Stellar Astrophysics Research Areas
Solar Physics
General Introduction
November 1995 seen the launch of ESA's first cornerstone mission in its
Horizon 2000 programme, i.e. the
Solar and Heliospheric Observatory (SOHO) .
This mission is part of ESA's Solar Terrestrial Science Programme and is the
only major solar satellite for the next decade. It's payload consists of 12
instruments which will be used to investigate the following items:
the nature of the solar corona
the acceleration of the solar wind, and
the internal structure of the Sun
It is towards the first of these topics that my effort is directed and, in
particular, the question of coronal heating.
It has being known for several decades that the solar corona has temperatures
in excess of one million degress, however, the mechanism by which this is maintained is still
an outstanding problem to-day. There have been many suggestions including,
Alfven wave dissipation, turbulent cascades, currents sheets dissipation or
nanoflaring and anomalous interruption of field-aligned currents. As of yet,
none of these have proven to be entirely satisfactory.
The choice of wavelength region for observing various parts of the solar
atmosphere is determined by the range of temperatures required to be studied.
For example, in order to study the solar corona and the chromospheric-coronal
transition region requires data shortward of 1600A region. On
SOHO the prime
instruments required to provide the necessary constraints in order to advance
our understanding of the coronal heating include,
The Coronal
Diagnostic Spectrometer (CDS) ,
The Solar Ultraviolet Measurement of Emitted
Radiation (SUMER) , and the
Extreme Ultraviolet Imaging
Telescope (EIT) .
From previous missions we know of plasma structures varying in size from a
solar radii to less than 1 arc sec (i.e. 725 km). For coronal heating the most
critical are the small scale structures, mostly in the form of loops.
Coordinated observations with the above suite of instruments (plus
ground-based coverage) has the capacity to provide invaluable and unique data
for solar physicists for the next decade which will enable major advancements
in this area.
Previous related work
Prior to 1975 the best UV and X-ray observations of the solar corona and the
transition region were those based on data obtained during the SKYLAB mission.
Since then there have been several important missions including, OSO 8, SMM,
various
HRTS
& NIXT rocket flights and
YOHKOH .
Based on the data obtained from
these missions we know that the intensities of chromospheric lines decreases
rapidly outside the chromospheric limb (as determined from say H alpha). In
the transition region, intensities persist to much higher altitudes, reaching
a maximum at ~3000km but remaining strong to altitudes of ~10,000km. From
differential emission measure analysis and using derived electron densities,
plane parallel model atmospheres calculations imply a transition region
thickness of only ~70km similar to that derived using opacity methods. Thus
the observed range of altitudes cannot be explained by plane parallel
atmospheres. A possible remedy is to extend the emission by assuming the
transition region gas is in thin structures, i.e. to assume filling factors
<< 1.
In addition to contributing to the picture of a finely structured transition
region,
HRTS has revealed the presence of a highly dynamic environment. For
example, it was shown that transition region line profiles cannot be
represented by a single Gaussian but instead showed a very complex velocity
structure. In many instances only an average velocity was computed with the
loss of relevant information on the dynamics. These multiple velocities were
observed in many different locations ranging from sunspots, to active regions
and sometimes in the so-called quiet regions. In fact, in `quiet' region,
these multiple velocities were observed ~15% of the time and this may be a
lower limit. The spectral lines concerned were those of lines formed at
temperatures of 80,000 to 100,000K. These lines showed both red- and
blue-shifted components and were present at both Sun center and at the limb.
Flow velocities were both sub- and supersonic. However, the mechanism causing
these flows and whether they extend to coronal temperatures is still a matter
of speculation.
Thus the picture emerging from these observations is that the transition
region should not be considered as static or stationary. Instead we may have a
transition region comprising of an ensemble of small, nearly isothermal loops.
The current array of instruments on
SOHO has the potential of providing
valuable observational data relating to temperatures structures ranging from
twenty thousand degrees to over a million degrees. Such data can be used to
provide important input not only for heating of the Sun's corona but to the
heating of stellar coronae in general.
Proposed programme of work
CDS has two spectrometers; the Normal Incidence Spectrometer covering the
wavelength ranges 310 - 380A and 517 - 633A and the Grazing Incidence
Spectrometer covering the ranges 155 - 224A, 261 - 346A, 396 - 496A and 662 -
787A. It's spectral resolution is of the order of 17 km/s. On the other-hand,
SUMER
will cover the wavelength range 800 - 1600A in first order and 500 -
800A in second order with a spectral resolution of 1 km/s. The major part of
our proposed programme is to use data from these two instruments, although
data from instruments such as
EIT will also be
used.
With high time resolution data over extended periods we can look at the
possibility of detecting loop oscillations. In fact, based on SMM
observations, oscillations in the UV continuum at 1370A have already being
observed. The observed periods are all in the range 4 to 7 mins, consistent
with the range of observed global photospheric periods. These observations
were obtained using a 13x13 pixel raster (each raster of 10x10 arc sec) with a
cycle time between rasters of 22 sec. It was normally found that only a small
numbers of pixels show a periodicity while adjacent pixels were void of any
periods. Such structures were therefore small, of the order of 10 to 20 arc
sec and the question we wish to ask is why only some regions showed a
periodicity. One can argue that these regions were
simply resonating with the under-lying photospheric period, however, why only
these regions and could this have implications for coronal heating. We
therefore plan an observational programme aimed at obtaining data as a
function of temperature using both rasters and line profiles modes (the SMM
data were taken only in the UV continuum at 1370A and therefore we do not have
sufficient information on the temperature and/or height of these oscillations).
The above programme will be directed towards active and non-active
regions, in particular loop structures. We plan to look at these structures
both on-disk and those approaching the limb. Using high spectral resolution of
selected lines with
SUMER
we can investigate profile changes, with
CDS we can
perform rasters plus line ratio diagnostics. With this data we can therefore
investigate the microscopic transition region structure which has to be
related to the local heating mechanism. This will then allow us to investigate
how this microscopic structure can explain such characteristics of the
transition region as the general redshift of EUV and UV spectral lines,
the large-scale temperature stratification and the universal emission measure
This will also involve the use of
atomic physics
in particular the data
required for the various diagnostics line ratios. Largely, this data already
exists in the literature, however since we have good working relationships
with the atomic physics group at
QUB , we would expect possible
collaborative projects to result from the application of new improved
calculations. The slaves involved in this work are my postdoc
(Dipankar Banerjee)
and my PhD students
(Luca Teriaca)
and
(Elena Perez) . Watch this space!
Chromospheric Modelling
The Sun, being our nearest star, is the only one which astronomers can
examine at sufficiently close range to resolve spatial features. However,
the complex processes of particle acceleration and energy release happen
even more dramatically elsewhere. It follows that the better we
understand the Sun, the more insight we have of activity in other stars.
One of the big issues in astrophysics is the role of stellar magnetic
activity, (e.g. what role do magnetic structures play in the storage and
release of energy).
Late-type dwarf stars also exhibit a range of phenomena collectively
called chromospheric activity. The chromosphere being a narrow part of
the atmosphere above the photosphere not normally visible without special
observing techniques. Its main characteristic is a rise in temperature
(from a few thousand degrees to over 20 thousand degrees) with height and
a complex time-changing structure. These include evidence of large-scale
spots analogous to sunspots, frequent optical flares analogous to solar
flares and strong emission lines in their optical spectra indicative of
the presence of heated outer atmospheres. On a solar model for these
phenomena, they would indicate the presence on the surfaces of these
stars of concentrations of strong magnetic fields.
In the absence of a comprehensive physical theory for the heating of the
chromosphere and corona, modelling work to date has mainly concentrated
on studying their structure as a means of placing constraints on heating
theories. In the optically thin conditions in the corona and most of the
transition region, emission measure analysis may be used. This derives
the column depth in a given species of excited atom from the total flux
observed in the line. By observing a sequence of suitable lines of
species of different excitation the run of volume emission measure may be
derived. Combining this with a number of density-diagnostic lines, a
reasonably complete model of the run of gas density versus temperature
can be derived.
In the optically thick chromosphere, however, the situation is much more
complex. In the bulk of the chromosphere, photons in the cores of the
lines of many species undergo repeated scattering and only those
generated in a thin layer near the top escape to infinity. Scattering in
the deep chromosphere, however, can result in photons from those layers
being shifted to the line wings where the optical depth is low and such
photons may escape. Thus the profile of a very optically thick line can
yield information on a range of depths in the chromosphere. The
derivation of this information requires detailed modelling of the process
of radiative transfer within the optically thick medium for comparison
with observation. Astronomers at Armagh Observatory have been involved in
a programme of such computations and the resulting comparison with data
for the past 8 years (
Gerry Doyle,
Darko Jevremovic).
Flare Dynamics
The solar cycle is vivid evidence that the Sun is not quiet, steady and
changeless. Its most violent outbursts are flare related events, which
comprise super-hot plasma which can cover several million square
kilometres of the star's surface. In general, solar flares are associated
with sunspots (i.e. the dark regions that occasionally appear on the
solar disc where strong magnetic fields emerge and where the normal rise
of hot gas from the solar interior is suppressed); in particular, the
sudden heating of material to extremely high temperatures is relevant to
theories of stellar flares and the loss of stellar matter (areas of
interest in which the astronomers at Armagh play a prominent role).
Solar flares, which are the basis of our models of stellar flares, are a
complex phenomenon and there is as yet no detailed description of the
physical processes involved with which everybody will agree.
Nevertheless, certain details are becoming increasingly clear. Many
flares occur within complexes of magnetic fields called active regions.
Most will agree that a flare occurs when an instability develops in one
or more magnetic loops which requires the magnetic configuration to
rearrange itself. As a by-product of this rearrangement, magnetic energy
is released. This energy may either produce local heating in the vicinity
of the instability or, as seems more likely on the basis of recent
evidence, it may either accelerate a beam of non-thermal electrons or
generate a conduction front. The electrons in such circumstances would be
constrained to move parallel to the magnetic lines of force i.e. along
the axis of the magnetic loop. The density of matter in the corona, where
the bulk of the loop exists, is sufficiently low for the electrons to
travel relatively unimpeded. On reaching the chromosphere at the base of
the loop the density rises steeply and the electrons undergo
collisional braking. In the process a substantial part of their energy
may be dissipated as heat. As a result, the formerly chromospheric
material is heated to coronal temperatures and expands rapidly to fill
the entire loop. Here it cools on a timescale of tens of minutes by
radiating in the soft X-ray part of the spectrum.
The detailed study of the flare process requires multiwavelength
observations that allow the extraction of the maximum amount of physics.
However, such data are only available for a limited number of events.
Valuable information can, however, be obtained from spectroscopic
observations alone.
The active dwarf M stars are characterized by their Balmer line emission
which is direct evidence for the existence of an active chromosphere,
although it should be noted that the less active dM stars also have
chromospheres. During flare activity both continuum and emission features
become enhanced and observations have been made of very large asymmetric
broadenings in the bases of chromospheric Balmer lines which are usually
interpreted as mass flow events. The first evidence of flare activity is
best observed in the ultra-violet continuum. The rise time is usually
much sharper in the late M dwarfs, perhaps a few seconds ranging to a few
minutes in the K or early M dwarfs. On the other hand, flare activity in
the RS CVn binaries tends to be more gradual in nature, sometimes lasting
several hours (
Gerry Doyle,
John Butler.
Brendan Byrne).
Atomic Physics
Accurate atomic data have always been essential for the realistic
modelling of astrophysical plasma. However major improvements in the last
few years in the variety and quality of the observational material
available from both ground and satellite based instrumentation has made
this requirement even more stringent. Over the past decade, several
computer codes have been developed at
Queen's University Belfast (QUB) to
calculate such data, including CIV3 and RMATRX, which are used for
generating radiative rates, and electron impact excitation rates, for
atoms and ions respectively. Results from these codes have been applied
to the analysis of a wide range of astronomical observations.
However probably the most successful application of the atomic data has
been the development of electron density and temperature diagnostic
emission line ratios for astrophysical plasmas, including the solar
transition region/corona and late-type stellar atmospheres. Many
diagnostics have been calculated over the past few years and applied to
solar UV, EUV and X-ray solar spectra (obtained with the
Solar Maximum
Mission and P78-1 satellites, the S082A,B and S-055 instruments on board
Skylab and the High Resolution Telescope and Spectrograph) on board
Spacelab 2), as well as UV observations of late-type stars obtained by
the
International Ultraviolet Explorer and
Extreme Ultraviolet Explorer
satellites (
Gerry Doyle). We have an ongoing programme with the QUB group in the
application of their atomic data to observations from the above missions
(
Gerry Doyle).
Environments Around Evolved Stars
Asymptotic Giant Branch (AGB) stars are evolved low or intermediate mass
objects which show a copious phenomenon of mass loss. Dust grains form in
the cool expanding circumstellar environment, absorb the optical
radiation of the parent star, and re-emit in the infrared (IR). The
chemical nature of dust grains depends on the composition of the stellar
photosphere: hence carbon compounds such as amorphous carbon (AC) and
Silicon Carbide (SiC) surround C-rich stars, while oxygen compounds
(silicate) constitute the main component of the circumstellar envelopes
around O-rich stars.
In the last decade it has been well established that many carbon stars -
especially those optically visible - show excess flux in the far-IR.
Explaining this excess emission is one of the problems being studied by
the group at Armagh. Our approach is to consider the observational data
spanning from 0.44mu to 1.0mm, involving a proper treatment of radiative
transfer effects. Modelling the observed spectral energy distribution of
these objects has a particular importance in the debate about the
evolution of Asymptotic Giant Branch stars. A model which includes a
large O-rich detached shell fitting the observational data better than a
shell of carbon compounds dust grains would support the first scenario.
Alternatively, finding that a relatively small shell composed of
carbon-compound dust grains allows a better explanation of the
observations would support the second scenario
(
Gerry Doyle,
Alex Lobel).
The Solar Cycle
The overall strength of the magnetic fields of the Sun wax and wane with
a period of about 11 years. At maximum magnetic activity the solar
surface is in turmoil with large cool areas called sunspots, frequent
explosive releases of energy called flares, and the expansion of its hot
outer atmosphere. Ongoing work at Armagh (
Climatology Research) links changes in the solar cycle to long-term trends in the Earth's weather.
Fast and Slow Rotators
Fast rotators exhibit all of the signs of chromospheric activity seen on
the Sun, but on a greatly enhanced scale. Flares are more frequent and
energetic; spots are larger and cooler, occupying up to 20% of the
star's surface area; and coronae are denser and hotter. Armagh
astronomers are involved in detailed investigations of stars rotating
more rapidly than the Sun, involving stellar flares (
Brendan Byrne,
Gerry Doyle), starspots
(
John Butler,
Brendan Byrne), and
stellar coronae (
Brendan Byrne).
Slow rotators, on the other hand, generate only weak magnetic fields
which transmit insufficient energy to significantly heat an outer
atmosphere. Theory indicates that such objects may still have
chromospheres, but that they should be very weak, being heated by
pressure (sound) waves. Armagh astronomers have found the first examples
of such objects (
Brendan Byrne,
Gerry Doyle) and are building mathematical models of their
atmospheres (
Gerry Doyle).
Young stars:
Most stars of the Sun's age are rotating slowly. The Sun itself
rotates only once per month. Young stars, on the other hand, are born
rotating extremely rapidly, typically once per 4-10 hours, and
subsequently brake with time. The classical theory of braking assumed
that this happened via a stellar "wind" leading to a
characteristically slow spindown with time. Recent results have
challenged the "classical" theory. During the reporting year the UK
Science and Engineering Research Council agreed to fund a major
programme of investigation of the nature of rotational spindown. The
work will be carried out by measuring the rotation of large numbers of
stars in a series of young star clusters with ages in a critical range
between about 30-200 million years. This work began during the
reporting year when our SERC-funded Post-Doctoral Research Assistant
made observations of the young open clusters IC2391 (Ref
PBB) and a Summer Vacation Student analysed
observations of NGC5460 (Ref
Brendan Byrne,
Armin Theissen).
Massive Stars
Most of the work described above applies to stars of about the same
mass as the Sun or less.
At the other end of the scale, stars with masses in excess of about
fifty times the mass of the
Sun are among the most luminous stars known in the Universe. Their
rate of internal energy
production is so great that they burn up their hydrogen fuel on an
astronomicaly short time-scale, a few hundred thousand
years. Furthermore, their gravities are barely able to hold them
together against the outward pressure of the energy streaming out from
the core. As a result they continually lose gas in a "stellar wind".
Hypergiant stars
Two of the brightest examples of such
"Hypergiant" stars are P Cygni and n Carinae. Both are being studied
at Armagh for signs of changes in their brightness on a time-scale of
a century which may indicate directly the stellar ageing process (Ref
Mart de Groot).
Cosmic abundances
When hydrogen is burned in the cores of
stars it is converted to heavier, more complex elements. The raw
material for most stars is the gas found in interstellar space, which
is predominantly hydrogen. As a star ages, the proportion of heavier
elements in its gas increases as a result of the nuclear
burning. Because massive stars live for a relatively short time, there
is relatively little time for heavier elements to build up. Thus their
chemical composition is essentially that of the gas from which they
formed. For this reason they are good probes of the chemical
composition of the interstellar medium. Worll; aimed at analyzing the
heavy element composition of the Galaxy and of our nearest neighbour
galaxies using massive star abundances is being carried on jointly
with astronomers at the Queen's University, Belfast.
Last Revised: 2010 February 22nd
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