Optical Images of Stellar Surfaces

Images a thousand times sharper than from Hubble

Current methods in optical astronomy barely start resolving stars as surface objects (not mere points of light), their disks perhaps flattened by rapid rotation, changing their size under pulsation, or in the process of ejecting gas clouds.

Improved resolution will reveal the diversity of stars but requires new techniques.  Conventional methods are limited by turbulence in air, which us why ambitious space missions have been proposed for both ESA and NASA.  However, their complexity means they will take time to become reality.

How can CTA make optical images?

The first kilometre-scale optical imager, enabling mapping of stellar surface structures, may be realised with CTA, using a technique known as intensity interferometry.  The great advantage of this technique is that it is not sensitive to either atmospheric turbulence or imperfections of the telescope mirrors.  The reason is that pairs of telescopes are connected only electronically (not by optical light), with time resolutions of nanoseconds, corresponding to light-travel distances of some metres.  Requirements are large-area light collectors with very fast detectors - precisely the features of CTA!

What we can Observe

Intensity interferometry is suitable for observing hot stars and gas clouds surrounding them.  Some examples are shown in the figure below, which shows types of objects that could be studied with CTA intensity interferometry. 

Objects that could be studied using CTA as an intensity interferometer. Top row: Stellar shapes and surfaces affected by rapid rotation; Middle row: Disks and winds of hot gas around active stars; Bottom row: Stellar surroundings such as shells, obscuring clouds, and jets.

 

How sharp could images be?

Computer modelling has been performed to understand how sharp images can be obtained with CTA, and a result is shown below.   

A simulation of the possibilities for a ground-based intensity interferometer. Here, the reconstructed image of a binary star (of assumed magnitude 6, barely visible to the naked eye) is shown.  The resolution approaches an angle of some 30 microarcseconds, a thousand times better than feasible with the Hubble Space Telescope.

 

Further Reading

Dravins et al.: Stellar Intensity Interferometry: Astrophysical targets for sub-milliarcsecond imaging, Proc. SPIE 7734, 77340A (2010); http://arxiv.org/abs/1009.5815 

Nuñez et al: Stellar Intensity Interferometry: Imaging capabilities of air Cherenkov telescope arrays, Proc. SPIE 7734, 77341C (2010); http://arxiv.org/abs/1009.5599 

LeBohec et al.: Stellar Intensity Interferometry: Experimental steps toward long-baseline observations, Proc. SPIE 7734, 77341D (2010); http://arxiv.org/abs/1009.5585 

Jensen et al.: Stellar Intensity Interferometry: Optimizing air Cherenkov telescope array layouts, Proc SPIE 7734, 77341T (2010); http://arxiv.org/abs/1009.5828