Impact of the convective mixing-length parameter α on stellar metallicity

2020 ◽  
Vol 635 ◽  
pp. A176 ◽  
Author(s):  
N. Song ◽  
S. Alexeeva ◽  
T. Sitnova ◽  
L. Wang ◽  
F. Grupp ◽  
...  
Keyword(s):  
Open Cluster ◽  
Synthesis Method ◽  
Stellar Atmospheres ◽  
High Signal ◽  
Mixing Length ◽  
Dwarf Stars ◽  
Giant Stars ◽  
Spectrum Synthesis ◽  
Mixing Length Theory ◽  
The Impact

Context. Mixing-length theory is used to treat stellar convection. As a simulation in one-dimensional stellar atmospheres models, the mixing-length parameter α is calibrated from the Sun and then applied to other stars. However, there is no strong evidence to suggest that α should be the same for stars of different evolutionary stages. Aims. We evaluate the impact of the α value on the metallicity of different types of stars and investigate the correlation between the metallicity discrepancy (Δ[Fe∕H]) and stellar parameters (Teff, log g). Methods. We selected ten well-studied field stars and one open cluster of nine members for which high-resolution and high signal-to-noise spectra are available. The model atmospheres were calculated with the code MAFAGS-OS. We derived iron abundances from Fe I and Fe II lines both under local thermodynamic equilibrium and non-LTE conditions using a spectrum synthesis method. After deriving [Fe/H] for each line, we calculated Δ[Fe∕H] with two different α values, fixed solar-calibrated α, and α obtained for each star individually. Finally, we investigated the correlation between Δ[Fe∕H] caused by revised α with stellar parameters. Results. For FGK dwarf stars, the Δ[Fe∕H] caused by the α correction is less than 0.02 dex, while for turn-off and giant stars, the Δ[Fe∕H] values are no more than 0.03 dex, which are lower than typical uncertainties in metallicity. For main-sequence stars, Δ[Fe∕H] versus Teff and Δ[Fe∕H] versus log g are well fit by linear relations.

NGC 6124: a young open cluster with anomalous- and fast-rotating giant stars

2021 ◽  
Author(s):  
N Holanda ◽  
N Drake ◽  
W J B Corradi ◽  
F A Ferreira ◽  
F Maia ◽  
...  
Keyword(s):  
Isotopic Ratio ◽  
Rotational Velocity ◽  
Open Cluster ◽  
Chemical Species ◽  
Stellar Atmospheres ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Analysis Technique ◽  
Young Open Cluster ◽  
Fast Rotating

Abstract We present the results of a chemical analysis of fast and anomalous rotator giants members of the young open cluster NGC 6124. For this purpose, we carried out abundances of the mixing sensitive species such as Li, C, N, Na and 12C/13C isotopic ratio, as well as other chemical species for a sample of four giants among the seven observed ones. This study is based on standard spectral analysis technique using high-resolution spectroscopic data. We also performed an investigation of the rotational velocity (v sin  i) once this sample exhibit abnormal values – giant stars commonly present rotational velocities of few km s−1. In parallel, we have been performed a membership study, making use of the third data release from ESA Gaia mission. Based on these data, we estimated a distance of d = 630 pc and an age of 178 Myr through isochrone fitting. After that procedure, we matched all the information raised and investigated the evolutionary stages and thermohaline mixing model through of spectroscopic Teff and log  g and mixing tracers, as 12C/13C and Na, of the studied stars. We derived a low mean metallicity of [Fe/H] = −0.13 ±0.05 and a modest enhancement of the elements created by the s-process such as Y, Zr, La, Ce, and Nd, which is in agreement of what has already been reported in the literature for young clusters. The giants analyzed have homogeneous abundances, except for lithium abundance [log  ε(Li)NLTE = 1.08±0.42] and this may be associated to a combination of mechanisms that act increasing or decreasing lithium abundances in stellar atmospheres.

scholarly journals New models for the convective flux in stellar atmospheres

1996 ◽  
Vol 176 ◽  
pp. 557-564 ◽  
Author(s):  
F. Kupka
Keyword(s):  
Stellar Evolution ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Turbulence Theory ◽  
Mixing Length ◽  
Convective Flux ◽  
The Past ◽  
New Models ◽  
F Stars ◽  
Mixing Length Theory

Over the past decades various forms of the mixing length theory (MLT) have been used to describe convection in stellar atmospheres. Recent advances in turbulence theory now allow for major improvements in modelling thermal convection. We review several models for convection which have been derived from turbulence theory, and describe one of them, the “CM model”, in detail. The CM model has been used in several stellar evolution and helioseismology codes during the last four years and has now been applied to model atmospheres. An overwiew comparing stellar atmosphere models based on the CM formulation with its MLT predecessors indicates improvements on model atmospheres for A and F stars.

scholarly journals The Gaia-ESO survey: Calibrating a relationship between age and the [C/N] abundance ratio with open clusters

2019 ◽  
Vol 629 ◽  
pp. A62 ◽  
Author(s):  
G. Casali ◽  
L. Magrini ◽  
E. Tognelli ◽  
R. Jackson ◽  
R. D. Jeffries ◽  
...  
Keyword(s):  
Open Cluster ◽  
Empirical Relationship ◽  
Star Clusters ◽  
Open Clusters ◽  
Open Star Clusters ◽  
Mixing Processes ◽  
Dwarf Stars ◽  
Additional Tool ◽  
Giant Stars ◽  
Stellar Ages

Context. In the era of large high-resolution spectroscopic surveys such as Gaia-ESO and APOGEE, high-quality spectra can contribute to our understanding of the Galactic chemical evolution by providing abundances of elements that belong to the different nucleosynthesis channels, and also by providing constraints to one of the most elusive astrophysical quantities: stellar age. Aims. Some abundance ratios, such as [C/N], have been proven to be excellent indicators of stellar ages. We aim at providing an empirical relationship between stellar ages and [C/N] using open star clusters, observed by the Gaia-ESO and APOGEE surveys, as calibrators. Methods. We used stellar parameters and abundances from the Gaia-ESO Survey and APOGEE Survey of the Galactic field and open cluster stars. Ages of star clusters were retrieved from the literature sources and validated using a common set of isochrones. We used the same isochrones to determine for each age and metallicity the surface gravity at which the first dredge-up and red giant branch bump occur. We studied the effect of extra-mixing processes in our sample of giant stars, and we derived the mean [C/N] in evolved stars, including only stars without evidence of extra mixing. By combining the Gaia-ESO and APOGEE samples of open clusters, we derived a linear relationship between [C/N] and (logarithmic) cluster ages. Results. We apply our relationship to selected giant field stars in the Gaia-ESO and APOGEE surveys. We find an age separation between thin- and thick-disc stars and age trends within their populations, with an increasing age towards lower metallicity populations. Conclusions. With this empirical relationship, we are able to provide an age estimate for giant stars in which C and N abundances are measured. For giant stars, the isochrone fitting method is indeed less sensitive than for dwarf stars at the turn-off. Our method can therefore be considered as an additional tool to give an independent estimate of the age of giant stars. The uncertainties in their ages is similar to those obtained using isochrone fitting for dwarf stars.

Study on the Influencing Factors of Gas Mixing Length in Nitrogen Displacement of Gas Pipeline

2013 ◽  
Vol 321-324 ◽  
pp. 299-304
Author(s):  
Huang Kun ◽  
Yan Xian ◽  
Kun Rong Shen
Keyword(s):  
Diffusion Theory ◽  
Back Pressure ◽  
Gas Pipeline ◽  
Impact Factors ◽  
Mixing Length ◽  
Gas Mixing ◽  
Pipe Length ◽  
Replacement Method ◽  
Mixing Length Theory ◽  
The Impact

The gas pipeline commissioning replacement is a vital section before the pipes being put into operation. It is related to the gas mixture length of segment regardless the replacement method, the gas mixing length theory formula is derived based on diffusion theory and convection diffusion equation. Taking the impact factors of gas movement into account, with software FLUENT, this paper simulated different flow regimes, pipe length, diameter, velocity, back pressure and temperature to influence the gas mixing length, the general influence law of various factors is identified. That is the gas mixing length increases with the increasing of pipe length, diameter, velocity, back pressure and temperature, the corresponding mixing length and displacement time are longer at laminar flow. The research results provide theoretical basis for the natural gas pipeline commissioning replacement to determine reasonable parameters.

scholarly journals Beryllium abundances along the evolutionary sequence of the open cluster IC 4651

2009 ◽  
Vol 5 (S268) ◽  
pp. 357-358 ◽  
Author(s):  
Rodolfo Smiljanic ◽  
L. Pasquini ◽  
C. Charbonnel ◽  
N. Lagarde
Keyword(s):  
Main Sequence ◽  
Open Cluster ◽  
High Signal ◽  
Evolutionary Sequence ◽  
Main Sequence Stars ◽  
Signal To Noise ◽  
Mixing Processes ◽  
Hydrodynamical Models ◽  
Spectrum Synthesis ◽  
Hertzsprung Gap

AbstractThe simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed features.

scholarly journals Precision angular diameters for 16 southern stars with VLTI/PIONIER

2020 ◽  
Vol 493 (2) ◽  
pp. 2377-2394
Author(s):  
Adam D Rains ◽  
Michael J Ireland ◽  
Timothy R White ◽  
Luca Casagrande ◽  
I Karovicova
Keyword(s):  
Broad Band ◽  
Unmet Need ◽  
High Signal ◽  
Correlated Errors ◽  
Single Star ◽  
Giant Stars ◽  
Stellar Diameters ◽  
Effective Temperatures ◽  
The Impact ◽  
Angular Diameters

ABSTRACT In the current era of Gaia and large, high signal-to-noise stellar spectroscopic surveys, there is an unmet need for a reliable library of fundamentally calibrated stellar effective temperatures based on accurate stellar diameters. Here, we present a set of precision diameters and temperatures for a sample of 6 dwarf, 5 sub-giant, and 5 giant stars observed with the PIONIER beam combiner at the VLTI. Science targets were observed in at least two sequences with five unique calibration stars each for accurate visibility calibration and to reduce the impact of bad calibrators. We use the standard PIONIER data reduction pipeline, but bootstrap over interferograms, in addition to employing a Monte Carlo approach to account for correlated errors by sampling stellar parameters, limb darkening coefficients, and fluxes, as well as predicted calibrator angular diameters. The resulting diameters were then combined with bolometric fluxes derived from broad-band Hipparcos–Tycho photometry and MARCS model bolometric corrections, plus parallaxes from Gaia to produce effective temperatures, physical radii, and luminosities for each star observed. Our stars have mean angular diameter and temperatures uncertainties of 0.8 per cent and 0.9 per cent, respectively, with our sample including diameters for 10 stars with no pre-existing interferometric measurements. The remaining stars are consistent with previous measurements, with the exception of a single star which we observe here with PIONIER at both higher resolution and greater sensitivity than was achieved in earlier work.

scholarly journals 3D non-LTE corrections for Li abundance and 6Li/7Li isotopic ratio in solar-type stars

2018 ◽  
Vol 618 ◽  
pp. A16 ◽  
Author(s):  
G. Harutyunyan ◽  
M. Steffen ◽  
A. Mott ◽  
E. Caffau ◽  
G. Israelian ◽  
...  
Keyword(s):  
Spectral Synthesis ◽  
Isotopic Ratio ◽  
Stellar Atmospheres ◽  
Line Profiles ◽  
Solar Mass ◽  
Lithium Abundance ◽  
Dwarf Stars ◽  
Link Type ◽  
Solar Type ◽  
The Impact

Context. Convective motions in solar-type stellar atmospheres induce Doppler shifts that affect the strengths and shapes of spectral absorption lines and create slightly asymmetric line profiles. One-dimensional (1D) local thermodynamic equilibrium (LTE) studies of elemental abundances are not able to reproduce this phenomenon, which becomes particularly important when modeling the impact of isotopic fine structure, like the subtle depression created by the 6Li isotope on the red wing of the Li I resonance doublet line. Aims. The purpose of this work is to provide corrections for the lithium abundance, A(Li), and the 6Li/7Li isotopic ratio that can easily be applied to correct 1D LTE lithium abundances in G and F dwarf stars of approximately solar mass and metallicity for three-dimensional (3D) and non-LTE (NLTE) effects. Methods. The corrections for A(Li) and 6Li/7Li are computed using grids of 3D NLTE and 1D LTE synthetic lithium line profiles, generated from 3D hydro-dynamical CO5BOLD and 1D hydrostatic model atmospheres, respectively. For comparative purposes, all calculations are performed for three different line lists representing the Li I λ670.8 nm spectral region. The 3D NLTE corrections are then approximated by analytical expressions as a function of the stellar parameters (Teff, log ℊ, [Fe/H], ν sin i, A(Li), 6Li/7Li). These are applied to adjust the 1D LTE isotopic lithium abundances in two solar-type stars, HD 207129 and HD 95456, for which high-quality HARPS observations are available. Results. The derived 3D NLTE corrections range between −0.01 and +0.11 dex for A(Li), and between −4.9 and −0.4% for 6Li/7Li, depending on the adopted stellar parameters. We confirm that the inferred 6Li abundance depends critically on the strength of the Si I 670.8025 nm line. Our findings show a general consistency with recent works on lithium abundance corrections. After the application of such corrections, we do not find a significant amount of 6Li in any of the two target stars. Conclusions. In the case of 6Li/7Li, our corrections are always negative, showing that 1D LTE analysis can significantly overestimate the presence of 6Li (up to 4.9% points) in the atmospheres of solar-like dwarf stars. These results emphasize the importance of reliable 3D model atmospheres combined with NLTE line formation for deriving precise isotopic lithium abundances. Although 3D NLTE spectral synthesis implies an extensive computational effort, the results can be made accessible with parametric tools like the ones presented in this work.

scholarly journals Rubidium abundances in solar metallicity stars

2021 ◽  
Vol 648 ◽  
pp. A107
Author(s):  
C. Abia ◽  
P. de Laverny ◽  
S. Korotin ◽  
A. Asensio Ramos ◽  
A. Recio-Blanco ◽  
...  
Keyword(s):  
Spectral Synthesis ◽  
Magnetic Activity ◽  
Point Of View ◽  
Asymptotic Giant Branch ◽  
High Signal ◽  
Asymptotic Giant Branch Stars ◽  
M Dwarfs ◽  
Giant Stars ◽  
Solar Metallicity ◽  
The Impact

Context. Rubidium is one of the few elements produced by the neutron capture s- and r-processes in almost equal proportions. Recently, a Rb deficiency ([Rb/Fe] < 0.0), amounting to a factor of about two with respect to the Sun, has been found in M dwarfs of near-solar metallicity. This stands in contrast to the close-to-solar [Sr, Zr/Fe] ratios derived in the same stars. This deficiency is difficult to understand from the point of view of observations and of nucleosynthesis. Aims. To test the reliability of this Rb deficiency, we study the Rb and Zr abundances in a sample of KM-type giant stars across a similar metallicity range, extracted from the AMBRE Project. Methods. We used high-resolution and high signal-to-noise spectra to derive Rb and Zr abundances in a sample of 54 bright giant stars with metallicities in the range of −0.6 ≲ [Fe/H] ≲ +0.4 dex, via spectral synthesis in both local and non-local thermodynamic equilibrium (LTE and NLTE, respectively). We also studied the impact of the Zeeman broadening in the profile of the Rb I at λ7800 Å line. Results. The LTE analysis also results in a Rb deficiency in giant stars, however, it is considerably lower than that obtained in M dwarfs. However, once NLTE corrections are performed, the [Rb/Fe] ratios are very close to solar (average −0.01 ± 0.09 dex) in the full metallicity range studied here. This stands in contrast to the value found for M dwarfs. The [Zr/Fe] ratios derived are in excellent agreement with those obtained in previous studies in FGK dwarf stars with a similar metallicity. We investigate the effect of gravitational settling and magnetic activity as possible causes of the Rb deficiency found in M dwarfs. Although the former phenomenon has a negligible impact on the surface Rb abundance, the presence of an average magnetic field with an intensity that is typical of that observed in M dwarfs may result in systematic Rb abundance underestimations if the Zeeman broadening is not considered in the spectral synthesis. This may explain the Rb deficiency in M dwarfs, but not fully. On the other hand, the new [Rb/Fe] and [Rb/Zr] versus [Fe/H] relationships can be explained when the Rb production by rotating massive stars and low-to-intermediate mass stars (these latter also producing Zr) are considered, without the need to deviate from the standard s-process nucleosynthesis in asymptotic giant branch stars, as suggested previously.

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