Hello! I’m studying the paper: GW170817: Measurements of Neutron Star Radii and Equation of State (arXiv:1805.11581) and got some doubts on the implied EOS from the GW data of this event.
If I’m not mistaken, exotic particles soften the EOS and from this paper the neutron stars seem to be better described by soft EOS like APR4 and WFF1, while H4 is ruled out. Right there is where I’m confused, because APR4 and WFF1 do not include exotic particles like H4, then why H4 is not favored? Is this event a counterexample maybe?
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It is true that in general the presence of exotic particles softens the EOS, but this is only in comparison with the same nucleonic EOS, not all other nucleonic EOSs: The G3/G4 nucleonic EOS that has the same parameters as the H4 EOS but without the hyperons is fairly stiff, so the H4 EOS is still fairly stiff in comparison with other EOSs (such as APR4 and WFF1). See Fig. 1 in [astro-ph/0507312] Observational constraints on hyperons in neutron stars for the mass-radius curves for the G3/G4 and H4 EOSs, as well as the other EOSs in this set.
Additionally, the hyperons are only present for sufficiently high densities, so they only affect the properties of the star significantly for sufficiently large masses–the aforementioned figure illustrates that the mass-radius curves for the G3/G4 and H4 EOSs are almost identical up to about 1.5 solar masses, so the hyperons do not have much of an effect at the masses that are probed by GW170817.
I hope that this resolves your confusion.
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Thank you! I´ll think it through and check carefully that paper. 
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