Simulating binary neutron star mergers with finite-temperature equations of state: The influences of the slope of the symmetry energy and artificial heating

We present a new set of numerical-relativity simulations of merging binary neutron stars, aiming to identify possible observable signatures of the slope of the symmetry energy <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>L</a:mi> <a:mrow> <a:mi>sym</a:mi> </a:mrow> </a:msub> </a:math> . To achieve this goal, we employ a set of equations of state based on a parametrization of the covariant density functional theory of nuclear matter that allows controlled variations of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>L</c:mi> <c:mrow> <c:mi>sym</c:mi> </c:mrow> </c:msub> </c:math> and the skewness <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msub> <e:mi>Q</e:mi> <e:mrow> <e:mi>sat</e:mi> </e:mrow> </e:msub> </e:math> , holding the latter fixed. For a set of our simulations, we identify a steep energy gradient in the equation of state at subsaturation densities, which acts as a source of heating with subsequent stiffening produced by thermal support. Accounting for related structural modifications in the tidal deformability reconciles our results with theoretical expectations. On the other hand, we show that gravitational waves are unlikely to distinguish the role of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>L</g:mi> <g:mrow> <g:mi>sym</g:mi> </g:mrow> </g:msub> </g:math> . In contrast to this, we find that the ejecta composition is significantly altered in our simulations, which employ an M1 moment scheme, when <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:msub> <i:mi>L</i:mi> <i:mrow> <i:mi>sym</i:mi> </i:mrow> </i:msub> </i:math> is varied. Based on our extracted dynamical ejecta properties, we compute r-process yields and find that they are distinct for the different <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:msub> <k:mi>L</k:mi> <k:mrow> <k:mi>sym</k:mi> </k:mrow> </k:msub> </k:math> , especially at lower mass numbers <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>A</m:mi> <m:mo>≲</m:mo> <m:mn>120</m:mn> </m:math> . This suggests that electromagnetic counterparts are more likely to exhibit signatures; however, a direct connection to <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:msub> <o:mi>L</o:mi> <o:mrow> <o:mi>sym</o:mi> </o:mrow> </o:msub> </o:math> remains a challenge, given the complex interplay between details of the ejecta properties and the kilonova signal.