scholarly journals Round Table Summary: Stellar Interferometry as a Tool to Investigate Atmospheres and to Compare Observations with Models

2003 ◽  
Vol 210 ◽  
pp. 313-321
Author(s):  
Markus Wittkowski
Keyword(s):  
Near Infrared ◽  
Round Table ◽  
Stellar Atmosphere ◽  
Stellar Model ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Long Baseline ◽  
Stellar Interferometry ◽  
Infrared Wavelengths ◽  
Spectrally Resolved

Long-baseline interferometry at optical and near-infrared wavelengths is an emerging technology which is quickly becoming a useful tool to investigate stellar atmospheres and to compare observations with models. Stellar atmosphere models have so far mainly been constrained by comparisons with stellar spectra which are integrated over the stellar disks. Interferometric observations provide spatially and spectrally resolved information and can thus provide important complementary observational information which can be compared to model predictions. Here, I summarize the different aspects on this topic which were discussed at a round table on Thursday, June 20, 2002, during IAU Symposium 210. This summary gives an overview on discussed interferometric facilities and techniques, concepts to study atmospheres by optical interferometry, and particular classes of objects. We conclude that more frequent interactions between the efforts of atmosphere modelling and interferometric observations promise to lead to increased confidence in stellar model atmospheres and to further advancement of the field in the next years.

scholarly journals Joint Discussion 10: 3D views on cool stellar atmospheres – theory meets observation

2009 ◽  
Vol 5 (H15) ◽  
pp. 331-343
Author(s):  
K.N. Nagendra ◽  
P. Bonifacio ◽  
H.-G. Ludwig
Keyword(s):  
Chemical Composition ◽  
High Efficiency ◽  
Big Bang ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Low Mass ◽  
Computational Resources ◽  
The Big Bang ◽  
The Universe

Much of what we know about the chemical composition of the Universe actually stems from the chemical composition of stars, which is often deciphered from the spectra emerging from their atmospheres. Cool, low-mass and long-living stars allow to study the evolution of the Universe's chemistry from a time shortly after the big bang until today. The observation and interpretation of stellar spectra is a classical field in astronomy but is still undergoing vivid developments. The enormous increase in available computational resources opened-up possibilities which led to a revolution in the degree of realism to which modelers can mimic Nature. High-resolution, high-stability, high-efficiency spectrographs are now routinely providing stellar spectra whose full information content can only be exploited if a very much refined description of a stellar atmosphere is at hand.

scholarly journals Presentation and Interpretation of High Resolution Infrared Spectra of Late-Type Stars

1974 ◽  
Vol 3 ◽  
pp. 255-268 ◽  
Author(s):  
R. I. Thompson
Keyword(s):  
Stellar Evolution ◽  
Carbon Star ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Stellar Spectra ◽  
Cool Star ◽  
Convective Mixing ◽  
Processed Material ◽  
Carbon Compounds ◽  
Red Giant

Current interest in stellar evolution is concentrated on the life of a star after it has left the main sequence. Of particular interest are the red giant or supergiant periods during the hydrogen and helium shell burning phases. Convective mixing during these stages can mix nuclear processed material to the surface where it may be viewed by spectroscopic methods. It is imperative that this rare chance to view processed material be exploited fully to increase our knowledge of stellar evolution.The observation and interpretation of cool star spectra has its own particular set of problems and advantages. A particular difficulty is the formation of molecules at the low temperatures which occur in the atmospheres of late stars. Not only must the particularly complex spectra of molecules be dealt with but the problem of chemical equilibrium in the atmosphere must be solved accurately before quantitative analysis may be performed. The formation of molecules, however, has one advantage in that it very dramatically separates those stars with carbon to oxygen ratios greater than one from those with ratios less than one. It is the very high dissociation energy of 11.1 eV for the CO molecule which performs this separation. If carbon is less abundant than oxygen all of the carbon is tied up in CO and only oxides are formed in the stellar atmosphere which produce typical M star spectra. If, however, carbon is more abundant than oxygen then carbon compounds such as C2 are formed in place of the oxides and a carbon star spectrum is formed. One of the great advantages of infrared stellar spectra is that it is the only ground based technique for observing CO in stellar atmospheres.

scholarly journals Radio Molecular Line Observations of Late Type Stars

1977 ◽  
Vol 42 ◽  
pp. 495-520 ◽  
Author(s):  
A. Winnberg
Keyword(s):  
Near Infrared ◽  
Spectral Lines ◽  
Late Type ◽  
Circumstellar Envelopes ◽  
Long Baseline ◽  
The Galaxy ◽  
Infrared Wavelengths ◽  
Opening Up ◽  
Excellent Tool ◽  
Geometry And Kinematics

Radio observations of spectral lines from OH, H2O, SiO and CO have put the spot-light on late-type stars and revived the interest in them. Especially they have helped the understanding of the outer envelopes of these stars and of their mass loss. They also have revealed late-type stars that are hidden from observation at optical and near-infrared wavelengths by their dense surrounding shells of gas and dust. The strong maser lines from OH, H2O and SiO have additionally given us an excellent tool in mapping the distribution of late-type variables throughout the Galaxy. On the other hand, the interpretation of the maser amplified lines in terms of a model is difficult. Very-long-baseline interferometry (VLBI) of these lines might be a way of studying the geometry and kinematics of the envelopes but there are a few problems connected with these observations which I will point out in this paper. However, recently “thermal” microwave lines of SiO and CO have been observed from late-type stars. These lines might be easier to interpret regarding physical processes in the outer circumstellar envelopes. As so often when a new field is opening up in astronomy the solutions to several old problems have been found but at the same time many more new problems have been brought up.

scholarly journals The CARMENES search for exoplanets around M dwarfs

2018 ◽  
Vol 615 ◽  
pp. A6 ◽  
Author(s):  
V. M. Passegger ◽  
A. Reiners ◽  
S. V. Jeffers ◽  
S. Wende-von Berg ◽  
P. Schöfer ◽  
...  
Keyword(s):  
High Resolution ◽  
Near Infrared ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Surface Gravity ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Target Sample ◽  
M Dwarf ◽  
The Phoenix

Context. The new CARMENES instrument comprises two high-resolution and high-stability spectrographs that are used to search for habitable planets around M dwarfs in the visible and near-infrared regime via the Doppler technique. Aims. Characterising our target sample is important for constraining the physical properties of any planetary systems that are detected. The aim of this paper is to determine the fundamental stellar parameters of the CARMENES M-dwarf target sample from high-resolution spectra observed with CARMENES. We also include several M-dwarf spectra observed with other high-resolution spectrographs, that is CAFE, FEROS, and HRS, for completeness. Methods. We used a χ2 method to derive the stellar parameters effective temperature Teff, surface gravity logg, and metallicity [Fe/H] of the target stars by fitting the most recent version of the PHOENIX-ACES models to high-resolution spectroscopic data. These stellar atmosphere models incorporate a new equation of state to describe spectral features of low-temperature stellar atmospheres. Since Teff, logg, and [Fe/H] show degeneracies, the surface gravity is determined independently using stellar evolutionary models. Results. We derive the stellar parameters for a total of 300 stars. The fits achieve very good agreement between the PHOENIX models and observed spectra. We estimate that our method provides parameters with uncertainties of σTeff = 51 K, σlog g = 0.07, and σ[Fe/H] = 0.16, and show that atmosphere models for low-mass stars have significantly improved in the last years. Our work also provides an independent test of the new PHOENIX-ACES models, and a comparison for other methods using low-resolution spectra. In particular, our effective temperatures agree well with literature values, while metallicities determined with our method exhibit a larger spread when compared to literature results.

scholarly journals Model Spectra for the Helium Cataclysmic Variable: AM CVn

1996 ◽  
Vol 158 ◽  
pp. 133-134
Author(s):  
S. Bard
Keyword(s):  
Radiative Transfer Equation ◽  
Accretion Rate ◽  
Stellar Atmosphere ◽  
Stellar Model ◽  
Accretion Disc ◽  
Stellar Atmospheres ◽  
Synthetic Spectrum ◽  
Cataclysmic Variable ◽  
Optical Telescope ◽  
International Ultraviolet Explorer

AbstractAccretion through Roche lobe overflow in the helium cataclysmic binary AM CVn is considered and its emitted spectrum is computed using model stellar atmospheres. The accretion disc in AM CVn is helium-rich and such discs have hitherto not been constructed. We have computed the models by numerically solving the radiative transfer equation for helium opacities. The method adopted is that of treating the accretion disc as a set of optically thick and geometrically thin concentric annuli. Each annulus is treated individually as a blackbody or as a stellar atmosphere. We find that our description of AM CVn is consistent with both photometric and spectroscopic measurements of the system which have been made using the Nordic Optical Telescope (NOT) and the International Ultraviolet Explorer (IUE). We find that the stellar model synthetic spectrum provides a better fit to the observed spectrum than the Planckian disc spectrum. Also, for a given accretion rate, the synthetic spectrum composed of the sum of a series of stellar atmospheres is flatter than the corresponding blackbody sum.

Photon Emission Spectral Signatures of AlGaN/GaN HEMT for Functional and Reliability Analysis

2008 ◽  
Author(s):  
Arkadiusz Glowacki ◽  
Christian Boit ◽  
Richard Lossy ◽  
Joachim Würfl
Keyword(s):  
Reliability Analysis ◽  
Gate Voltage ◽  
Near Infrared ◽  
Photon Emission ◽  
Electrical Characterization ◽  
Spectral Signatures ◽  
Operating Modes ◽  
Gan Hemt ◽  
Spectrally Resolved ◽  
Performance Variations

Abstract Non-degraded and degraded AlGaN/GaN HEMT devices have been characterized electrically and investigated in various operating modes using integral and spectrally resolved photon emission (PE). In degraded devices the PE dependence on the gate voltage differs from the non-degraded devices. Various types of dependencies on the gate voltage have been identified when investigating local degradation sites. PE spectroscopy was performed at various bias conditions. For both devices broad spectra have been obtained in a wavelength regime from visible to near-infrared, including local performance variations. Signatures of the degradation have been determined in the electrical characterization, in integral PE distribution and in the PE spectrum.

scholarly journals Electromechanically reconfigurable optical nano-kirigami

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Shanshan Chen ◽  
Zhiguang Liu ◽  
Huifeng Du ◽  
Chengchun Tang ◽  
Chang-Yin Ji ◽  
...  
Keyword(s):  
Near Infrared ◽  
Large Range ◽  
Three Dimensional ◽  
High Contrast ◽  
Si Substrate ◽  
High Modulation ◽  
Gold Nanostructure ◽  
Infrared Wavelengths ◽  
On Chip ◽  
Nano Electromechanical System

AbstractKirigami, with facile and automated fashion of three-dimensional (3D) transformations, offers an unconventional approach for realizing cutting-edge optical nano-electromechanical systems. Here, we demonstrate an on-chip and electromechanically reconfigurable nano-kirigami with optical functionalities. The nano-electromechanical system is built on an Au/SiO2/Si substrate and operated via attractive electrostatic forces between the top gold nanostructure and bottom silicon substrate. Large-range nano-kirigami like 3D deformations are clearly observed and reversibly engineered, with scalable pitch size down to 0.975 μm. Broadband nonresonant and narrowband resonant optical reconfigurations are achieved at visible and near-infrared wavelengths, respectively, with a high modulation contrast up to 494%. On-chip modulation of optical helicity is further demonstrated in submicron nano-kirigami at near-infrared wavelengths. Such small-size and high-contrast reconfigurable optical nano-kirigami provides advanced methodologies and platforms for versatile on-chip manipulation of light at nanoscale.

scholarly journals Comparison of fringe-tracking algorithms for single-mode near-infrared long-baseline interferometers

2014 ◽  
Vol 569 ◽  
pp. A2 ◽  
Author(s):  
É. Choquet ◽  
J. Menu ◽  
G. Perrin ◽  
F. Cassaing ◽  
S. Lacour ◽  
...  
Keyword(s):  
Near Infrared ◽  
Single Mode ◽  
Long Baseline ◽  
Tracking Algorithms ◽  
Fringe Tracking

Spectral characteristics of leafy spurge (Euphorbia esula) leaves and flower bracts

Weed Science ◽  
2004 ◽  
Vol 52 (4) ◽  
pp. 492-497 ◽  
Author(s):  
E. Raymond Hunt ◽  
James E. McMurtrey ◽  
Amy E. Parker Williams ◽  
Lawrence A. Corp
Keyword(s):  
Remote Sensing ◽  
High Performance ◽  
Near Infrared ◽  
Spectral Characteristics ◽  
Leafy Spurge ◽  
Hyperspectral Remote Sensing ◽  
Total Carotenoids ◽  
Physiological Basis ◽  
Infrared Wavelengths ◽  
Pigment Concentrations

Leafy spurge can be detected during flowering with either aerial photography or hyperspectral remote sensing because of the distinctive yellow-green color of the flower bracts. The spectral characteristics of flower bracts and leaves were compared with pigment concentrations to determine the physiological basis of the remote sensing signature. Compared with leaves of leafy spurge, flower bracts had lower reflectance at blue wavelengths (400 to 500 nm), greater reflectance at green, yellow, and orange wavelengths (525 to 650 nm), and approximately equal reflectances at 680 nm (red) and at near-infrared wavelengths (725 to 850 nm). Pigments from leaves and flower bracts were extracted in dimethyl sulfoxide, and the pigment concentrations were determined spectrophotometrically. Carotenoid pigments were identified using high-performance liquid chromatography. Flower bracts had 84% less chlorophylla, 82% less chlorophyllb, and 44% less total carotenoids than leaves, thus absorptance by the flower bracts should be less and the reflectance should be greater at blue and red wavelengths. The carotenoid to chlorophyll ratio of the flower bracts was approximately 1:1, explaining the hue of the flower bracts but not the value of reflectance. The primary carotenoids were lutein, β-carotene, and β-cryptoxanthin in a 3.7:1.5:1 ratio for flower bracts and in a 4.8:1.3:1 ratio for leaves, respectively. There was 10.2 μg g−1fresh weight of colorless phytofluene present in the flower bracts and none in the leaves. The fluorescence spectrum indicated high blue, red, and far-red emission for leaves compared with flower bracts. Fluorescent emissions from leaves may contribute to the higher apparent leaf reflectance in the blue and red wavelength regions. The spectral characteristics of leafy spurge are important for constructing a well-documented spectral library that could be used with hyperspectral remote sensing.

Sign in / Sign up

Export Citation Format

Share Document