3.3 Personal Overview: Dainis Dravins

Summary of impressions of the UKATC Workshop on Science with ELTs, 25-26 September 2000

1. Remarks on the conclusions and direction of the meeting

Although there can of course be no doubt about the great scientific value of an ELT, I still feel that the final "conclusions" were too conservative and to some extent lacking the definition of some really fundamental milestone goals that would exclusively require ELTs. In my subjective view, I perceived most suggestions as being merely "incremental" in character, i.e. adding quantitively to studies of a nature that is qualitatively similar as before. Perhaps this is the nature of astronomy that "we" have often in the past been unable to predict novel discoveries enabled by new instruments, and perhaps this is bound to be different from the situation in, e.g., particle physics. Still, having been on various committees recommending future astronomical space missions, I feel that in some of those cases it was possible to more clearly define what imaginative new domains would become feasible to study, not previously or otherwise reachable. I think something similar should be attempted also for ELTs.

Really imaginative new vistas must be fascinating also to non-specialists in any particular field: perhaps one should even make it a criterion to have such goals defined by persons not working in that field. Of course, the limited time made this not possible during the recent workshop, when rather each participant was invited to elaborate on the potential in his/her own specific field. However, almost by definition, this leads to a discussion of enhancements in those (already established) fields.

2. Topics where dramatic advances are in prospect

Although it may not be trivial to identify thresholds that arguably will be passed when going from 8-10 m to 30-100 m telescopes, one can think of some whose potential could be further explored than I felt was the case at the Workshop, such as:

2.1. Stellar surfaces

Many stars become surface objects and will no longer be only point sources. The "Catalogue of Apparent Diameters and Absolute Radii of Stars" (Fracassini et al., 1988) lists on the order of 100 stars with diameters of 10 milliarcsec or greater. An ELT with resolution 1 mas will thus produce at least 100-pixel-images of the *photospheres* of each of these. Most of these stars are K and M giants, many of which in addition have extended atmospheres, dust shells, and other circumstellar items that can be imaged. An integral-field spectrometer will give high-resolution spectra in different polarizations for each spatially resolved surface element, enabling studies such as the dynamic mapping of stellar oscillations, magnetic-field studies of starspots and flares; and the equator-pole differences in the acceleration of stellar winds.

2.2. Neutron stars

These have hitherto been studied mainly in radio and X-rays. However, to study the neutron star itself (i.e. not the accretion-flow emission in an X-ray binary), optical, highly time-resolved spectroscopy from an *isolated* neutron star may be the optimum. These objects are faint (likely candidates have V = 24 or 26), and a search for e.g. their nonradial oscillations (predicted periods in the range of milliseconds or 100's of microseconds) obviously require ELTs. Although, at first sight, it could appear that X-ray observations perhaps would be more suitable, their potential is limited by the limited number of X-ray photons that realistically can be collected over such short timescales by foreseen space instruments. A detailed probing of neutron-star interiors would enable a better understanding of baryonic matter and is, of course, of considerable interest also outside astronomy proper. Other neutron-star related observations include the optical counterpatrts of millisecond pulsars, of relevance for pulsar physics and branches of radio astronomy.

2.3. Quantum optics

All classical astronomical instruments (spectrometers, imagers, photometers, polarimeters, interferometers) measure quantities that can be deduced from the spatial and/or temporal first-order coherence of light. In a quantum description this corresponds to properties that can be ascribed to groups of individual photons. However, light also carries other properties (studied in laboratory quantum optics) such as the second- (and higher)-order coherence, properties that can be ascribed to groups of pairs (or a greater number) of photons. This "entropy" of light (equivalent to the ordering of photons in time) in principle carries information of how the photons have been created (spontaneous or stimulated emission), and how they have since been scattered in angle or frequency. It is possible (at least in principle) to segregate between two otherwise identical spectral line profiles, one where the Doppler broadening has come from those atoms that emitted the original photons, and another where the broadening has come from motions of intervening atoms that have since scattered already existing photons. While such phenomena are measured in the laboratory, their astrophysical observability is not yet known, and the potential thus somewhat speculative. However, it is clear that the signal-to-noise ratio for such measurements improves drastically for ELTs since the signal is proportional to the square of the light-collecting power, i.e. diameter**4 (for two-photon properties) or an even higher power for multi-photon correlations: this would be non-linear optics applied to astronomy.

2.4. Solar-system planets:

A spatial resolution of 1 mas resolves a few km on Jupiter or on Jovian satellites such as Io, enabling systematic studies of the atmospheric and surface conditions on all major and many minor planets, and planetary moons. I believe this capability alone can easily justify an ELT - to obtain the same with spacecraft would require a very large armada of them. Specific challenges during the next few decades would include monitoring how the atmopshere of Pluto is frozen out as Pluto's eccentric orbit carries it further away from the Sun. Spatially resolved spectroscopy would reveal how this freezing-out begins at the planet's poles (or does it?). Unfortunately, it appears that there was really nobody really knowledgeable about planets among the participants at the recent meeting: since this may be a field where the ELTs may have their greatest impact, I believe such persons should be invited to the later meetings.