Magnetars are compact stars which are observationally determined to have very strong surface magnetic fields of the order of 10 14 − 10 15 G. The centre of the star can potentially have a magnetic field several orders of magnitude larger. We study the effect of the field on the mass and shape of such a star. In general, we assume a non-uniform magnetic field inside the star which varies with density. The magnetic energy and pressure as well as the metric are expanded as multipoles in spherical harmonics up to the quadrupole term. Solving the Einstein equations for the gravitational potential, one obtains the correction terms as functions of the magnetic field. Using a nonlinear model for the hadronic EoS the excess mass and change in equatorial radius of the star due to the magnetic field are quite significant if the surface field is 10 15 G and the central field is about 10 18 G. For a value of the central magnetic field strength of 1.75 × 10 18 G, we find that both the excess mass and the equatorial radius of the star changes by about 3 − 4% compared to the spherical solution.

No abstract

Recent observation of pulsar PSR J1614-2230 with mass about 2 solar masses poses a severe constraint on the equations of state (EOS) of matter describing stars under extreme conditions. Neutron stars (NS) can reach the mass limits set by PSR J1614-2230. But stars having hyperons or quark stars (QS) having boson condensates, with softer EOS can barely reach such limits and are ruled out. QS with pure strange matter also cannot have such high mass unless the effect of strong coupling constant or color superconductivity are considered. In this work I try to calculate the upper mass limit for a hybrid stars (HS) having a quark-hadron mixed phase. The hadronic matter (having hyperons) EOS is described by relativistic mean field theory and for the quark phase I use the simple MIT bag model. I construct the intermediate mixed phase using Glendenning construction. HS with a mixed phase cannot reach the mass limit set by PSR J1614-2230 unless I assume a density dependent bag constant. For such case the mixed phase region is small. The maximum mass of a mixed hybrid star obtained with such mixed phase region is 2.01M ⊙ .

We investigate the General Relativistic (GR) effects on the conversion from nuclear to two-flavour quark matter in compact stars, both static as well as rotating. We find that GR effects lead to qualitative differences in rotating stars, indicating the inadequacy of non-relativistic (NR) or even Special Relativistic (SR) treatments for these cases. PACS numbers:Strange Quark Matter (SQM), consisting of approximately equal numbers of up (u), down (d) and strange (s) quarks, is the putative true ground state of strong interaction [1], a conjecture supported by model calculations for certain ranges of values for strange quark mass and strong coupling constant [2]. There have been concerted efforts at confirming the existence of Quark-Gluon Plasma (QGP) and SQM, though transiently, in ultra relativistic collisions. On the other hand, QGP and SQM could naturally occur in the cores of compact stars, where central densities of about an order of magnitude higher than the nuclear matter saturation density are expected. Given the very low strangeness fraction in normal nuclear matter (NM), it is almost inevitable that a transition from nuclear (hadronic) to quark matter should proceed through a conversion to an initial stage of (metastable) two flavour quark matter, which should decay to the stable SQM. Thus, neutron stars with sufficiently high central densities ought to get converted to strange, or at least hybrid, stars. These transitions could have observable signatures in the form of a jump in the breaking index and gamma ray bursts [3,4]. On the other hand a full quark star may explain the phenomena of observed quasi periodic oscillations [5].There are several plausible scenarios where neutron stars could convert to quark stars, through a "seed" of external SQM [6], or triggered by the rise in the central density due to a sudden spin-down in older neutron stars [7]. Several authors have studied the conversion of nuclear matter to strange matter under different assumptions [8,9,10,11,12,13,14,15,16,17,18]. They have been summarized in a recent work of ours [19] and for the constraint of space, we do not repeat them here, except to mention that Tokareva et al [15] have lately modelled the hadron to SQM conversion as a single step * Electronic address: abphy@caluniv.ac.in † Electronic address: sanjay@bosemain.boseinst.ac.in ‡ Electronic address: ritam@bosemain.boseinst.ac.in § Electronic address: sibaji@bosemain.boseinst.ac.in

Recent results and data suggest that high magnetic fields in neutron stars (NS) strongly affect the characteristics (radius, mass) of the star. Such stars are even separated into a class known as magnetars, for which the surface magnetic field is greater than 1014 G. In this work we discuss the effect of such a high magnetic field on the phase transition of a NS to a quark star (QS). We study the effect of magnetic field on the transition from NS to QS including the magnetic‐field effect in the equation of state (EoS). The inclusion of the magnetic field increases the range of baryon number densities for which the flow velocities of the matter in the respective phase are finite. The magnetic field helps in initiation of the conversion process. The velocity of the conversion front, however, decreases due to the presence of the magnetic field, as the presence of the magnetic field reduces the effective pressure (P). The magnetic field of the star is decreased by the conversion process, and the resultant QS has lower magnetic field than the initial NS.