AR-2019-2020

Figure 19: (Left): The profile of the CaII K line in HD 74194 obtained during 1993 (black line) by Cha and Sembach is superposed on the profile obtained on 2019 May 17 with the SALT (blue line). (Right): Profile of NaI D2 line in HD 74194 obtained in 1996 by Cha and Sembach (black line) shown superposed on the profile of NaI D2 line observed on 2019 May 17 with the SALT (red line). The red arrows indicate the changes to the components between Cha and Sembachs and SALT observations of line profile changes which are seen along the sight-lines of HD74194 and HD72350. Compact stars and the nuclear equation of state Neutron stars are among the densest objects in the Universe. Under the extreme densities present in their core, strangeness-carrying particles (such as hyperons, kaons or deconfined quark matter) could appear. The presence of such “exotic” matter could influence neutron star phenomena, and their understanding is crucial for the correct interpretation of astrophysical observables. Multi- messenger observations (gravitational wave, multi-wavelength electromagnetic) from mergers of neutron stars provide us with information rich in physics under such extreme conditions. The stability of the merger remnant depends crucially on the underlying nuclear equation of state (EoS) and thus, provides a method to probe the nature of dense matter. Since the detection of GWs from the NS binary merger event GW170817, the fate of the binary remnant remains a mystery. As no evidence of a remnant has yet been found from post-merger searches by the LIGO-VIRGO collaboration, one may study different possibilities theoretically. One likely outcome of the merger is a metastable differentially rotating hot hypermassive neutron star. As the stability (dynamical and secular) and time of subsequent collapse of the remnant depend on its interior composition, it opens the possibility to constrain the dense matter EoS from its stability analysis. In a recent publication, Debarati Chatterjee and her collaborators Sarmistha Banik and Kr- ishna Prakash Nunna, searched for possible signatures of strangeness in the neutron star interior on the secular stability of the merger remnant. They calculated the onset of secular instability for different realistic (phenomenological) EoSs with and without strangeness using the Turning Point criterion, and investigated the maximum mass that may be supported by differential rotation and thermal effects. They found that inclusion of thermal effects reduced the maximum mass of the differentially rotating configurations. This is interesting as the hypermassive remnant is conjectured to be hot, and hence, thermal effects cannot be neglected. This was the first realistic study of the secular stability of neutron star merger remnants that used consistent hot and cold EoSs of dense matter. Using realistic EoSs including hyperonic and kaonic degrees of freedom, they also found that the maximum supported mass obtained depends both on the EoS and the degree of differential rotation. They further investigated the effect of strangeness on the collapse time of the merger remnant. They considered the scenario in which the hypermassive NS merger remnant rapidly loses angular