87 research outputs found
A geometrical origin for the covariant entropy bound
Causal diamond-shaped subsets of space-time are naturally associated with
operator algebras in quantum field theory, and they are also related to the
Bousso covariant entropy bound. In this work we argue that the net of these
causal sets to which are assigned the local operator algebras of quantum
theories should be taken to be non orthomodular if there is some lowest scale
for the description of space-time as a manifold. This geometry can be related
to a reduction in the degrees of freedom of the holographic type under certain
natural conditions for the local algebras. A non orthomodular net of causal
sets that implements the cutoff in a covariant manner is constructed. It gives
an explanation, in a simple example, of the non positive expansion condition
for light-sheet selection in the covariant entropy bound. It also suggests a
different covariant formulation of entropy bound.Comment: 20 pages, 8 figures, final versio
Geometric entropy, area, and strong subadditivity
The trace over the degrees of freedom located in a subset of the space
transforms the vacuum state into a density matrix with non zero entropy. This
geometric entropy is believed to be deeply related to the entropy of black
holes. Indeed, previous calculations in the context of quantum field theory,
where the result is actually ultraviolet divergent, have shown that the
geometric entropy is proportional to the area for a very special type of
subsets. In this work we show that the area law follows in general from simple
considerations based on quantum mechanics and relativity. An essential
ingredient of our approach is the strong subadditive property of the quantum
mechanical entropy.Comment: Published versio
Derivation and comparison of formulae for the adjustment of total calcium
Background: Free ionized calcium (Ca2+) is the biologically active component of total calcium (TCa) and hence responsible for its biological action. TCa is routinely adjusted for albumin using several formulae (e.g. James, Orell, Payne and Berry) to more closely reflect Ca2+. Here, we derive a novel formula to estimate Ca2+ and compare its performance to established formulae. Methods: Cohort for prediction of Ca2+: 2806 serum samples (TCa) taken contemporaneously with blood gas samples (Ca2+) at Imperial College Healthcare NHS Trust were used to derive formulae to estimate Ca2+ using multivariable linear regression. Cohort for prediction of PTH: Performance of novel and existing formulae to predict PTH in 5510 patients was determined by Spearman correlation. Results: Ca2+ prediction Cohort: Adjusted calcium (r2 = 0.269) was less strongly associated with Ca2+, than TCa (r2 = 0.314). Prediction of Ca2+ from a newly derived formula incorporating TCa, potassium, albumin, and hematocrit had an improved r2 of 0.327, whereas inclusion of all available parameters increased the r2 further to 0.364. Of the established formulae, James performed best in predicting Ca2+ (r2 = 0.27). PTH prediction cohort: Berry resulted in higher whereas Orell in lower adjusted calcium levels. Prediction of PTH was strongest in the setting of hypercalcemia, with James having the highest Spearman correlation coefficient (+0.496) similar to including all parameters (+0.499). Conclusion: Adjustment of calcium for albumin using established formulae does not always outperform unadjusted TCa in the reflection of Ca2+. Further prospective studies are needed to optimise adjustment of TCa and to establish bounds for validity
Development of a UHPLC-MS/MS (SRM) method for the quantitation of endogenous glucagon and dosed GLP-1 from human plasma
© 2017 Future Science Ltd. Aim: The performance of glucagon and GLP-1 immunoassays is often poor, but few sensitive LC-MS/MS methods exist as alternatives. Experimental: A multiplexed LC-MS/MS method using a 2D extraction technique was developed. Results: The method was established for the quantitation of endogenous glucagon (LLOQ: 15 pg/ml) and dosed GLP-1 (LLOQ: 25 pg/ml) in human plasma, and is the first such method avoiding immunoenrichment. Specificity of endogenous glucagon quantitation was assured using a novel approach with a supercharging mobile phase additive to access a sensitive qualifier SRM. Endogenous glucagon concentrations were within the expected range, and showed good reproducibility after extended sample storage. A cross-validation against established immunoassays using physiological study samples demonstrated some similarities between methods. Conclusion: The LC-MS/MS method offers a viable alternative to immunoassays for quantitation of endogenous glucagon, dosed glucagon and/or dosed GLP-1
A spectral survey of an ultra-hot Jupiter: Detection of metals in the transmission spectrum of KELT-9 b
Context: KELT-9 b exemplifies a newly emerging class of short-period gaseous
exoplanets that tend to orbit hot, early type stars - termed ultra-hot
Jupiters. The severe stellar irradiation heats their atmospheres to
temperatures of K, similar to the photospheres of dwarf stars. Due
to the absence of aerosols and complex molecular chemistry at such
temperatures, these planets offer the potential of detailed chemical
characterisation through transit and day-side spectroscopy. Studies of their
chemical inventories may provide crucial constraints on their formation process
and evolution history.
Aims: To search the optical transmission spectrum of KELT-9 b for absorption
lines by metals using the cross-correlation technique.
Methods: We analyse 2 transits observed with the HARPS-N spectrograph. We use
an isothermal equilibrium chemistry model to predict the transmission spectrum
for each of the neutral and singly-ionized atoms with atomic numbers between 3
and 78. Of these, we identify the elements that are expected to have spectral
lines in the visible wavelength range and use those as cross-correlation
templates.
Results: We detect absorption of Na I, Cr II, Sc II and Y II, and confirm
previous detections of Mg I, Fe I, Fe II and Ti II. In addition, we find
evidence of Ca I, Cr I, Co I, and Sr II that will require further observations
to verify. The detected absorption lines are significantly deeper than model
predictions, suggesting that material is transported to higher altitudes where
the density is enhanced compared to a hydrostatic profile. There appears to be
no significant blue-shift of the absorption spectrum due to a net day-to-night
side wind. In particular, the strong Fe II feature is shifted by km~s, consistent with zero. Using the orbital velocity of the
planet we revise the steller and planetary masses and radii.Comment: Submitted to Astronomy and Astrophysics on January 18, 2019. Accepted
on May 3, 2019. 26 pages, 11 figure
Stellar surface magneto-convection as a source of astrophysical noise II. Center-to-limb parameterisation of absorption line profiles and comparison to observations
Manifestations of stellar activity (such as star-spots, plage/faculae, and
convective flows) are well known to induce spectroscopic signals often referred
to as astrophysical noise by exoplanet hunters. For example, setting an
ultimate goal of detecting true Earth-analogs demands reaching radial velocity
(RV) precisions of ~9 cm/s. While this is becoming technically feasible with
the latest generation of highly stabilised spectrographs, it is astrophysical
noise that sets the true fundamental barrier on attainable RV precisions. In
this paper we parameterise the impact of solar surface magneto-convection on
absorption line profiles, and extend the analysis from the solar disc centre
(Paper I) to the solar limb. Off disc-centre, the plasma flows orthogonal to
the granule tops begin to lie along the line-of-sight and those parallel to the
granule tops are no longer completely aligned with the observer. Moreover, the
granulation is corrugated and the granules can block other granules, as well as
the intergranular lane components. Overall, the visible plasma flows and
geometry of the corrugated surface significantly impact the resultant line
profiles and induce centre-to-limb variations in shape and net position. We
detail these herein, and compare to various solar observations. We find our
granulation parameterisation can recreate realistic line profiles and induced
radial velocity shifts, across the stellar disc, indicative of both those found
in computationally heavy radiative 3D magnetohydrodynamical simulations and
empirical solar observations.Comment: 17 pages, 14 figures, accepted to Ap
Unsigned magnetic flux as a proxy for radial-velocity variations in Sun-like stars
We estimate disc-averaged RV variations of the Sun over the last magnetic
cycle, from the single Fe I line observed by SDO/HMI, using a physical model
for rotationally modulated magnetic activity that was previously validated
against HARPS-N solar observations. We estimate the disc-averaged, unsigned
magnetic flux and show that a simple linear fit to it reduces the RMS of RV
variations by 62%, i.e. a factor of 2.6. We additionally apply the FF' method,
which predicts RV variations based on a star's photometric variations. At cycle
maximum, we find that additional physical processes must be at play beyond
suppression of convective blueshift and velocity imablances resulting from
brightness inhomogeneities, in agreement with recent studies of solar RV
variations. By modelling RV variations over the magnetic cycle using a linear
fit to the unsigned magnetic flux, we recover injected planets at an orbital
period of about 300 days with RV semi-amplitudes down to 0.3 m/s. To reach
semi-amplitudes of 0.1 m/s, we will need to identify and model additional
physical phenomena that are not well traced by the unsigned magnetic flux or
FF'. The unsigned magnetic flux is an excellent proxy for rotationally
modulated, activity-induced RV variations, and could become a key tool in
confirming and characterising Earth analogs orbiting Sun-like stars. The
present study motivates ongoing and future efforts to develop observation and
analysis techniques to measure the unsigned magnetic flux at high precision in
slowly rotating, relatively inactive stars like the Sun.Comment: 25 pages, 11 figures, 3 tables, submitted to Ap
Rotation of planet-harbouring stars
The rotation rate of a star has important implications for the detectability,
characterisation and stability of any planets that may be orbiting it. This
chapter gives a brief overview of stellar rotation before describing the
methods used to measure the rotation periods of planet host stars, the factors
affecting the evolution of a star's rotation rate, stellar age estimates based
on rotation, and an overview of the observed trends in the rotation properties
of stars with planets.Comment: 16 pages, 4 figures: Invited review to appear in 'Handbook of
Exoplanets', Springer Reference Works, edited by Hans J. Deeg and Juan
Antonio Belmont
A spectral survey of an ultra-hot Jupiter
Context. KELT-9 b exemplifies a newly emerging class of short-period gaseous exoplanets that tend to orbit hot, early type stars â termed ultra-hot Jupiters. The severe stellar irradiation heats their atmospheres to temperatures of ~4000 K, similar to temperatures of photospheres of dwarf stars. Due to the absence of aerosols and complex molecular chemistry at such temperatures, these planets offer the potential of detailed chemical characterization through transit and day-side spectroscopy. Detailed studies of their chemical inventories may provide crucial constraints on their formation process(es) and evolution history.
Aims. We aim to search the optical transmission spectrum of KELT-9 b for absorption lines by metals using the cross-correlation technique.
Methods. We analysed two transit observations obtained with the HARPS-N spectrograph. We used an isothermal equilibrium chemistry model to predict the transmission spectrum for each of the neutral and singly ionized atoms with atomic numbers between three and 78. Of these, we identified the elements that are expected to have spectral lines in the visible wavelength range and used those as cross-correlation templates.
Results. We detect (>5Ï) absorption by NaâŻI, CrâŻII, ScâŻII and YâŻII, and confirm previous detections of MgâŻI, FeâŻI, FeâŻII, and TiâŻII. In addition, we find evidence of CaâŻI, CrâŻI, CoâŻI, and SrâŻII that will require further observations to verify. The detected absorption lines are significantly deeper than predicted by our model, suggesting that the material is transported to higher altitudes where the density is enhanced compared to a hydrostatic profile, and that the material is part of an extended or outflowing envelope. There appears to be no significant blue-shift of the absorption spectrum due to a net day-to-night side wind. In particular, the strong FeâŻII feature is shifted by 0.18 ± 0.27 km sâ1, consistent with zero. Using the orbital velocity of the planet we derive revised masses and radii of the star and the planet: M* = 1.978 ± 0.023 Mâ, R* = 2.178 ± 0.011 Râ, mp = 2.44 ± 0.70 MJ and Rp = 1.783 ± 0.009 RJ
The Rossiter-McLaughlin effect in Exoplanet Research
The Rossiter-McLaughlin effect occurs during a planet's transit. It provides
the main means of measuring the sky-projected spin-orbit angle between a
planet's orbital plane, and its host star's equatorial plane. Observing the
Rossiter-McLaughlin effect is now a near routine procedure. It is an important
element in the orbital characterisation of transiting exoplanets. Measurements
of the spin-orbit angle have revealed a surprising diversity, far from the
placid, Kantian and Laplacian ideals, whereby planets form, and remain, on
orbital planes coincident with their star's equator. This chapter will review a
short history of the Rossiter-McLaughlin effect, how it is modelled, and will
summarise the current state of the field before describing other uses for a
spectroscopic transit, and alternative methods of measuring the spin-orbit
angle.Comment: Review to appear as a chapter in the "Handbook of Exoplanets", ed. H.
Deeg & J.A. Belmont
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