scholarly journals Carbon, oxygen, and iron abundances in disk and halo stars

2019 ◽  
Vol 630 ◽  
pp. A104 ◽  
Author(s):  
A. M. Amarsi ◽  
P. E. Nissen ◽  
Á. Skúladóttir
Keyword(s):  
Radiative Transfer ◽  
Galactic Disk ◽  
Three Dimensional ◽  
Stellar Atmosphere ◽  
Late Type ◽  
One Dimensional ◽  
Halo Stars ◽  
Low Metallicity ◽  
Halo Population ◽  
High Metallicity

The abundances of carbon, oxygen, and iron in late-type stars are important parameters in exoplanetary and stellar physics, as well as key tracers of stellar populations and Galactic chemical evolution. However, standard spectroscopic abundance analyses can be prone to severe systematic errors, based on the assumption that the stellar atmosphere is one-dimensional (1D) and hydrostatic, and by ignoring departures from local thermodynamic equilibrium (LTE). In order to address this, we carried out three-dimensional (3D) non-LTE radiative transfer calculations for C I and O I, and 3D LTE radiative transfer calculations for Fe II, across the STAGGER-grid of 3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE abundance corrections can be as severe as − 0.3 dex for C I lines in low-metallicity F dwarfs, and − 0.6 dex for O I lines in high-metallicity F dwarfs. The 3D LTE versus 1D LTE abundance corrections for Fe II lines are less severe, typically less than + 0.15 dex. We used the corrections in a re-analysis of carbon, oxygen, and iron in 187 F and G dwarfs in the Galactic disk and halo. Applying the differential 3D non-LTE corrections to 1D LTE abundances visibly reduces the scatter in the abundance plots. The thick disk and high-α halo population rise in carbon and oxygen with decreasing metallicity, and reach a maximum of [C/Fe] ≈ 0.2 and a plateau of [O/Fe] ≈ 0.6 at [Fe/H] ≈ −1.0. The low-α halo population is qualitatively similar, albeit offset towards lower metallicities and with larger scatter. Nevertheless, these populations overlap in the [C/O] versus [O/H] plane, decreasing to a plateau of [C/O] ≈ −0.6 below [O/H] ≈ −1.0. In the thin-disk, stars having confirmed planet detections tend to have higher values of C∕O at given [O/H]; this potential signature of planet formation is only apparent after applying the abundance corrections to the 1D LTE results. Our grids of line-by-line abundance corrections are publicly available and can be readily used to improve the accuracy of spectroscopic analyses of late-type stars.

Three Dimensional Radiative Transfer in ICON-LEM

2020 ◽  
Author(s):  
Fabian Jakub ◽  
Bernhard Mayer
Keyword(s):  
Radiative Transfer ◽  
Detrimental Effect ◽  
Energy Transport ◽  
Three Dimensional ◽  
Cloud Formation ◽  
One Dimensional ◽  
3D Effects ◽  
Computational Simplicity ◽  
Boundary Layer Dynamics ◽  
Side Illumination

<pre class="moz-quote-pre">Recent studies have shown that the effects of three dimensional radiative transfer may impact cloud formation and precipitation. While one-dimensional solvers are favoured due to their computational simplicity, they do however neglect any horizontal energy transport. In particular, the 1D approximation neglects 3D effects such as cloud side illumination and the displacement of the cloud's shadow at the surface which are relevant whenever the sun is not in the zenith. This has a detrimental effect on the results of high resolution simulations. 3D radiative transfer has the potential to considerably change the boundary layer dynamics, the evolution of clouds, their lifetime and precipitation onset. To this date, studies that investigate the influence of 3D effects on realistic NWP settings are rare, primarily because there haven't been 3D radiative transfer solvers around that were fast enough to be run interactively in a forecast simulation. For that purpose we adapted the TenStream solver (parallel 3D radiative transfer solver for LES) to unstructured meshes and coupled it to ICON-LEM. We will present the new solver in the context of ICON-LEM simulations, the methodologies used and its characteristics.</pre>

scholarly journals Three-dimensional effects in polarization signatures as observed from precipitating clouds by low frequency ground-based microwave radiometers

2006 ◽  
Vol 6 (3) ◽  
pp. 5427-5456
Author(s):  
A. Battaglia ◽  
C. Simmer ◽  
H. Czekala
Keyword(s):  
Radiative Transfer ◽  
Monte Carlo Model ◽  
Three Dimensional ◽  
Low Frequency ◽  
Radiative Transfer Model ◽  
Observation Angle ◽  
Transfer Model ◽  
One Dimensional ◽  
Dimensional Effects ◽  
The Impact

Abstract. Consistent negative polarization differences (i.e. differences between the vertical and the horizontal brightness temperature) are observed when looking at precipitating systems by ground-based radiometers at slant angles. These signatures can be partially explained by one-dimensional radiative transfer computations that include oriented non-spherical raindrops. However some cases are characterized by polarization values that exceed differences expected from one-dimensional radiative transfer. A three-dimensional fully polarized Monte Carlo model has been used to evaluate the impact of the horizontal finiteness of rain shafts with different rain rates at 10, 19, and 30 GHz. The results show that because of the reduced slant optical thickness in finite clouds, the polarization signal can strongly differ from its one-dimensional counterpart. At the higher frequencies and when the radiometer is positioned underneath the cloud, significantly higher negative values for the polarization are found which are also consistent with some observations. When the observation point is located outside of the precipitating cloud, typical polarization patterns (with troughs and peaks) as a function of the observation angle are predicted. An approximate 1-D slant path radiative transfer model is considered as well and results are compared with the full 3-D simulations to investigate whether or not three-dimensional effects can be explained by geometry effects alone. The study has strong relevance for low-frequency passive microwave polarimetric studies.

scholarly journals Reduction of radiation biases by incorporating the missing cloud variability by means of downscaling techniques: a study using the 3-D MoCaRT model

2012 ◽  
Vol 5 (9) ◽  
pp. 2261-2276 ◽  
Author(s):  
S. Gimeno García ◽  
T. Trautmann ◽  
V. Venema
Keyword(s):  
Radiative Transfer ◽  
Spatial Resolution ◽  
Heterogeneous Media ◽  
Monte Carlo Model ◽  
Computational Cost ◽  
Three Dimensional ◽  
Accurate Solution ◽  
Stochastic Methods ◽  
Climate Modelling ◽  
One Dimensional

Abstract. Handling complexity to the smallest detail in atmospheric radiative transfer models is unfeasible in practice. On the one hand, the properties of the interacting medium, i.e., the atmosphere and the surface, are only available at a limited spatial resolution. On the other hand, the computational cost of accurate radiation models accounting for three-dimensional heterogeneous media are prohibitive for some applications, especially for climate modelling and operational remote-sensing algorithms. Hence, it is still common practice to use simplified models for atmospheric radiation applications. Three-dimensional radiation models can deal with complex scenarios providing an accurate solution to the radiative transfer. In contrast, one-dimensional models are computationally more efficient, but introduce biases to the radiation results. With the help of stochastic models that consider the multi-fractal nature of clouds, it is possible to scale cloud properties given at a coarse spatial resolution down to a higher resolution. Performing the radiative transfer within the cloud fields at higher spatial resolution noticeably helps to improve the radiation results. We present a new Monte Carlo model, MoCaRT, that computes the radiative transfer in three-dimensional inhomogeneous atmospheres. The MoCaRT model is validated by comparison with the consensus results of the Intercomparison of Three-Dimensional Radiation Codes (I3RC) project. In the framework of this paper, we aim at characterising cloud heterogeneity effects on radiances and broadband fluxes, namely: the errors due to unresolved variability (the so-called plane parallel homogeneous, PPH, bias) and the errors due to the neglect of transversal photon displacements (independent pixel approximation, IPA, bias). First, we study the effect of the missing cloud variability on reflectivities. We will show that the generation of subscale variability by means of stochastic methods greatly reduce or nearly eliminate the reflectivity biases. Secondly, three-dimensional broadband fluxes in the presence of realistic inhomogeneous cloud fields sampled at high spatial resolutions are calculated and compared to their one-dimensional counterparts at coarser resolutions. We found that one-dimensional calculations at coarsely resolved cloudy atmospheres systematically overestimate broadband reflected and absorbed fluxes and underestimate transmitted ones.

scholarly journals Analysis of the Discrete Theory of Radiative Transfer in the Coupled “Ocean–Atmosphere” System: Current Status, Problems and Development Prospects

2020 ◽  
Vol 8 (3) ◽  
pp. 202 ◽  
Author(s):  
Viktor P. Afanas’ev ◽  
Alexander Yu. Basov ◽  
Vladimir P. Budak ◽  
Dmitry S. Efremenko ◽  
Alexander A. Kokhanovsky
Keyword(s):  
Radiative Transfer ◽  
Three Dimensional ◽  
Operator Method ◽  
Current Status ◽  
One Dimensional ◽  
Current State ◽  
The Matrix ◽  
The One ◽  
Discrete Theory ◽  
Development Prospects

In this paper, we analyze the current state of the discrete theory of radiative transfer. One-dimensional, three-dimensional and stochastic radiative transfer models are considered. It is shown that the discrete theory provides a unique solution to the one-dimensional radiative transfer equation. All approximate solution techniques based on the discrete ordinate formalism can be derived based on the synthetic iterations, the small-angle approximation, and the matrix operator method. The possible directions for the perspective development of radiative transfer are outlined.

Resistance of superconducting-normal interfaces at 0 K

1984 ◽  
Vol 391 (1801) ◽  
pp. 255-268 ◽  
Keyword(s):  
Radiative Transfer ◽  
Dimensional Analysis ◽  
Andreev Reflection ◽  
Three Dimensional ◽  
Linear Variation ◽  
Dimensional Theory ◽  
Milne Problem ◽  
One Dimensional ◽  
Interface Resistance ◽  
Bismuth Content

The model proposed by G. L. Harding, A. B. Pippard and J. R. Tomlinson (Harding, Pippard & Tomlinson, Proc . R . Soc . Lond . A340, 1 (1974)) to explain their observation of an enhanced interface resistance at a junction of copper and lead, when bismuth is added to the lead, is developed into a more satisfactory three-dimensional theory. The model for the resistance at 0 K is shown to resemble the Milne problem in radiative transfer, as treated by Chandrasekhar, but considerably modified by Andreev reflection. The theory predicts a linear variation of resistance with bismuth content, as is not incompatible with the measurements, but the magnitude of the calculated resistance, though considerably less than what was found in the original one-dimensional analysis, is still at least 50 % too high. Factors are suggested that should be taken into account in a thorough treatment of the interface, but without any assurance that they would eliminate the discrepancy.

scholarly journals Three-dimensional effects in polarization signatures as observed from precipitating clouds by low frequency ground-based microwave radiometers

2006 ◽  
Vol 6 (12) ◽  
pp. 4383-4394 ◽  
Author(s):  
A. Battaglia ◽  
C. Simmer ◽  
H. Czekala
Keyword(s):  
Radiative Transfer ◽  
Monte Carlo Model ◽  
Three Dimensional ◽  
Low Frequency ◽  
Radiative Transfer Model ◽  
Observation Angle ◽  
Transfer Model ◽  
One Dimensional ◽  
Dimensional Effects ◽  
The Impact

Abstract. Consistent negative polarization differences (i.e. differences between the vertical and the horizontal brightness temperature) are observed when looking at precipitating systems by ground-based radiometers at slant angles. These signatures can be partially explained by one-dimensional radiative transfer computations that include oriented non-spherical raindrops. However some cases are characterized by polarization values that exceed differences expected from one-dimensional radiative transfer. A three-dimensional fully polarized Monte Carlo model has been used to evaluate the impact of the horizontal finiteness of rain shafts with different rain rates at 10, 19, and 30 GHz. The results show that because of the reduced slant optical thickness in finite clouds, the polarization signal can strongly differ from its one-dimensional counterpart. At the higher frequencies and when the radiometer is positioned underneath the cloud, significantly higher negative values for the polarization are found which are also consistent with some observations. When the observation point is located outside of the precipitating cloud, typical polarization patterns (with troughs and peaks) as a function of the observation angle are predicted. An approximate 1-D slant path radiative transfer model is considered as well and results are compared with the full 3-D simulations to investigate whether or not three-dimensional effects can be explained by geometry effects alone. The study has strong relevance for low-frequency passive microwave polarimetric studies.

scholarly journals Observational constraints on the origin of the elements

2020 ◽  
Vol 634 ◽  
pp. A55 ◽  
Author(s):  
A. J. Gallagher ◽  
M. Bergemann ◽  
R. Collet ◽  
B. Plez ◽  
J. Leenaarts ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Three Dimensional ◽  
Solar Spectrum ◽  
Random Errors ◽  
Exchange Reactions ◽  
One Dimensional ◽  
Optical Region ◽  
Non Local ◽  
First Time ◽  
Model Atom

Context. The pursuit of more realistic spectroscopic modelling and consistent abundances has led us to begin a new series of papers designed to improve current solar and stellar abundances of various atomic species. To achieve this, we have begun updating the three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer code, MULTI3D, and the equivalent one-dimensional (1D) non-LTE radiative transfer code, MULTI 2.3. Aims. We examine our improvements to these codes by redetermining the solar barium abundance. Barium was chosen for this test as it is an important diagnostic element of the s-process in the context of galactic chemical evolution. New Ba II + H collisional data for excitation and charge exchange reactions computed from first principles had recently become available and were included in the model atom. The atom also includes the effects of isotopic line shifts and hyperfine splitting. Methods. A grid of 1D LTE barium lines were constructed with MULTI 2.3 and fit to the four Ba II lines available to us in the optical region of the solar spectrum. Abundance corrections were then determined in 1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium abundance was computed from these corrections. Results. We present for the first time the full 3D non-LTE barium abundance of A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual fully consistent barium lines. Errors here represent the systematic and random errors, respectively.

scholarly journals Reduction of radiation biases by incorporating the missing cloud variability via downscaling techniques: a study using the 3-D MoCaRT model

2012 ◽  
Vol 5 (1) ◽  
pp. 1543-1573
Author(s):  
S. Gimeno García ◽  
T. Trautmann ◽  
V. Venema
Keyword(s):  
Radiative Transfer ◽  
Spatial Resolution ◽  
Heterogeneous Media ◽  
Climate Modeling ◽  
Computational Cost ◽  
Three Dimensional ◽  
Accurate Solution ◽  
Stochastic Methods ◽  
One Dimensional ◽  
The One

Abstract. To handle complexity to the smallest detail in atmospheric radiative transfer models is in practice unfeasible. On the one hand, the properties of the interacting medium, i.e. the atmosphere and the surface, are only available at a limited spatial resolution. On the other hand, the computational cost of accurate radiation models accounting for three-dimensional heterogeneous media are prohibitive for some applications, esp. for climate modeling and operational remote sensing algorithms. Hence, it is still common practice to use simplified models for atmospheric radiation applications. Three-dimensional radiation models can deal with much more complexity than the one-dimensional ones providing a more accurate solution of the radiative transfer. In turn, one-dimensional models introduce biases to the radiation results. With the help of stochastic models that consider the multi-fractal nature of clouds, it is possible to scale cloud properties given at a coarse spatial resolution down to a finer resolution. Performing the radiative transfer within the spatially fine-resolved cloud fields noticeably helps to improve the radiation results. In the framework of this paper, we aim at characterizing cloud heterogeneity effects on radiances and broadband flux densities, namely: the errors due to unresolved variability (the so-called plane parallel homogeneous, PPH, bias) and the errors due to the neglect of transversal photon displacements (independent pixel approximation, IPA, bias). First, we study the effect of the missing cloud variability on reflectivities. We will show that the generation of subscale variability by means of stochastic methods greatly reduce or nearly eliminate the reflectivity biases. Secondly, three-dimensional broadband flux densities in the presence of realistic inhomogeneous cloud fields sampled at fine spatial resolutions are calculated and compared to their one-dimensional counterparts at coarser resolutions.

scholarly journals Bimodal chemical evolution of the Galactic disk and the Barium abundance of Cepheids

2013 ◽  
Vol 9 (S298) ◽  
pp. 86-91 ◽  
Author(s):  
Jacques R.D. Lépine ◽  
Sergei Andrievky ◽  
Douglas A. Barros ◽  
Thiago C. Junqueira ◽  
Sergio Scarano
Keyword(s):  
Chemical Evolution ◽  
Star Formation Rate ◽  
Galactic Disk ◽  
Formation Rate ◽  
Abundance Distribution ◽  
The Galaxy ◽  
Low Metallicity ◽  
The Difference ◽  
Two Sides ◽  
High Metallicity

AbstractIn order to understand the Barium abundance distribution in the Galactic disk based on Cepheids, one must first be aware of important effects of the corotation resonance, situated a little beyond the solar orbit. The thin disk of the Galaxy is divided in two regions that are separated by a barrier situated at that radius. Since the gas cannot get across that barrier, the chemical evolution is independent on the two sides of it. The barrier is caused by the opposite directions of flows of gas, on the two sides, in addition to a Cassini-like ring void of HI (caused itself by the flows). A step in the metallicity gradient developed at corotation, due to the difference in the average star formation rate on the two sides, and to this lack of communication between them. In connection with this, a proof that the spiral arms of our Galaxy are long-lived (a few billion years) is the existence of this step. When one studies the abundance gradients by means of stars which span a range of ages, like the Cepheids, one has to take into account that stars, contrary to the gas, have the possibility of crossing the corotation barrier. A few stars born on the high metallicity side are seen on the low metallicity one, and vice-versa. In the present work we re-discuss the data on Barium abundance in Cepheids as a function of Galactic radius, taking into account the scenario described above. The [Ba/H] ratio, plotted as a function of Galactic radius, apparently presents a distribution with two branches in the external region (beyond corotation). One can re-interpret the data and attribute the upper branch to the stars that were born on the high metallicity side. The lower branch, analyzed separately, indicates that the stars born beyond corotation have a rising Barium metallicity as a function of Galactic radius.

scholarly journals Spectroscopy with Three-Dimensional Model Atmospheres of Late-Type Stars

2015 ◽  
Vol 11 (A29B) ◽  
pp. 151-153
Author(s):  
Remo Collet
Keyword(s):  
Three Dimensional ◽  
Spectral Lines ◽  
Differential Analysis ◽  
Late Type ◽  
Dimensional Model ◽  
Three Dimensional Model ◽  
One Dimensional ◽  
Carbon Abundance

AbstractIn this contribution, I present the results of a differential analysis of CH spectral lines for the determination of carbon abundances with three- and one-dimensional model atmospheres of late-type stars at various metallicities. 3D-1D abundance corrections are found to be significant particularly at low metallicities and for turn-off stars. The dependence of 3D-1D carbon abundance corrections on the stellar C/O ratio is also discussed.

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