test of this evidence. Towards that end, we
observed equatorial quasar HS 15491919 with three telescopes: the Very Large
Telescope, Keck and, for the first time in such analyses, Subaru. By directly
comparing these spectra to each other, and by `supercalibrating' them using
asteroid and iodine-cell tests, we detected and removed long-range distortions
of the quasar spectra's wavelength scales which would have caused significant
systematic errors in our measurements. For each telescope we measure
the relative deviation in from the current laboratory value,
, in 3 absorption systems at redshifts
, 1.342, and 1.802. The nine measurements of
are all consistent with zero at the 2- level,
with 1- statistical (systematic) uncertainties 5.6--24 (1.8--7.0) parts
per million (ppm). They are also consistent with each other at the 1-
level, allowing us to form a combined value for each telescope and, finally, a
single value for this line of sight: ppm, consistent with both zero and
previous, large samples. We also average all Large Programme results measuring
ppm.
Our results demonstrate the robustness and reliability at the 3 ppm level
afforded by supercalibration techniques and direct comparison of spectra from
different telescopes.Comment: 24 pages, 11 figures, 9 table
Most precise single redshift bound to the variability of the fine-structure constant
Verification of theoretical predictions of an oscillating behavior of the
fine-structure constant, alpha, with cosmic time requires high precision
measurements at individual redshifts, while in earlier studies the mean Delta
alpha/alpha values averaged over wide redshift intervals were usually reported.
This requirement can be met via the Single Ion Differential alpha Measurement
(SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped
Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted
mean calculated on base of carefully selected 34 FeII pairs is <Delta
alpha/alpha>=(-0.07+/-0.84)10^{-6}. The precision of this estimate represents
the absolute improvement with respect to what has been done in the measurements
of Delta alpha/alpha.Comment: 2 pages, to appear in the Proceed. of IAU Symp.232 "The Scientific
Requirements for Extremely Large Telescopes", eds. P. Whitelock, B.
Leibundgut, and M. Dennefel
Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
Ultraviolet absorption from interstellar 12CO and 13CO was detected toward
rho Oph A and chi Oph. The measurements were obtained at medium resolution with
the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column
density ratios, N(12CO)/N(13CO), of 125 \pm 23 and 117 \pm 35 were derived for
the sight lines toward rho Oph A and chi Oph, respectively. A value of 1100 \pm
600 for the ratio N(12C16O)/N(12C18O) toward rho Oph A was also obtained.
Absorption from vibrationally excited H_2 (v" = 3) was clearly seen toward this
star as well.
The ratios are larger than the isotopic ratios for carbon and oxygen
appropriate for ambient interstellar material. Since for both carbon and oxygen
the more abundant isotopomer is enhanced, selective isotopic photodissociation
plays the key role in the fractionation process for these directions. The
enhancement arises because the more abundant isotopomer has lines that are more
optically thick, resulting in more self shielding from dissociating radiation.
A simple argument involving the amount of self shielding [from N(12CO)] and the
strength of the ultraviolet radiation field premeating the gas (from the amount
of vibrationally excited H_2) shows that selective isotopic photodissociation
controls the fractionation seen in these two sight lines, as well as the sight
line to zeta Oph.Comment: 40 pages, 8 figures, to appear in 10 July 2003 issue of Ap
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