**5. Conclusions**

In this work we investigate the stability of *ud*QM and the properties of *ud*QM nuggets, *ud*QM stars, and white dwarfs comprised of *ud*QM nuggets. The properties of *ud*QM are obtained based on equivparticle model [62,63,68], where an additional term is introduced to account for the quark matter symmetry energy. By comparing the binding energies of *ud*QM nuggets and the heaviest *β*-stable nucleus <sup>266</sup>Hs, the stability window for *ud*QM is obtained, where there exists a minimum surface tension value below which finite nuclei cannot decay into *ud*QM nuggets. The properties of *ud*QM nuggets are then examined considering the effects of charge screening and electrons [78]. By adopting the minimum possible surface tension values, there are *ud*QM nuggets at certain size that are more stable than others if a large symmetry energy is adopted. In such cases, a crust made of *ud*QM nuggets is expected in *ud*QM stars, where we have obtained the EOSs adopting Gibbs construction. It is found that the mass-radius relation of *ud*QM stars resembles that of traditional neutron stars. Meanwhile, similar to the cases of strange stars [97], a new family of white dwarfs comprised entirely of *ud*QM nuggets and electrons is obtained, where its maximum mass approaches to the Chandrasekhar limit due to the large charge-to-mass ratio of *ud*QM nuggets.

**Author Contributions:** Conceptualization, C.-J.X., J.-F.X., G.-X.P. and R.-X.X.; Data curation, C.-J.X.; Funding acquisition, C.-J.X.; Investigation, J.-F.X.; Methodology, G.-X.P. and R.-X.X.; Writing—original draft preparation, L.W. and J.H.; Writing—review and editing, J.-F.X., G.-X.P. and R.-X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by by National SKA Program of China No. 2020SKA0120300, National Natural Science Foundation of China (Grant Nos. 11705163, 12005005, 11947098, 11875052, and 11673002), Ningbo Natural Science Foundation (Grant Nos. 2019A610066 and 2019A610092), and key research projects of universities in Henan province (Grant No. 20A140003).

**Institutional Review Board Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
