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Concept# Céphéide

Résumé

thumb|Une céphéide variant sur une période de quelques jours.
Une céphéide est une étoile variable, géante ou supergéante jaune, de 4 à 15 fois plus massive que le Soleil et de plus lumineuse, dont l'éclat varie de 0,1 à 2 magnitudes selon une période bien définie, comprise entre 1 et 135 jours, d'où elle tire son nom d'étoile variable. Elles ont été nommées d'après le prototype que constitue l'étoile δ de la constellation de Céphée. L'Étoile polaire est une céphéide (du moins jusqu'en 1994 où il est apparu que son éclat était devenu stable, sans explication à ce changement — voir Alpha Ursae Minoris).
Histoire
L'archétype des céphéides est δ Cephei dans la constellation de Céphée. Elle est découverte variable par John Goodricke en .
Dès 1897, Michel Luizet, de l'Observatoire de Lyon, étudie les étoiles variables ; avec plus de observations à son actif, il présente une thèse sur « les Céphéides considérées comme étoiles doubles, avec une monographie de l’étoile variable

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Galaxie

vignette|redresse=1.5|M51, la Galaxie du Tourbillon, un exemple typique de galaxie spirale.
Une galaxie est une structure cosmique formée par le rassemblement d'étoiles et de leurs planètes éventuel

Étoile

vignette|Le Soleil, l’étoile la plus proche de la Terre, vu lors d'une éruption en ultraviolets avec de fausses couleurs.
Une étoile est un corps céleste plasmatique qui rayonne sa propre lumière par

Voie lactée

La Voie lactée, aussi nommée la Galaxie (avec une majuscule), est une galaxie spirale barrée qui comprend entre 200 et d'étoiles, et sans doute plus de de planètes. Elle abrite le Système solaire e

Cours associés (7)

PHYS-209: Astrophysics I: introduction to astrophysics

Ce cours décrit de façon simple les processus physiques qui expliquent l'univers dans lequel nous vivons. En couvrant une large gamme de sujets, le but du cours est aussi de donner un aperçu général des objets astrophysiques qui nous entourent.

PHYS-101(h): General physics : mechanics (SV I)

Le but du cours de physique générale est de donner à l'étudiant les notions de base nécessaires à la compréhension des phénomènes physiques. L'objectif est atteint lorsque l'étudiant est capable de prévoir quantitativement les conséquences de ces phénomènes avec des outils théoriques appropriés.

PHYS-402: Astrophysics IV : observational cosmology

Cosmology is the study of the structure and evolution of the universe as a whole. This course describes the principal themes of cosmology, as seen
from the point of view of observations.

Séances de cours associées (7)

Masses of classical Cepheids of 3-11 M are predicted by theory but those measured clump between 3.6-5 M. As a result, their mass-luminosity relation is poorly constrained, impeding our understanding of basic stellar physics and the Leavitt Law. All Cepheid masses come from the analysis of 11 binary systems, including only five that are double lined and well suited for accurate dynamical mass determination. We present a project to analyze a new, numerous group of Cepheids in double-lined binary (SB2) systems to provide mass determinations in a wide mass interval and study their evolution. We analyze a sample of 41 candidate binary LMC Cepheids spread along the P-L relation, which are likely accompanied by luminous red giants, and present indirect and direct indicators of their binarity. In a spectroscopic study of a subsample of 18 brightest candidates, for 16 we detected lines of two components in the spectra, already quadrupling the number of Cepheids in SB2 systems. Observations of the whole sample may thus lead to quadrupling all the Cepheid mass estimates available now. For the majority of our candidates, erratic intrinsic period changes dominate over the light-travel-time effect due to binarity. However, the latter may explain the periodic phase modulation for four Cepheids. Our project paves the way for future accurate dynamical mass determinations of Cepheids in the LMC, Milky Way, and other galaxies, which will potentially increase the number of known Cepheid masses even 10-fold, hugely improving our knowledge about these important stars.

2021The Hubble constant H0 is one of the most important parameters in cosmology, as it encodes the age of the Universe and is necessary for any distance determination at a cosmological scale. It is, however, only poorly constrained by traditional methods. The current favored value, H0 = 72±8 km s-1 Mpc-1, is provided by the HST Hubble constant Key Project (Freedman et al. 2001), which combines several Cepheid-calibrated distance indicators. This roughly 10% error nevertheless denotes only the statistical uncertainty in the determination of H0, while the possible systematical errors in the first step of the distance ladder (the distance to the Large Magellanic Cloud) may be of the same order of magnitude. Time delays between gravitationally lensed images of distant quasars can yield a more precise measurement of the Hubble constant, on a truly cosmic scale, and independently of any local distance calibrator. At the beginning of this thesis, time delays had been measured in only ten lensed systems, nine of which gave H0 estimates. However before 2004, no concerted and long term action has succeeded to apply the time delay method at a level of precision really competitive with other techniques. The major difficulties arise from the modeling of the lens mass distribution, and from the uncertainty on the time delay measurement itself, which was typically of about 10% in past monitoring programs. COSMOGRAIL (COSmological MOnitoring of GRAvItational Lenses) is an international collaboration initiated in April 2004 at the Laboratory of Astrophysics of EPFL, and which aims at measuring precise time delays for most known lensed quasars, in order to determine the Hubble constant down to an uncertainty of a few percent. This thesis took place at the beginning of COSMOGRAIL and consisted in setting up this large photometric monitoring. It addressed both issues of carrying out accurate photometry of faint blended sources and of obtaining well sampled light curves, in order to measure precise time delays. As part of the COSMOGRAIL project, I have been managing the monitoring of over twenty gravitationally lensed quasars with the 1-2m telescopes involved both in the Northern and Southern hemispheres, and organizing the data. The first crucial work of this thesis was then to develop an automated reduction pipeline able to produce an homogeneous data set from images acquired with very different telescopes. This pipeline was also needed to perform aperture photometry of all lensed quasars, in order to study their variability and define the monitoring priorities. The powerful MCS deconvolution algorithm (Magain, Courbin, & Sohy 1998) was greatly used in this work and allowed to highly improve the image resolution, with the aim of obtaining accurate photometric measurements of the individual quasar lensed images. I have finally tested and improved three different numerical techniques to determine time delays between the quasar components from their light curves. In this thesis, time delays have been determined in four systems. The first one was measured in the doubly imaged quasar SDSS J1650+4251, after two years of monitoring with the 1.5m telescope of Maidanak Observatory, in Uzbekistan. The quadruply lensed system RXS J1131–1231 was then studied and three time delays determined from 3-year observations with the Swiss Euler 1.2m telescope located at La Silla, in Chile. The photometric monitoring of the quadruple WFI J2033–4723 was also carried out with the Euler telescope, and data were then merged with those obtained by a second monitoring group, with the SMARTS 1.3m telescope at the Cerro Tololo Interamerican Observatory (CTIO), also located in Chile. Two time delays were measured in this system, after three years of observations, the close pair A1 – A2 remaining unresolved. Three time delays were determined in the quadruply imaged quasar HE0435–1223, after four years of optical monitoring with Euler, Mercator and Maidanak telescopes, to which photometric measurements by SMARTS 1.3m telescope were added. Euler and SMARTS merged data for the doubly imaged quasar QJ0158–4325 were also analysed, the size of the source accretion disk was measured, but we failed to determine a time delay due to the high amplitude of the microlensing variability in this system. The accuracies on time delay measurements reached in this thesis are of the order of 3-4% and show a clear improvement from the typical 10% uncertainties of past monitoring programs. These results were finally converted into estimates of the Hubble constant following different models of the lensing mass potential. The H0 mean value obtained when considering the individual determinations from twelve gravitationally lensed quasars with known time delays is H0 = 60 ± 7 km s-1 Mpc-1. This result is consistent with the current favored value, and above all promising, as including additional systems to this ensemble will surely provide tighter bounds on H0. In conclusion, the increasing number of time delay measurements and improvements in lens modeling should reduce the errors on the Hubble constant estimate provided by gravitational lensing. Conversely, the determination of more time delays should put further constraints on lens galaxy density profiles when using a prior on H0 from other studies.

David Andrew Barry, Caitlin Ellen Barry, Nicolas Le Dantec, Ulrich Lemmin, Amir Mehdi Razmi

Lake Geneva (length 74 km on the long east-west axis, surface area 562 km2, volume 89 km3) is a freshwater lake bordered by Switzerland and France. The lake’s hydrodynamics are forced principally by wind and seasonality, with inflows and the Coriolis effect playing relatively minor roles. Of the major forcings, wind is highly variable due to the rapid changes in topography around the lake, with mountains in the east and relatively gentle landscapes in the west. Numerous field investigations have revealed that the lake’s currents, which are dominated by the wind, are likewise highly variable. In particular, analysis of field measurements of Lake Geneva’s wind and currents found that the lake’s currents during the summer stratification period are consistent with diurnal winds and long-fetch synoptic events. Obviously, a quantitative understanding of the wind forcing is a prerequisite for evaluating the current patterns in the lake. Hourly wind patterns (produced using the non-hydrostatic, fully compressible COSMO model) at 10 m above the lake were provided by MeteoSuisse (the Swiss meteorological service) on a 2.2 km2 grid for 2009-2010. These patterns were categorized using the k-means data-mining method, with each pattern assigned an arbitrary integer index 1, 2, 3, etc., along with the pattern's frequency. For later use, all wind fields corresponding to a given pattern were grouped into bins. It was found that the index frequencies could be approximated by a Poisson distribution with a characteristic temporal autocorrelation time of around 15-20 hours. More specifically, the wind pattern autocorrelation has an initial rapid, power law-like decline (~αt, where α ≈ 0.8 and t is the lag in hours) for about 24 hours, then a slow decay. The main features of this behavior (Poisson process with a power-law autocorrelation) were captured by an integer auto-regressive process, the INAR(1) model. This model was used as a stochastic generator of wind-pattern indices, i.e., the INAR(1) model produces a sequence of integers, each of which corresponds to a wind pattern. For a given index, the aforementioned binned COSMO wind fields were sampled randomly to produce the stochastic wind-field sequence.

2011