  Citation: Celi, M.O.; Mariani, M.; Orsaria, M.G.; Tonetto, L. Oscillating Magnetized Color Superconducting Quark Stars. Universe 2022, 8, 272. https://doi.org/10.3390/ universe8050272 Academic Editor: Nicolas Chamel Received: 29 March 2022 Accepted: 4 May 2022 Published: 6 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). universe Article Oscillating Magnetized Color Superconducting Quark Stars Marcos Osvaldo Celi 1,2, * , Mauro Mariani 1,2 , Milva Gabriela Orsaria 1,2 and Lucas Tonetto 3,4 1 Grupo de Gravitación, Astrofísica y Cosmología, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, La Plata 1900, Argentina; mmariani@fcaglp.unlp.edu.ar (M.M.); morsaria@fcaglp.unlp.edu.ar (M.G.O.) 2 Consejo Nacional de Investigaciones Científicas y Técnicas, Godoy Cruz 2290, Ciudad Autónoma de Buenos Aires 1425, Argentina 3 Dipartimento di Fisica, “Sapienza” University of Rome, Piazzale A. Moro, 5, 00185 Roma, Italy; lucas.tonetto@roma1.infn.it 4 Istituto Nazionale di Fisica Nucleare, Sezione di Roma, Piazzale A. Moro, 5, 00185 Roma, Italy * Correspondence: mceli@fcaglp.unlp.edu.ar Abstract: The main objective of this work is to study the structure, composition, and oscillation modes of color superconducting quark stars with intense magnetic fields. We adopted the MIT bag model within the color superconductivity CFL framework, and we included the effects of strong magnetic fields to construct the equation of state of stable quark matter. We calculated observable quantities, such as the mass, radius, frequency, and damping time of the oscillation fundamental f mode of quark stars, taking into account current astrophysical constraints. The results obtained show that color superconducting magnetized quark stars satisfy the constraints imposed by the observations of massive pulsars and gravitational wave events. Furthermore, the quantities associated with the oscillation f mode of these objects fit the universal relationships for compact objects. In the context of the new multi-messenger gravitational wave astronomy era and the future asteroseismology of neutron stars, we hope that our results contribute to the understanding of the behavior of dense matter and compact objects. Keywords: neutron stars; quark stars; color superconductivity; di-quarks; magnetic field; oscillations (including pulsations); equation of state; dense matter 1. Introduction Strange matter, the possibility that the fundamental state of matter at high densities is a system of quarks in equilibrium against weak interactions, has been the subject of research in a wide variety of scenarios [1–7]. Probably one of the most favorable situations for the appearance of this type of matter, even without being absolutely stable, occurs during the last stages of the evolution of massive stars, associated with the explosion of type II supernovae, and during the formation of neutron stars (NSs) [8–12]. The latter are the densest objects in the universe, with radii of just over a dozen kilometers and masses of around 1.4 M ⊙ . NSs, in addition to having high densities, have magnetic fields (MFs) from 10 8 up to 10 15 Gauss at their surface; in particular, the objects with the greatest MF values, ∼ 10 14−15 Gauss, are the so-called magnetars. Under these extreme conditions, QCD predicts that hadronic matter may undergo a transition to a strange matter phase or a color superconducting phase [13]. This prediction, together with the stability of the strange matter hypothesis, opens up the possibility of the formation of compact objects composed entirely of quark matter: quark stars (QSs). The theoretical existence of such compact objects raises questions about what the true composition of NSs is, how astrophysical objects containing strange matter can be detected, and what are the main observational characteristics that differentiate them from objects made of non-strange matter. During the last decade, some NSs of 2 M ⊙ were detected, establishing new restrictions to the equation of state (EoS) of the matter that composes these objects and that is still Universe 2022, 8, 272. https://doi.org/10.3390/universe8050272 https://www.mdpi.com/journal/universe