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