Abstract
We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of neutral and singly ionized iron lines and their impact on spectroscopic stellar parameters. The ...calculations were performed for an extensive grid of 1D marcs models of metal-rich and metal-poor late-type dwarfs and giants. We find that iron abundances derived from Fe i lines are increasingly underestimated in hotter, lower surface gravity and more metal-poor stars, in a simple and well-defined pattern, while LTE is usually a realistic approximation for Fe ii lines. For the vast majority of dwarfs and giants, the perturbed ionization balance of Fe i and Fe ii is the main relevant non-LTE effect to consider in the determination of spectroscopic parameters, while for extremely metal-poor stars and hot giant stars significant impact is seen also on the excitation balance and on the microturbulence determination from Fe i lines.
Aims. Aluminium plays a key role in studies of the chemical enrichment of the Galaxy and of globular clusters. However, strong deviations from LTE (non-LTE) are known to significantly affect the ...inferred abundances in giant and metal-poor stars. Methods. We present non-local thermodynamic equilibrium (NLTE) modeling of aluminium using recent and accurate atomic data, in particular utilizing new transition rates for collisions with hydrogen atoms, without the need for any astrophysically calibrated parameters. For the first time, we perform 3D NLTE modeling of aluminium lines in the solar spectrum. We also compute and make available extensive grids of abundance corrections for lines in the optical and near-infrared using one-dimensional model atmospheres, and apply grids of precomputed departure coefficients to direct line synthesis for a set of benchmark stars with accurately known stellar parameters. Results. Our 3D NLTE modeling of the solar spectrum reproduces observed center-to-limb variations in the solar spectrum of the 7835 Å line as well as the mid-infrared photospheric emission line at 12.33 μm. We infer a 3D NLTE solar photospheric abundance of A(Al) = 6.43 ± 0.03, in exact agreement with the meteoritic abundance. We find that abundance corrections vary rapidly with stellar parameters; for the 3961 Å resonance line, corrections are positive and may be as large as +1 dex, while corrections for subordinate lines generally have positive sign for warm stars but negative for cool stars. Our modeling reproduces the observed line profiles of benchmark K-giants, and we find abundance corrections as large as −0.3 dex for Arcturus. Our analyses of four metal-poor benchmark stars yield consistent abundances between the 3961 Å resonance line and lines in the UV, optical and near-infrared regions. Finally, we discuss implications for the galactic chemical evolution of aluminium.
As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using ...archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic stagger model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe i/Fe ii excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is overestimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves. We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally and temporally averaged 3D stagger model atmospheres.
Neutral sodium is a minority species in the atmospheres of late-type stars, and line formation in local thermodynamic equilibrium (LTE) is often a poor assumption, in particular for strong lines. We ...present an extensive grid of non-LTE calculations for several Na I lines in cool stellar atmospheres, including metal-rich and metal-poor dwarfs and giants. For the first time, we constructed a Na model atom that incorporates accurate quantum mechanical calculations for collisional excitation and ionisation by electrons as well as collisional excitation and charge exchange reactions with neutral hydrogen. Similar to Li I, the new rates for hydrogen impact excitation do not affect the statistical equilibrium calculations, while charge exchange reactions have a small but non-negligible influence. The presented LTE and non-LTE curves-of-growth can be interpolated to obtain non-LTE abundances and abundance corrections for arbitrary stellar parameter combinations and line strengths. The typical corrections for weak lines are −0.1... −0.2 dex, whereas saturated lines may overestimate the abundance in LTE by more than 0.5 dex. The non-LTE Na abundances appear very robust with respect to uncertainties in the input collisional data.
Massive sets of stellar spectroscopic observations are rapidly becoming available and these can be used to determine the chemical composition and evolution of the Galaxy with unprecedented precision. ...One of the major challenges in this endeavour involves constructing realistic models of stellar spectra with which to reliably determine stellar abundances. At present, large stellar surveys commonly use simplified models that assume that the stellar atmospheres are approximately in local thermodynamic equilibrium (LTE). To test and ultimately relax this assumption, we have performed non-LTE calculations for 13 different elements (H, Li, C, N, O, Na, Mg, Al, Si, K, Ca, Mn, and Ba), using recent model atoms that have physically-motivated descriptions for the inelastic collisions with neutral hydrogen, across a grid of 3756 1D
MARCS
model atmospheres that spans 3000 ≤
T
eff
∕K ≤ 8000, − 0.5 ≤log
g
∕cm s
−2
≤ 5.5, and − 5 ≤ Fe/H ≤ 1. We present the grids of departure coefficients that have been implemented into the GALAH DR3 analysis pipeline in order to complement the extant non-LTE grid for iron. We also present a detailed line-by-line re-analysis of 50 126 stars from GALAH DR3. We found that relaxing LTE can change the abundances by between − 0.7 dex and + 0.2 dex for different lines and stars. Taking departures from LTE into account can reduce the dispersion in the A/Fe versus Fe/H plane by up to 0.1 dex, and it can remove spurious differences between the dwarfs and giants by up to 0.2 dex. The resulting abundance slopes can thus be qualitatively different in non-LTE, possibly with important implications for the chemical evolution of our Galaxy. The grids of departure coefficients are publicly available and can be implemented into LTE pipelines to make the most of observational data sets from large spectroscopic surveys.
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 SMSS0313-6708by means of the most realistic methods available, and compare the results to predicted supernova yields. Methods. A 3D hydrodynamical Staggermodel 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 = 1051 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.
Abstract
We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of Fe for a number of well-studied late-type stars in different evolutionary stages. A new model of ...the Fe atom was constructed from the most up-to-date theoretical and experimental atomic data available so far. Non-LTE (NLTE) line formation calculations for Fe were performed using 1D hydrostatic marcs and mafags-os model atmospheres, as well as the spatial and temporal average stratifications from full 3D hydrodynamical simulations of stellar convection computed using the stagger code. It is shown that the Fe i/Fe ii ionization balance can be well established with the 1D and mean 3D models under NLTE including calibrated inelastic collisions with H i calculated from Drawin's formulae. Strong low-excitation Fe i lines are very sensitive to the atmospheric structure; classical 1D models fail to provide consistent excitation balance, particularly so for cool metal-poor stars. A better agreement between Fe i lines spanning a range of excitation potentials is obtained with the mean 3D models. Mean NLTE metallicities determined for the standard stars using the 1D and mean 3D models are fully consistent. Moreover, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g. the infrared flux method and parallaxes, if one of the stellar parameters is constrained independently.
ABSTRACT
We report the discovery of SMSS J160540.18−144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure $\left\rm {Fe}/\rm {H}\right= -6.2 \pm 0.2$ (1D ...LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, $\left\rm {C}/\rm {Fe}\right = 3.9 \pm 0.2$, while other abundances are compatible with an α-enhanced solar-like pattern with $\left\rm {Ca}/\rm {Fe}\right = 0.4 \pm 0.2$, $\left\rm {Mg}/\rm {Fe}\right = 0.6 \pm 0.2$, $\left\rm {Ti}/\rm {Fe}\right = 0.8 \pm 0.2$, and no significant s- or r-process enrichment, $\left\rm {Sr}/\rm {Fe}\right \lt 0.2$ and $\left\rm {Ba}/\rm {Fe}\right \lt 1.0$ (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about $10\, \rm M_\odot$ imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly $20\, \rm M_\odot$ are incompatible with the observed abundance pattern.
ABSTRACT
We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, ...we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity (Fe/H) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have Fe/H ≤ −2.75 and only approximately seven per cent have Fe/H > −2.0 dex. The most metal-poor candidate in the sample has Fe/H < −4.75 and is notably carbon rich. Except at the lowest metallicities (Fe/H < −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with C/Fe1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus Fe/H1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE > 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE < 7.3 and C/Fe1D,LTE > +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed sample has a power-law slope of Δ(Log N)/ΔFe/H = 1.5 ± 0.1 dex per dex for −4.0 ≤ Fe/H ≤ −2.75, but appears to drop abruptly at Fe/H ≈ −4.2, in line with previous studies.
Context. Un-evolved, very metal-poor stars are the most important tracers of the cosmic abundance of lithium in the early universe. Combining the standard Big Bang nucleosynthesis model with Galactic ...production through cosmic ray spallation, these stars at Fe / H < − 2 are expected to show an undetectably small 6Li / 7Li isotopic signature. Evidence to the contrary may necessitate an additional pre-galactic production source or a revision of the standard model of Big Bang nucleosynthesis. It would also cast doubts on Li depletion from stellar atmospheres as an explanation for the factor 3–5 discrepancy between the predicted primordial 7Li from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. Aims. We revisit the isotopic analysis of four halo stars, two with claimed 6Li-detections in the literature, to investigate the influence of improved model atmospheres and line formation treatment. Methods. For the first time, a combined 3D, non-local thermodynamic equilibrium (NLTE) modelling technique for Li, Na, and Ca lines is utilised to constrain the intrinsic line-broadening and to determine the Li isotopic ratio. We discuss the influence of 3D NLTE effects on line profile shapes and assess the realism of our modelling using the Ca excitation and ionisation balance. Results. By accounting for NLTE line formation in realistic 3D hydrodynamical model atmospheres, we can model the Li resonance line and other neutral lines with a consistency that is superior to LTE, with no need for additional line asymmetry caused by the presence of 6Li. Contrary to the results from 1D and 3D LTE modelling, no star in our sample has a significant (2σ) detection of the lighter isotope in NLTE. Over a large parameter space, NLTE modelling systematically reduces the best-fit Li isotopic ratios by up to five percentage points. As a bi-product, we also present the first ever 3D NLTE Ca and Na abundances of halo stars, which reveal significant departures from LTE. Conclusions. The observational support for a significant and non-standard 6Li production source in the early universe is substantially weakened by our findings.
PDF
