Pulsars are highly magnetised, spinning compact stars that
emit intense bipolar beams of electromagnetic radiation. A pulse is observed
when such a beam happens to sweep pass the Earth, hence the term ‘pulsars’.
These objects are the superdense and compact remnant cores of massive stars
that have gone supernova. A typical compact star packs as much mass as the Sun
within an object just several kilometres in size.
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Figure 1: Schematic view of a pulsar. The sphere in the
middle represents the compact star; the curves indicate the magnetic field
lines and the protruding bipolar cones represent the beam emission zones.
Credit: Roy Smits.
A paper by M. Bailes et al. (2011) reported on the detection
of a Jupiter-mass object in a very close-in orbit around the pulsar PSR
J1719-1438. This Jupiter-mass object is so close to the pulsar that its density
has to be greater than 23 grams per cubic centimetre for it to not get torn
apart by the pulsar’s titanic gravity. Such a high density is not typical for
Jupiter-mass objects. In comparison, Jupiter has a mean density of only 1.3
grams per cubic centimetre. It is theorised that the Jupiter-mass object around
PSR J1719-1438 is the dense remnant core of a carbon white dwarf that had its
outer layers stripped away and accreted by the pulsar. An object like this
would easily have the high density necessary to avoid being tidally torn apart
by the pulsar’s gravity.
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Figure 2: Artist’s impression of the orbit of the
Jupiter-mass object around PSR J1719-1438.
Nevertheless, a paper by J.E. Horvath (2012) suggests that
the Jupiter-mass object around PSR J1719-1438 might not be the dense remnant
core of a white dwarf. Instead, it could be something more exotic. The proposed
solution is that the object is an exotic Jupiter-mass clump of superdense
strange quark matter. An object like this is so compact; it would measure a
mere ~1 km in radius.
There are a number of mechanisms that can explain the origin
of a Jupiter-mass quark matter object. In one such process, the pulsar itself
is already comprised of quark matter (i.e. the pulsar is a quark star) and its formation
involved the conversion of neutrons to quarks in an explosive and turbulent
manner. The turbulence can eject sufficient quantities of quark matter to form
a Jupiter-mass quark matter object in a very close-in orbit around the pulsar.
Another process involves the merger of two low-mass quark stars. A. Bauswein et
al. (2009) show that a merger event like this can easily eject enough quark
matter to form a Jupiter-mass object.
Future observations might reveal whether or not the
Jupiter-mass object around PSR J1719-1438 is indeed an exotic object. For
example, a Jupiter-mass quark matter object measuring ~1 km in radius would be
too small to generate a detectable photometric signal (i.e. reflected light
from the pulsar). Moreover, an exotic quark matter object is not expected to
produce any detectable evaporation signature since there would be no “normal
matter” for the pulsar’s energetic radiation to strip off into space. For this
reason, the non-detection of circumstellar material around PSR J1719-1438 can
be evidence that is consistent with the pulsar’s companion being an exotic
quark matter object.
References:
- M. Bailes et al. (2011), “Transformation of a Star into a
Planet in a Millisecond Pulsar Binary”, arXiv:1108.5201 [astro-ph.SR]
- J.E. Horvath (2012), “The nature of the companion of PSR
J1719-1438: a white dwarf or an exotic object?”, arXiv:1205.1410 [astro-ph.HE]
- A. Bauswein et al. (2009), “Mass ejection by strange star
mergers and observational implications”, arXiv:0812.4248 [astro-ph]