A&A 411, 543-552 (2003)
DOI: 10.1051/0004-6361:20031491
A. Maeder - G. Meynet
Geneva Observatory 1290 Sauverny, Switzerland
Received 28 May 2003 / Accepted 15 September 2003
Abstract
We compare the current effects of rotation in stellar evolution to those of
the magnetic field created by the Tayler instability. In stellar
regions, where a magnetic field can be generated by the dynamo due to differential
rotation (Spruit 2002), we find that the growth rate
of the magnetic instability is much faster
than for the thermal instability. Thus, meridional circulation is small
with
respect to the magnetic fields, both for the transport of
angular momentum and of chemical elements. Also, the horizontal coupling by
the magnetic field, which reaches values of a few 105 G, is much more important than the effects
of the horizontal turbulence. The field, however, is not sufficient to distort the shape
of the equipotentials. We impose the condition that the energy of the magnetic field
created by the Tayler-Spruit
dynamo cannot be larger than the energy excess present in the differential
rotation. This leads to a criterion for the existence of the magnetic field
in stellar interiors.
Numerical tests are made in a rotating star model of
rotating with
an initial velocity of 300 km s-1. We find that the coefficients of diffusion for the transport
of angular momentum by the magnetic field are several orders of magnitude
larger than the transport coefficients for meridional circulation and shear mixing.
The same applies to the diffusion coefficients for the chemical elements; however,
very close to the core, the strong
-gradient reduces the mixing by the magnetic instability
to values not too different from the case without magnetic field. We also find that magnetic instability
is present throughout the radiative envelope, with the exception of the very outer
layers, where differential rotation is insufficient to build the field, a fact consistent
with the lack of evidence of strong fields at the surface of massive stars.
However, the equilibrium situation reached by a rotating star with magnetic field
and rotation is still to be ascertained.
Key words: stars: rotation - stars: magnetic field - stars: evolution
The inclusion of new physical effects in stellar evolution greatly improves the comparisons with observations. About twenty years ago the impact of mass loss by stellar winds on the evolution was found to be large. However, some significant discrepancies remained and the inclusion of rotation has enabled substantial progress in the comparisons with observed chemical abundances, with number counts and with chemical evolution of galaxies (cf. Langer et al. 1999; Maeder & Meynet 2000). The magnetic field is the next, but certainly not the last, in this series of effects which may influence all the outputs of stellar evolution.
In this work, we focus mainly on the relative importance of the effects
of the magnetic field and of rotational instabilities to try to determine which effects can be let aside
and what must be considered a priority. Section 2 summarizes the main effects of the
magnetic field we are considering here following Spruit (1999, 2002).
Section 3 compares
the characteristic times of meridional circulation and of magnetic field
instabilities. Section 4 considers what happens to the horizontal turbulence
in presence of magnetic fields. Section 5 shows a new physical limit on the occurrence
of the magnetic field
in rotating stars. Section 6 gives some numerical values on the size of magnetic and
rotational effects in the
case of a
star. Section 7 presents the conclusions.
Let us collect here some basic expressions and concepts we need below. Spruit (1999, 2002) has shown that the magnetic field can be created in radiative layers of stars in differential rotation. Even a small toroidal field is subject to an instability (called Tayler instability by Spruit), which creates a vertical field component. Differential rotation winds up this vertical component, so that many new horizontal field lines are produced. These horizontal field lines become progressively closer and denser in a star in a state of differential rotation, and therefore a much stronger horizontal field is built. This is the dynamo processs described by Spruit. The Tayler instability is a pinch-type instability. As shown by Spruit (1999), it has a very low threshold and is characterized by a short timescale. Also it is the first instability to occur. The magnetic shear instability may be present, but it is of much less importance (Spruit 1999).
The instability occurs in radiative zones and two cases are
considered by Spruit (1999,
2002) depending on the thermal and -gradients
through the associated oscillation frequencies,
The growth rate of the magnetic instability is
![]() |
(4) |
The magnetic instability works only if the unstable displacements do
not lose too much energy against the stable stratification. For this
to be the case, the radial displacements (against the buoyancy force)
must be small compared to the horizontal displacements. Taking ras a maximum for the horizontal displacements, this sets an upper
limit on the radial length scale
lr0, 1 of the displacements,
Spruit (2002; Eqs. (18) and (19))
considers the amplification timescale necessary to double the component
starting from the radial component
over the largest
characteristic lengths defined by Eqs. (5) and (6). He assumes the equality
of the amplification timescale
with the timescale for the damping by magnetic diffusivity over the above
lenghtscales. In this way, he obtains
the expressions for the
Alfvén frequency. In the first case, where
dominates, this is
Table 1:
Structural parameters of the model of
with
km s-1 when
.
Table 2:
Diffusion coefficients of the model of
with
km s-1 when
.
A major question arises concerning a rotating star with a magnetic field. What happens to the meridional circulation in presence of the magnetic field of the Tayler-Spruit dynamo? Basically, meridional circulation occurs because thermal equilibrium cannot be achieved on an equipotential inside a rotating star. Thus, we may wonder whether the horizontal breakdown of thermal equilibrium in a rotating star can be compensated by a magnetic stress on an equipotential.
Let us define a velocity
characterizing the radial growth of the magnetic
instability. In the two cases 0 and 1, we consider the ratio
of the appropriate maximum lengths given by
Eqs. (5) and (6) to the characteristic time
,
These velocities have to be compared with the radial component
of the radial part of the velocity of
the meridional circulation U(r), as given by Maeder & Zahn (1998;
Eq. (4.38)). If the circulation velocity would largely dominates, this would mean that the magnetic
field has effects which are too weak to influence the meridional
circulation and the circulation would develop as usually supposed.
If on the contrary, one has
,
this means
that the magnetic instability develops much faster than the thermal instability
at the origin of meridional circulation.
If so, this means that the thermal instability created by rotation on an equipotential
will interact firstly with the magnetic field. Tayler
instability, which has the shortest timescale, may possibly develop
and create a magnetic field which will
introduce some stress horizontally on the equipotential.
Detailed calculations must be done with
accounting for the effects of the magnetic field in the equation for the
entropy conservation at the basis for the calculations of meridional
circulation, (this has been made for the effects of horizontal turbulence
on the meridional circulation
by Maeder & Zahn 1998).
For now, we assume in this
case, in the whole region where
,
that the usual circulation velocity must be set to zero.
This working hypothesis is largely confirmed by the comparison below (cf. Tables 1 and 2)
of the velocities U(r)
of meridional circulation to the velocities
characterizing the growth of the magnetic instability, the
last ones being 4 to 7 orders of a magnitude larger than the first one.
Before looking more to the numerical values, we note that
some caution has to be taken so that the comparisons of
and U(r) are done in a
consistent way. The expression
for the transfer of angular momentum
by circulation and diffusion D is (see Zahn 1992;
Maeder & Zahn 1998)
Below in Sect. 6,
a star model with an initial mass of
,
a composition given by
X=0.705 and Z=0.02 and an initial rotation velocity of 300 km s-1 has been
computed. The prescriptions for rotation are those in Maeder & Meynet (2001).
Tables 1 and 2 show the main parameters when the central H-content is
at an age of
yr.
We see that the velocities
are
to
times larger that the velocity U(r) of meridional circulation.
This shows that, even if present, the velocity of meridional circulation
is negligible with respect to the corresponding velocity characterizing the
transport of angular momentum by the magnetic field. Thus the question
whether or not meridional circulation appears in the presence of magnetic
field is of little relevance, since meridional circulation would anyway be
totally negligible in comparison of the effect of magnetic instability.
The same remark arises when one compares the magnetic
diffusion coefficients for the angular momentum as given in
Eqs. (40), (42) and (44) with the corresponding expression (28) for meridional circulation.
Thus, we suggest that in the stellar regions where magnetic field is present, and only there (see Sect. 5), the meridional circulation may be neglected with respect to magnetic field effects for the transport of angular momentum. As is shown in Sect. 4 below, the above conclusion also applies to the transport of the chemical elements.
The turbulence in rotating stars is highly anisotropic and has a strong horizontal component,
described by a diffusion coefficient ,
because vertically the thermal gradient stabilizes turbulence. This horizontal turbulence
strongly reduces the horizontal differential rotation, so that rotation varies only
radially. The rotation is therefore said to be "shellular'' (Zahn 1992,
uniform
at the surface of isobaric shells).
The coefficient
also plays a role in the mixing of the chemical elements and
meridional circulation (Maeder & Zahn 1998). A first expression for
was given by Zahn (1992). Recently another expression of this coefficient
has been obtained (Maeder 2003),
The question is now: what happens to this horizontal turbulence, even if it is much larger
than initially supposed,
in the presence of the magnetic field? According to Spruit (2002),
the Tayler instability and the associated dynamo leads to horizontal field
components in cases 0 and 1,
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= | ![]() |
(22) |
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= | ![]() |
(23) |
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(24) |
Now, the question is how does the horizontal coupling due to the magnetic field
compares with the horizontal turbulence characterized
by
.
Spruit (2002; Eqs. (31) and (32)) has given an estimate of the coefficient
of the vertical coupling by magnetic field. However, we cannot use it here,
because the field is very anisotropic. The
coefficient
for the horizontal coupling must be much larger
than the vertical one. At low rotation,
we suggest to take the following estimate,
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(25) |
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(26) |
In models with rotation and without magnetic fields, the combined effect
of meridional circulation and horizontal turbulence may be treated
as a diffusion with a coefficient usually called
(cf. Chaboyer & Zahn 1992),
![]() |
(29) |
![]() |
(30) |
In a radiative zone, the magnetic field created by the Tayler-Spruit instability arises
from differential rotation.
Therefore, we must impose the condition that the
energy in the magnetic field cannot be larger than the excess energy in differential
rotation. Tayler (1973) and Pitts & Tayler (1986) have
studied the conditions for the appearance of the magnetic instabilities,
which have very short growth times. The above condition is different, it is an
energy condition, which must be satisfied on longer timescales.
Due to the magnetic diffusivity, the field once created tends to disappear.
Taking the values of
given for example in Fig. 6 (see also Tables 1 and 2), which are between 1010.5 and 1012 cm2 s-1 over most of the star except very close to the convective
core, we find that the timescale
for the diffusion of the magnetic field is of the order of
to
yr, which is short with respect to the
stellar lifetimes. This means that the field created by Tayler-Spruit dynamo
will exist only if the energy of the magnetic field is
continuously replenished by differential rotation during the MS evolution. Therefore, we
need to apply the above condition.
For a magnetic instability with a displacement
of amplitude ,
the kinetic energy EB by unit of mass is
From these two geometrical remarks, we obtain
the necessary condition for the existence
of a magnetic field generated by the Tayler-Spruit dynamo as follows,
We can go a step further, since the Alfvén frequency
is a function of rotation and differential parameter |q|.
For the case 0, where the
-gradient dominates,
the ratio
is given by the above Eq. (10). Thus, the above condition (37)
becomes in this case
We now consider case 1 with thermal diffusion, the Alfvén frequency
is given by Eq. (11). With the above condition (37), one obtains
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Figure 1:
Internal H-profile in the
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Let us collect here the various expressions we have for the
diffusion coefficients by the Tayler-Spruit dynamo in radiative zones.
Case 0 applies when
.
Magnetic field is
present only when the criterion given by Eq. (38) is satisfied. Then
the diffusion coefficients for the transport of the angular momentum
and chemical elements are respectively,
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Figure 2:
The oscillation frequencies in the model of Fig. 1.
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For case 1, we compare the coefficient with indices "1'' and "1P''
and must take the larger ones. From Fig. (5) below, we see that the coefficients "1P'' may be larger than the coefficients "1''
in some parts of the star. Of course, we have also to test whether
the magnetic field can be created from differential rotation. For such zones of case 1P, the
criterion for the existence of the magnetic field is evidently the following one
We calculate evolutionary models of a
star with initial composition
X=0.705 and Z=0.02. The physics of the models (opacities, nuclear reactions, mass loss rates,
treatment of rotation, increase of the mass loss rates with rotation, etc.)
are the same as in Meynet & Maeder (2003).
We compute the MS evolution of a model with
an initial velocity of 300 km s-1, which leads to an average velocity during the MS phase
of about 220 km s-1, which corresponds to the observed average rotation velocity.
We now consider in detail the properties of a particular model with rotation to see
the various coefficients and criteria characterizing the growth
of the magnetic field.
We take the model at an age
yr with a central
H-content
.
The H-profile inside the star is illustrated in
Fig. 1 and the oscillation frequencies
and
in Fig. 2. We see that case 0 applies between
and 1.59, which corresponds to mass coordinates
5.53 and
respectively. The internal profile
is illustrated
in Fig. 3.
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Figure 3: The distribution of the angular velocity in the reference model of Fig. 1 with rotation (continuous line). |
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Figure 4:
The diffusion coefficients
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The diffusion coefficients
and
are illustrated
in Fig. 4. These coefficients grow very fastly at the edge of the core
since
decreases very fastly there and the dependence in the ratio
goes like the power 4 and 6 for the two coefficients
respectively. These coefficients apply only in the rising
part between
and 1.59 as indicated. Above this value, the main
restoring force is no longer the
-gradient, but the stable temperature gradient.
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Figure 5:
The diffusion coefficients for angular momentum
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In case 1, where thermal gradients dominate, the diffusion coefficients are
illustrated in Fig. 5. We see that for the transports of
angular momentum and of chemical elements, the coefficients with indices "1''
dominate in the external parts of the star above
.
However, we also notice that in a sizeable
region, i.e. between
and 2.69,
the coefficients with indices "
'' dominate
over those with "1''. This occurs, as expected, at some limited
distance above the edge of the convective core, at the place where the
-gradient
becomes small enough, but is still different from zero. In these regions, more general developments
having case 0 and 1 as limiting cases would be a progress. We see that the differences between
the two cases "
'' and "1'' amounts to a maximum of 0.4 dex and 0.6 dex
for the transport of angular momentum and chemical elements respectively. This is limited, but
non negligible, and it may justify a further study of the physics of the
general case. However, we notice that this difference is small when compared
to the differences resulting from the inclusion of the magnetic field
or not, which as shown below amounts to several orders of magnitude.
Thus, we conclude that the present coefficients of diffusion
need to be further improved, but they nevertheless describe correctly the
main results of the inclusion of the magnetic field.
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Figure 6:
The figure shows the complete description
of the diffusion coefficient
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Tables 1 and 2 provides some useful structural parameters, the
diffusion coefficients and the velocities of meridional circulation and of magnetic
instability at 3 locations
in the reference model of
with
an initial velocity of 300 km s-1 at an age
yr.
The three levels considered illustrate the case 0,
1P and 1 respectively. These Tables permit further quantitative analysis
of the various terms intervening in the equations.
Figure 6 shows the comparison of the diffusion coefficients due
to the magnetic field compared to the diffusion coefficient
by shear
instability in the rotating star and to the thermal diffusivity K.
The transport of angular momentum by the
magnetic field is 6-7 orders of magnitude stronger than by shear instability in a rotating star.
Similarly, as discussed in Sect. 4.2, the magnetic transport of angular momentum
is also 4-7 orders of magnitude larger than by meridional circulation. Therefore, we conclude
that transport of angular momentum by magnetic field is totally dominating, if magnetic field is present.
For the transport of chemical elements, the difference between the two diffusion coefficients
amount to 3-5 orders of magnitude in favour of the transport by magnetic field. The
difference is especially large in deep regions at some distance of
the convective core. At the very edge of the convective core,
the dependence of the coefficient
in the power 6 of
reduces the magnetic diffusion
drastically, so that as mentioned in Sect. 4.2 the ratio
of the transport of chemical elements by the magnetic instability to the transport by
circulation may amount to about 1 order of magnitude.
Some tests indicate that this makes the chemical enrichments in helium and nitrogen at the
stellar surface are stronger, but not too different,
from those without magnetic field, despite the fact that
the diffusion coefficients with magnetic field are orders
of magnitude larger over most of the stellar interior.
On the whole, we see that for chemical mixing also, the magnetic
instability plays a great role.
It is also interesting to see that diffusion coefficients by the magnetic field are almost equal (transport of chemical elements) or even larger (transport of the angular momentum) than the thermal diffusivity. This means that magnetic effects by the Tayler-Spruit dynamo are in general equal or larger than thermal effects. Globally thermal effects may be relatively more significant in the outer layers.
An important question in the models is to determine at each shell mass
whether differential rotation described by parameter qis sufficient to create the Tayler-Spruit dynamo
to produce a magnetic field. We examine here whether these conditions are
fulfilled in the model with rotation only studied in the previous
subsection. Firstly, we examine the regions adjacent
to the core between
and 1.59,
where case 0 applies, since
dominates.
Figure 7 shows the difference
,
we see that in the concerned region between
and 1.59,
this difference is negative, thus magnetic field is present there. This is an interesting result
because it means that despite the strong restoring buoyancy force due to the
very large
-gradient, the differential rotation is high enough to
develop magnetic instability.
At each time step during evolution, such tests need to be performed.
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Figure 7:
The difference
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Secondly, we examine the zone above
and up to
,
where case 1P applies. There, we check for the difference
.
If this expression is negative,
magnetic field is created. This expression lies between
-0.40 and -0.50 in this whole intermediate region. Thus, we conclude that
magnetic field is also present there.
The criterion for the existence of the magnetic field in the external zone,
which corresponds to case 1 is given by Eq. (39). From Fig. 8,
we see that in this zone
which lies between
and the surface, the difference
is generally
positive, which means that magnetic field is present over that region. However,
we notice that very close to the surface this difference goes to zero. This means
that at the surface, differential rotation is becoming insufficient to generate the
magnetic instability. This is interesting because it may explain why there is in general no
strong magnetic field observed at the surface of OB stars (cf. Mathys 2003), despite
the likely existence of a strong internal field created the Tayler-Spruit instability.
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Figure 8:
The quantity
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These results show that in a rotating star, the conditions on the differential rotation for the growth of the magnetic field are largely realized, with the possible exception of the superficial layers.
The main conclusion is that the Tayler instability and the Tayler-Spruit dynamo are of major importance for stellar evolution, both for the transport of angular momentum and for the transport of chemical elements. Future evolutionary models applying the results of this work will be made to study the results on tracks, surface composition, rotation, etc.
It is likely that in a rotating star calculated with magnetic field from the beginning,
the differential rotation is very much reduced by the magnetic transport
of angular momentum described above. We may suspect that differential
rotation will be reduced down to a stage where the criterion (37) discussed
in Sect. 5.1 is just marginally satisfied, i.e.
Note added in proof: The above calculations determine the magnetic field
which develops
in a rotating star, which had no field until a considered specific evolutionary stage.
Recent calculations have confirmed, as suggested in the conclusions, that models with
magnetic field included throughout MS evolution reach an equilibrium situation with
very little differential rotation. This happens in turn to make a feedback on the field
amplitude and on the velocity of meridional circulation. This has two consequences:
a) the internal magnetic fields are reduced to a few 104 G; b) the velocity of meridional
circulation is increased to about
.
This confirms that magnetic fields
play an essential role. However, in the equilibrium models some effects
of meridional circulation could also influence the internal profile of
due to the above feedback.
Acknowledgements
We express our best thanks to Dr. H.C. Spruit for very useful comments. The most valuable remarks of an unknown referee are also acknowledged with thanks.