Letters of Intent received in 2015

LoI 2017-283
Stars with a stable magnetic field: from pre-main sequence to compact remnants

Date:	28 August 2017 to 1 September 2017
Category:	Non-GA Symposium
Location:	Brno, Czech Republic
Contact:	Jiri Krticka (krticka@physics.muni.cz)
Coordinating division:	Division G Stars and Stellar Physics
Other divisions:
Chair of SOC:	Ernst Paunzen (Masaryk University)
Chair of LOC:	Ernst Paunzen (Masaryk University)

 

Topics

Characteristics of surface magnetic fields in early-type stars
Surface structure formation in the presence of magnetic field
Magnetic fields and the stellar structure and evolution
Magnetism, accretion and braking of PMS stars
Magnetic field in connection with diffusion and accretion
Magnetic field origin and stability
Magnetically-confined winds
Stellar pulsations in the presence of global magnetic fields
Descendants of early-type magnetic stars
Final phases of stellar evolution : magnetism in compact objects
The future of magnetic field measurements in hot stars

 

Rationale

Characteristics of surface magnetic fields in early-type stars

Significant progress has been made during recent years in
characterising the surface magnetic field topologies of
early-types stars. On the one hand, detailed studies of magnetic
field geometries using spectropolarimetric observations in all
four Stokes parameters revealed some unexpected complexity and
the presence of small-scale magnetic field structures in addition
to previously known large-scale field topologies. On the other
hand, large surveys, especially of massive stars, have expanded
dramatically the number of known magnetic objects. These surveys
have demonstrated that the phenomenon of fossil stellar magnetism
encompasses essentially all types of early-type stars, stretching
over a wide range of masses and evolutionary stages. Furthermore,
the existence of a new, theoretically unexpected, type of very
weak magnetic fields in intermediate-mass stars has been
established. A comprehensive discussion of all these
observational results is timely and essential for inferring
universal magnetic field properties that should be interpreted by
theoretical studies.

Surface structure formation in the presence of magnetic field

Many early-type magnetic stars exhibit chemical abundance
inhomogeneities believed to be produced by an anisotropic atomic
diffusion. These chemical spots, taken as a proxy of the field
presence, come in different varieties and shapes. Theoretical studies
were not able to explain this diversity. Recent investigations have
substantially extended the number of chemical spot maps, allowing to
probe their dependence on the magnetic field characteristics and
stellar parameters. Furthermore, weak abundance inhomogeneities were
discovered on seemingly non-magnetic stars, which questions the
validity of the basic physical process assumed to be responsible for
the formation of these spots. In the light of these results, a
discussion of the observational and theoretical aspects of the
surface structure formation in early-type stars appears to be
interesting and timely.

Magnetic fields and the stellar structure and evolution

The stellar evolution was within the last few decades supposed to be
governed mainly by the stellar mass. At the turn of the millennium it
become clear that also stellar rotation and magnetic fields
significantly influence the stellar evolution. From the example of
our Sun it was also clear that both processes are closely
coupled. That is why the first evolutionary models that included the
stellar rotation soon accounted also for the magnetic fields. The
application of the internal magnetic field using so-called
Tayler-Spruit dynamo theory showed importance of the magnetic fields
for the transport of angular momentum and of chemical elements. The
existence of the dynamo was questioned leading to a decline of the
interest in the study of internal magnetic fields. We plan to re-open
a discussion about the importance of the magnetic fields for the
stellar structure, namely for the internal transport. This could be
crucial for the understanding peculiar abundances observed on the
surface of OB stars, or for the generation of magnetic field in the
neutron stars.

The magnetized hot stars may lose angular momentum via their winds,
what is yet another example of the coupling of the magnetic fields
and stellar rotation. This process seems to be observationally tested
by extremely long rotational periods (of the orders of tens of days to
years) of magnetic O stars. We plan to discuss further observational
implications of rotational braking by the magnetized winds and
possibilities of the testing the evolutionary models with rotation
and magnetic fields.

Magnetism, accretion and braking of PMS stars

Observations indicate that stars are born even today in magnetized
rotating cores of molecular clouds. Centrifugal and electromagnetic
forces lead then to the formation of a disk-like structure during the
gravitational collapse of the molecular cloud cores. The evolution of
accretion disks depends on the efficiency of angular momentum
removal. Turbulence, magnetic braking and outflows are the most
important mechanisms of angular momentum transfer in accretion disks
of young stars. Turbulence in accretion disks comes probably from
magnetorotational instability. The magnetic braking mechanism is
based on the process of angular momentum transfer by torsional Alfven
waves. Centrifugally driven winds arise when ordered magnetic field
lines are inclined more than 30 degrees from vertical. Efficiency of
angular momentum transport depends on strength and geometry of the
magnetic field. Numerical simulations indicate that the initial
magnetic flux of molecular clouds cores is partially conserved during
the process of star formation. It was shown that the initially
uniform magnetic field acquires hour-glass geometry during
protostellar cloud collapse. Collapsing protostellar cloud with
magnetic field evolves into flat structure according to numerical
simulations. It was shown that the collapse of the clouds with strong
magnetic field switches to the magnetostatic contraction into oblate
self-gravitating structures. These numerical simulations show that
accretion disks of young stars should have fossil magnetic field. In
other words, the magnetic flux of accretion disks is the relic of the
parent protostellar clouds magnetic flux.

Ohmic dissipation (OD), magnetic ambipolar diffusion (MAD), turbulent
diffusion and buoyancy are the basic dissipation mechanisms limiting
the fossil magnetic field during protostellar cloud collapse and
subsequent accretion. Efficiency of OD and MAD depends on the
ionization fraction. There are three important non-ideal
magnetohydrodynamical effects operating in accretion disks: OD, MAD,
and Hall effect. Efficiency of the MAD and Hall effect depends on
magnetic field strength. Geometry of the magnetic field also plays
crucial role in the producing of MRI-induced turbulence. I was shown
that the vertical magnetic field enhances turbulent stresses
comparing to the case with the purely toroidal magnetic field.

The complexity of the fossil magnetic field geometry in accretion
disks of young stars makes its investigation with Zeeman experiments
and polarization measurements difficult. It requires a resolution of
0.1 AU at minimal distance of 1 pc that corresponds to angular
resolution 10^−6 angular seconds.

Magnetic field in connection with diffusion and accretion

The chemically peculiar (CP) stars of the upper main sequence have
been targets for astrophysical studies since the discovery of these
object by the American astronomer Antonia Maury in 1897. The main
characteristics of the classical CP stars are: peculiar and often
variable line strengths, quadrature of line variability with radial
velocity changes, photometric variability with the same periodicity
and coincidence of extrema. Slow rotation was inferred from the
sharpness of spectral lines. Overabundances of several orders of
magnitude compared to the Sun were derived for heavy elements such as
Silicon, Chromium, Strontium and Europium.

In 1947, Babcock, discovered a global dipolar magnetic field in the
star 78 Virginis followed by a catalog of similar stars. This catalog
also described the variability of the field strength in many CP
stars, including even a reversal of magnetic polarity, leading to the
Oblique Rotator concept of slowly rotating stars with non-coincidence
of the magnetic and rotational axes. This model produces variability
and reversals of the magnetic field strength similar to a light
house. Due to the chemical abundance concentrations at the magnetic
poles also spectral and the related photometric variabilities are
easily understood, as well as radial velocity variations of the
appearing and receding patches on the stellar surface. The CP stars
can be divided into magnetic and non-magnetic subgroups.

The peculiar (surface) abundances for the CP stars have been
explained by the following two theories. The first theory is based on
the diffusion of chemical elements depending on the balance between
gravitational pull and uplift by the radiation field through
absorption in spectral lines. This should modify the chemical surface
composition in case of sufficient stability in these layers.

While diffusion seems to be appropriate for both the magnetic and
non-magnetic stars to explain spectral peculiarity, it is not yet
clear to which extent the interaction with the interstellar medium
via accretion and transport of angular momentum may modify the
effects of diffusion and break the stellar rotational velocities
during main sequence life time. A most important point is the origin
of the global stellar magnetic fields for CP stars.

Magnetic field origin and stability

The origin of the magnetic field in hot stars is still unclear.
Magnetic field may be generated in the convective cores of these
stars and transported to the stellar surface by buoyancy. However,
the timescale of this process may be prohibitively long.
Consequently, the dynamo processes may be important only in the
shallow subsurface convective zones, creating weak and variable
fields, that still await their observational verification. Moreover,
the possibility of the existence of the dynamo in the radiative zone
is extensively discussed in the literature.

The fossil theory therefore provides explanation of the origin of the
magnetic field in massive stars which is more favoured in the
community. The fossil field theory comes in two flavors. The observed
fields may be direct descendants of the weak interstellar magnetic
fields amplified during the process of the star formation. However,
this is complicated by a neutral nature of the interstellar medium
and by the existence of the convection in the pre-main sequence
phase. Consequently, the interstellar magnetic field may provide just
a seed magnetic field, which is amplified during the convective
phases of the star formation. The fossil field theory is supported by
the observations of predecessor of massive stars, Herbig Ae/Be stars.

The problem of the stability of internal magnetic field is crucial
for understanding the structure and evolution of internal magnetic
fields. While the pure poloidal field, which is manifested
observationally, as well as pure toroidal field provide
configurations which are not stable, the only stable magnetic field
configurations come from the combination of the poloidal and toroidal
fields. All these issues have to be thoroughly discussed both from
observational and theoretical point of view.

Magnetically-confined winds

A striking consequence of magnetism in hot OB stars relates to the
interaction between the magnetic field and their powerful,
radiatively-driven stellar winds, which peel away their outer layers
and critically determine the final stages of a massive star’s life.
If a magnetic field has a large-scale component extending above the
stellar surface – as is the case for about 70 known magnetic OB
stars, it channels these strong winds, creating a structured
circumstellar magnetosphere that modifies the mass and angular
momentum loss, hence greatly changing their evolution and ultimate
demises.

A key aspect of the presence of these magnetospheres is the
alteration of the stellar spectroscopic characteristics at all
wavelengths. As the magnetic field’s symmetry axis is generally not
aligned with the star’s rotational axis, observations at various
rotational phases provide important clues about the detailed physics
governing the magnetospheres, and also about the critical topic of
mass-loss of massive stars. As recent large surveys (e.g. MiMeS,
BinaMIcS, BOB) are discovering more and more magnetic stars, a better
picture of wind confinement is emerging thanks to extensive
observational and theoretical efforts. The proposed conference will
be an excellent opportunity to bring together researchers
specializing in various wavelength ranges, as the community is aiming
toward a global understanding of these fascinating magnetospheres
across the whole electromagnetic spectrum.

Stellar pulsations in the presence of global magnetic fields

A handful of A-type magnetic stars (so-called "rapidly oscillating
Ap" or roAp stars) pulsate in high-overtone non-radial p-modes, with
periods on the order of 10 minutes. These pulsations, associated with
the presence of strong magnetic fields in stellar envelopes, enable
asteroseismic analysis that provides useful information about
fundamental stellar parameters. Besides the classical
asteroseismology high-resolution spectroscopy of roAp pulsations can
provide a remarkably detailed view of the propagation of
magneto-hydrodynamic waves in stellar atmospheres. Besides the Sun,
this is the only class of stars for which such detailed vertical
analysis of non-radial pulsations is possible.

From the theoretical point of view, convection is assumed to be
suppressed in the stellar envelope. For example, using a two solar
mass main-sequence model, the presence of a magnetic field always
stabilizes low-order acoustic modes. All the low-order modes of the
model that are excited by the kappa-mechanism in the He II ionization
zone in the absence of a magnetic field are found to be stabilized if
the polar strength of the dipole magnetic field is larger than about
1 kG. For high-order p modes, on the other hand, distorted dipole and
quadrupole modes excited by the kappa-mechanism in the H ionization
zone remain overstable, even in the presence of a strong magnetic
field. It was found, however, that all the distorted radial
high-order modes are stabilized by the effect of the magnetic field.
Thus, a non-adiabatic analysis suggests that distorted dipole modes
and distorted quadrupole modes are most likely excited in rapidly
oscillating Ap stars. The latitudinal amplitude dependence is found
to be in reasonable agreement with the observationally determined
ones. Finally, the expected amplitude of magnetic perturbations at
the surface is found to be very small.

Descendants of early-type magnetic stars

The unique large-scale organization of the magnetic fields in O-, B-,
and A-, which in many cases appears to occur essentially under the
form of a single large dipole located close to the centre of the
star, contrasts with the magnetic field of late-type stars, which is
most probably sub-divided in a large number of small dipolar elements
scattered across the stellar surface. To properly understand the
physics of these stars it is not only particularly important to know
the origin of magnetic fields, but also their evolution on the
main-sequence and even further in the giant phase and beyond.

Stars with a mass up to a few solar masses are one of the main
contributors to the enrichment of the interstellar medium in dust and
heavy elements. The process of the mass-loss responsible for this
enrichment is still not exactly known and forces beyond radiation
pressure might be required. The shaping process of often strongly
a-spherical planetary nebula is equally elusive. Both binaries and
magnetic fields have been suggested to be possible reasons although a
combination of both might also be a natural explanation. There is
evidence for magnetic fields around AGB and post-AGB stars
pre-Planetary Nebulae and PNe themselves. Magnetic fields appear to
be a common characteristics in the envelopes of apparently single
stars. There are also strong indications of magnetically collimated
outflows from post-AGB/pre-PNe objects supporting a significant role
in shaping the circumstellar envelope.

Final phases of stellar evolution : magnetism in compact objects

The majority of stars will evolve into a white dwarf star. A
significant fraction (~10 to 20%) of white dwarfs harbours a magnetic
field that ranges from a few kG to about 1000 MG. The progenitors of
magnetic white dwarfs are often assumed to be magnetic Ap and Bp
stars, where under the assumption of magnetic flux conservation, the
magnetic field strengths observed in Ap stars would correspond to
white dwarf fields in excess of 10 MG. However, recently other
mechanisms for producing magnetic fields in white dwarfs have been
proposed. For example, strong fields may be created in evolving
binaries via a dynamo during a common envelope phase. The binary star
origin was proposed due to the lack of close white dwarf plus cool
main-sequence stars compared to the 25% incidence of magnetic
cataclysmic variables.

Recently, a high incidence of magnetism was found among cool polluted
white dwarfs. The accepted explanation for the pollution of the white
dwarf atmosphere is that they have accreted planetary debris.
Although the reason for this higher incidence of magnetism still
lacks an explanation, it maybe that magnetism was generated by an
early merger event in a densely populated system.

The recent observational discoveries (e.g., magnetic polluted white
dwarfs) and also the development of the binary origin for magnetism
makes a symposium on magnetic stars timely. A symposium that combines
the studies of magnetic white dwarfs with other magnetic objects will
contribute toward our understanding the origin of magnetic fields in
white dwarfs, which may have multiple origins and how they relate to
other stars.

Neutron stars have been known to be magnetic since the first
discovery of a pulsar, however the origin of the magnetic field is
still being debated. Similarly to white dwarfs, one of the origins
proposed is that it is a fossil field where the magnetic flux is
conserved from the main-sequence phase to the neutron star. Another
origin proposed is that the field is generated by a dynamo process in
the core of a supernova progenitor. Recent work, have shown that the
fossil field hypothesis can explain the highly magnetic “magnetars”
but cannot explain the population of the lower field neutron stars
and hence need an alternative origin. A binary origin for magnetic
fields in neutron stars is also a possibility. Magnetic fields may be
generated by more than one way, and therefore there may be
similarities in the formation of magnetic fields in neutron stars,
white dwarfs and even possibly main-sequence stars. Hence studies of
magnetic fields in main-sequence stars, white dwarfs and neutron
stars will help us understand how some of these stars become
magnetic.

The future of magnetic field measurements in hot stars

The recent unprecedented development of our understanding of the
stellar magnetic fields is closely connected with appearance of new
instruments with higher resolution and stability. The future
facilities, which may not be for the first time confined to the
ground-based instruments, may improve our understanding of the
stellar magnetic field even more. A careful planning of new
instruments and observational strategies is necessary to maximize the
output of future instruments. All these issues have to be thoroughly
discussed in a broad scientific community.