Apparent magnitude | EPFL Graph Search

Apparent magnitude (m) is a measure of the brightness of a star or other astronomical object. An object's apparent magnitude depends on its intrinsic luminosity, its distance, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer. The word magnitude in astronomy, unless stated otherwise, usually refers to a celestial object's apparent magnitude. The magnitude scale dates back to the ancient Roman astronomer Claudius Ptolemy, whose star catalog listed stars from 1st magnitude (brightest) to 6th magnitude (dimmest). The modern scale was mathematically defined in a way to closely match this historical system. The scale is reverse logarithmic: the brighter an object is, the lower its magnitude number. A difference of 1.0 in magnitude corresponds to a brightness ratio of \sqrt[5]{100}, or about 2.512. For example, a star of magnitude 2.0 is 2.512 times as bright as a star of magnitude 3.0, 6.31 times as bri

Future trends in wind resources and their consistency in the Indian sub-continent

Michael Lehning, Adrien Michel, Yasmine Zakari

We use the latest Coordinated Regional Dynamical Experiment regional climate models over South Asia at 0.22 degrees grid spacing (WAS-22) under two Representative Concentration Pathways scenarios (RCP 2.6 and RCP 8.5) to evaluate their performance against their predecessor at 0.44 degrees grid spacing (WAS-44) and investigate the impact of climate change on future wind resources in the Indian sub-continent. The comparison of WAS-22 with the previous WAS-44 regional climate models looks at their ability to reproduce annual and seasonal wind speed climatology and trends with respect reanalysis datasets and finds that WAS-22 models achieve the best results. Hence, we use WAS-22 models as a source of climate information, correlate them to local station measurements and wind farm operation data at three sites in the study domain. We then estimate future annual and seasonal wind trends in the Indian sub-continent. Our findings on future trends suggest increasing annual wind speeds in the north-west (+ 0.04 m/s per decade (+/- 8%)), the center (+ 0.03 m/s per decade (+/- 11%)) and the east (+ 0.02 m/s per decade (+/- 19%)) of the Indian sub-continent under the RCP 8.5 scenario at the end of the century, with changes occurring more substantially during the last 30 years of the century. The results project future perturbations during the pre-monsoon season occurring throughout India and trends with the highest magnitude concentrated during the monsoon season. An additional contribution of this investigation is a decision-making tool enabling the quantification of the spatio-temporal consistency of wind trends using a structure-function.

ELSEVIER2022

Electronic properties of nano-crystalline titanium dioxide thin films

Alain Bally

Titanium dioxide is a cheap, chemically stable and non-toxic material. However its electrical properties are unstable and it is a modest semiconductor and a mediocre insulator. For several applications it would be interesting to make it either more insulating or more conducting. The goal of this work was to modify the electrical properties of nano-crystalline TiO2 thin films deposited by reactive sputtering and to understand the mechanism leading to these modifications. The principal factors that influence the electrical conductivity are on the one hand the concentration and nature of the chemical impurities incorporated in TiO2, and on the other hand the morphology of the thin films. The study was split into two parts. The fist part describes the modifications of the sputtered material obtained by chemical doping of the TiO2 thin films. Niobium, cerium, iron, and fluorine were incorporated successfully in TiO2. The second part describes the modifications obtained by modifying the reactive gas used during the sputtering process: thus oxygen was substituted with water vapor. Several analysis techniques have been used to characterize the TiO2 thin films. They are essentially divided in four categories. The chemical analyses included electron probe microanalyses, x-ray photoemission spectroscopy, and secondary ion mass spectrometry. The structure and morphology analyses of thin films were carried out with x-ray diffraction and atomic force microscopy. The electrical properties in dc or ac mode were measured between room temperature and 350°C. Finally optical transmission provided information on the electronic states and morphology of the thin films. With an appropriate choice of impurities, titanium dioxide can be made more insulating (TiO2:Ce), more conducting with an n-type electrical conductivity (TiO2:Nb) or p-type electrical conductivity (TiO2:Fe). Chemical doping is also the cause of important structure and morphology changes, for example it can force the transformation from the anatase to the rutile structure. It is shown in particular that dopant atoms generate defects such as oxygen vacancies. These defects impede the variation of the electrical conductivity produced by the impurities, thus, in spite of dopant concentration in the percent range, the variation of the electrical conductivity is an increase or a decrease of three order of magnitude at most when compared with undoped TiO2. Thin films deposited with water vapor as the reactive gas present an increase in the electrical conductivity by height order of magnitude compared to samples prepared with oxygen in similar conditions. No atom other than titanium or oxygen could be detected in the thin films in significant concentration. The dramatic conductivity increase is due to the injection inside TiO2 grains of electrons donated by unsaturated titanium atoms lying on the grain surface. A nanometric grain size is a requirement to make such high doping levels possible. Depending on the impurities or reactive gas selected, titanium dioxide can be made either into a good insulator with a high breakdown field and a high permittivity or into a reasonably conducting, transparent film. The electrical conductivity of the samples prepared for this study cover the range from 10-10 to 103 S m-1.

EPFL1999

Measurement of time-dependent CP violation in B0->Dpi decays and optimisation of flavour tagging algorithms at LHCb

Vincenzo Battista

This thesis presents the results of a time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays using

3~\rm fb^{-1}

of proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider during Run 1 with a centre-of-mass energy of

7

(2011) and

8

(2012) TeV. The LHCb experiment is dedicated to the study of the properties of

b

-flavoured hadrons, in particular

CP

violation in the

B

meson system. The Standard Model of Particle Physics describes very precisely the mechanism and the amount of

CP

violation expected in the Universe. However, the observed matter-antimatter asymmetry is larger by several order of magnitude compared to the predictions. This could be explained by the existence of a new source of

CP

violation, originating in New Physics beyond the Standard Model. The time-dependent analysis of

B^0\to D^{\mp}\pi^{\pm}

decays provides constraints on the angle

\gamma

of the Unitarity Triangle, one of the fundamental parameters of the Standard Model related to

CP

violation. Since no sizeable high-order Standard Model processes are expected to contribute, any deviation from the predictions would be an unambiguous signature of New Physics. The current experimental precision on

\gamma

is significantly lower than that of theoretical predictions. This motivates the effort for new experimental determinations of

\gamma

in order to reduce its uncertainty. The analysis of

\Bz\to\Dmp\pipm

decays, although not as sensitive as that obtained from decays of charged

B

mesons into

D^{(*)0}K^{(*)+}

final states, represents an independent and uncorrelated estimation of

\gamma

that contributes to the global combination of all

\gamma

measurements. The result obtained in this thesis is more precise than previous determinations from other experiments (BaBar, Belle) using

B^0\to D^{\mp}\pi^{\pm}

decays. Although based on a very large sample of about half a million signal events, it is still dominated by statistical uncertainties, indicating good prospects for future improvements in precision with more data from Run 2 and beyond. In addition to the

B^0\to D^{\mp}\pi^{\pm}

analysis, this thesis also summarizes the studies to improve the performances of the flavour tagging algorithms used by the LHCb collaboration to infer the flavour of neutral

B

mesons in time-dependent analyses. The performance of these algorithms, being correlated with the kinematics of the reconstructed particles as well as the complexity of the event (number of tracks and primary vertices), showed a significant decrease on Run 2 data (2015--2018), which were collected at a centre-of-mass energy of

13~\rm TeV

. Thanks to new implementations, these algorithms now have a performance similar to that obtained with Run 1 data.

EPFL2018