Objectives

The science cases of VEGA are those of a new but already existing and tested instrument: the REGAIN focal instrument which was designed and built for the GI2T but of course boosted by the possibilities offered by the CHARA array. This instrument, in his CHARA adaptation, called VEGA will open new fields in a wide range of Astrophysical topics only addressable in the visible domain. It will provide a spectral resolution up to 30000 within the spectral range 0.4-0.9 µm and a spatial resolution of less than 1mas for up to 4 telescopes in its X-l special configuration. A polarimetric device (SPIN) measuring simultaneously the polarization in 2 directions either circular or linear is also implemented in this instrument. Since VEGA was already tested on the sky on 1.5 m telescopes it is also very well suited for the 1m CHARA array and will only need minor adaptations for the injection of the CHARA beams. The decision to stop GI2T/REGAIN is due to the great ageing of the telescopes as well as the non optimal seeing conditions in Calern. These two issues will be solved on CHARA and it allows us to focus our attention on its implementation in the CHARA environment and the correct interfaces for a fast scientific exploitation. This instrument will first offer the combination of 2 telescopes and will be extended to 3 and 4 telescopes in a second step. The design and building of the REGAIN spectrograph have been made in the context of three telescopes and the development for CHARA for this implementation is mainly a matter of a better control of the photon counting detector (sampling issues) as well as a development of the data reduction pipeline (closure phase and spectro-spatial encoding of fringes). Future improvements (extension to three and four telescopes) will multiply by a factor of 4 to 10 the scientific output of VEGA.

Estimations of limiting magnitude have been made and show that for a 30mn exposure time, a signal to noise ratio of 10 and the medium seeing conditions in Mt Wilson, VEGA reach the magnitude 6 (resp. 9) with the R=30000 (resp. R=1500) spectral resolution. It should be noted that these calculations are made with the same equations than the ones used and calibrated on the GI2T. Infrared and visible measurements are complementary for a correct and complete radiative transfer modeling. On one hand, many of the astrophysical processes involved in stellar physics and investigated by interferometry have stronger signatures at visible wavelengths. On the other hand, new physical processes will be investigated, broadening the field of astrophysical domains covered by the CHARA array. The spatial resolution will reach 0.3 mas at 0.6 µm with the longest baseline. Moreover, the high spectral resolution will allow to study with unprecedented details the kinematics of stellar sources. The main scientific goals of VEGA for the partners of the projects are:

·        Active hot stars: the envelopes of Be stars have been one of the favorite topics for visible interferometric observations. However, the mechanisms causing the mass loss and shaping the resulting circumstellar envelopes remain unclear as well as the formation and the vanishing of the circumstellar disk has a function of time. Be stars are also an excellent test bench for the interferometric study of many other astrophysical problems

·        Cepheids: The observation of Cepheids in low resolution and long baselines will allow increasing the sample of Cepheids observed by VINCI/VLTI, AMBER/VLTI and FLUOR/CHARA. Expecting a 2% precision on Cepheids distances, it will be possible to calibrate the zero-point of the Cepheids P-L relation at a level of 0.01 magnitude. By using the high resolution (R=30000) and high baselines (B=330) of VEGA/CHARA it will be also possible to constrain the projection factor for several Cepheids.

·        Hot emission-line stars in binaries: Spectrointerferometry also provides a unique chance to study the role of duplicity for the Be phenomenon. Many bright Be stars were found to be components of binary systems and the number of such discoveries is increasing. Spectrointerferometry can therefore not only allow to detect companions to many Be stars but also to find out their properties and nature. It will also be possible to see how the geometry of Be envelopes in binaries differs from that in single stars.

·        Mira stars and related objects: at least three kinds of programs can be addressed by VEGA/CHARA, and only by it. One is the measurement of the diameter of a large sample of Miras, including Population II ones. The second program aims at studying the surface inhomogeneities of the closest Miras. Finally, a third program would be highly useful to constrain model atmosphere of Miras: the measurement of the center-to-limb darkening (or possibly brightening, depending on the wavelength).

Through the collaborations within the VEGA consortium and the CHARA collaboration, we foresee also a certain number of others scientific topics, such as 1) Fundamental stellar parameters, 2) Stellar activity, spots and Doppler Imaging, 3) Differential rotation and stellar inclination, 4) Asteroseismology and 5) WR stars.