Black hole-neutron star mergers with massive neutron stars in numerical relativity

We study the merger of black hole-neutron star (BH-NS) binaries in numerical relativity, focusing on the properties of the remnant disk and the ejecta, varying the mass of compactness of the NS and the mass and spin of the BH. We find that within the precision of our numerical simulations, the remnant disk mass and ejecta mass normalized by the NS baryon mass (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mover accent="true"><a:mi>M</a:mi><a:mo stretchy="false">^</a:mo></a:mover><a:mi>rem</a:mi></a:msub></a:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mover accent="true"><e:mi>M</e:mi><e:mo stretchy="false">^</e:mo></e:mover><e:mi>eje</e:mi></e:msub></e:math>, respectively), and the cutoff frequency <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>f</i:mi><i:mi>cut</i:mi></i:msub></i:math> normalized by the initial total gravitational mass of the system at infinite separation approximately agree among the models with the same NS compactness <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:msub><k:mi>C</k:mi><k:mi>NS</k:mi></k:msub><k:mo>=</k:mo><k:msub><k:mi>M</k:mi><k:mi>NS</k:mi></k:msub><k:mo>/</k:mo><k:msub><k:mi>R</k:mi><k:mi>NS</k:mi></k:msub></k:math>, mass ratio <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>Q</m:mi><m:mo>=</m:mo><m:msub><m:mi>M</m:mi><m:mi>BH</m:mi></m:msub><m:mo>/</m:mo><m:msub><m:mi>M</m:mi><m:mi>NS</m:mi></m:msub></m:math>, and dimensionless BH spin <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msub><o:mi>χ</o:mi><o:mi>BH</o:mi></o:msub></o:math> irrespective of the NS mass <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msub><q:mi>M</q:mi><q:mi>NS</q:mi></q:msub></q:math> in the range of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mrow><s:mn>1.092</s:mn><s:mo>−</s:mo><s:mn>1.691</s:mn><s:msub><s:mi>M</s:mi><s:mo stretchy="false">⊙</s:mo></s:msub></s:mrow></s:math>. This result shows that the merger outcome depends sensitively on <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mi>Q</v:mi></v:math>, <x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:msub><x:mi>χ</x:mi><x:mrow><x:mi>BH</x:mi></x:mrow></x:msub></x:math>, and <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:msub><z:mi>C</z:mi><z:mi>NS</z:mi></z:msub></z:math> but only weakly on <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:msub><bb:mi>M</bb:mi><bb:mi>NS</bb:mi></bb:msub></bb:math>. This justifies the approach of studying the dependence of NS tidal disruptions on the NS compactness by fixing the NS mass but changing the EOS. We further perform simulations with massive NSs of <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:msub><db:mi>M</db:mi><db:mi>NS</db:mi></db:msub><db:mo>=</db:mo><db:mn>1.8</db:mn><db:msub><db:mi>M</db:mi><db:mo stretchy="false">⊙</db:mo></db:msub></db:math>, and compare our results of <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"><gb:msub><gb:mover accent="true"><gb:mi>M</gb:mi><gb:mo stretchy="false">^</gb:mo></gb:mover><gb:mi>rem</gb:mi></gb:msub></gb:math> and <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"><kb:msub><kb:mover accent="true"><kb:mi>M</kb:mi><kb:mo stretchy="false">^</kb:mo></kb:mover><kb:mi>eje</kb:mi></kb:msub></kb:math> with those given by existing fitting formulas to test their robustness for more compact NSs. We find that the fitting formulas obtained in the previous studies are accurate within the numerical errors assumed, while our results also suggest that further improvement is possible by systematically performing more precise numerical simulations.