In this contribution, I explore the possible open issues that
an instrument such as PUEO'NUI at CFHT could help solving in
the study of pre-main sequence binary systems. PUE'O-NUI is a
possible upgrade of the current adaptive optics system PUE'O
at CFHT. The proposed instrument would provide high Strehl
ratio images in the near-infrared (90%) and
diffraction-limited images in the visible, down to band at
least. I only consider here young stars that are
yr-old at most and focus on and band
observations, with a consideration of the benefits of
observing at visible wavelengths.
The first point to have in mind regarding PUE'O-NUI is that its main advantage over the current PUE'O system is in the separation range 0.1''-1'', where it will dramatically reduce the ``speckle noise''. Outside of this radius (on order of the seeing), the gain produced by the new system is negligible. One must therefore focus on this separation range to evaluate the scienfic gain of an instrument like PUE'O-NUI.
Systematic surveys for tight binary systems among young stars with PUE'O (and ADONIS on a similar-size telescope) has shown that tight stellar companions are very frequent among solar-type and lower mass objects. Indeed, based on the observed number of companions at separations 1'' and the large range of flux ratios, one can expect many companions with large flux ratios -8mag) at separation as small as 0.1'', where they are currently not detectable with PUE'O or other instruments. It is therefore worth investigating what kind of companions could be found with PUE'O-NUI and what open issue that would help solving.
>From the current simulations, the objects for which PUE'O-NUI will provide a significant gain over PUE'O are relatively bright, typically . With such a brightness criterion, only a handful - 20 or so - of young solar-type stars (TTauri stars) are concerned. On the other hand, more massive objects are brighter and there are more than 100 stars that could be observed with PUE'O-NUI. They would be either Herbig AeBe stars (2-5) or OB stars (); their distance range from 100pc to 2kpc.
Regarding TTauri stars, it is important to note that the vast majority of known binaries have moderate flux ratios (mag), essentially because the mass-luminosity relation at such young ages is quite shallow. Even substellar objects are quite bright when they are only a few Myr old and they are in principle easily detectable at almost all separations 0.1'' with PUE'O. In the visible, PUE'O-NUI would produce diffraction-limited images, which PUE'O cannot do at the moment. This could be scientifically useful, both in imaging (spectral energy distribution determination) and spectroscopic mode (spectral type, hence mass, could be derived) assuming that the latter be provided as a mode of PUE'O-NUI. However, with a 0.06'' resolution at band and a moderate flux ratio (40%), such observations could also be conducted with HST, which offers a somewhat poorer resolution but a much better image quality. Overall, the gain of PUE'O-NUI over already existing instruments as far as observations of binary TTauri stars is concerned is very limited.
While the mass-luminosity relation is relatively shallow below 1-2 at an age of a few Myr, it is very steep for more massive objects. With PUE'O, a 0.08 companion located 0.1-0.2'' away from a 2Myr-old star can be detected only if the latter is less massive than 1. With PUE'O-NUI, this would be possible at almost any separation 0.1'' for primaries up to . This would represent a major improvement, in that it would allow a complete census of stellar companions to massive stars. There are a variety of questions that would be solved with such observations:
In summary, the most exciting scientific case for an
instrument like PUE'O-NUI as far as young binary systems is
concerned is the determination of all low-mass stellar and
some substellar companions to high-mass (OBA) stars in various
environments. Such observations will provide extremely
valuable information that will help improving our knowledge of
the star formation process.