199 research outputs found
A new TiO line list
No full textA new line list for 9 electronic transitions of TiO (alpha, beta, gamma, gamma', delta, epsilon, phi, a-f, and E-B) is presented and made publicly available. It is the latest and much improved version of a suite of TiO line lists inaugurated in Plez et al</p
Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O-M stars
No full textBroad band colors and bolometric corrections in the Johnson-Cousins-Glass system (Bessell, 1990; Bessell & Brett, 1988) have been computed from synthetic spectra from new model atmospheres of Kurucz (1995a), Castelli (1997), Plez, Brett & Nordlund (1992),</p
Cool star model atmospheres for Gaia : ATLAS, MARCS, and PHOENIX
No full textInternational audienceI present the widely used model atmosphere codes ATLAS, MARCS, and PHOENIX, and I compare their output model structures and spectra for cool stars of FGKM-types. While model atmosphere stratifications agree closely with each other in the f-D approximation, this is not the case for spectra. Differences between model spectra from different codes are largest in the blue-UV, but smaller differences appear in all regions, especially in the molecular features of cooler model spectra. I recommend the groups to try to solve these discrepancies together. In the meantime, users must be careful when using these spectra in regimes where they differ. I discuss here only comparisons of spectra at solar metallicity, and this should be extended to other metallicities. Detailed comparisons with carefully calibrated spectrophotometric data, and high resolution spectra for stars with well known parameters are also of prime importance. It appears that we still need better line positions for molecules. Finally we should remember that f-D models are only a step towards a better representation of reality, and we should keep developing, and carefully test 3-D, NLTE models
Non-LTE iron abundance determination in cool stars : the role of hydrogen collisions
No full textLa détermination d'abondances stellaires très précises a toujours été et reste un point clé de toute analyse spectroscopique.Cependant, de nombreuses études ont montré que l'hypothèse de l'équilibre thermodynamique local (ETL), largement utilisée dans les analyses spectroscopiques est inadéquate pour déterminer les abondances et les paramètres stellaires des étoiles géantes et pauvres en métaux où les effets hors-ETL dominent. C'est pourquoi, une modélisation hors-ETL des spectres stellaires est cruciale afin de reproduire les observations et ainsi déterminer avec précision les paramètres stellaires.Cette modélisation hors-ETL nécessite l'utilisation d'un grand jeu de données atomiques, qui ne sont pas toujours connues avec certitude. Dans les étoiles froides, les taux de collisions de l'atome d'hydrogène sont une des principales sources d'incertitudes. Ces taux sont souvent calculés en considérant une approche classique (l'approximation de Drawin) pour les transitions permises lié-lié et les transitions d'ionisations. Cette approche classique tend à surestimer les taux de collisions et ne reproduit pas correctement le comportement avec les énergies.Dans cette thèse, nous démontrons que l'approximation de Drawin ne peut pas décrire les taux de collisions dans le cas de l'atome d'hydrogène. Nous présentons une nouvelle méthode pour estimer ces taux, par le biais d'ajustement sur des taux quantiques existant pour d'autres éléments.Nous montrons que cette méthode d'ajustement quantique (MAQ) est satisfaisante pour les modélisations hors-ETL lorsque les taux quantiques dédiés ne sont pas effectivement disponibles.Nous testons cette nouvelle méthode, avec le modèle d'atome de Fer que nous avons développé, sur des étoiles de référence issues « du Gaia-ESO survey ».En partant de paramètres photosphériques non-spectroscopiques connus, nous déterminons les abondances (1D) en fer de ces étoiles de référence dans les cas ETL et hors-ETL .Nos résultats dans le cas hors ETL conduisent à un excellent accord entre les abondances de FeI et FeII avec de faibles écarts types de raies à raies, particulièrement dans le cas des étoiles pauvres en métaux.Notre méthode est validée par comparaison avec de nouveaux calculs quantiques préliminaires sur l'atome de Fe I et d'hydrogène, dont les ajustements sont en excellent accord avec les nôtres.Determination of high precision abundances has and will always be an important goal of all spectroscopic studies. The use of LTE assumption in spectroscopic analyses has been extensively shown in the literature to badly affect the determined abundances and stellar parameters, especially in metal-poor and giant stars which can be subject to large non-LTE effects. Non-LTE modeling of stellar spectra is therefore essential to accurately reproduce the observations and derive stellar abundances. Non-LTE calculations require the inputof a bulk of atomic data, which may be subject to uncertainties. In cool stars, hydrogen collisional rates are a major source of uncertainty, which are often approximated using a classical recipe (the Drawin approximation) for allowed bound-bound, and ionization transitions only. This approximation has been shown to overestimate the collisional rates, and does not reproduce the correct behavior with energies. We demonstrate in this dissertation the inability of the Drawin approximation to describe the hydrogen collisional rates.We introduce a new method to estimate these rates based on fitting the existing quantum rates of other elements. We show that this quantum fitting method (QFM) performs well in non-LTE calculations when detailed quantum rates are not available. We test the newly proposed method, with a complete iron model atom that we developed, on a reference set of stars from the Gaia-ESO survey. Starting from well determined non-spectroscopic atmospheric parameters, we determine 1D, non-LTE, and LTE iron abundances for this set ofstars. Our non-LTE results show excellent agreement between Fe I and Fe II abundances and small line-by-line dispersions, especially for the metal-poor stars. Our method is validated upon comparison with new preliminary Fe I+H quantum calculations, whose fits show an excellent agreement with ours
MARCS model atmospheres
No full textIn this review presented at the Symposium A Stellar Journey in Uppsala, June 2008, I give an account of the historical development of the MARCS code, and its premises from the first version published in 1975 to the 2008 grid. The primary driver for the development team who constantly strive to include the best possible physical data, is the science that can be done with the models. A few preliminary comparisons of M star model spectra to spectrophotometric observations are presented. Particular results related to opacity effects are discussed. The size of errors in spectral energy distribution (SED) and model thermal stratification is estimated for different densities of wavelength sampling. The number of points used in the MARCS 2008 grid (108 000) is large enough to ensure errors of only a few K in all models of the grid, except the optically very thin layers of metal-poor stars. Errors in SEDs may reach about 10% locally in the UV. The published sampled SEDs are thus adequate to compute synthetic broadband photometry, but higher resolution spectra will be computed in the near future and published as well on the MARCS site (marcs.astro.uu.se). Test model calculations with TiO line opacity accounted for in scattering show significant cooling of the upper atmospheric layers of red giants. Rough estimates of radiative and collisional time scales for electronic transitions of TiO indicate that scattering may well be the dominant mechanism in these lines. However, models constructed with this hypothesis are incompatible with optical observations of TiO (Arcturus) or IR observations of OH (Betelgeuse), although they may succeed in explaining H2O line observations. More work is needed in that direction.</p
High-precision photometry in the era of ZTF and LSST cosmological surveys : characterization of instrumental and atmospheric transmissions.
No full textLes supernovae de type 1a (SNe Ia) sont des sondes cosmologiques essentielles pour étudier l'Univers récent et approfondir notre compréhension de l'énergie noire. Leur luminosité régulière, combinée aux observations réalisées par les grands relevés photométriques terrestres multi-bandes, permet d'estimer leurs distances et de contraindre les paramètres cosmologiques. Toutefois, pour les relevés actuels et futurs comme le textit{Zwicky Transient Facility} (ZTF) et le textit{Legacy Survey of Space and Time} (LSST), les incertitudes de calibration photométrique surpasseront les incertitudes statistiques dans les analyses cosmologiques basées sur les SNe Ia, ce qui limitera la précision des résultats cosmologiques. Pour atteindre les objectifs fixés par la collaboration textit{Dark Energy Science Collaboration} du relevé textit{LSST}, il est donc nécessaire d'améliorer la précision de la calibration des flux photométriques d'un ordre de grandeur, jusqu'à 0.1%.Pour obtenir des rapports de distance précis entre les observations de SNe Ia à différents textit{redshifts}, une calibration rigoureuse des couleurs est essentielle, car toute erreur dans la réponse chromatique des instruments peut fausser notre compréhension de l'expansion de l'univers. Par conséquent, une connaissance précise de la transmission totale, incluant les effets instrumentaux et atmosphériques à chaque époque du relevé, est indispensable pour atteindre le niveau de précision requis.Les travaux menés au cours de cette thèse se concentrent sur le développement de nouvelles méthodes pour améliorer la caractérisation de ces deux fonctions de transmission, formant ainsi deux axes de recherche complémentaires intégrés au projet de métrologie StarDICE qui vise à relever le défi de la calibration photométrique en utilisant des sources lumineuses calibrées en laboratoire pour étendre la calibration absolue à un catalogue d'étoiles standards, observées par les télescopes terrestres participant à des relevés cosmologiques.Le premier axe de travail concerne la conception d'une version transportable du textit{Collimated Beam Projector}, un instrument de laboratoire destiné à déterminer avec précision les réponses spectrales des télescopes effectuant des relevés photométriques au sol. J'ai contribué à la mise au point d'un prototype sur banc expérimental, démontrant qu'il est possible d'atteindre une précision de l'ordre d'une millimagnitude sur l'intégrale des fonctions de transmission instrumentales des filtres textit{g} et textit{r} du système photométrique de ZTF.Le second axe repose sur l'exploration d'une méthode inédite de détermination de l'extinction grise atmosphérique par des mesures radiométriques fournies par une caméra infrarouge thermique. Déployée avec succès à l'Observatoire de Haute-Provence, cette approche s'avère prometteuse pour qualifier la structure spatiale de l'extinction grise lors de conditions d'observation variables et corriger les mesures photométriques avec une précision supérieures aux méthodes déjà existantes.Ces deux efforts ont montré des résultats encourageants pour les développements futurs, tant pour la mesure de la réponse instrumentale du télescope du relevé ZTF que pour la caractérisation de l'extinction atmosphérique grise du relevé LSST qui mènera sa campagne d'observation même en conditions atmosphériques suboptimales.Type Ia supernovae (SNe Ia) are crucial cosmological probes for studying the recent Universe and deepening our understanding of dark energy. Their consistent luminosity, combined with observations from large ground-based multi-band photometric surveys, allows for distance estimation and the constraining of cosmological parameters. However, for current and future surveys such as the textit{Zwicky Transient Facility} (ZTF) and the textit{Legacy Survey of Space and Time} (LSST), photometric calibration uncertainties will surpass statistical uncertainties in cosmological analyses based on SNe Ia, limiting the precision of model inferences. To meet the goals set by the textit{Dark Energy Science Collaboration} of the textit{LSST} survey, improving photometric flux calibration accuracy by an order of magnitude, down to 0.1%, is essential.As the universe expands, the colors of SNe Ia shift toward the red: high-redshift SNe Ia are observed in the red bands of telescopes, while low-redshift ones are seen in bluer bands. Precise color calibration is crucial for obtaining accurate distance ratios between these observations, as any error in the instrumental chromatic response can distort our understanding of the universe's expansion. Consequently, a precise understanding of total transmission, including instrumental and atmospheric effects at each epoch of the survey, is necessary to achieve the required level of precision.This thesis focuses on developing new methods to improve the characterization of these two transmission functions, forming two complementary research axes within the StarDICE metrology project. StarDICE aims to address the challenge of photometric calibration by using laboratory-calibrated light sources to extend absolute calibration to a catalog of standard stars observed by ground-based telescopes engaged in cosmological surveys.The first research axis involves the design of a transportable version of the textit{Collimated Beam Projector}, a laboratory instrument designed to precisely determine the spectral responses of ground-based survey telescopes. I contributed to the development of a prototype on an experimental bench, demonstrating that a precision of the order of a millimagnitude can be achieved on the integral of the instrumental transmission functions for the textit{g} and textit{r} filters of the ZTF photometric system.The second axis explores a novel method for determining atmospheric gray extinction through radiometric measurements provided by a thermal infrared camera. Successfully deployed at the Haute-Provence Observatory, this approach is promising for qualifying the spatial structure of grey extinction under variable observation conditions, and for correcting photometric measurements more accurately than existing methods.These two approaches have shown promising results for future developments, both in measuring the instrumental response of the ZTF survey telescope and in characterizing atmospheric conditions for the LSST survey, where optimizing observation time will require data collection even under suboptimal conditions
Photospheric Constraints, Current Uncertainties in Models of Stellar Atmospheres, and Spectroscopic Surveys
No full textInternational audienc
UBVJHK synthetic photometry of galactic O stars
No full textInternational audienceAims.The development of powerful infrared observational techniques enables the study of very extincted objects and young embedded star-forming regions. This is especially interesting in the context of massive stars that form and spend a non negligible fraction of their life still enshrouded in their parental molecular cloud. Spectrophotometric calibrations are thus necessary to constrain the physical properties of heavily extincted objects. Methods: Here, we derive UBVJHK magnitudes and bolometric corrections from a grid of atmosphere models for O stars. Bessel passbands are used. Bolometric corrections (BC) are derived as a function of Teff and are subsequently used to derive BC -spectral type (ST) and absolute magnitudes- ST relations. Results: Infrared magnitudes and, for the first time, bolometric corrections are given for the full range of spectral types and luminosity classes. Infrared colors are essentially constant, and (H-K)0 is 0.05 mag bluer than previously proposed. Optical calibrations are also provided and are similar to previous work, except for (B-V)0 which is found to be at minimum -0.28 for standard O stars, slightly higher (0.04 mag) than commonly accepted. Conclusions: We present a consistent set of photometric calibrations of optical and infrared magnitudes and bolometric corrections for Galactic O stars as a function of Teff and spectral type based on non-LTE atmosphere models including winds and line-blanketing
The impact of winds on the spectral appearance of red supergiants
No full textInternational audienceABSTRACT The rate at which mass is lost during the red supergiant (RSG) evolutionary stage may strongly influence how the star appears. Though there have been many studies discussing how RSGs appear in the mid- and far-infrared (IR) as a function of their mass-loss rate, to date, there have been no such investigations at optical and near-IR wavelengths. In a preliminary study, we construct model atmospheres for RSGs that include a wind, and use these models to compute synthetic spectra from the optical to the mid-IR. The inclusion of a wind has two important effects. First, higher mass-loss rates result in stronger absorption in the TiO bands, causing the star to appear as a later spectral type despite its effective temperature remaining constant. This explains the observed relation between spectral type, evolutionary stage, and mid-IR excess, as well as the mismatch between temperatures derived from the optical and IR. Secondly, the wind mimics many observed characteristics of a ‘MOLsphere’, potentially providing an explanation for the extended molecular zone inferred to exist around nearby RSGs. Thirdly, we show that wind fluctuations can explain the spectral variability of Betelgeuse during its recent dimming, without the need for dust.</jats:p
Red supergiant star studies with CO5BOLD and Optim3D
No full textTo be published on the proceedings of the CO5BOLD workshop 2012 on Memorie della SAIt Supplementi, Vol. 24, E. Caffau & L. Sbordone eds. (http://www.lsw.uni-heidelberg.de/co5bold/workshop/)International audienceWe describe recent work focused towards a better understanding of red supergiant stars using 3D radiative-hydrodynamics (RHD) simulations with CO5BOLD. A small number of simulations now exist that span up to seven years of stellar time, at various numerical resolutions. Our discussion concentrates on interferometric and spectroscopic observations. We point out a number of problems, in particular the line depth and line width that are not well reproduced by simulations. The most recent introduction of a non-grey treatment of the radiation field dramatically improved the match with observations, without solving all difficulties. We also review the newly revived effective temperature scale controversy, and argue that it will only be solved using 3D RHD models
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