Stability and instability of strange dwarfs

Abstract

Aims. More than 20 years ago, the existence of stable white dwarfs with a core of strange quark matter was proposed. More recently, via the study of radial modes, it has been concluded instead that such objects are unstable. We aim to clarify this issue. Methods. We investigated the stability of these objects by looking at their radial oscillations while incorporating boundary conditions at the quark–hadron interface, which correspond to either a rapid or a slow conversion of hadrons into quarks. Results. Our analysis shows that objects of this type are stable if the star is not strongly perturbed and ordinary matter cannot transform into strange quark matter because of the Coulomb barrier separating the two components. On the other hand, ordinary matter can be transformed into strange quark matter if the star undergoes a violent process, as in the preliminary stages of a type Ia supernova, and this causes the system to become unstable and collapse into a strange quark star. In this way, the accretion-induced collapse of strange dwarfs can be facilitated, and kilometre-sized objects with sub-solar masses can be produced.

Is the Compact Object Associated with HESS J1731-347 a Strange Quark Star? A Possible Astrophysical Scenario for Its Formation

“…Concerning the first point, the density of DM in the Galactic bulge can easily be 1-2 orders of magnitude larger than in the area where the Sun is located (Nesti & Salucci 2013;Salucci 2019), and this can be a relevant factor, as we already suggested in the case of accretion-induced collapse of strange dwarfs (Di Clemente et al 2023). Concerning the time interval between the formation of a dense core inside the progenitor and the moment of its collapse, it depends on the mass of the progenitor.…”

“…Therefore, the cooling curves of QSs and of slowly cooling NSs can be almost indistinguishable (Schaab et al 1997;Weber 2005). In Di Clemente et al (2023) we have suggested that SAX J1808.4-3658 is also a QS, since it has been indicated that its mass could be smaller than 1 M e (Di Salvo et al 2019). On the other hand, the thermal emission of that object indicates that some form of enhanced cooling takes place in the star (Heinke et al 2009).…”

“…In this way the total BE of QSs can be significantly larger than that of NSs. This important point was used in our recent paper, Di Clemente et al (2023), where we suggested a mechanism to produce compact objects having a mass smaller than 1 M e via accretion-induced collapse of white dwarfs containing strange quark matter at their center. Clearly, this mechanism for producing QSs does not apply to the case of HESS J1731-347, since the central object is embedded in a dust shell of ∼2 M e expelled at the moment of the SN explosion (Doroshenko et al 2016).…”

“…Instead, in order to satisfy the limit on the mass of HESS J1731-347, we have explicitly used the significantly larger value of the binding energy of a QS. In the last decade we have developed a scheme, named the two-families scenario, in which NSs and QSs coexist (Berezhiani et al 2003;Drago et al 2014a) and we have discussed in several papers the pathways leading to the formation of a QS (Wiktorowicz et al 2017;De Pietri et al 2019;Di Clemente et al 2023). The large range of masses of QSs does not therefore rule out the possible existence of NSs.…”

Two-flavor Color Superconducting Quark Stars May Not Exist

“…The possible existence of ultralow-mass compact stars has sparked interest in speculating about self-bound quark matter as the potential physical nature of pulsar-like objects (Bodmer 1971;Witten 1984). For example, based on the mass-radius relations obtained from the MIT bag model and Nambu-Jona-Lasinio (NJL) model, as well as the constant speed of sound parameterization, Di Clemente et al (2022) demonstrated that this compact star might be a strange star, while further indicating an existing astrophysical path that could lead to the formation of such an object. Nonstrange quark stars compatible with the compact object HESS J1731-347 were studied in Restrepo et al (2023).…”