A&A 450, 747-758 (2006)
DOI: 10.1051/0004-6361:20054260
J. L. Zdunik1 - M. Bejger1 - P. Haensel1 - E. Gourgoulhon2
1 - N. Copernicus Astronomical
Center, Polish
Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
2 - LUTH, UMR 8102 du CNRS, Observatoire de Paris, 92195 Meudon Cedex, France
Received 27 September 2005 / Accepted 6 January 2006
Abstract
Aims. We analyze potentially observable phenomena during spin evolution of isolated pulsars, such as back bending and corequakes resulting from instabilities, which could result from phase transitions in neutron star cores.
Methods. We study these aspects of spin evolution of isolated compact stars by means of analytical models of equations of state, for both constant-pressure phase transitions and the transitions through the mixed-phase region. We use high-precision 2-D multi-domain spectral code LORENE for the calculation of the evolutionary sequences of rotating neutron stars. This allows us to search the parameter space for possible instability regions, and possible changes in the stability character of rotating stars with phase transitions in their cores.
Results. We determine the conditions on the density jump in constant-pressure phase transitions which leads to the back bending phenomena or to the existence of the unstable segments in the evolutionary sequences of spinning down isolated normal neutron stars. We formulate the conjectures concerning the existence of two disjoint families of non-rotating and rotating stationary configurations of neutron stars. To clarify the effect of rotation on the stability of neutron star we present the particular case of EOSs leading to marginal instability of static and rotating configurations: marginal instability point in non-rotating configurations continues to exist in all evolutionary spin-down tracks. We discuss the fate of rotating stars entering the region of instability calculating the change in radius, energy release, and spin-up associated with the corequake in rotating neutron star, triggered by the instability. The energy release is found to be very weakly dependent on the angular momentum of collapsing star.
Key words: dense matter - equation of state - pulsars: general - stars: neutron - stars: rotation
Particularly interesting method of searching for the phase transition in neutron star cores via pulsar timing was proposed by Glendenning et al. (1997). As a pulsar spins down, its central density increases, and for a certain density a new phase of matter can appear. In the case considered by Glendenning et al., the new phase consisted of quark matter. The authors suggested, that the softening of the EOS, induced by the formation of the new dense phase, leads to a temporary spin-up era, the phenomenon called back-bending. Originally, the name comes from nuclear physics, where the phenomenon of "back-bending'' was observed in the systematics of the moment of inertia of excited states of rapidly rotating nuclei, see e.g., Ring & Schuck (1980). The calculations of Glendenning et al. were performed within the slow-rotation approximation (Hartle & Thorne 1968; Hartle 1967), supplemented with additional relations resulting from accounting for the rotational stretching and frame-dragging effects (Weber & Glendenning 1992,1991).
Several other authors (Chubarian et al. 2000; Heiselberg & Hjorth-Hensen 1998) carried out their calculations of the back-bending phenomenon using the slow rotation approximation of Hartle. However, as shown by Salgado et al. (1994), the Hartle method, when compared with results of exact 2-D numerical codes, breaks down for angular velocity close to the Keplerian one.
First calculations concerning back-bending based on exact 2-D code were performed by Cheng et al. (2002). These authors used the version of KEH code (Komatsu et al. 1989a,b), improved by Stergioulas & Friedman (1995) (see also references therein). In their work, Cheng et al. focused on the role of the crust for the very existence of the back-bending. Indeed, as they show, even a slight change in the physical state of the crust (for example, a change in the crust-core transition pressure) may significantly affect the results. This shows that high precision is mandatory for reliable calculation of the back-bending phenomenon.
Another important article containing results based on 2-D computations with the Stergioulas & Friedman (1995) code, was published by Spyrou & Stergioulas (2002). They showed that the results obtained by Glendenning et al. (1997) are plagued by large numerical uncertainties. For example, the very same EOS as that used by Glendenning et al. (EOS from Table 9.2 of Glendenning 2000) did not yield the back-bending phenomenon at all in Spyrou & Stergioulas (2002)! It became evident that the back-bending problem is much subtler than previously considered, and that it requires careful handling as well as high-precision 2-D computations. Also, Spyrou & Stergioulas pointed out some errors in previous papers on back-bending. For example, the formula for braking index must be corrected by taking into account the rotational flattening of the star (Sect. 6 in Spyrou & Stergioulas 2002).
Most recently, Zdunik et al. (2004) showed that the back-bending phenomenon can also occur for the EOSs different from the mixed-phase one. They also pointed out importance of the stability with respect to the axi-symmetric perturbations. The appearance of hyperons in the dense matter (Balberg & Gal 1997) softens some EOSs so much, that pulsars losing angular momentum actually spin up during a period of time. Paradoxically, during this spin-up phase pulsars could lose a significant amount of their angular momentum.
It should be stressed that all previous works (except of
Zdunik et al. 2004) considered back-bending as a feature of the
the dependence of the moment of inertia, I, on the
rotation frequency,
:
I=I(f). As shown recently by Zdunik et al. (2004), this can easily
lead to incorrect determination of the stability of rotating
stars. Zdunik et al. (2004) pointed out that many cases claimed before
to correspond to the back-bending, actually cannot be realized in
nature because of the instability with respect to the
axi-symmetric perturbations.
One of the aims of the present work is to determine reliably and precisely the stability regions on the back bending segments of the spin evolution tracks. In order to avoid any precision problem, and to investigate large and possibly complete parameter space, we will work with analytical EOSs of dense matter exhibiting a softening at supra-nuclear density. Two examples of softening by a phase transition will be considered. We will study EOSs with constant-pressure phase transitions, characterized by a density jump obtained using the Maxwell construction. Second, we will use EOSs with phase transition extending over a finite pressure range in which two pure phases coexist forming a mixed-phase state. Such EOSs are obtained for the first-order phase transition between two pure phases by relaxing the condition of local electric charge neutrality and replacing it by less stringent condition of the global neutrality (Glendenning 1992). Mixed-phase state can be realized provided the surface tension at the interface between the two pure phases is not too large.
The plan of the article is as follows. In Sect. 2 we present various types of analytical EOSs used in the calculation of the spin evolution tracks. Numerical methods used in exact 2-D calculations are briefly presented in Sect. 3. Our numerical results are described in Sect. 4. We first describe the general criteria for the back-bending and the stability for spinning-down stars. Then, the results for the EOSs with a mixed-phase segment are studied in Sect. 4.1, and those with constant pressure phase transition with density jump are reviewed in Sect. 4.2. In Sect. 5 we describe a link between the existence of unstable segments in the families of static and rotating configurations of neutron stars. Change in neutron star parameters, accompanying transitions between two rotating configurations, triggered by instabilities of isolated rotating neutron stars, are studied in Sect. 6. Modifications in the pulsar timing and pulsar age evaluations, due to phase transitions in spinning-down isolated neutron stars are studied in Sect. 7. A summary of our results and their discussion is presented in final Sect. 8. Formulae referring to analytical models of EOSs with phase transitions are collected in the Appendix.
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Figure 1:
Two examples of EOSs employed in our calculations:
constant-pressure phase transition (dashed line, EOS with
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Table 1:
Main parameters of the EOSs with mixed-phase
segment. Below the mixed-phase transition point n1 a
polytropic EOS with
is used. Mixed phase
extends within
,
and is described by a
polytrope with adiabatic index
.
Above the density n2 we assume pure quark matter with MIT bag model
EOS
.
In all
cases the dimensionless polytropic pressure coefficient Kwas equal 0.025
(see Appendix A for details).
and
denote
the maximum allowable baryon and gravitational mass of the
non-rotating star. The EOSs are labeled as follows: MSt
produces a stable back bending, MUn - an unstable one, and MM
produces a marginally stable case
(for more details see the text).
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Figure 2: Examples of the three EOSs with phase transition through the mixed-phase state, considered in the present paper. The parameters of the EOSs are given in Table 1. |
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Figure 3:
Total angular momentum versus rotation frequency f
( left panels),
and moment of inertia
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In the present paper we will show examples of the EOS with constant-pressure (density-jump) phase transitions and mixed-phase transitions for which one of the two situations occurs:
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Figure 4:
Upper panel: stellar baryon mass vs radius for fixed frequency (dashed
lines) and fixed total angular momentum (solid lines), for the
model MM of mixed-phase EOS. Bottom panel: stellar angular momentum
as a function of the rotational frequency for fixed baryon mass
(indicated as a label, in solar masses) for the same MM EOS. This
EOS corresponds to the marginal case from the point of view of
stability - the curves
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Figure 5: The same as in Fig. 4 but for the MSt EOS. Phase transition does not result in the stability loss - all configurations are stable. |
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Figure 6:
The same as in Fig. 4 but for MUn EOS.
Upper panel: Phase transition
results in the stability loss, i.e., there exists a region where ![]() ![]() ![]() |
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In Figs. 4-6 we present the results obtained for three different choices of the EOS's parameters describing mixed phase phase transitions (Table 1).
Model MM has been chosen to be a marginal one - there exists a
region where the curve
(or
)
is
locally horizontal, which means the marginal stability. The
function
for non-rotating star (f=0) has an
inflection point slightly above the configuration where phase
transition to the mixed phase occurs. At a fixed f, the
condition for an inflection point reads,
Model MSt produces a set of stable configurations in
the region of phase transition (Fig. 5). Here, is an increasing function of
for non-rotating
configurations, as well as along all rotating sequences of
configurations at fixed J, terminating at the global maximum
mass configurations. The back-bending phenomenon is in this
case limited to the baryon masses larger than that for which
the curve
(or equivalently
)
at
fixed f has a flat, horizontal region (strictly speaking,
where
the Eq. (1) is fulfilled).
The numerical values for the MSt model
are
and f>400 Hz (see for example
the bottom panel of Fig. 5, where the curve for
has almost vertical part).
For the MUn model there exists a region for which configurations are
unstable, i.e. the baryon mass is decreasing function of
at fixed J (Fig. 6).
In some sense this instability is not very strong - the difference
between the maximum (local) and minimum mass is of the order of 0.3%. However this feature (existence of instability region)
is characteristic to all
rotational frequencies - for all values of angular momentum (J) fixed along
the curve, the baryon mass has local maximum and local minimum connected by
an unstable sequence of stellar configurations.
For non-rotating configurations, the reaction of the star to a
constant pressure (first order) phase transition has been
studied in detail in the second half of 1980s (see
Zdunik et al. 1987, and references therein). The appearance of a
new, dense phase in the center of the star results in the
change of the derivatives of the global stellar parameters
with respect to
(see the formula B6 in the appendix
of an article by Zdunik et al. 1987). Two important dimensionless
parameters are: fractional density jump
and the relativistic parameter
.
There exists a
critical value of
,
,
such that for
configurations with an infinitesimally small B-phase core are
unstable with respect to collapse into a new configuration
with a large core of the dense phase. Putting it differently,
a phase transition with
destabilizes the star at central pressure
at which the phase transition occurs. It should be
stressed that while
guarantees
stability of small-core configurations, it does not assure the
stability of configurations with a finite, or - in an extreme
case - a large core. In such a case the instability would
result from the softness of the
phase somewhat above
and not directly from an over-critical
.
In other words the compressibility of a matter
leads to the larger mean density in the core than the
value
at the phase boundary. The response of the
whole star to the appearance of the dense core built of the
B-phase of the matter is determined by the mass and radius of
this core (strictly speaking, this statement is true for
non-rotating configurations, see Zdunik et al. (1987);
for rotating ones also rotation
rate and resulting oblateness play role). As a
result even if
the first order phase
transition can lead to the unstable configurations for finite
size of the core. As a result for the given model of the
matter in the phases
and
there exist the maximum value
of density jump
for which all configurations below
maximum mass are stable. Of course
and the
difference between
and
is larger for softer
EOS in the phase
.
Table 2:
Selected sets
of EOSs with a
constant pressure phase transition. For all cases,
and
(see the text and Appendix A.1).
Numerical results for a collection of sets of EOSs with constant
pressure phase transition are collected in
Table 2. The parameters presented in this table
correspond to the onset of back bending, i.e., the rotational
frequency and baryon mass for which the curve
or
starts to have a flat region. More
precisely, at these values of frequency and mass, an inflection
point appears in the curves under consideration. We also included
parameters of those EOSs for which all non-rotating stars with
are stable. This means that for such
EOSs the
curve for static configurations
increases monotonically up to
.
The parameter
gives then the maximum value of the density
jump for a fixed set of other EOS parameters (adiabatic indices
and
,
number density threshold
)
for which
this property of neutron stars is valid; in other words
corresponds to the "marginally stable'' case. Increasing
implies increasing softening of the EOS by the phase
transition. If
,
the phase transition leads to the existence of an
unstable branch of the non-rotating stellar configurations.
This unstable branch separates stable family of neutron stars with
A-phase cores from a second family of superdense neutron
stars with B-phase cores: these are two distinct neutron-star
families.
It should be mentioned that this feature (existence of the unstable region) does not
depend on rotation - the unstable branches exist also for rotating configurations
(strictly speaking for any value of a total angular momentum of the star J there exist
a region with
). We have
tested this feature (existence of or the lack of unstable regions) for very small
departures from marginally stable case (
).
From numerical results it follows that if
all
rotating configurations are stable (before loosing stability at maximum mass
point) and if
we have two branches of stable
configuration for rotating stars (for any J).
Picking up the onset parameters is visualized in
Fig. 7 where we display
for one of the EOSs from Table 2. The curves are
plotted for three frequencies, with middle one corresponding to the
back bending onset,
.
Last column of Table 2
gives the maximum allowable baryon mass for static configurations,
.
We restrict ourselves to
back bending for the normal (non
supra-massive) stars, which appears during the spin-down evolution which terminates
eventually by a non-rotating stable configuration. The dependence between the
back-bending onset parameters -
,
corresponding baryon mass, and the
intrinsic parameters of the EOS -
the density jump
,
as well as the "departure'' from the
critical configuration (
)
is presented in Fig. 8. The three families of curves visualize
the data from Table 2 (solid lines for
,
dotted for
and dashed for
).
The value
defines the onset of back-bending at the limit f=0;
in this case the back-bending phenomenon is present for any rotational
frequency.
As it can be seen on the left panel, the onset frequency
depends very weakly on the EOS in the dense core - the main parameter describing the
reaction of the star to the appearance of this phase transition is the density jump.
The right panel presents the same data not normalized with respect to the
maximum density jump
- the results can be very well approximated by
the dependence
.
These two plots can be treated as
a slice through the parameter space to search for regions of the back-bending appearance
-
in the right panel, the back-bending is present, for a particular model, above
its curve.
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Figure 7:
The definition of the frequency of the onset of
back-bendingphenomenon,
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Figure 8:
The onset frequency of the back-bending phenomenon
as a function of the departure of ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Rotation can influence stability of a star of a given baryon
mass. In particular, stars with
cannot exist without rotation and collapse into black hole as
soon as their rotation frequency falls below a certain minimum
value. Here, however, we will restrict to a different problem
of stability, which will concern the normal configurations
only.
In what follows we will use the term "stability'' in a restricted sense. Namely, by stability (instability) of an equilibrium configuration we will mean stability (instability) with respect to radial perturbations in the non-rotating case, and with respect to axi-symmetric perturbations for rotating configurations.
We studied a very large set of EOSs with phase transitions at
constant pressure, as well as those with transition through a
mixed phase state. We then produced static sequences and normal
rotating sequences for these EOSs. Our calculations were very
precise, because we used analytic forms of the EOSs. The results
for both constant pressure phase transitions, and those proceeding
through mixed phase, turned out to be qualitatively the same. In
all cases, if non-rotating configuration were stable
(monotonically increasing
and
), then for
any value of the total angular momentum J the functions
and
were monotonically
increasing, too. Thus, when all non-rotating configuration with
were stable (with respect to radial
perturbations), all normal rotating configurations were stable too
(with respect to axi-symmetric perturbations). On the other hand,
if for non-rotating stars there existed a region with decreasing
and
,
even extremely small one with a
very shallow minimum, then an unstable region persisted within the
rotating configurations, at each value of J. These two cases are
illustrated in the Figs. 5 and 6.
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Figure 9:
The mass M of the star as a function of
central baryon number density ![]() |
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We studied also the case of marginally stable EOS. An
inflection point, witnessing marginal stability, present in
the
or
curves for non-rotating stars continued to exist in
the
or
curves for normal rotating stars
(Figs. 4, 9).
The analysis of numerical results leads us to an interesting conclusion. Namely, for an EOS with a phase transition (constant pressure one or through mixed-phase state), rotation neither stabilizes nor destabilizes normal sequences of stationary configurations based on this EOS. We define a family of configurations as a compact set (in mathematical sense) of configurations. Similarly, an EOS leading to a marginally stable point for non-rotating stars, produces also spin-down evolution tracks with a marginally stable point. Our result can be formulated as three conjectures:
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Figure 10: Evolution of an isolated pulsar loosing angular momentum, after it reaches the instability region in J-f plane and then collapses. Arrows lead from unstable configuration to a collapsed stable one, with the same baryon mass and angular momentum. Dotted lines - unstable configurations. |
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Figure 11: Changes of stellar parameters of a rotating solitary neutron star, due to a collapse which occurs after a pulsar loosing angularmomentum reaches an unstable configuration. |
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Figure 12: Total gravitational mass of the star as a function of its angular momentum, for fixed baryon number of the star for our EOS model MUn. The central density is increasing along this curve as marked by the arrows. The upper segment (dotted) corresponds to the unstable configurations. Two cusps reflect strict property that the mass and angular momentum have simultaneous extrema along the path with fixed baryon number. |
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As was shown on many occasions in the preceding sections, an
isolated neutron star, loosing its angular momentum, moves
down along the line of fixed
in the J(f) plane, and
can at some moment reach the instability point (i)
(the minimum of J(f)at fixed
in Fig. 10).
The subsequent behavior of the star cannot be described by our
stationary rigidly rotating model. In real world, the star has
to collapse, rearranging the angular momentum distribution in
its interior. What we can do, is determining the final
stationary state (f), which by construction will be a stable
rigidly rotating neutron star. We assume, that the transition
conserves the baryon number of the star and is sufficiently
rapid so that the angular momentum loss can be neglected.
Therefore, the final stable configuration will have the same
and J as the unstable initial one,
,
.
The difference
between the parameters of these configurations (mass,
equatorial radius, moment of inertia) gives us the energy
release, and changes in equatorial radius and rotation
frequency, due to the collapse implied by the
instability.
Examples are shown in Figs. 10-12
for our EOS model MUn. As the star becomes more compact,
collapse is accompanied by the decrease of the equatorial
radius (by a few kilometers) and by a significant spin up. For
a given EOS, the changes in radius, energy release, and
spin-up are function of the angular momentum at the
instability point:
,
,
.
As we see in
Fig. 11, the energy release depends rather
weakly on the rotation of the unstable configuration (i.e., on
the value of
). It should be mentioned that
our MUn model is only an example of the EOS resulting in the
instability region within the hydrostatic equilibria. For EOSs
with a weaker phase transition this instability region would
be narrower and the changes of stellar parameters in the
collapse would be smaller. However an approximate
constancy of the energy release (i.e., its very weak
dependence on
)
seems to be a generic
property of rotating neutron stars undergoing a collapse
due to a first order phase transition.
In order to discuss in more detail the energy release during
collapses
,
we plotted in Fig. 12 the gravitational mass of the star, M, as a function
of angular momentum, J, at fixed baryon mass:
.
Consider an initial configuration C1. As the star looses
angular momentum, it moves down along line a, and reaches
eventually the cusp
(corresponding to the value of
). To continue moving on the dotted segment
,
the star would have to gain angular momentum and energy!
As we already mentioned, the evolution of the star beyond the
instability point cannot be described by our hydrostationary model. The star
can only collapse to the final configuration ,
with the same
values of
and J, i.e. along vertical arrow in Fig. 12. Then, it evolves down the line b.
We notice a very special role played by the point at which line a and b cross. This is a degeneracy point,
which corresponds to two very different configurations
of the same
,
M, and J. However, transitions
between these two configuration are prevented by the huge
energy barrier.
The existence of the sharp cusps at
and C2 on
the
track is a very stringent test of the
precision of the numerical code: it means that mass and
angular momentum extrema (for fixed baryon mass) are reached
exactly at the same point. This property follows from the
general relativistic relation
(Bardeen 1972). Here,
,
and
is the "injection energy
per unit mass''.
This relation has to be strictly
fulfilled by the stationary configurations.
A graph, analogous to Fig. 12,
can be plotted in the
plane
for configurations with fixed angular momentum J. Also in
this case, the existence of sharp cusps proves the correctness
of the numerical code.
It has been already pointed out by Spyrou & Stergioulas (2002), that the
back bending phenomenon, resulting from the growth of a
dense-phase core, can lead to significant difference between
the actual pulsar age,
,
and that
inferred from the measurements of the period P and period derivative,
,
and denoted
.
The calculation of
is based on quite strong assumptions. Firstly,
pulsar kinetic energy loss due to radiation is given by the magnetic
dipole formula. Secondly, non-relativistic approximation is used,
with pulsar kinetic energy given by
,
where
and pulsar moment of inertia is constant,
independent of
.
Following Spyrou & Stergioulas (2002), we will use general relativistic
notion of total pulsar energy, Mc2. Then the pulsar energy
balance is
Let us consider the increase of stellar energy due to a spin up
to frequency f at constant .
In general relativity, the
increase is given by
.
In the standard model, we neglect the effect of f on stellar
structure, so that
.
This is a good approximation when rotation is
slow and EOS is smooth (no phase transition).
Equation (2) can be then rewritten as
In the case when angular momentum loss leads to
a phase transition at the stellar
center, the situation is much more complicated, because
of the strong f-dependence of the pulsar structure in
the vicinity of the phase transition. This difference
is illustrated in Fig. 13, where we plotted
the quantity
resulting from
our calculations, and compared it with results
given by standard non-relativistic model with constant I.
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Figure 13:
The relative mass-energy increase due to rotation of
the star at fixed baryon mass,
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Figure 14:
The evolution of the pulsar period when the energy
loss is described by the magnetic dipole braking with
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We limited ourselves to the case of normal rotating configurations, which are connected with the non-rotating ones by the angular momentum loss. We considered two types of instabilities which bound the sets of stationary configurations: mass shedding and instability with respect to the axisymmetric perturbations. The EOSs split into two sets: those producing a single family of stable stationary configurations (static and rotating) of neutron stars and those producing two disjoint families of stable stationary configurations. Conjectures concerning normal configurations based on EOSs with a phase transition has been formulated. If an EOS yields two disjoint families of static configurations containing "twin neutron stars'' of the same baryon mass but different radius, then also stable normal rotating configurations form two disjoint regions in the mass - equatorial radius plane, which contain "twin neutron stars'' of the same baryon mass but of different compactness.
Very often, neutron stars are called a second family of compact stars, the first family being composed of white dwarfs. Therefore, our conjecture means that an EOS which produces a third (disjoint) family of static compact stars, produces also a third (disjoint) family of rotating normal stars.
We have also shown the existence of a very special class of "fine tuned EOSs'' with phase transitions which produce marginally stable stationary configurations of normal neutron stars, which form a boundary separating stable stationary configurations (a line in the mass-equatorial radius plane).
Conditions on the density jump in constant-pressure phase transitions were derived, under which their presence in the EOS produces the back bending phenomenon in the spin down evolution.
The case when a spinning down normal neutron star reaches an unstable configuration was studied in detail. The instability leads to neutron star collapse, associated with an energy release in a "corequake'', decrease of radius, increase of central density, and spin up of the star. We have shown that the energy release associated with such a "corequake'' depends rather weakly on the initial rotation frequency at the instability point. In our examples, energy release was of the order of a few times 1050 erg.
In the present paper we put accent on the numerical precision and mathematical strictness. We hope that in this way we prepared ground for further studies of the impact of the phase transitions in dense matter on the structure, evolution, and dynamics of rotating neutron stars. These further studies will be performed using realistic EOSs available in the literature and taking into account important microscopic aspects of the phase transitions. The kinetics of the phase transition coupled with stellar spin-down, and the ensuing neutron star corequake are now being studied. These topics will be the subject of our subsequent papers.
Acknowledgements
This work was partially supported by the Polish MNiI grant no. 1P03D-008-27 and by the PAN/CNRS LEA Astro-PF.
Dense matter is strongly
degenerate, so that the T=0 approximation is valid.
First Law of Thermodynamics implies then expression for energy
per baryon
The energy density
is thus given by
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Figure A.1: Examples of phase transitions considered in the text; constant pressure phase transition ( left), and the phase transition through the mixed-phase state ( right). |
From the continuity of pressure, baryon density, and energy density
at the
transition point, we get