Close-in Planets Around Pulsars And Their Astrophysical Implications

Till now,nearly 3000 pulsars have been observed in our Galaxy.Among these pulsars,only a few are found to be accompanied by one or several planets.But interestingly,in these pulsar planets,at least one object(PSR J1719-1438 b)is a close-in planet.In fact,the orbital radius of PSR J1719-1438 b is only 6 × 1010 cm,and the orbital period is correspondingly 7837 s.We suggest that close-in pulsar planets are very valuable objects that deserve being paid special attention to.They can help us to probe the internal structure of pulsars,and they may also be closely related to fast radio bursts.In this thesis,we will use close-in pulsar planets to study the existence of strange quark stars,and to explain the trigger mechanism of repeating fast radio bursts.Under the condition of high density,strange quark matter composed of up,down and strange quarks may have a lower energy per baryon than ordinary neutron matter.As a result,strange quark matter may be the real ground state of baryonic matter at high density.Based on this hypothesis,a series of strange quark objects composed of strange quark matter can exist stably.They include strange quark stars(strange stars),strange quark dwarfs(strange dwarfs)and strange quark planets(strange planets).Pulsars,often thought of as neutron stars,may actually be strange stars.However,strange stars and neutron stars do not differ much in appearance and are very similar in observational aspects.To distinguish between strange stars and neutron stars observationally so as to find the real ground state of baryonic matter is still an unsolved problem in astrophysics.In our studies,we noticed that the structure of smaller strange quark objects(such as strange dwarfs and strange planets)is very different from that of normal matter counterparts.For example,the densities of strange dwarfs and strange planets can be as high as that of nucleus so that their radii are very small.Especially,strange quark planets will not be torn apart by the tidal force of their host pulsars.They can safely move around pulsars in very close-in orbits and produce intense gravitational wave emission,all of which can be tested by observation.Therefore,the existence of strange quark stars can be confirmed by searching for close-in strange quark planets.On the other hand,compact stars(especially pulsars and black holes)can produce intensive bursts at various wavelengths.For example,supernovae,gamma-ray bursts,and X-ray bursts are all connected with compact stars.Most interestingly,a new kind of transients,fast radio bursts(FRBs),were discovered in 2007.They provide a valuable opportunity for studying compact stars from a very different aspect.Many fundamental problems related to fast radio bursts are still unclear: how are they triggered? Why coherent emission can be produced? In this thesis,we argued that a close-in planet moving around a pulsar in a highly elliptical orbit can give a natural explanation for repeating fast radio bursts.The planet will be partially torn apart every time it passes by the periastron.The clumps produced during this process will fall toward the pulsar and collide with the compact star to give birth to fast radio bursts as observed from some repeating FRB sources.The main content of this thesis is related to the study of close-in pulsar planets.We use these interesting objects to probe the existence of strange quark objects,and to explain the trigger mechanism of fast radio bursts.The structure of my thesis is organized as follows.In Chapter 1,we briefly introduce the background related to the research content of this paper.In the second chapter,we review the basic contents of strange quark matter and strange stars,including the research history,the basic properties of strange stars,and possible methods to distinguish strange stars from neutron stars.Chapter 3 presents a short review of FRBs.The first discovery of FRBs is described.The observational features are introduced.Physical conditions for producing coherent radio emission are summarized.The trigger mechanisms of FRBs are also described and various models are compared.Finally,we also address possible application of FRBs as a cosmological probe.In Chapter 4,we use close-in pulsar planets to study the internal composition of pulsars.The basic idea is that strange planets have an extremely high density so that they can resist the tidal force from their host pulsar.For a normal planet,the density can hardly be larger than 30 g cm-3.When the orbital radius is less than ? 5.6 × 1010 cm or the orbital period is less than ? 6100 s,a normal planet will be completely torn apart by tidal forces.On the contrary,due to the extremely high density,an SQM planet can survive safely even when it is very close to its host star.In our study,we have examined all the possible pulsar planets observed till now and found several new SQM planet candidates which are all close-in objects.Gravitational wave emission from these planetary systems are calculated and compared with the sensitivity curves of various gravitational wave detectors.Our study clearly shows that close-in pulsar planets can effectively help us identify SQM objects in our Galaxy.Chapter 5 presents another study that is also closely related to strange quark objects,i.e.searching for strange dwarfs.A strange dwarf has a mass similar to that of a normal white dwarf,but it possesses an extremely dense SQM core(with a density of 4 × 1014 g cm-3)and thus has a correspondingly smaller radius.Since strange dwarfs and white dwarfs have a different mass-radius relation,we try to search for strange dwarfs among the so called “white dwarfs” observed till now.In this way,we have identified eight possible candidates of strange dwarfs.Further observational studies on these interesting objects are solicited.In chapter 6,we present a new model for repeating fast radio bursts that involves a close-in planet moving around a pulsar in a highly elliptical orbit.Fast radio bursts are still mysterious astronomical phenomena nowadays.Repeating fast radio bursts can provide us valuable information about their origin.The two most famous recurrent bursting sources are FRBs 121102 and 180916,with a period of 157 days and 16.35 days,respectively.Previous studies have shown that the periodicity of FRBs may be related to some astrophysical processes in neutron star binary systems.In our model,we argue that periodic repeating fast radio bursts could be generated by the interaction between a pulsar and its close-in planet in a highly elliptical orbit.Every time the planet passes by the pulsar,it will be partially torn apart by the tidal force from its host star.Clumps produced during this process will fall toward the pulsar and eventually collide with it,producing the observed FRBs.This model can naturally explain repeating FRBs with a period ranging from a few days to several hundred days.It can also account for many other main features of FRBs 121102 and 180916.Finally,in Chapter 7,we summarize the contents of this thesis and address some unsolved issues in the field.Especially,taking into account the existing and planned observational equipments,we present a short prospect on what could be done in the near future.