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Register a new userTwo coexisting families of compact stars: observational implications for millisecond pulsars
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Added by
Sudip Bhattacharyya
Publication date
2017
fields
Physics
and research's language is
English
Authors
Sudip Bhattacharyya
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It is usually thought that a single equation of state (EoS) model correctly represents cores of all compact stars. Here we emphasize that two families of compact stars, viz., neutron stars and strange stars, can coexist in nature, and that neutron stars can get converted to strange stars through the nucleation process of quark matter in the stellar center. From our fully general relativistic numerical computations of the structures of fast-spinning compact stars, known as millisecond pulsars, we find that such a stellar conversion causes a simultaneous spin-up and decrease in gravitational mass of these stars. This is a new type of millisecond pulsar evolution through a new mechanism, which gives rise to relatively lower mass compact stars with higher spin rates. This could have implication for the observed mass and spin distributions of millisecond pulsars. Such a stellar conversion can also rescue some massive, spin-supported millisecond pulsars from collapsing into black holes. Besides, we extend the concept of critical mass $M_{rm cr}$ for the neutron star sequence (Berezhiani et al. 2003; Bombaci et al. 2004) to the case of fast-spinning neutron stars, and point out that neutron star EoS models cannot be ruled out by the stellar mass measurement alone. Finally, we emphasize the additional complexity for constraining EoS models, for example, by stellar radius measurements using X-ray observations, if two families of compact stars coexist.
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The spin-down of a neutron star, e.g. due to magneto-dipole losses, results in compression of the stellar matter and induces nuclear reactions at phase transitions between different nuclear species in the crust. We show that this mechanism is effective in heating recycled pulsars, in which the previous accretion process has already been compressing the crust, so it is not in nuclear equilibrium. We calculate the corresponding emissivity and confront it with available observations, showing that it might account for the likely thermal ultraviolet emission of PSR J0437-4715.
An understanding of spin frequency ($ u$) evolution of neutron stars in the low-mass X-ray binary (LMXB) phase is essential to explain the observed $ u$-distribution of millisecond pulsars (MSPs), and to probe the stellar and binary physics, including the possibility of continuous gravitational wave emission. Here, using numerical computations we conclude that $ u$ can evolve in two distinctly different modes, as $ u$ may approach a lower spin equilibrium value ($ u_{rm eq,per}$) for persistent accretion for a long-term average accretion rate ($dot{M}_{rm av}$) greater than a critical limit ($dot{M}_{rm av,crit}$), and may approach a higher effective spin equilibrium value ($ u_{rm eq,eff}$) for transient accretion for $dot{M}_{rm av} < dot{M}_{rm av,crit}$. For example, when $dot{M}_{rm av}$ falls below $dot{M}_{rm av,crit}$ for an initially persistent source, $ u$ increases considerably due to transient accretion, which is counterintuitive. We also find that, contrary to what was suggested, a fast or sudden decrease of $dot{M}_{rm av}$ to zero in the last part of the LMXB phase is not essential for the genesis of spin-powered MSPs, and neutron stars could spin up in this $dot{M}_{rm av}$-decreasing phase. Our findings imply that the traditional way of $ u$-evolution computation is inadequate in most cases, even for initially persistent sources, and may not even correctly estimate whether $ u$ increases or decreases.
We analyse the phenomenological implications of the two-families scenario on the merger of compact stars. That scenario is based on the coexistence of both hadronic stars and strange quark stars. After discussing the classification of the possible mergers, we turn to detailed numerical simulations of the merger of two hadronic stars, i.e., first family stars in which delta resonances and hyperons are present, and we show results for the threshold mass of such binaries, for the mass dynamically ejected and the mass of the disk surrounding the post-merger object. We compare these results with those obtained within the one-family scenario and we conclude that relevant signatures of the two-families scenario can be suggested, in particular: the possibility of a rapid collapse to a black hole for masses even smaller than the ones associated to GW170817; during the first milliseconds, oscillations of the postmerger remnant at frequencies higher than the ones obtained in the one-family scenario; a large value of the mass dynamically ejected and a small mass of the disk, for binaries of low total mass. Finally, based on a population synthesis analysis, we present estimates of the number of mergers for: two hadronic stars; hadronic star - strange quark star; two strange quark stars. We show that for unequal mass systems and intermediate values of the total mass, the merger of a hadronic star and a strange quark star is very likely (GW170817 has a possible interpretation into this category of mergers). On the other hand, mergers of two strange quark stars are strongly suppressed.
The maximum mass of a neutron star has important implications across multiple research fields, including astrophysics, nuclear physics and gravitational wave astronomy. Compact binary millisecond pulsars (with orbital periods shorter than about a day) are a rapidly-growing pulsar population, and provide a good opportunity to search for the most massive neutron stars. Applying a new method to measure the velocity of both sides of the companion star, we previously found that the compact binary millisecond pulsar PSR J2215+5135 hosts one of the most massive neutron stars known to date, with a mass of 2.27$pm$0.16 M$_odot$ (Linares, Shahbaz & Casares, 2018). We reexamine the properties of the 0.33 M$_odot$ companion star, heated by the pulsar, and argue that irradiation in this redback binary is extreme yet stable, symmetric and not necessarily produced by an extended source. We also review the neutron star mass distribution in light of this and more recent discoveries. We compile a list of all (nine) systems with published evidence for super-massive neutron stars, with masses above 2 M$_odot$. We find that four of them are compact binary millisecond pulsars (one black widow, two redbacks and one redback candidate). This shows that compact binary millisecond pulsars are key to constraining the maximum mass of a neutron star.
Bose-Einstein condensates (BECs) have been proposed as candidate states of matter for the interior of neutron stars. Specifically, Chavanis and Harko obtained the mass-radius relation for a BEC star and proposed that the recently discovered neutron stars with masses around 2$M_odot$ are BEC stars. They employed a barotropic equation of state (EOS), with one free parameter, that was first found by Colpi, Wasserman, and Shapiro (CSW), to describe them and derive stable equilibrium configurations of spinning BEC stars in General Relativity. In this work we show that while it is true that BECs allow for compact object masses as heavy as the heaviest observed ones, such stars cannot simultaneously have radii that are small enough to be consistent with the latest observations, in spite of the flexibility available in the EOS in the form of the free parameter. In fact, our conclusion applies to any spinning relativistic boson star that obeys the CSW EOS.
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