The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Wolfgang Hayek - One of the best experts on this subject based on the ideXlab platform.
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the stagger grid a grid of 3d stellar atmosphere models ii horizontal and temporal averaging and Spectral Line formation
Astronomy and Astrophysics, 2013Co-Authors: Zazralt Magic, Wolfgang Hayek, Remo Colle, Marti AsplundAbstract:Aims. We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmo- spheres computed with the Stagger-code. The temporally and spatially averaged (hereafter denoted as � 3D� ) models are explored in the light of local thermodynamic equilibrium (LTE) Spectral Line formation by comparing spectrum calculations using full 3D atmo- sphere structures with those from � 3Daverages. Methods. We explored methods for computing mean � 3Dstratifications from the Stagger-grid time-dependent 3D radiative hydro- dynamical atmosphere models by considering four different reference depth scales (geometrical depth, column-mass density, and two optical depth scales). Furthermore, we investigated the influence of alternative averages (logarithmic, enforced hydrostatic equilib- rium, flux-weighted temperatures). For the Line formation we computed curves of growth for Fe i and Fe ii Lines in LTE. Results. The resulting � 3Dstratifications for the four reference depth scales can be very different. We typically find that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are high- est, the differences become considerable and increase for higher Teff ,l ower logg ,a nd lower (Fe/H). The differential comparison of Spectral Line formation shows distinctive differences depending on which � 3Dmodel is applied. The averages over layers of constant column-mass density yield the best mean � 3Drepresentation of the full 3D models for LTE Line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to increasing interference by reversed granulation towards higher effective temperature, in particular at low metallicity.
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the stagger grid a grid of 3d stellar atmosphere models ii horizontal and temporal averaging and Spectral Line formation
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: Zazralt Magic, Remo Collet, Wolfgang Hayek, Martin AsplundAbstract:We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmospheres computed with the Stagger-code. The temporally and spatially averaged (hereafter denoted as ) models are explored in the light of local thermodynamic equilibrium (LTE) Spectral Line formation by comparing spectrum calculations using full 3D atmosphere structures with those from averages. We explore methods for computing mean stratifications from the Stagger-grid time-dependent 3D radiative hydro- dynamical atmosphere models by considering four different reference depth scales (geometrical depth, column-mass density, and two optical depth scales). Furthermore, we investigate the influence of alternative averages (logarithmic or enforced hydrostatic equilibrium, flux-weighted temperatures). For the Line formation we compute curves of growth for Fe i and Fe ii Lines in LTE . The resulting stratifications for the four reference depth scales can be considerably different. We find typically that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are highest, the differences become considerable and increase for higher Teff, lower logg, and lower [Fe/H]. The differential comparison of Spectral Line formation shows distinctive differences depending on which model is applied. The averages over layers of constant column-mass density yield the best mean representation for LTE Line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to the increasing interference of the reversed granulation towards higher effective temperature, in particular at low metallicity.
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3d lte Spectral Line formation with scattering in red giant stars
arXiv: Solar and Stellar Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:We investigate the effects of coherent isotropic continuum scattering on the formation of Spectral Lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model atmospheres of red giant stars. Continuum flux levels, Spectral Line profiles and curves of growth for different species are compared with calculations that treat scattering as absorption. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE Lines: continua gain more brightness than Line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in Line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the Line profiles, leading to larger abundance corrections for stronger Lines, which reach -0.5 dex at 3000 A for Fe II Lines in 3D with excitation potential 2 eV at [Fe/H]=-3.0. The corrections are less severe for low-excitation Lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. 1D calculations exhibit similar scattering abundance corrections for weak Lines, but those for strong Lines are generally smaller compared to 3D models and depend on the choice of microturbulence.
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3D LTE Spectral Line formation with scattering in red giant stars
Astronomy & Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:Aims. We investigate the effects of coherent isotropic continuum scattering on the formation of Spectral Lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and ID hydrostatic model atmospheres of red giant stars. Methods. Detailed radiative transfer with coherent and isotropic continuum scattering is computed for 3D hydrodynamical and 1D hydrostatic models of late-type stellar atmospheres using the SCATE code. Opacities are computed in LTE, while a coherent and isotropic scattering term is added to the continuum source function. We investigate the effects of scattering by comparing continuum flux levels, Spectral Line profiles and curves of growth for different species with calculations that treat scattering as absorption. Results. Rayleigh scattering is the dominant source of scattering opacity in the continuum of red giant stars. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of λ ≲ 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE Lines: continua gain more brightness than Line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in Line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the Line profiles, leading to larger abundance corrections for stronger Lines, which reach -0.5 dex at 3000 A for Fe II Lines in 3D with excitation potential χ = 2 eV at [Fe/H] = -3.0. The corrections are less severe for low-excitation Lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. ID calculations exhibit similar scattering abundance corrections for weak Lines, but those for strong Lines are generally smaller compared to 3D models and depend on the choice of microturbulence. Conclusions. Continuum scattering should be taken into account for computing realistic Spectral Line profiles at wavelengths λ ≲ 4000 A in metal-poor giant stars. Profile shapes are strongly affected by velocity fields and horizontal inhomogeneities, requiring a treatment based on 3D hydrodynamical rather than classical 1D hydrostatic model atmospheres.
Åke Nordlund - One of the best experts on this subject based on the ideXlab platform.
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3d lte Spectral Line formation with scattering in red giant stars
arXiv: Solar and Stellar Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:We investigate the effects of coherent isotropic continuum scattering on the formation of Spectral Lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model atmospheres of red giant stars. Continuum flux levels, Spectral Line profiles and curves of growth for different species are compared with calculations that treat scattering as absorption. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE Lines: continua gain more brightness than Line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in Line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the Line profiles, leading to larger abundance corrections for stronger Lines, which reach -0.5 dex at 3000 A for Fe II Lines in 3D with excitation potential 2 eV at [Fe/H]=-3.0. The corrections are less severe for low-excitation Lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. 1D calculations exhibit similar scattering abundance corrections for weak Lines, but those for strong Lines are generally smaller compared to 3D models and depend on the choice of microturbulence.
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3D LTE Spectral Line formation with scattering in red giant stars
Astronomy & Astrophysics, 2011Co-Authors: Wolfgang Hayek, Martin Asplund, Remo Collet, Åke NordlundAbstract:Aims. We investigate the effects of coherent isotropic continuum scattering on the formation of Spectral Lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and ID hydrostatic model atmospheres of red giant stars. Methods. Detailed radiative transfer with coherent and isotropic continuum scattering is computed for 3D hydrodynamical and 1D hydrostatic models of late-type stellar atmospheres using the SCATE code. Opacities are computed in LTE, while a coherent and isotropic scattering term is added to the continuum source function. We investigate the effects of scattering by comparing continuum flux levels, Spectral Line profiles and curves of growth for different species with calculations that treat scattering as absorption. Results. Rayleigh scattering is the dominant source of scattering opacity in the continuum of red giant stars. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of λ ≲ 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE Lines: continua gain more brightness than Line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in Line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the Line profiles, leading to larger abundance corrections for stronger Lines, which reach -0.5 dex at 3000 A for Fe II Lines in 3D with excitation potential χ = 2 eV at [Fe/H] = -3.0. The corrections are less severe for low-excitation Lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. ID calculations exhibit similar scattering abundance corrections for weak Lines, but those for strong Lines are generally smaller compared to 3D models and depend on the choice of microturbulence. Conclusions. Continuum scattering should be taken into account for computing realistic Spectral Line profiles at wavelengths λ ≲ 4000 A in metal-poor giant stars. Profile shapes are strongly affected by velocity fields and horizontal inhomogeneities, requiring a treatment based on 3D hydrodynamical rather than classical 1D hydrostatic model atmospheres.
Ana G Perez - One of the best experts on this subject based on the ideXlab platform.
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the sdss iii apogee Spectral Line list for h band spectroscopy
Astrophysical Journal Supplement Series, 2015Co-Authors: Matthew D. Shetrone, Katia Cunha, Verne V. Smith, Allende C Prieto, Dmitry Bizyaev, Jennifer A Johnson, Jon A. Holtzman, J E Lawler, Ana G PerezAbstract:We present the H-band Spectral Line lists adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The APOGEE Line lists comprise astrophysical, theoretical, and laboratory sources from the literature, as well as newly evaluated astrophysical oscillator strengths and damping parameters. We discuss the construction of the APOGEE Line list, which is one of the critical inputs for the APOGEE Stellar Parameters and Chemical Abundances PipeLine, and present three different versions that have been used at various stages of the project. The methodology for the newly calculated astrophysical Line lists is reviewed. The largest of these three Line lists contains 134,457 molecular and atomic transitions. In addition to the format adopted to store the data, the Line lists are available in MOOG, Synspec, and Turbospectrum formats. The limitations of the Line lists along with guidance for its use on different Spectral types are discussed. We also present a list of H-band Spectral features that are either poorly represented or completely missing in our Line list. This list is based on the average of a large number of Spectral fit residuals for APOGEE observations spanning a wide range of stellar parameters.
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the sdss iii apogee Spectral Line list for h band spectroscopy
arXiv: Instrumentation and Methods for Astrophysics, 2015Co-Authors: Matthew D. Shetrone, Katia Cunha, Verne V. Smith, Carlos Allende Prieto, Dmitry Bizyaev, Jennifer A Johnson, Jon A. Holtzman, J E Lawler, Ana G Perez, Szabolcs MeszarosAbstract:We present the $H$-band Spectral Line lists adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). The APOGEE Line lists comprise astrophysical, theoretical, and laboratory sources from the literature, as well as newly evaluated astrophysical oscillator strengths and damping parameters. We discuss the construction of the APOGEE Line list, which is one of the critical inputs for the APOGEE Stellar Parameters and Chemical Abundances PipeLine, and present three different versions that have been used at various stages of the project. The methodology for the newly calculated astrophysical Line lists is reviewed. The largest of these three Line lists contains 134,457 molecular and atomic transitions. In addition to the format adopted to store the data, the Line lists are available in MOOG, Synspec and Turbospectrum formats. We also present a list of $H$-band Spectral features that are either poorly represented or completely missing in our Line list. This list is based on the average of a large number of Spectral fit residuals for APOGEE observations spanning a wide range of stellar parameters.
Andrew M Weiner - One of the best experts on this subject based on the ideXlab platform.
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Spectral Line by Line shaping for optical and microwave arbitrary waveform generations
Laser & Photonics Reviews, 2008Co-Authors: Chen-bin Huang, Z Jiang, Daniel E Leaird, Jose Caraquitena, Andrew M WeinerAbstract:Spectral Line-by-Line shaping is a key enabler towards optical arbitrary waveform generation, which promises broad impact both in optical science and technology. In this paper, generation of optical and microwave arbitrary waveforms using the Spectral Line-by-Line shaping technique is reviewed. Compared to conventional pulse shaping, significant new physics arises in the Line-by-Line regime, where the shaped pulse fields generated from one laser pulse now overlap with those generated from adjacent pulses. This leads to coherent interference effects related to the properties of optical frequency combs which serve as the source in these experiments. We explore such effects in a series of experiments using several different high-repetition-rate optical combs, including harmonically mode-locked lasers and continuous-wave lasers that are externally phase modulated either with or without the help of an optical cavity. As an application of Line-by-Line pulse shaping, we describe generation of microwave electrical arbitrary waveforms that can be reprogrammed at rates approaching 10 GHz.
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Spectral Line by Line pulse shaping on an optical frequency comb generator
IEEE Journal of Quantum Electronics, 2007Co-Authors: Z Jiang, Chen-bin Huang, Daniel E Leaird, Houxun Miao, M Kourogi, Kazuhiro Imai, Andrew M WeinerAbstract:We demonstrate optical processing based on Spectral Line-by-Line pulse shaping of a frequency comb generated by an optical frequency comb generator (OFCG). The OFCG is able to generate a smooth, broad, stable, known-phase frequency comb, which is ideal for recently developed Spectral Line-by-Line pulse shaping technology applications. We demonstrate Line-by-Line pulse shaping on 64 Lines at 10-GHz Line spacing, generate transform-limited 1.6-ps short pulses at 10 GHz by combining two pulses in each period directly from the OFCG, and show various examples for optical arbitrary waveform generation. Further, we demonstrate that these pulse sources are of sufficient quality to support optical fiber communication applications as confirmed by bit error rate measurements.
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optical processing based on Spectral Line by Line pulse shaping on a phase modulated cw laser
IEEE Journal of Quantum Electronics, 2006Co-Authors: Z Jiang, Daniel E Leaird, Andrew M WeinerAbstract:We demonstrate optical processing based on Spectral Line-by-Line pulse shaping of a frequency comb generated by phase modulating a CW laser and show various applications including CW-to-pulse conversion, width/wavelength tunable return-to-zero pulse generation, pulse-to-CW conversion, wavelength conversion, and microwave photonics. The correlations between optical spectra, time-domain waveforms, and RF spectra are quantitatively studied
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Optical arbitrary waveform generation and characterization using Spectral Line-by-Line control
2006 Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, 2006Co-Authors: Zhi Jiang, Daniel E Leaird, Andrew M WeinerAbstract:We demonstrate optical arbitrary waveform generation and characterization using Spectral Line-by-Line control with high resolution grating based pulse shapers. Such integrated capabilities are the enabling techniques for high-fidelity optical arbitrary waveform generation.
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Optical arbitrary waveform generation and characterization using Spectral Line-by-Line control
Journal of Lightwave Technology, 2006Co-Authors: Zhi Jiang, Daniel E Leaird, Andrew M WeinerAbstract:This paper demonstrates optical arbitrary waveform generation (O-AWG) and its characterization using Spectral Line-by-Line control with high-resolution grating-based pulse shapers. Such integrated capabilities are the enabling techniques for high-fidelity O-AWG.
N A Patel - One of the best experts on this subject based on the ideXlab platform.
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an interferometric Spectral Line and imaging survey of vy canis majoris in the 345 ghz band
Astrophysical Journal Supplement Series, 2013Co-Authors: T Kaminski, C A Gottlieb, K Young, K M Menten, N A PatelAbstract:A Spectral Line survey of the oxygen-rich red supergiant VYCanis Majoris was made between 279 and 355GHz with the Submillimeter Array. Two hundred twenty three Spectral features from 19 molecules (not counting isotopic species of some of them) were observed, including the rotational spectra of TiO, TiO2, and AlCl for the first time in this source. The parameters and an atlas of all Spectral features is presented. Observations of each Line with a synthesized beam of ∼0. 9, reveal the complex kinematics and morphology of the nebula surrounding VYCMa. Many of the molecules are observed in high lying rotational levels or in excited vibrational levels. From these, it was established that the main source of the submillimeter-wave continuum (dust) and the high excitation molecular gas (the star) are separated by about 0. ′′ 15. Apparent coincidences between the molecular gas observed with the SMA, and some of the arcs and knots observed at infrared wavelengths and in the optical scattered light by the Hubble Space Telescope are identified. The observations presented here provide important constraints on the molecular chemistry in oxygen-dominated circumstellar environments and a deeper picture of the complex circumstellar environment of VYCMa.
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an interferometric Spectral Line and imaging survey of vy canis majoris in the 345 ghz band
arXiv: Solar and Stellar Astrophysics, 2013Co-Authors: T Kaminski, C A Gottlieb, K Young, K M Menten, N A PatelAbstract:A Spectral Line survey of the oxygen-rich red supergiant VY Canis Majoris was made between 279 and 355 GHz with the Submillimeter Array. Two hundred twenty three Spectral features from 19 molecules (not counting isotopic species of some of them) were observed, including the rotational spectra of TiO, TiO2, and AlCl for the first time in this source. The parameters and an atlas of all Spectral features is presented. Observations of each Line with a synthesized beam of ~0.9 arcsec, reveal the complex kinematics and morphology of the nebula surrounding VY CMa. Many of the molecules are observed in high lying rotational levels or in excited vibrational levels. From these, it was established that the main source of the submillimeter-wave continuum (dust) and the high excitation molecular gas (the star) are separated by about 0.15 arcsec. Apparent coincidences between the molecular gas observed with the SMA, and some of the arcs and knots observed at infrared wavelengths and in the optical scattered light by the Hubble Space Telescope are identified. The observations presented here provide important constraints on the molecular chemistry in oxygen-dominated circumstellar environments and a deeper picture of the complex circumstellar environment of VY CMa.
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an interferometric Spectral Line survey of irc 10216 in the 345 ghz band
Astrophysical Journal Supplement Series, 2011Co-Authors: N A Patel, K Young, C A Gottlieb, P Thaddeus, R W Wilson, K M Menten, M J Reid, Michael C Mccarthy, J Cernicharo, Sandra BrunkenAbstract:We report a Spectral-Line survey of the extreme carbon star IRC+10216 carried out between 293.9 and 354.8 GHz with the Submillimeter Array. A total of 442 Lines were detected, more than 200 for the first time; 149 are unassigned. Maps at an angular resolution of similar to 3 '' were obtained for each Line. A substantial new population of narrow Lines with an expansion velocity of similar to 4 km s(-1) (i.e., approximate to 30% of the terminal velocity) was detected. Most of these are attributed to rotational transitions within vibrationally excited states, emitted from energy levels above the v = 0, J = 0 ground state with excitation energy of 1000-3000 K. Emission from these Lines appears to be centered on the star with an angular extent of < 1 ''. We use multiple transitions detected in several molecules to derive physical conditions in this inner envelope of IRC+10216.
