45 research outputs found
Non-LTE abundances of zinc in different spectral type stars and the Galactic [Zn/Fe] trend based on quantum-mechanical data on inelastic processes in zinc-hydrogen collisions
We present a new model atom of Zn I-II based on the most up-to-date
photoionisation cross-sections, electron-impact excitation rates, and rate
coefficients for the Zn I + H I and Zn II + H- collisions. The latter were
calculated using the multi-channel quantum asymptotic treatment based on the
Born-Oppenheimer approach. Non-LTE analysis was performed for the first time
for lines of Zn I and Zn II in the ultraviolet (UV) spectra of two very
metal-poor reference stars, HD 84937 and HD 140283. We found consistent non-LTE
abundance from the resonance Zn I 2138 A line, the subordinate lines, and the
lines of Zn II. In both stars, non-LTE leads to 0.17 dex higher average
abundance from Zn I, while, for Zn II lines, non-LTE corrections are minor and
do not exceed 0.06 dex. Using lines of Zn I in the high-resolution spectra, we
determined the non-LTE abundances for a sample of 80 stars in the -2.5 < [Fe/H]
< 0.2 metallicity range. The [Zn/Fe] versus [Fe/H] diagram reveals a dip, with
[Zn/Fe] = 0.3 in the most metal-poor stars, a close-to-solar value for [Fe/H] =
-1.2, and increasing [Zn/Fe] up to 0.3 in the thick disk stars. The
close-to-solar metallicity stars have subsolar [Zn/H] = -0.1, on average.
Non-LTE abundances of zinc were derived for the first time for seven reference
F to B-type stars. We provide a grid of the non-LTE abundance corrections.Comment: 15 pages, 9 figures, accepted to MNRA
Systematic NLTE study of the -2.6 < [Fe/H] < 0.2 F and G dwarfs in the solar neighbourhood. I. Stellar atmosphere parameters
We present atmospheric parameters for 51 nearby FG dwarfs uniformly
distributed over the -2.60 < [Fe/H] < +0.20 metallicity range that is suitable
for the Galactic chemical evolution research. Lines of iron, Fe I and Fe II,
were used to derive a homogeneous set of effective temperatures, surface
gravities, iron abundances, and microturbulence velocities. We used
high-resolution (R>60000) Shane/Hamilton and CFHT/ESPaDOnS observed spectra and
non-local thermodynamic equilibrium (NLTE) line formation for Fe I and Fe II in
the classical 1D model atmospheres. The spectroscopic method was tested with
the 20 benchmark stars, for which there are multiple measurements of the
infrared flux method (IRFM) Teff and their Hipparcos parallax error is < 10%.
We found NLTE abundances from lines of Fe I and Fe II to be consistent within
0.06 dex for every benchmark star, when applying a scaling factor of S_H = 0.5
to the Drawinian rates of inelastic Fe+H collisions. The obtained atmospheric
parameters were checked for each program star by comparing its position in the
log g-Teff plane with the theoretical evolutionary track in the Yi et al.
(2004) grid. Our final effective temperatures lie in between the T_IRFM scales
of Alonso et al. (1996) and Casagrande et al. (2011), with a mean difference of
+46 K and -51 K, respectively. NLTE leads to higher surface gravity compared
with that for LTE. The shift in log g is smaller than 0.1 dex for stars with
either [Fe/H] > -0.75, or Teff 4.20. NLTE analysis is
crucial for the VMP turn-off and subgiant stars, for which the shift in log g
between NLTE and LTE can be up to 0.5 dex. The obtained atmospheric parameters
will be used in the forthcoming papers to determine NLTE abundances of
important astrophysical elements from lithium to europium and to improve
observational constraints on the chemo-dynamical models of the Galaxy
evolution.Comment: 18 pages, 14 figures, accepted for publication in Ap
The Pristine survey -- XXII. A serendipitous discovery of an extremely Li-rich very metal-poor giant and a new method of Li/Li isotope measurement
We report the serendipitous discovery of a very metal-poor (VMP) Li-rich
giant star ( = 469080 K, log g = 1.340.13, [Fe/H] =
0.07). We analyse the Li I 6103 and 6707 \r{A} lines accounting for
departures from local thermodynamic equilibrium (NLTE) and correcting for 3D
effects using literature data, which yields a lithium abundance
. Comparing lithium abundances from the two
lines, in 1D NLTE we measure the isotope ratio Li/Li =
1.64 %. When correcting for 3D effects, we detect the fragile
Li isotope at -sigma level and the ratio Li/Li =
5.65 %. To our knowledge, this is the first Li/Li
measurement in an extremely Li-rich VMP star. The Cameron-Fowler mechanism,
which is proposed to produce Li-rich stars, does not imply Li production
and is therefore inconsistent with our measurement when applying 3D
corrections. We also derive NLTE abundances for 16 elements, most of which show
similar abundances to those found in VMP stars. Sodium is an exception:
[Na/Fe] = 0.07 , which is 0.5 dex higher than what
is typical for VMP stars. This star joins the sample of rare Li-rich VMP stars,
and we offer a novel way to constrain the source of lithium in such stars
through isotope ratio measurements.Comment: accepted for publication in MNRA
Fundamental parameters of the Ap-stars GO And, 84 UMa, and Îș Psc
Aims. The aim of this work is to determine fundamental parameters of three Ap stars, GO And (HD 4778), Îș Psc (HD 220825), and 84 UMa (HD 120198), using spectroscopic techniques. By analysing these stars, we complete the sample of Ap stars for which fundamental parameters have additionally been derived by means of interferometry. This enables a cross-comparison of results derived by direct and indirect methods. Methods. Our study is based on the analysis of high-resolution spectra with a high signal-to-noise ratio that were obtained with ESPaDOnS spectrograph. We used an iterative method of fundamental parameter determinations that includes self-consistent modelling of the stellar atmosphere, taking individual abundances of chemical elements into account and subsequently fitting a theoretical spectral energy distribution to the observed distribution. The quality of the spectroscopic determinations was evaluated through a comparison with the interferometric results. Results. For all investigated stars, we determined fundamental parameters and derived chemical abundances that are typical for Ap stars. The abundances are mainly characterised by a gradual increase of heavy element atmospheric abundances from an order of magnitude for iron peak elements up to very significant excesses of 3-4 dex of the rare-earth elements relative to the solar values. The only exception is Ba, whose abundance is close to the solar abundance. There is also a significant He deficiency in the atmospheres of HD 120198 and HD 220825, whereas the He abundance in HD 4778 is close to the solar abundance. We do not find significant Fe and Cr stratification. Using these abundances, we constructed self-consistent atmospheric models for each star. The effect of the surface chemical inhomogeneity on the derived fundamental parameters did not exceed ±100 K in effective temperature, which lies within the range of errors in similar self-consistent analyses of Ap stars. Finally, we compared spectroscopically derived effective temperatures, radii, and luminosity for 13 out of 14 Ap stars in a benchmark sample with the interferometric results. While radii and luminosity agree within the quoted errors of both determinations, the spectroscopic effective temperatures are higher than the interferometric temperatures for stars with Teff > 9000 K. The observed hydrogen line profiles favour the spectroscopically derived temperatures. © ESO 2021.This research has made use of the data from Gaia DR2 catalogues through the VizieR catalogue access tool. The use of the VALD database is acknowledged. This work is based on observations obtained with
ESpAdOnS spectropolarimeter. We use the archived data from EsPaDOnS spectrograph with programme ID: ID 09BQ78 (HD 4778), ID 16AC27 (HD 120198), and ID 18BC22 (HD 220825). A.R. acknowledges the financial support from the grant RFBR, project number 19-32-90147. D.S. acknowledges the financial
support from the State Agency for Research of the Spanish MCIU through the âCenter of Excellence Severo Ochoaâ award to the Instituto de AstrofĂsica de AndalucĂa (SEV-2017-0709).Peer reviewe