209 research outputs found
The fundamental constants and their variation: observational status and theoretical motivations
This article describes the various experimental bounds on the variation of
the fundamental constants of nature. After a discussion on the role of
fundamental constants, of their definition and link with metrology, the various
constraints on the variation of the fine structure constant, the gravitational,
weak and strong interactions couplings and the electron to proton mass ratio
are reviewed. This review aims (1) to provide the basics of each measurement,
(2) to show as clearly as possible why it constrains a given constant and (3)
to point out the underlying hypotheses. Such an investigation is of importance
to compare the different results, particularly in view of understanding the
recent claims of the detections of a variation of the fine structure constant
and of the electron to proton mass ratio in quasar absorption spectra. The
theoretical models leading to the prediction of such variation are also
reviewed, including Kaluza-Klein theories, string theories and other
alternative theories and cosmological implications of these results are
discussed. The links with the tests of general relativity are emphasized.Comment: 56 pages, l7 figures, submitted to Rev. Mod. Phy
Spatial variation in the fine-structure constant -- new results from VLT/UVES
(abridged) We present a new analysis of a large sample of quasar
absorption-line spectra obtained using UVES (the Ultraviolet and Visual Echelle
Spectrograph) on the VLT (Very Large Telescope) in Chile. In the VLT sample
(154 absorbers), we find evidence that alpha increases with increasing
cosmological distance from Earth. However, as previously shown, the Keck sample
(141 absorbers) provided evidence for a smaller alpha in the distant absorption
clouds. Upon combining the samples an apparent variation of alpha across the
sky emerges which is well represented by an angular dipole model pointing in
the direction RA=(17.3 +/- 1.0) hr, dec. = (-61 +/- 10) deg, with amplitude
(0.97 +0.22/-0.20) x 10^(-5). The dipole model is required at the 4.1 sigma
statistical significance level over a simple monopole model where alpha is the
same across the sky (but possibly different to the current laboratory value).
The data sets reveal a number of remarkable consistencies: various data cuts
are consistent and there is consistency in the overlap region of the Keck and
VLT samples. Assuming a dipole-only (i.e. no-monopole) model whose amplitude
grows proportionally with `lookback-time distance' (r=ct, where t is the
lookback time), the amplitude is (1.1 +/- 0.2) x 10^(-6) GLyr^(-1) and the
model is significant at the 4.2 sigma confidence level over the null model
[Delta alpha]/alpha = 0). We apply robustness checks and demonstrate that the
dipole effect does not originate from a small subset of the absorbers or
spectra. We present an analysis of systematic effects, and are unable to
identify any single systematic effect which can emulate the observed variation
in alpha.Comment: 47 pages, 35 figures. Accepted for publication by Monthly Notices of
the Royal Astronomical Society. Please see
http://astronomy.swin.edu.au/~mmurphy/pub.html for an ASCII version of table
A1 and the full set of Voigt profile fits for appendix
Probing variations in the fundamental constants with quasar absorption lines
Precision cosmology challenges many aspects of fundamental physics. In particular, quasar absorption lines test the assumed constancy of fundamental constants over cosmological time-scales and distances. Until recently, the most reliable technique was the alkali doublet (AD) method where the measured doublet separation probes variations in the fine-structure constant, . However, the recently introduced many-multiplet (MM) method provides several advantages, including a demonstrated ≈10-fold precision gain. This thesis presents detailed MM analyses of 3 independent Keck/HIRES samples containing 128 absorption systems with . We find statistical evidence for a smaller α in the absorption clouds: . All three samples separately yield consistent, significant . The data marginally prefer constant rather than constant . The two-point correlation function for α and the angular distribution of give no evidence for spatial variations. We also analyse 21 Keck/HIRES Si iv doublets, obtaining a 3-fold relative precision gain over previous AD studies: for . Our statistical evidence for varying α requires careful consideration of systematic errors. Modelling demonstrates that atmospheric dispersion is potentially important. However, the quasar spectra suggest a negligible effect on . Cosmological variation in Mg isotopic abundances may affect at . Galactic observations and theory suggest diminished 25,26Mg abundances in the low metallicity quasar absorbers. Removing 25,26Mg isotopes yields more negative values. Overall, known systematic errors can not explain our results. We also constrain variations in , comparing H I 21-cm and millimetre-wave molecular absorption in 2 systems. Fitting both the H I and molecular lines yields the tightest, most reliable current constraints: and at and 0.6847 respectively. Possible line-of-sight velocity differences between the H I and molecular absorbing regions dominate these errors. A larger sample of mm/H I comparisons is required to reliably quantify this uncertainty and provide a potentially crucial check on the MM result
Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra
[Abridged] We previously presented evidence for a varying fine-structure
constant, alpha, in two independent samples of Keck/HIRES QSO spectra. Here we
present a detailed many-multiplet analysis of a third Keck/HIRES sample
containing 78 absorption systems. We also re-analyse the previous samples,
providing a total of 128 absorption systems over the redshift range
0.2<z_abs<3.7. All three samples separately yield consistent, significant
values of da/a. The analyses of low- and high-z systems rely on different
ions/transitions with very different dependencies on alpha, yet they also give
consistent results. We identify additional random errors in 22 high-z systems
characterized by transitions with a large dynamic range in apparent optical
depth. Increasing the statistical errors on da/a for these systems gives our
fiducial result, a weighted mean da/a=(-0.543+/-0.116)x10^-5, representing
4.7-sigma evidence for a smaller weighted mean alpha in the absorption clouds.
Assuming that da/a=0 at z_abs=0, the data marginally prefer a linear increase
in alpha with time: dota/a=(6.40+/-1.35)x10^-16 yr^-1. The two-point
correlation function for alpha is consistent with zero over 0.2-13 Gpc comoving
scales and the angular distribution of da/a shows no significant dipolar
anisotropy. We therefore have no evidence for spatial variations in da/a. We
extend our previous searches for possible systematic errors, identifying
atmospheric dispersion and isotopic structure effects as potentially the most
significant. However, overall, known systematic errors do not explain the
results. Future many-multiplet analyses of QSO spectra from different
telescopes and spectrographs will provide a now crucial check on our Keck/HIRES
results.Comment: 31 pages, 25 figures (29 EPS files), 8 tables. Accepted by MNRAS.
Colour versions of Figs. 6, 8 & 10 and text version of Table 3 available at
http://www.ast.cam.ac.uk/~mim/pub.htm
Highly Variable Extinction and Accretion in the Jet-driving Class I Type Young Star PTF 10nvg (V2492 Cyg, IRAS 20496+4354)
We report extensive new photometry and spectroscopy of the highly variable
young stellar object PTF 10nvg including optical and near-infrared time series
data as well as mid-infrared and millimeter data. Following the previously
reported 2010 rise, during 2011 and 2012 the source underwent additional
episodes of brightening and dimming events including prolonged faint states.
The observed high-amplitude variations are largely consistent with extinction
changes having a 220 day quasi-periodic signal. Spectral evolution includes not
only changes in the spectral slope but correlated variation in the prominence
of TiO/VO/CO bands and atomic line emission, as well as anticorrelated
variation in forbidden line emission which, along with H_2, dominates optical
and infrared spectra at faint epochs. Neutral and singly-ionized atomic species
are likely formed in an accretion flow and/or impact while the origin of
zero-velocity atomic LiI 6707 in emission is unknown. Forbidden lines,
including several rare species, exhibit blueshifted emission profiles and
likely arise from an outflow/jet. Several of these lines are also seen
spatially offset from the continuum source position, presumably in a shocked
region of an extended jet. CARMA maps resolve on larger scales a spatially
extended outflow in mm-wavelength CO. We attribute the observed photometric and
spectroscopic behavior in terms of occultation of the central star as well as
the bright inner disk and the accretion/outflow zones that renders shocked gas
in the inner part of the jet amenable to observation at the faint epochs. We
discuss PTF 10nvg as a source exhibiting both accretion-driven (perhaps
analogous to V1647 Ori) and extinction-driven (perhaps analogous to UX Ori or
GM Cep) high-amplitude variability phenomena.Comment: accepted to AJ - in press (74 pages
Near Infrared Spectroscopy of the Ultracompact HII region G45.12+0.13
We present complete, low resolution IJHK spectroscopy of the ultracompact HII
region, G45.12+0.13. From the observed HI line strengths, we derive a near
infrared extinction law that is slightly steeper than the average. After
correction with this extinction law, we find good agreement between the
observed line ratios of HeI, Fe+, Fe++, S+ and S++ and the available atomic
data. Our data show that the density within the core of G45.12+0.13 must be at
least 10^4/cm^3. This is consistent with the known radio structure of the HII
region and in considerable disagreement with previous work using mid and far
infrared lines. There must also be considerable opacity in the HeI 2 3P-2 3S
transition, and we show how the observed strengths of the other HeI lines are
consistent with this. From modelling the photoionisation structure, we find
good agreement with most of the observed data if the hottest star present has
Teff < 42000K. Consideration of the helium ionisation state places a lower
limit on this value so that we can also constrain Teff > 38000K. Discrepancies
still exist between some of the observed and model line ratios, but the most
obvious tend to be the mid-IR observations.Comment: 17 pages Latex source, 5 postscript figures and macros. gzipped tar
file. TO appear in Monthly Notices of the Royal Astronimcal Society. Also
available by anonymous ftp from ftp://aaoepp.aao.gov.au/local/sll/g45.uu
(uuencoded gzipped tar file
Physical Conditions in Shocked Interstellar Gas Interacting with the Supernova Remnant IC 443
We present the results of a detailed investigation into the physical
conditions in interstellar material interacting with the supernova remnant IC
443. Our analysis is based on a comprehensive examination of high-resolution
far-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph
onboard the Hubble Space Telescope of two stars behind IC 443. One of our
targets (HD 43582) probes gas along the entire line of sight through the
supernova remnant, while the other (HD 254755) samples material located ahead
of the primary supernova shock front. We identify low velocity quiescent gas in
both directions and find that the densities and temperatures in these
components are typical of diffuse atomic and molecular clouds. Numerous high
velocity components are observed in the absorption profiles of neutral and
singly-ionized atomic species toward HD 43582. These components exhibit a
combination of greatly enhanced thermal pressures and significantly reduced
dust-grain depletions. We interpret this material as cooling gas in a
recombination zone far downstream from shocks driven into neutral gas clumps.
The pressures derived for a group of ionized gas components at high positive
velocity toward HD 43582 are lower than those of the other shocked components,
pointing to pressure inhomogeneities across the remnant. A strong very high
velocity component near -620 km/s is seen in the absorption profiles of
highly-ionized species toward HD 43582. The velocity of this material is
consistent with the range of shock velocities implied by observations of soft
thermal X-ray emission from IC 443. Moderately high-velocity gas toward HD
254755 may represent shocked material from a separate foreground supernova
remnant.Comment: 88 pages, 27 figures, accepted for publication in Ap
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