J/A+A/618/A141 Inelastic H-atom collisions NLTE corrections (Ezzeddine+, 2018)
An empirical recipe for inelastic hydrogen-atom collisions in non-LTE
calculations.
Ezzeddine R., Merle T., Plez B., Gebran M., Thevenin F., Van der Swaelmen M.
<Astron. Astrophys. 618, A141 (2018)>
=2018A&A...618A.141E 2018A&A...618A.141E (SIMBAD/NED BibCode)
ADC_Keywords: Atomic physics
Keywords: atomic processes - line: formation - stars: abundances -
stars: atmospheres - stars: late-type
Abstract:
Determination of high-precision abundances of late-type stars has been
and always will be an important goal of spectroscopic studies, which
requires accurate modeling of their stellar spectra with non-local
thermodynamic equilibrium (NLTE) radiative transfer methods. This
entails using up-to-date atomic data of the elements under study,
which are still subject to large uncertainties. We investigate the
role of hydrogen collisions in NLTE spectral line synthesis, and
introduce a new general empirical recipe to determine inelastic charge
transfer (CT) and bound-bound hydrogen collisional rates. This recipe
is based on fitting the energy functional dependence of published
quantum collisional rate coefficients of several neutral elements
(BeI, NaI, MgI, AlI, SiI and CaI) using simple polynomial equations.
We perform thorough NLTE abundance calculation tests using our method
for four different atoms, Na, Mg, Al and Si, for a broad range of
stellar parameters. We then compare the results to calculations
computed using the published quantum rates for all the corresponding
elements. We also compare to results computed using excitation
collisional rates via the commonly used Drawin equation for different
fudge factors, SH, applied. We demonstrate that our proposed method is
able to reproduce the NLTE abundance corrections performed with the
quantum rates for different spectral types and metallicities for
representative NaI and AlI lines to within 0.05dex and 0.03dex,
respectively.
For MgI and SiI lines, the method performs better for the cool giants
and dwarfs, while larger discrepancies up to 0.20dex could be
obtained for some lines for the subgiants and warm dwarfs. We obtained
larger NLTE correction differences between models incorporating Drawin
rates relative to the quantum models by up to 0.40dex. These large
discrepancies are potentially due to ignoring either or both CT and
ionization collisional processes by hydrogen in our Drawin models.Our
general empirical fitting method (EFM) for estimating hydrogen
collision rates performs well in its ability to reproduce, within
narrow uncertainties, the abundance corrections computed with models
incorporating quantum collisional rates. It performs generally best
for the cool and warm dwarfs, with slightly larger discrepancies
obtained for the giants and subgiants. It could possibly be extended
in the future to transitions of the same elements for which quantum
calculations do not exist, or, in the absence of published quantum
calculations, to other elements as well.
Description:
Tables of NLTE corrections obtained for selected lines of NaI, MgI,
AlI and SiI computed for each element with four different atoms of
different implementations of hydrogen collisions using (i) the
published quantum rates (QM), (ii) our proposed fitting method (EFM),
(iii) the Drawin equation with SH=0.1 (DRW,SH=0.1) (iv) the Drawin
equation with SH=1.0 (DRW,SH=1.0), as well as (v) the Drawin
equation with SH=0.002 (DRW,SH=0.002) for AlI, for a range of
stellar parameters.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 79 162 NLTE corrections obtained for NaI, MgI, AlI and
SiI at [Fe/H]=+0.0 and [Fe/H]=-1.0 for
Teff=4500 and logg=2.5, logg=3.5 and logg=4.5
tablea2.dat 79 164 NLTE corrections obtained for NaI, MgI, AlI and
SiI at [Fe/H]=+0.0 and [Fe/H]=-1.0 for
Teff=5500 and logg=3.5 and logg=4.5 and for
Teff=6500 and logg=4.5
tablea3.dat 79 162 NLTE corrections obtained for NaI, MgI, AlI and
SiI at [Fe/H]=-2.0 and [Fe/H]=-3.0 for
Teff=4500 and logg=2.5, logg=3.5 and logg=4.5
tablea4.dat 79 162 NLTE corrections obtained for NaI, MgI, AlI and
SiI at [Fe/H]=-2.0 and [Fe/H]=-3.0 for
Teff=5500 and logg=3.5 and logg=4.5 and for
Teff=6500 and logg=4.5
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Byte-by-byte Description of file: tablea?.dat
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Bytes Format Units Label Explanations
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1- 4 I4 K Teff Effective Temperature
9- 11 F3.1 [cm/s2] logg Surface Gravity
15- 19 F5.2 [-] [Fe/H] Metallicity
23- 25 A3 --- El Elemental Species
29- 36 F8.3 nm lam Wavelength
40- 45 F6.3 --- Del(QM) NLTE abundance correction using QM
48- 53 F6.3 --- Del(EFM) NLTE abundance correction using EFM
56- 61 F6.3 --- Del(DRW-0.1) NLTE abundance correction using
DRW,SH=0.1
64- 69 F6.3 --- Del(DRW-1.0) NLTE abundance correction using
DRW,SH=1.0
74- 79 F6.3 --- Del(DRW-0.002) ? NLTE abundance correction using
DRW,SH=0.002
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Acknowledgements:
Rana Ezzeddine, ranae(at)mit.edu
(End) Patricia Vannier [CD] 13-Sep-2018