Swinburne Astronomy Online
Stars are the most fascinating objects in Nature. Thanks to them matter is synthesised and we owe them our existence. In their evolution very extreme physical situations happen that stars will probably be the best laboratories of physics at mankind reach.
Human
knowledge has in some way defined the basic concepts of birth, life, and death
of many of these objects. Yet, there is a lot to be discovered, mainly in the
aspects where observation does not follow theory, either by disagreement or by
lack of data.
This
text refers precisely one of these points, in which theory predicts the
existence of stars containing a certain form of
matter, the strange quark matter.
But, it has not yet been done observationally in a concrete way the
discovery of any of those stars. Yet, surprisingly, it is probable that those
stars are already well known to us...
According
to the Standard Model of particle physics, matter is formed by elementary
particles, six leptons and six quarks, that obey the
Fermi-Dirac statistics and Pauli´s
principle of exclusion. These particles are known as fermions. The Standard
Model also includes four forces that act on the fermions: gravity, strong
nuclear force, weak force, and electromagnetic force. These forces are
transmitted by another kind of particles, the bosons, that obey the
Bose-Einstein statistics and do not obey the Pauli´s
principle of exclusion, i.e., several can occupy the same quantum state. Until
now, it was proved the existence of the photon, the electromagnetic boson,
eight gluons, responsible for the strong nuclear force, the Z, W+
and W- bosons that transmit the weak force. The graviton, responsible for the gravity,
has not yet been discovered.
There
are three families of leptons (electron-electron neutrino, muon-muon
neutrino, and tau-tau neutrino) and three families of
quarks (up-down, charm-strange, and top-bottom). Each quark has electric charge
(u, c, t have +2/3 and d, s, b have –1/3) and colour charge related to the
strong force (red, blue and green). All the families have their anti-particles
with opposite charges.
Hadrons
(baryons and mesons) are particles made of quarks and gluons. According to the
Standard Model hadrons cannot carry colour charge. For this to happen, in a
baryon, made of three quarks, the quarks must have different colour charges; in
a meson, made of a quark and an anti-quark, the colours must be complementary.
Ordinary
atomic nuclei are made of protons and neutrons. A proton is made of two up and
one down quarks resulting in an electric charge of +1. A neutron is made of two
down and one up quarks having neutral electric charge. In both cases, the
quarks are bound by gluons.
Neutrons
and protons are bound in the atomic nucleus by the strong force transmitted by pions which are mesons formed by combinations of up,
anti-up, down, and anti-down quarks. Ordinary matter as we know it from Nature is then made of
up and down quarks and respective anti-particles, and electron family. But,
what will happen if the ordinary matter if submitted to pressures able to brake neutrons in their constituents, the quarks?
Strange Quark Matter
At high enough pressure, ordinary nuclei may dissolve their nuclear boundaries and transform in a called quark-gluon plasma, also known as quark matter. This plasma will be composed of quarks a locally deconfined, gluons and some electrons. Up and down quarks can convert into other flavours via the weak interaction. However, “in practice, only up, down and strange quarks occur in quark matter, because other quark flavours have masses much larger than the chemical potentials involved” [19]. Strange quark matter is then made of roughly equal numbers of deconfined up, down and strange quarks [2] [8] [19], some electrons and gluons. The electrons guarantee the charge neutrality.
“At
any pressure, three flavour quark matter is
energetically favoured over two flavour quark matter” [19] [7].
According to the called “strange matter hypothesis “, strange matter is absolutely stable; the energy per baryon of strange quark matter if less than the lowest energy per baryon found in the nuclei of iron 56. Should this hypothesis be true, then the ordinary state of matter, the hadronic state, is a metastable state [8]. This means that in favourable circumstances, ordinary matter will tend to transform into strange quark matter!
Strange
matter may have been produced in early phases of the hot Universe, but may have
evaporated completely as the Universe cooled [19]. Being this so, where comes
from the strange matter necessary to build a strange star?
Neutron
stars may have the exact conditions to build strange matter in their interiors,
and hence, neutron stars may be converted into strange stars [2][3]. Some authors as
The
discovery of quark matter in the core of neutron stars would be the proof that
the strange quark matter isn’t absolutely stable [9]. In fact, the conversion
of a neutron star into a strange star may occur in a time varying from 1ms to 1
second, which means that the transition phase is very short, assuming the
strange quark matter hypothesis [8].
The
conversion of neutron stars into strange stars may happen through several
mechanisms, most of them related with a “seed” of strange quark matter in
contact with free neutrons. The “seeds” may be created in the high pressures
and temperatures inside the neutron stars, such as the creation of L (uds) baryons that may convert directly into
strange matter. Otherwise, “seeds” may come from space, if there are small
lumps in the Galaxy [9] [19].
The
conversion of neutron stars into strange stars may liberate some 10 53 erg
of energy, and may be responsible for some g-ray bursts detected at Z»1-3
[4].
The
temperature of a strange star may be as high as a few times 1011K
when it forms [6].
Strange
stars may exist with or without a crust.
A
bare strange star has a quark matter surface and a cloud of electrons around
it. The electrons are held electromagnetically to the surface
and extends several hundred fermis above it.
The surface is held by the strong force and its “integrity is greater than for
any other astrophysical object known” [19]. Bare strange stars are
thought to be bad radiators of thermal X-ray photons [6] [19]. However, due to
the huge electric field around the surface, pairs of electron-positron are
emitted inducing hard X-ray emission if the surface temperature is higher than
5x108K. In bare strange stars the surface temperature is close to
the core temperature. A strange star remains nearly bare when the surface
temperature is higher than 3x107 K.
A strange star may have a thin crust of “normal matter” supported by the huge
outward electric field, as long as this matter doesn’t have free neutrons. If
free neutrons exist in that crust, the contact with the strange matter will
convert them immediately into strange matter. This implies that a strange star
can only support a crust with the density below the neutron drip.
Neutron
stars have a crust with two layers, the outer being a solid lattice of
neutron-rich nuclei neutralised by electrons, and the inner one containing in addition
a degenerate gas of free neutrons [20]. Strange stars can only support the
correspondent outer layer, since ions do not react with the strange quark
matter. The electromagnetic field forces a gap between the crust and the
strange quark matter surface, preventing strong interactions between the two
[9] [19]. The minimum radius of a strange star with maximal crust is 5.5 km
[5]. Pulsars, neutron stars with fast rotation, suffer from glitches, changes
in the rotational speed, due to differences of moment of inertia between the
core and the crust. However, “one cannot say definitely that strange stars can
account for any complete set of glitch observations for a particular pulsar”
[9]. Strange stars with crust have the
same relationship between the core temperature and the surface temperature as
neutron stars.
Since
the theory aims to the possibility that neutron stars convert into strange
stars, it is interesting to point out the similarities and differences between
them.
Neutron
stars have a minimum mass to exist, about 0.1 solar masses. Strange stars do
not have a low mass limit.
Above
about 3 solar masses, neutron stars cannot exist. Strange stars have a
predicted maximum mass of 1.8 solar masses.
In
neutron stars, the radius decreases with increasing mass. Strange stars with
masses below 1 solar mass, has its mass proportional to the cube of its radius.
Above 1 solar mass,
the mass curve diverges from cubed radius. For a mass roughly
equal to 1.4 solar masses, the radius of a strange star is similar to that of a
neutron star.
Neutron
stars are bound by gravity. Strange stars are self-bounded objects, via the
strong interaction.
In
low mass specimens, the moment of inertia of strange stars is small compared to
the moment of inertia of neutron stars.
Strange
stars cool more rapidly than neutron stars, within the first thirty years after
birth [17], because quark matter is a more effective emitter of neutrinos than
neutron matter. Quark matter cools via the reactions:
d ® u +
e- + anti ne
u + e- ® d + ne
s ® u + e- + anti ne
u +
e- ® s + ne
The
detection of strange stars will not be an easy task. Some authors defend that
“the population of quark stars can easily be as large as the population of
black holes “ [11] [16]. Others say that 10% of the
twenty five known ms pulsars may be in the transition phase to strange stars
[9]. Others, more radical, defend that pulsars are not neutron stars but
strange stars [10]. Others, on the contrary, say that “there is no compelling reason
for the existence of strange pulsars” [14].
So
far, the signatures of strange stars
relatively to neutron stars are based on cooling efficiency differences,
rotational differences in pulsars, and on the determination of radius versus
mass [6] [15]. Other signatures could be :
-
an oscilation of about 250 GHz due to the radiation of
currents generated in the crust [15]
-
protostrange
stars can be convective. The dynamo effect could (?) be an element of
distinction between strange stars and neutron stars [12].
-
bare strange stars can show a thermal featureless spectrum that can be a new
proof to identify them.
-
may be that the detection of
small lumps of strange quark matter, strangelets, be
achieved in an experience that will be done in the International Space Station
scheduled for 2005.
For
the moment, the list of candidates for strange stars is very reduced: the X-ray
burster 4U 1820 – 30, the ms pulsar SAX J1808.4 –
3658, the atoll source 4U 1728 – 34, the neutron star candidate J1856.5 – 3754
that may have a radius of 3.8 to 8.2 km, the X-ray pulsar Her X – 1, and the
bursting X-ray pulsar GRO J1744 – 28.
Strange stars are most likely to exist if the
strange quark matter hypothesis is correct.
However
our ability to detect them is much limited.
Experiments
on this subject are presently taking place at RHIC and at LHC laboratories.
This
fact could bring some danger to us, since charged strangelets
could trigger the disruption of our planet.
A
part of strange matter bodies may account for the Galactic dark matter, in the
form of strange dwarfs and even strange planets.
Speculating
a little on this, the Jupiter-like exoplanets found
in orbits of 0.3 astronomical units would match the strange quark matter
scenario, maintaining our theories of solar system intact.
[1]
Fierce Flash, Strange Star
Charles Seife, Science Now,
[2]
Pulsar may be strange star
Physics world, Feb 2000
[3]
Quark star glimmers
John Whitfield, Nature,
[4] The
quark strange star in the enlarged Nambu-Jona-Lasinio
model
Ryszard Manka et al., New Journal
of Physics 4 (2002) 14.1-14.18
[5] On
the minimum radius of strange stars with crust
J.L. Zdunik, arXiv:astro-ph/0208334
v1
[6]
Thermal evolution and Light Curves of Young Bare Strange Stars
Dany Page et al., arXiv:astro-ph/0204275
v2
[7] Strangelets and their possible astrophysical origin
Luis Maspri,
arXiv:astro-ph/0202096 v1 5 Feb 2002
[8]
Strange star candidates
Ignazio Bombaci, arXiv:astro-ph/0201369 v1
[9]
From Neutron
Stars to Strange Stars
Fridolin Weber, arXiv:astro-ph/0112058
v1
[10]
Are pulsars strange?
R.C. Kapoor et al.
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2001BASI...29...347K
[11]
Population Synthesis of neutron stars, strange (quark) stars and black
holes
Belczynski, K et al., ADS, bibcode:2001ESASP.459..219B
[12]
The birth of strange stars and their dynamo-originated magnetic fields
Xu, R..X et al. ADS,
bibcode:2001A&A...371..963X
[13]
Strange Quark Stars: Structural Properties and Possible Signatures for
Their
Existence
Bombaci, Ignazio,
ADS, bibcode:2001pnsi.conf..253B
[14]
Whither strange pulsars?
Sushan Konar
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=2000BASI...28..299K
[15] Millimeter-Wave Signature of strange Matter Stars
John J. Broderick et al., The Astrophysical Journal, 492:L71-L74,1998 Jan 1
[16]
Low-Mass Normal-Matter Atmospheres of Strange Stars and their Radiation
Usov, Vladimir V. ADS, bibcode:1997ApJ...481L.107U
[17]
Differences in the cooling Behaviour of Strange Quark Matter Stars and
Neutron Stars
Schaab, Christoph et
al. ADS, bibcode:1997ApJ...480L.111S
[18]
From Strange Stars to Strange Dwarfs
N.K. Glendenning et al., The Astrophysical
Journal, 450:253-261,1995 Sep 1
[19]
Strange Stars
Charles Alcock et al., The
Astrophysical Journal, 310:261-272, 1986 Nov 1
[20] An
Introduction to Modern Stellar Astrophysics
[21] A
Thermal Featureless Spectrum: Evidence for Bare Strange Stars?
Xu, R.X., ADS, bibcode:
2002ApJ...570L..65X