First discovery of trans-iron elements in a DAO-type white dwarf (BD−22°3467)

ABSTRACT We have identified 484 lines of the trans-iron elements (TIEs) Zn, Ga, Ge, Se, Br, Kr, Sr, Zr, Mo, In, Te, I, Xe, and Ba, for the first time in the ultraviolet spectrum of a DAO-type white dwarf (WD), namely BD−22°3467, surrounded by the ionized nebula Abell 35. Our TIE abundance determination shows extremely high overabundances of up to 5 dex – a similar effect is already known from hot, H-deficient (DO-type) WDs. In contrast to these where a pulse-driven convection zone has enriched the photosphere with TIEs during a final thermal pulse and radiative levitation has established the extreme TIE overabundances, here the extreme TIE overabundances are exclusively driven by radiative levitation on the initial stellar metallicity. The very low mass ($0.533^{+0.040}_{-0.025}\, \mathrm{M}_\odot$) of BD−22°3467 implies that a third dredge-up with enrichment of s-process elements in the photosphere did not occur in the asymptotic giant branch (AGB) precursor.

A Bayesian direct method implementation to fit emission line spectra: application to the primordial He abundance determination

ABSTRACT This work presents a Bayesian algorithm to fit the recombination and collisionally excited line spectra of gas photoionized by clusters of young stars. The current model consists of 14 dimensions: two electron temperatures, one electron density, the extinction coefficient, the optical depth on the $\rm He\, \small {I}$ recombination lines, and nine ionic species. The results are in very good agreement with those previously published using the traditional methodology. The probabilistic programming library pymc3 was chosen to explore the parameter space via an NUTs sampler. These machine learning tools provided excellent convergence quality and speed. The primordial helium abundance measured from a multivariable regression using oxygen, nitrogen, and sulfur was $Y_{\mathrm{ P},\, \mathrm{ O}\!-\!\mathrm{ N}\!-\!\mathrm{ S}}=0.243\pm 0.005$ in agreement with a standard Big Bang scenario.

Possibility of chromospheric back-radiation influencing the lithium line formation in Spite plateau stars

Context. Spectroscopically determined lithium abundances of metal-poor turn-off dwarfs are known to be nearly constant (Spite plateau), but manifestly lower than the primordial value expected from the standard cosmological model. However, abundance determination by using conventional model atmospheres may not necessarily be correct since the existence of high-temperature chromosphere even in very old stars has been confirmed. Aims. The aim of this study is to examine how the extra UV flux possibly irradiated from the chromosphere could affect the formation of the Li I 6708 line, and whether or not its influence might lead to a solution of the Li abundance discrepancy. Methods. A simple model chromosphere of a uniform thin gray slab emitting only thermal radiation is assumed, characterized by optical thickness and temperature. By taking into account this incident radiation in the surface boundary condition, non-local thermodynamical equilibrium calculations for neutral Li atoms are carried out in order to see how the equivalent widths and the resulting abundances are affected by these parameters. Results. If the parameters are appropriately chosen, the strength of the Li I 6708 line can be reduced by a factor of ~2–3 due to overionization caused by enhanced UV radiation, leading to an apparent lowering of the abundance by ~0.3–0.5 dex, which is consistent with the discrepancy in question. Moreover, the observed slight metallicity-dependent trend of the plateau can also be reproduced as a result of the change in atmospheric transparency. Conclusions. Superficial underestimation of Li abundances due to considerable overionization caused by chromospheric radiation may be regarded as a ponderable interpretation for the cosmological Li problem. The touchstone to verify this model would be to check the existence of significantly enhanced UV radiation in these Spite plateau stars, which should be detected if this scenario is valid, although very few such UV spectrophotometric observations have been done to date.

On the method of precise abundance determination of isotopologues in a gas mixture

A method is presented which allows one to derive partial pressures of isotopologue molecules in a gaseous mixture under the conditions of rapid isotope exchange.

NLTE abundances of C, O, Ca, Ti, and Fe in the reference BAF-type stars

We present accurate methods of abundance determination based on the non-local thermodynamic equilibrium (NLTE) line formation for carbon, oxygen, calcium, titanium, and iron in the atmospheres of BAF-type stars. For C I-II, O I, Ca I-II, and Ti I-II, their comprehensive model atoms were described in our previous papers. A fairly complete model atom of Fe I-II is first applied in this study. We determine the NLTE abundances of the nine BAF-type stars with well-determined atmospheric parameters, using high-resolution and high signal-to-noise ratio spectral observations in the broad wavelength range, from the UV to the IR. For C, Ca, Ti, and Fe, NLTE leads to consistent abundances from the lines of the two ionisation stages. The C I, Ca II, and Fe II emission lines were detected in the near IR spectrum of the late B-type subgiant star HD 160762. They are well reproduced in the classical hydrostatic model atmosphere, when applying our NLTE methods.

Mega (metal-poor) not so much : Non-LTE spectroscopic stellar parameters and abundance determination of Ultra metal-poor stars

We present Non-Local Thermodynamic Equilibrium (Non-LTE) abundance corrections for Mg, Ca, and Fe in 12 ultra metal-poor (UMP) stars ([Fe/H] < −4.00). We show that they increase in absolute value toward the lower metallicity up to 0.45 dex for Mg, 0.30 dex for Ca, and 1.00 dex for Fe. This represents a first step toward a full Non-LTE analysis of chemical species in the UMP stars that will enable us to put useful constraints on the properties of the “First” stars.