Stars and Planets:  Overview of stellar atronomy

Contributed by J. Drew

The "Stars and Planets" group was one of three groups comprising the Edinburgh workshop.  At an early stage the main group divided itself into two, allowing those whose science interests are entirely focused on star formation problems to pursue that very obvious avenue .

The second group, which I  report on here, tried to take a broader perspective across the whole of stellar astrophysics, in order that nothing was missed there. We were Penny Sackett, Dainis Dravins, Pat Roche and I (Janet Drew) as I recall (with Adrian Russell present much of the time).

 The structure of accretion disks and testing models for the anomalous viscosity:

Accretion disks are commonplace in the Universe and exist on a range of spatial scales: active nuclei in galaxies are believed to be fed by matter transported through them; accretion disks lie at the heart of the star and planet formation processes and become important again in the late stages of stellar evolution in a range of interacting binaries. Most recently, the formation of an accretion disk has been mooted as part of the mechanism for creating `hypernovae'.

A long-standing problem has been the origin of the viscous shear that is central to the inward migration of matter through an accretion disk, and the outward transfer of angular momentum. It was recognised over two decades ago that `normal' viscosity on the molecular level was orders of magnitude too weak to explain the timescales observed in astrophysical disks. Currently there is much interest in the role that turbulent MHD processes may play in creating this `anomalous' viscosity. But there are problems within even this concept (e.g. how to sustain the viscosity in the absence of significant gas ionization) and it is clear that the work is cut out for the astronomical community to devise observational tests of this model and subsequent variants. What this amounts to is that observations are needed that can characterise the interior structure of accretion disks.

Consider the case of a disk around a compact object such as a white dwarf or neutron star (objects of mass of order 1 Msun). Currently feasible spectroscopy on 4 -- 10-metre telescopes using the integrated light from such systems has failed to teach us much at all about the interior structure of such disks. Partly this is for technical theoretical reasons (realistic modelling is still beyond us). But it is also because existing data only tell us about grossly time-averaged spectral properties and there is also the fundamental limitation that much of the emergent light may only originate in superficial layers that tell us little. To do better than this, and really prise open the interior workings of accretion, a proper exploration of the time domain is needed. The relevant timescales for accreting compact objects are short: the period of the innermost Keplerian orbit around a 1-Msun white dwarf is about 10 secs. It is plausible that phenomena associated with the turbulence implicated in the viscosity exhibit timescales this short or even shorter. White light observations of flickering have already detected timescales in this regime. What would we find if we could take spectra, well-enough resolved in wavelength to reveal the details of spectral line shapes, at a rate of 1 per second? It might then be possible to construct and analyse lightcurves for specific doppler shifted wavelengths within spectral lion profiles (specific locations within the accretion disk observed, in effect). Data of this nature amounts to the much-needed window onto the dynamic processes we expect to be at work in such environments.

A related class of time domain observations would be reverberation observations of a more refined kind than are already undertaken on today's telescopes. Light travel times across WD/NS/BH interacting binaries are on the order of 1 sec.

The accretion disk in star and planet formation is a problem that captures most imaginations much more vividly. The analogous experiment to the compact-binary time-resolved spectroscopy, in this setting, could be carried out in a directly spatially-resolved fashion with an ELT. Since the disks of young stars are absolutely much larger, the timescales of interest will be correspondingly longer. The problem will be more complex in that the disk constituents are a mixture of gas, dust and larger rocks. The reader is referred to other submissions that will deal with the quantitative issues appropriate here.

Technical implications:

no constraint on achievable spatial resolution - only a minimum requirement on telescope sensitivity at optical wavelengths (and fast readout of detectors).

Figures to calculate on: V mags in the range 12 to 20, to be observed at a sampling rate of 1 sec or better, at a spectral resolving power of 5000-10000.

The optical is needed in order to emphasise the accretion process rather than any companion object. (In the best of all possible worlds one would probably most like to access the space UV!)

For the study of disks around young stars, spatial resolution down to 1 mas is desirable at least at IR wavelengths. Coronography is also required to suppress the light of the central star. This last is something a wide range of stellar astrophysics applications will need - hence some hard data on the degree of nulling achievable will be useful.