Day 1467

Monday.

1809.07960
The AMBRE Project: r-process elements in the Milky Way thin and thick disks
Guiglion, et al

The chemical evolution of neutron capture elements in the MW disc is still a matter of debate.  Aim to understand the chemical evolution of r-process elements in MW disc.  Focus on 3 pure r-process elements Eu, Gd, and Dy.  Using high-resolution FEROS, HARPS, and UVES spectra from the ESO archive, perform a homogeneous analysis on 6500 FGK MW stars, thanks to the automatic optimization pipeline GAUGUIN.  Present abundances of Ba (5057 stars), Eu (6268 stars), Gd (5431 stars) and Dy (5479 stars).  Chemically characterize the thin and the thick disks, and a metal-rich alpha-rich population.  Find that the [Eu/Fe] ratio follows a continuous sequence from the thin disc to the thick disc as a function of the metallicity.  In thick disc stars, the [Eu/Ba] ratio is found to be constant, while the [Gd/Ba] and [Dy/Ba] ratios decrease as a function of the metallicity. These observations clearly indicate a different nucleosynthesis history in the thick disc between Eu and Gd-Dy.  Also find that the alpha-rich metal-rich stars are also enriched in r-process elements (like thick disc stars), but their [Ba/Fe] is very different from thick disc stars.  Finally, find that the [r/alpha] ratio tends to decrease with metallicity, indicating that SNe of different properties probably contribute differently to the synthesis of r-process elements and alpha-elements.  Provide average abundance trends for [Ba/Fe] and [Eu/Fe] with rather small dispersions, and for the first time for [Gd/Fe] and [Dy/Fe].  This data may help to constrain chemical evolution models of MW r- and s-process elements and the yields of massive stars.  Including yields of neutron-star or BH mergers is now crucial if we want to quantitatively compare observations to Galactic chemical evolution models.


1809.08126
Dense matter equation of state for neutron star mergers
Lalit, et al

In simulations of binary neutron star mergers, the dense matter equation of state (EOS) is required over wide ranges of density and temperature as well as under conditions in which neutrinos are trapped, and the effects of magnetic fields and rotation prevail.  Here, assess the status of dense matter theory and point out the success and limitations of approaches currently in use.  A comparative study of the excluded volume (EV) and viral approaches for the np alpha system using the equation of state of Akmal, Pandharipande and Ravenhall for interacting nucleons is presented in the sub-nuclear density regime.  Owing to the excluded volume of the alpha-particles, their mass fraction vanishes in the EV approach below the baryon density 0.1 fm^{-3}, whereas it continues to rise due to the predominantly attractive interactions in the viral approach.  The EV approach of Lattimer et al. is extended here to include clusters of light nuclei such as d, 3H and 3He ain addition to alpha particles.  Results of the relevant state variables from this development are presented and enable comparisons with related by slightly different approaches in the literature.  Also comment on some of the sweet and sour aspects of the supra-nuclear EOS.  The extent to which the gravitational and baryon masses vary due to thermal effects, neutrino trapping, magnetic fields and rotation are summarized from earlier studies in which the effects from each of these sources were considered separately.  Increases of about 25% (50%) occur for rigid (differential) rotation with comparable increases occurring in the presence of magnetic fields only for fields in excess of 1e18 Gauss.  Comparatively smaller changes occur due to thermal effects and neutrino trapping.  Some future studies to gain further insight into the outcome of dynamical simulations are suggested.