The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

Osorio, Yeisson; Barklem, P. S.; Lind, Karin; Asplund, Martin

doi:10.1051/0004-6361/201016418

Cited by 12 publications

(23 citation statements)

References 32 publications

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“…The level energies and the spontaneous radiative decay rate are taken from Klevas et al (2016). We only consider electron impact as the collisional excitation process and its rate coefficients are taken from Osorio et al (2011). The cooling rate including the line self-absorption β esc Λ Li,thin = n(Li)n(e)L Li /ρ (erg g −1 s −1 ) is calculated for given sets of the temperature T, the electron number density n(e) and the lithium column density N(Li).…”

Section: Summary and Discussionmentioning

confidence: 99%

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

Matsukoba

Takahashi

Sugimura

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Supermassive stars (SMSs) with mass ∼ 10 5 M are promising candidates for the origin of supermassive black holes observed at redshift 6. They are supposed to form as a result of rapid accretion of primordial gas, although it can be obstructed by the time variation caused by circum-stellar disc fragmentation due to gravitational instability. To assess the occurrence of fragmentation, we study the structure of marginally gravitationally unstable accretion discs, by using a steady one-dimensional thin disc model with detailed treatment of chemical and thermal processes. Motivated by two SMS formation scenarios, i.e., those with strong ultraviolet radiation background or with large velocity difference between the baryon and the dark matter, we consider two types of flows, i.e., atomic and molecular flows, respectively, for a wide range of the central stellar mass 10 − 10 5 M and the accretion rate 10 −3 − 1 M yr −1 . In the case of a mostly atomic gas flowing to the disc outer boundary, the fragmentation condition is expressed as the accretion rate being higher than the critical value of 10 −1 M yr −1 regardless of the central stellar mass. On the other hand, in the case of molecular flows, there is a critical disc radius outside of which the disc becomes unstable. Those conditions appears to be marginally satisfied according to numerical simulations, suggesting that disc fragmentation can be common during SMS formation.

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Section: Summary and Discussionmentioning

confidence: 99%

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

Matsukoba

Takahashi

Sugimura

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…For lithium, the dominant electron collisional rates come from Park (1971). While the R-matrix calculations performed by Osorio et al (2011) differed significantly, the dominant transitions happen to agree sufficiently well that the effects on the resonance line strength are smaller than 0.01 dex.…”

Section: Lithium and Sodiummentioning

confidence: 93%

3D NLTE analysis of the most iron-deficient star, SMSS0313-6708

Nordlander

Amarsi

Lind

et al. 2016

A&A

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104

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Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1−10 M ), to massive (10−100 M ), or even supermassive (100−1000 M ). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE) are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 by means of the most realistic methods available, and compare the results to predicted supernova yields. Methods. A 3D hydrodynamical Stagger model atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < −6.53 (3σ), than previous estimates based on 3D NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D− 3D NLTE differences, all of which lead to a higher abundance determination. Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20−60 M exploding with low energies (1−2 B, where 1 B = 10 51 erg), we also find good fits for progenitors of 10 M , with very low explosion energies (<1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae.

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“…Before calculating the RM data, the data of Park (1971) were used in non-LTE modelling, and these data typically agree to within 80% of the RM data, significantly better than the vR or IP methods. Thus, the new RM data for electron collisions did not have a strong effect on non-LTE modelling, since the existing data were relatively good; see Osorio et al (2011) for Li, Lind et al (2011) for Na. Data for hydrogen collision processes, including charge transfer, have been calculated for Li by Barklem et al (2003), and for Na by Barklem et al (2010), and used in non-LTE studies by Barklem et al and Lind et al (2009) in the case of Li and Lind et al (2011) for Na.…”

Section: Discussionmentioning

confidence: 99%

“…RM calculations have been performed for excitation of low-lying levels of Li and Na by electron collisions by Osorio et al (2011) and Gao et al (2010), respectively. Before calculating the RM data, the data of Park (1971) were used in non-LTE modelling, and these data typically agree to within 80% of the RM data, significantly better than the vR or IP methods.…”

Section: Discussionmentioning

confidence: 99%

Mg line formation in late-type stellar atmospheres

Osorio

Barklem

Lind

et al. 2015

A&A

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130

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Context. Magnesium is an element of significant astrophysical importance, often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from local thermodynamic equilibrium (LTE). The importance of Mg, together with the unique range of spectral features in late-type stars probing different parts of the atom, as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in stellar atmospheres. Previous non-LTE modelling of spectral line formation has, however, been subject to uncertainties due to lack of accurate data for inelastic collisions with electrons and hydrogen atoms. Aims. In this paper we build and test a Mg model atom for spectral line formation in late-type stars with new or recent inelastic collision data and no associated free parameters. We aim to reduce these uncertainties and thereby improve the accuracy of Mg non-LTE modelling in late-type stars.Methods. For the low-lying states of Mg i, electron collision data were calculated using the R-matrix method. Hydrogen collision data, including charge transfer processes, were taken from recent calculations by some of us. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. This model was then employed in the context of standard non-LTE modelling in 1D (including average 3D) model atmospheres in a small set of stellar atmosphere models. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed. Results. The modelled spectra agree well with observed spectra from benchmark stars, showing much better agreement with line profile shapes than with LTE modelling. The line-to-line scatter in the derived abundances shows some improvements compared to LTE (where the cores of strong lines must often be ignored), particularly when coupled with averaged 3D models. The observed Mg emission features at 7 and 12 μm in the spectra of the Sun and Arcturus, which are sensitive to the collision data, are reasonably well reproduced. Charge transfer with H is generally important as a thermalising mechanism in dwarfs, but less so in giants. Excitation due to collisions with H is found to be quite important in both giants and dwarfs. The R-matrix calculations for electron collisions also lead to significant differences compared to when approximate formulas are employed. The modelling predicts non-LTE abundance corrections ΔA(Mg) NLTE−LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Conclusions. Our results emphasise ...

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The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

Cited by 12 publications

References 32 publications

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

Gravitational stability and fragmentation condition for discs around accreting supermassive stars

3D NLTE analysis of the most iron-deficient star, SMSS0313-6708

Mg line formation in late-type stellar atmospheres

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