Probing up-down quark matter via gravitational waves

Recently, it was shown that quark matter with only $u$, $d$ quarks ($ud$QM) can be the ground state of matter for baryon number $A>A_\textrm{min}$ with $A_{\rm min}\gtrsim 300$. In this paper, we explore $ud$ quark stars ($ud$QSs) that are composed of $ud$QM, in the context of the two-families scenario in which $ud$QSs and hadronic stars (HSs) can co-exist. Distinct signatures are discussed compared to the conventional study regarding strange quark stars (SQSs). We show that the requirements of $A_{\rm min}\gtrsim 300$ and the most-massive compact star observed being $ud$QS together put stringent constraints on the allowed parameter space of $ud$QSs. Then we study the related gravitational-wave probe of the tidal deformability in binary star mergers, including the $ud$QS-$ud$QS and $ud$QS-HS cases. The obtained values of the tidal deformability at 1.4 solar mass and the average tidal deformability are all in good compatibility with the experimental constraints of GW170817/AT2017gfo. This study points to a new possible interpretation of the GW170817 binary merger event, where $ud$QS may be at least one component of the binary system detected.