Magnetism
in active stars: The study of
magnetic fields in
cool active stars (Zeeman-Doppler imaging) is fundamental to understanding their activity and the
dynamo processes they
trigger, and to enabling us to
see our Sun in an evolutionary context. The magnetic fields so far found are sufficiently
intricate
that the circular polarisation signal (sensitive to the vector properties of the
magnetic field) they induce in spectral lines is tiny.
Co-addition of
the signals from thousands of spectral lines permits major
reductions in the threshold for detection and for the magnitude limit to which it is
practical to observe, and greatly facilitates
mapping of the
field structure.
The next step is to attempt field
detections in new
classes of objects, such as stars of young open clusters, classical
T Tauri stars featuring magnetically channelled accretion and ejection,
accretion discs of young stars and
cataclysmic variables,
or hot stars with azimuthally
structured stellar winds (e.g. O, Herbig Ae/Be, Wolf-Rayet or
Be stars)
Magnetic
field in chemically peculiar A and B stars: Numerous
upper main sequence stars are known to have well ordered kG
magnetic fields.
As yet little is know about the detailed structure of such fields,
essential information for understanding their intense interaction with the stellar
atmosphere, envelope and wind. Observing the circular and linear
polarisation
signatures due to the Zeeman effect in many
lines should make possible detailed mapping of the field structure for the first
time and test in detail the presumption that such magnetic structures are fossil
remnants from a previous evolutionary stage.
Geometry
and chemistry of circumstellar scattering matter: Asymmetric
circumstellar material (e.g. in an accretion disc near a T Tauri star, or the
circumstellar disc of a Be star) is
able to polarise, both
linearly and circularly, scattered light from the stellar
photosphere. In particular, study of how this polarisation varies through emission
lines can greatly help to constrain the general geometry of the scattering environment
and its relationship to gaseous emission regions.
Asteroseismology of hot stars: Numerous rapidly rotating
hot stars (delta Scuti, Herbig Ae/Be, Be,
O, roAp stars) pulsate in a variety of
non-radial modes which furnish invaluable information about the
structure of the stellar envelope. High degree modes can be
detected through small line profile variations and will be very useful additional
constraints for identifying pulsation modes on targets to be observed with the very
first asteroseismological spacecrafts
COROT/MOST are
built by France/Canada. Addition of
information from
many lines
can make possible detection of these tiny distortions in spite of exposure times short
enough to avoid phase smearing.
Asteroseismology of solar-type stars:
Our knowledge of stellar interiors can be greatly aided by study of the weak
non-radial pulsations in Sun-like stars. Low degree modes can be
observed through the tiny radial velocity variations they cause. By combining information
from many spectral
lines simultaneously, radial velocity variations as low as a
few m/s per observation should be achievable for the brightest objects, yielding
noise levels of a few cm/s in Fourier space in just a few nights.
Atmosphere dynamics of pulsating stars:
The atmospheric structure of many
radially pulsating stars differs greatly from that of a static star. Study of the
response of spectral lines of various elements and ions can greatly help to
understand the propagation of waves and shocks through the atmosphere.
Doppler imaging of stars having surface brightness/abundance
inhomogeneities:
Surface variations of brightness/abundance lead to line profile variations that
can be observed and modelled to obtain
maps. The
ability to do this for many lines greatly increases the number of elements
for which brightness/abundance maps can be derived as well as the accuracy of each map,
considerably enhances the constraints on the structure of starspots, as well
as allow a accurate estimate of the short-term evolution of the
starspot distribution (e.g.
differential
rotation).
Tomography of accretion discs and winds:
Observation of profile
variations of emission lines makes possible the
mapping of the
structured discs and/or flows of circumstellar material, much as in
Doppler imaging.
This should be possible for many types of stars, including
cataclysmic variables,
classical T Tauri and Herbig Ae/Be stars.
Search for extra-solar planets:
Detection of
planets
of Jupiter
type around other solar-type stars demands extremely
precise radial velocity measures. The multiplex advantage of using
many lines
at once can dramatically increase the available precision over measurements
based on only a few lines, and can greatly increase the sample of
stars that can be studied. Another potential and very challenging
application is to attempt detecting the tiny spectral signature
of these
extrasolar planets.
Abundance studies:
It goes almost without saying that this
instrument will be outstanding for chemical abundance studies of
many kinds of stars, especially relatively faint ones.
As often mentioned above, many of the scientific programmes listed above can make use of multi-line techniques to boost their sensitivity by a considerable amount, by a factor of 60 in S/N in the particular case of Zeeman-Doppler or Doppler imaging of cool stars for instance. One would need to use a telescope 60 times larger in diameter to be able to do the same from a single line analysis. We estimate that such multi-line techniques could for instance allow us to:
detect and map stellar
magnetic fields
of cool active stars up to mV = 14;
map brightness spots
and detect
differential
rotation of cool active stars up to mV = 18;
measure chemical abundances
of a few species (independently of stellar rotation rate) for stars up to
mV = 18.