Les étoiles de petite masse comme traceurs des populations de la voie lactée : étude des effets de la métallicité dans les atmosphères froides

Very Low-Mass stars (M dwarfs) are an important source of information for probing the lowmass end of the main sequence, down to the hydrogen burning limit. They are the dominantstellar component of the Galaxy and make up the majority of baryonic matter in the Galaxy.Moreover, an increasing number of M dwarfs are now known to host exoplanets, includingsuper-Earth exoplanets. The determination of accurate fundamental parameters for M dwarfshas therefore relevant implications for both stellar and Galactic astronomy as well as planetology.Despite their large number in the Galaxy, M dwarfs remain elusive objects and themodelling of their photosphere has long remained a challenge (molecular opacities, dust cloudformation). The description of these stars therefore need a strong empirical basis, or validation.In particular, the effect of metallicity on the physics of cool atmospheres are still poorly known,even for early-type M-dwarfs.[...]Les étoiles de petite masse (naines M) forment la composante stellaire dominante de notre galaxie et contribuent à la majeure partie de la matière baryonique dans la galaxie. Notre compréhension de la galaxie repose donc sur la connaissance de cette composante peu lumineuse. Par ailleurs, un nombre grandissant d’exoplanètes sont découverte autour de naines M, y compris des super-terres. L’étude des naines M a ainsi des implications importantes sur la physique stellaire, permettant de comprendre les processus en jeu dans ces atmosphères froides. Ces astres restent cependant peu connus du fait de leur faible luminosité intrinsèque. La description de ces étoiles nécessite une validation empirique fort, notamment en ce qui concerne les effets de l’abondance chimique sur la physique des atmosphères froides.Le but de cette thèse est de déterminer les paramètres fondamentaux des naines M et de tester les modèles d’atmosphères d’étoiles froides.Dans la première partie nous comparons des spectres calibrés dans le visible et l’infrarouge des composantes de système triple LHS 1070 avec des spectres synthétiques. Cette étude permet d’améliorer notre compréhension de la formation de la poussière dans les atmosphères très froides. Ce travail est étendu à un échantillon de naines M et permet de déterminer l’échelle de température des naines M. La seconde partie présente l’analyse des spectres à haute résolution de 21 naines M de faible métallicité (sous naines). Nous analysons en détail les signature moléculaires et atomiques dans le spectre. La comparaison avec des spectres synthétiques permet de mesurer les paramètres stellaires des étoiles et de déterminer avec précision leur métallicité. Ce travail doit initier des recherches futures. En particulier nous voulons étendre cette étude dans le domaine proche-infrarouge pour s’assurer que les résultats obtenus sont cohérents du domaine optique jusqu’à l’infrarouge proche

The effect of general anaesthesia versus conscious sedation in dosimetric distribution of intracavitary radiotherapy in cervical cancer patients

Background: Majority of Indian patients presents in locally advanced stage and most of them treated by combination of external teletherapy and intracavitary brachytherapy (ICRT). Because of deficient infrastructure, the waiting period is generally long at existing caner canters. Hence ICRT may be done in conscious sedation to treat more patients by avoiding time consuming general anaesthesia. The aim of this study is to know the effect of general anaesthesia vs. conscious sedation in dosimetric distribution in brachytherapy and its feasibility.Methods: Total 80 ICRT applications were randomized to general anaesthesia (GA) and conscious sedation (CS) groups. Fletcher suit type of applicators was used and dose delivery equipment was cobalt 60 high dose rate remote after loading brachytherapy unit. In CS group, injection midazolam 0.5-8mg (median 2.5mg) in the form of slow i.v. infusion was used along with antiemetic support.Results: Total 6 parameters were analyzed. e.g., Dose to point A1, Dose to point A2, Bladder max dose, Bladder mean dose, Rectal max dose and Rectal mean dose. The dose distribution was found similar both   groups and it did not depend on type of anaesthesia.Conclusions: The high volume centers of developing countries are most suitable candidate to opt conscious sedation to perform ICRT to treat more cancer cervix patients in same time frame

Comparison of 2D and 3D gamma evaluation method in patient specific intensity-modulated radiotherapy quality assurance

Background: In this study we have compared 2D and 3D gamma pass percentage for a variety of acceptance criteria for 40 step-and-shoot IMRT (intensity-modulated radiotherapy) plans. Methods: Treatment planning was done for 40 patient including head and neck, abdomen and pelvis simulated on the Siemens Healthcare GmBH CT simulator with images of 3 mm slice thickness using treatment planning system (TPS) (Monaco Version 5.11.03, Elekta medical system) using Monte Carlo algorithm. The gamma evaluation was done using PTW VeriSoft 8.1 which allowed us to perform 2D and 3D gamma index calculation, slice-by-slice comparison of measured and calculated dose distributions, measured dose was compared against the calculated DICOMRT dose on the OCTAVIUS 3D phantom from TPS. Results: The average 3D and 2D gamma passing in coronal planes were 96.61±0.45% and 96.27±0.78% for 5 mm/5% criteria, 93.74±4.17% and 91.9±4.88% for 3 mm/3% criteria, 85.83±7.58% and 82.41±8.06% for 2 mm/2% criteria and 62.8±9.42% and 59.18±9.52% for 1 mm/1% criteria respectively for all cases. The average gamma passing rate for 3D gamma analysis was 0.35%, 1.97 %, 3.97% and 5.78% higher when compared with 2D coronal planar analyses for 5 mm/5%, 3 mm/3%, 2 mm/2% and 1 mm/1% DTA criteria respectively. Conclusions: It is concluded in the study that 3 D gamma passing rate is higher compared to 2D gamma passing for head and neck, abdomen and pelvis cases

Energy-Dependent Timing of Thermal Emission in Solar Flares

We report solar flare plasma to be multi-thermal in nature based on the theoretical model and study of the energy-dependent timing of thermal emission in ten M-class flares. We employ high-resolution X-ray spectra observed by the Si detector of the "Solar X-ray Spectrometer" (SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral evolution of the X-ray line and continuum emission flux F(\epsilon) from the flare by integrating a series of isothermal plasma flux. We find that multi-temperature integrated flux F(\epsilon) is a power-law function of \epsilon with a spectral index (\gamma) \approx -4.65. Next, based on spectral-temporal evolution of the flares we find that the emission in the energy range E= 4 - 15 keV is dominated by temperatures of T= 12 - 50 MK, while the multi-thermal power-law DEM index (\gamma) varies in the range of -4.4 and -5.7. The temporal evolution of the X-ray flux F(\epsilon,t) assuming a multi-temperature plasma governed by thermal conduction cooling reveals that the temperature-dependent cooling time varies between 296 and 4640 s and the electron density (n_e) varies in the range of n_e= (1.77-29.3)*10^10 cm-3. Employing temporal evolution technique in the current study as an alternative method for separating thermal from non-thermal components in the energy spectra, we measure the break-energy point ranging between 14 and 21\pm1.0 keV.Comment: Solar Physics, in pres

The very low mass multiple system LHS\,1070 -- a testbed for model atmospheres for the lower end of the main sequence

Accepted in Astronomy and Astrophysics, 15 pages, 11 figuresInternational audienceLHS1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres.This work aims at determining the fundamental stellar parameters of LHS1070 and to test recent model atmospheres: BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have performed a chi^2-minimization comparing well calibrated optical and infrared spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters Teff, radius, and log g for each of the three components of LHS1070

A SPACE-BASED UV-MISSION TO INVESTIGATE THE HISTORY OF STAR FORMATION IN THE UNIVERSE

• AEGIS – Advanced European Galaxy Imager and Spectrograph • Student project began in 2011 at the Alpbach Summerschool in Austria, and was continued at DLR Bremen (CEF). • Motivation: Lack of UV-Data. We want to cover the so called “redshift desert” to close the gap between low redshift and high redshift observations • Investigate the evolution of the star formation rate of the universe in the redshift range 0.5 < z < 1.5 (4Gyr – 8Gyr) to better understand the expected peak at z ≈