Stars and phenomena observable with the hypertelescope

Like traditional telescopes, hypertelescopes will obtain direct images of a wide range of astrophysical objects. However, while the Hubble space telescope has a 40 milliseconds resolution with a 2,4m mirror, the hypertelescope in Ubaye should achieve 2 milli-arc-seconds in yellow light in its 57m meta-aperture version, 0,5 milli-arc-seconds in the 200m version and 0,1 milli-arc-seconds in the ELHyT version. As for the resolution of the space versions of 100 km or 100 000 km foreseen in the future, it should be of the order of micro or nano-arc-seconds.

The maximum angular dimension λ/s of the sources, for which direct imaging is possible with a hypertelescope, is in principle limited to about 20 arch milli-arc-seconds in yellow light for an arrangement of mirrors distanced at 5m. Nevertheless, the efficiency of image deconvolution methods recently developed by C. Aime, D. Mary and P. Nunez allows overcoming this limitation and in consequence obtaining angular dimensions greater than 20 milli-arc-seconds. With these improvements, the largest and closets giant superstars, like Betelgeuse, become accessible to direct imaging. In the case of clusters, where the stars are distanced of λ/d or more – say at least 100 milliseconds -, a hypertelescope can, with a clean field covering a few arch seconds, provide simultaneous images of the entire cluster. Each of the images covering the cluster can be advantageously obtained with a “hyperspectral” camera, i.e. providing a spectrum for each resolved element or resolution element.

Practiced for almost a century on the Sun, this method will allow obtaining, for close-by stars, observations concerning their corona, chromosphere, photosphere and even, via the astero-seismology techniques, their internal structure.

Existing interferometers - with only a small number of apertures - are starting to produce reconstructed images obtained from repeated observations over time. On the other hand, due to its large number of apertures, the hypertelescope produces a direct instantaneous image, which makes possible to study rapidly evolving stellar phenomena. In addition, the concentration of light during the peak of interference enhances the sensitivity in terms of the magnitude limit.

Magnitude limit of the hypertelescope

The limiting magnitude accessible with a hypertelescope is identical to that of a monolithic telescope having the same collecting surface than the meta-aperture of the hypertelescope. It is important to note that for both instruments the theoretical limiting magnitude is accessible only if adaptive optics techniques are implemented for cophasing, which requires the installation of a laser-guide star (not needed in the case of a space instrument). Given that current ground-based interferometers are not equipped with laser-guide stars, the gain in sensitivity with a hypertelescope equipped with a laser-guide star is greater than the gain resulting only from direct imaging.

Direct imaging and scientific goals, some examples

Exo-planets

  • Exo-planets currently known in order to detect in their surface changes in colour or spectrum suggesting seasonal variations
  • Search for exo-planets around stars farther away from the sun than currently known exo-planets
  • Search for exo-planets closer to their star than those detected by other means
  • Transits of exo-planets appearing on the resolved image of their parent star

Stars

  • Structures of proto-planetary discs in order to specify the conditions for the formation of exo-planets (Benisty & al. 2010)
  • Surface of nearby stars in order to detect phenomena analogous to solar flares and spots
  • Surface of chemically peculiar stars (CP stars) in order to resolve spots due to chemical abundance inhomogeneities
  • Neutron stars such as the Crab pulsar (in principle observable with a meta-aperture of 100 000 km feasible in space)
  • Optical afterglow of gamma ray bursts, as observed in external galaxies

Galaxies

  • Active galactic nuclei, particularly in the Milky Way, suitable for all hypertelescopes, and already addressed by the interferometers confirming the presence of a giant black hole
  • Clusters of galaxies
  • Gravitational lenses
  • More distant galaxies than those observed with the Hubble telescope, thus giving access to an even younger Universe