scholarly journals Main-sequence and sub-giant stars in the globular cluster NGC 6397: The complex evolution of the lithium abundance

2009 ◽  
Vol 5 (S268) ◽  
pp. 257-261
Author(s):  
J. I. González Hernández ◽  
P. Bonifacio ◽  
E. Caffau ◽  
M. Steffen ◽  
H.-G. Ludwig ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
3D Models ◽  
Stellar Atmospheres ◽  
Evolutionary Status ◽  
Thermodynamical Equilibrium ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Effective Temperatures ◽  
Hydrodynamical Simulations

AbstractThanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. We also detect a positive slope of Li abundance with effective temperature, the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide an unambiguous evidence that the Li abundance changes with evolutionary status.The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.

scholarly journals Doppler beaming factors for white dwarfs, main sequence stars, and giant stars

2020 ◽  
Vol 641 ◽  
pp. A157
Author(s):  
A. Claret ◽  
E. Cukanovaite ◽  
K. Burdge ◽  
P.-E. Tremblay ◽  
S. Parsons ◽  
...  
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Main Sequence ◽  
Theoretical Calculations ◽  
Three Dimensional ◽  
Black Body ◽  
3D Models ◽  
Giant Stars ◽  
Effective Temperatures ◽  
Limb Darkening

Context. Systematic theoretical calculations of Doppler beaming factors are scarce in the literature, particularly in the case of white dwarfs. Additionally, there are no specific calculations for the limb-darkening coefficients of 3D white dwarf models. Aims. The objective of this research is to provide the astronomical community with Doppler beaming calculations for a wide range of effective temperatures, local gravities, and hydrogen/metal content for white dwarfs as well as stars on both the main sequence and the giant branch. In addition, we present the theoretical calculations of the limb-darkening coefficients for 3D white dwarf models for the first time. Methods. We computed Doppler beaming factors for DA, DB, and DBA white dwarf models, as well as for main sequence and giant stars covering the transmission curves of the Sloan, UBVRI, HiPERCAM, Kepler, TESS, and Gaia photometric systems. The calculations of the limb-darkening coefficients for 3D models were carried out using the least-squares method for these photometric systems. Results. The input physics of the white dwarf models for which we have computed the Doppler beaming factors are: chemical compositions log [H/He] = −10.0 (DB), −2.0 (DBA), and He/H = 0 (DA), with log g varying between 5.0 and 9.5 and effective temperatures in the range 3750–100 000 K. The beaming factors were also calculated assuming non-local thermodynamic equilibrium for the case of DA white dwarfs with Teff >  40 000 K. For the mixing-length parameters, we adopted ML2/α = 0.8 (DA case) and 1.25 (DB and DBA). The Doppler beaming factors for main sequence and giant stars were computed using the ATLAS9 version, characterized by metallicities ranging from [−2.5, 0.2] solar abundances, with log g varying between 0 and 5.0 and effective temperatures between 3500 and 50 000 K. The adopted microturbulent velocity for these models was 2.0 km s−1. The limb-darkening coefficients were computed for three-dimensional DA and DB white dwarf models calculated with the CO5BOLD radiation-hydrodynamics code. The parameter range covered by the three-dimensional DA models spans log g values between 7.0 and 9.0 and Teff between 6000 and 15 000 K, while He/H = 0. The three-dimensional DB models cover a similar parameter range of log g between 7.5 and 9.0 and Teff between 12 000 and 34 000 K, while logH/He = −10.0. We adopted six laws for the computation of the limb-darkening coefficients: linear, quadratic, square root, logarithmic, power-2, and a general one with four coefficients. Conclusions. The beaming factor calculations, which use realistic models of stellar atmospheres, show that the black body approximation is not accurate, particularly for the filters u, u′, U, g, g′, and B. The black body approach is only valid for high effective temperatures and/or long effective wavelengths. Therefore, for more accurate analyses of light curves, we recommend the use of the beaming factors presented in this paper. Concerning limb-darkening, the distribution of specific intensities for 3D models indicates that, in general, these models are less bright toward the limb than their 1D counterparts, which implies steeper profiles. To describe these intensities better, we recommend the use of the four-term law (also for 1D models) given the level of precision that is being achieved with Earth-based instruments and space missions such as Kepler and TESS (and PLATO in the future).

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 Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours

2018 ◽  
Vol 613 ◽  
pp. A24 ◽  
Author(s):  
A. Kučinskas ◽  
J. Klevas ◽  
H.-G. Ludwig ◽  
P. Bonifacio ◽  
M. Steffen ◽  
...  
Keyword(s):  
Main Sequence ◽  
Broad Band ◽  
Stellar Atmospheres ◽  
Spectral Energy ◽  
Energy Distributions ◽  
Different Types ◽  
Convective Motions ◽  
Effective Temperatures ◽  
Red Giant ◽  
Colour Indices

Aims. We studied the influence of convection on the spectral energy distributions (SEDs), photometric magnitudes, and colour indices of different types of stars across the H–R diagram. Methods. The 3D hydrodynamical CO5BOLD, averaged ⟨3D⟩, and 1D hydrostatic LHD model atmospheres were used to compute SEDs of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M∕H] = 0.0 and − 2.0. Using the obtained SEDs, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHKs, and the medium-band Strömgren uvby photometric systems. Results. The 3D–1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed ± 0.03 mag. Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach ≈−0.2 mag at [M∕H] = 0.0 and ≈−0.1 mag at [M∕H] = −2.0. Generally, the 3D–1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D–1D differences at [M∕H] = 0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D–1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures.

scholarly journals The Gravity-Darkening of the Main-Sequence Components of Spectral Types A, F and G in Detached Close Binary Systems

1989 ◽  
Vol 107 ◽  
pp. 353-353
Author(s):  
Y. Nakamura ◽  
M. Kitamura
Keyword(s):  
Structural Change ◽  
Binary Systems ◽  
Main Sequence ◽  
Stellar Atmospheres ◽  
Close Binary ◽  
Close Binary Systems ◽  
Correction Process ◽  
Sequence Components ◽  
Effective Temperatures ◽  
Ellipticity Effect

Abstract.From analysis of the photometric ellipticity effect in seven well-understood detached close binary systems, empirical values of the exponent α of gravity-darkening have been practically deduced for eleven main-sequence components of spectral types A, F and G which should cover the range of structural change (from radiative to convective) in stellar atmospheres. The result indicate that values of the exponent diminish gradually with decreasing effective temperatures from α ~ 1.0 for radiative atmospheres with T > 8500 K to α =0.2 ~ 0.3 for convective atmospheres with T < 6500 K, in spite of some uncertainty in the reflection correction process.

scholarly journals Lithium abundance patterns of late-F stars: an in-depth analysis of the lithium desert

2018 ◽  
Vol 614 ◽  
pp. A55 ◽  
Author(s):  
Claudia Aguilera-Gómez ◽  
Iván Ramírez ◽  
Julio Chanamé
Keyword(s):  
Effective Temperature ◽  
Main Sequence ◽  
Atmospheric Parameters ◽  
Lithium Abundance ◽  
Temperature Range ◽  
Abundance Patterns ◽  
Depth Analysis ◽  
Effective Temperatures ◽  
F Stars

Aims. We address the existence and origin of the lithium (Li) desert, a region in the Li–Teff plane sparsely populated by stars. Here we analyze some of the explanations that have been suggested for this region, including mixing in the late main sequence, a Li dip origin for stars with low Li abundances in the region, and a possible relation with the presence of planets. Methods. To study the Li desert, we measured the atmospheric parameters and Li abundance of 227 late-F dwarfs and subgiants, chosen to be in the Teff range of the desert and without previous Li abundance measurements. Subsequently, we complemented those with literature data to obtain a homogeneous catalog of 2318 stars, for which we compute masses and ages. We characterize stars surrounding the region of the Li desert. Results. We conclude that stars with low Li abundances below the desert are more massive and more evolved than stars above the desert. Given the unexpected presence of low Li abundance stars in this effective temperature range, we concentrate on finding their origin. We conclude that these stars with low Li abundance do not evolve from stars above the desert: at a given mass, stars with low Li (i.e., below the desert) are more metal-poor. Conclusions. Instead, we suggest that stars below the Li desert are consistent with having evolved from the Li dip, discarding the need to invoke additional mixing to explain this feature. Thus, stars below the Li desert are not peculiar and are only distinguished from other subgiants evolved from the Li dip in that their combination of atmospheric parameters locates them in a range of effective temperatures where otherwise only high Li abundance stars would be found (i.e., stars above the desert).

scholarly journals Improving spectroscopic lithium abundances

2020 ◽  
Vol 638 ◽  
pp. A58 ◽  
Author(s):  
A. Mott ◽  
M. Steffen ◽  
E. Caffau ◽  
K. G. Strassmeier
Keyword(s):  
Equivalent Width ◽  
3D Models ◽  
Stellar Atmospheres ◽  
Lithium Content ◽  
Lithium Abundance ◽  
Analytical Functions ◽  
Open Questions ◽  
Wide Range ◽  
1D Model ◽  
Host Stars

Context. Accurate spectroscopic lithium abundances are essential in addressing a variety of open questions, such as the origin of a uniform lithium content in the atmospheres of metal-poor stars (Spite plateau) or the existence of a correlation between the properties of extrasolar planetary systems and the lithium abundance in the atmosphere of their host stars. Aims. We have developed a tool that allows the user to improve the accuracy of standard lithium abundance determinations based on 1D model atmospheres and the assumption of local thermodynamic equilibrium (LTE) by applying corrections that account for hydrodynamic (3D) and non-LTE (NLTE) effects in FGK stars of different metallicity. Methods. Based on a grid of CO5BOLD 3D models and associated 1D hydrostatic atmospheres, we computed three libraries of synthetic spectra of the lithium λ 670.8 nm line for a wide range of lithium abundances, accounting for detailed line formation in 3D NLTE, 1D NLTE, and 1D LTE, respectively. The resulting curves-of-growth were then used to derive 3D NLTE and 1D NLTE lithium abundance corrections. Results. For all metallicities, the largest corrections are found at the coolest effective temperature, Teff = 5000 K. They are mostly positive, up to + 0.2 dex, for the weakest lines (lithium abundance A(Li)1DLTE = 1.0), whereas they become more negative towards lower metallicities, where they can reach − 0.4 dex for the strongest lines (A(Li)1DLTE = 3.0) at [Fe/H] = − 2.0. We demonstrate that 3D and NLTE effects are small for metal-poor stars on the Spite plateau, leading to errors of at most ± 0.05 dex if ignored. We present analytical functions evaluating the 3D NLTE and 1D NLTE corrections as a function of Teff [5000…6500 K], log g [3.5…4.5], and LTE lithium abundance A(Li) [1.0…3.0] for a fixed grid of metallicities [Fe∕H] [ − 3.0…0.0]. In addition, we also provide analytical fitting functions for directly converting a given lithium abundance into an equivalent width, or vice versa, a given equivalent width (EW) into a lithium abundance. For convenience, a Python script is made available that evaluates all fitting functions for given Teff, log g, [Fe∕H], and A(Li) or EW. Conclusions. By means of the fitting functions developed in this work, the results of complex 3D and NLTE calculations are made readily accessible and quickly applicable to large samples of stars across a wide range of metallicities. Improving the accuracy of spectroscopic lithium abundance determinations will contribute to a better understanding of the open questions related to the lithium content in metal-poor and solar-like stellar atmospheres.

scholarly journals Spectroscopy of main-sequence and subgiant stars in globular star clusters

1984 ◽  
Vol 105 ◽  
pp. 139-142
Author(s):  
James E. Hesser ◽  
Gretchen L.H. Harris ◽  
R.A. Bell ◽  
R.D. Cannon
Keyword(s):  
Globular Cluster ◽  
Main Sequence ◽  
Star Clusters ◽  
Observational Constraints ◽  
Spectral Features ◽  
Small Temperature ◽  
Giant Stars ◽  
Magnitude Diagram ◽  
Subgiant Stars ◽  
Complex Subject

Since the mid-1970's it has been apparent that giant stars of similar V and B-V within a “normal” globular cluster [i.e., one with a narrow giant branch in its color-magnitude diagram (CMD)] exhibit a perplexing range of strengths for such spectral features as CH, CN and NH. This complex subject has been reviewed by Kraft (1979), McClure (1979) and Freeman and Norris (1981). DDO photometry first revealed star-to-star differences of CN strengths at MV>+1, where observational confusion between asymptotic and first-ascent giant stars is removed (Hesser, Hartwick and McClure 1976, 1977; Hesser 1978). Subsequently, we have sought to place observational constraints on possible mechanisms by studying such questions as: At what MV's do spectral differences first become observable? Do spectral features other than those from CNO-based molecules vary from star-to-star? Can small temperature or gravity differences produce the observed ranges?

scholarly journals Mathematical Assessment of Physical and Chemical Processes from the Middle B to the Early F Type Main Sequence Stars

2017 ◽  
Vol 12 (S330) ◽  
pp. 362-363
Author(s):  
Kutluay Yüce ◽  
Saul J. Adelman
Keyword(s):  
Main Sequence ◽  
Stellar Atmospheres ◽  
Stellar Winds ◽  
Main Sequence Stars ◽  
Energy Distributions ◽  
Elemental Abundances ◽  
Effective Temperatures ◽  
Evolutionary Paths ◽  
Physical And Chemical ◽  
Hr Diagram

AbstractThe middle B to the early F Main Sequence stars have some of the most quiet stellar atmospheres. In this part of the HR diagram we find stars with atmospheres in radiative equilibrium. They lack the convective circulations of the middle F and cooler stars and the massive stellar winds of hotter stars. When stars of different mass evolve off the Main Sequence in this part of the HR Diagram their evolutionary paths do not cross initially. Thus stars with the same effective temperature and surface gravity have the same luminosity and mass. By comparing their elemental abundances, we might be able to identify physical processes which cause any differences in their abundances. Here we begin with stars whose effective temperatures and surface gravities are similar, and which have been analyzed by us using spectra obtained from the Dominion Astrophysical Observatory (DAO). Improvements in our knowledge of the energy distributions of stars (for example via GAIA measurements) should lead to improved estimates of stellar effective temperatures and surface gravities.

scholarly journals Turbulence and the Lithium Abundance in Giants and Main-Sequence Disk and Halo Stars

1991 ◽  
Vol 145 ◽  
pp. 327-340
Author(s):  
S. Vauclair
Keyword(s):  
Main Sequence ◽  
Present Situation ◽  
Point Of View ◽  
Theoretical Point ◽  
Main Sequence Stars ◽  
Lithium Abundance ◽  
Giant Stars ◽  
Halo Stars

The present situation concerning the observations of lithium in subgiant and giant stars is reviewed. These observations, compared to the determinations of lithium in main-sequence stars, can be used to obtain more stringents constraints on the dynamics of the stellar gas in the main-sequence as well as in the subgiant and giant stages. The macroscopic motions expected in stars are discussed from a theoretical point of view and compared to the observations, with special focusing on the theories of rotation-induced turbulence.

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