To achieve these science goals, we propose to study a new visible beam combiner for the CHARA array, called CHARA/SPICA. It is designed for the measurement of the angular diameters of 1000 stars and will provide a major step forward in terms of magnitude and precision with respect to the present situation.

The combination of the six CHARA telescopes equipped with AO with a low-resolution spectrograph (50 channels over 600–900 nm) will yield 750 different spatial frequencies in one single observation and thus will allow precise determinations of angular diameters.

For a subsample of bright stars, a medium spectral resolution mode (200 channels over 40 nm) will allow spectral imaging of stellar surfaces and environments for higher accuracy on the stellar/planetary parameters and studies on the kinematics of environments and atmospheres.

To push the limiting magnitude in the different modes and give access to a large sample of stars, CHARA/SPICA is assisted by a fringe tracking system in the H band to reach “long” (200 ms–few s) exposures of the fringe signals in the visible. The design is based on the pdfFRIEND prototype extended to 6T. A great advantage with a fibered instrument is that the injection module and the instrument itself are completely decoupled. This is not only interesting for implementation but also for the alignment where all the efforts could be focused on the injection system.



Current design of the SPICA/CHARA instrument from the input beams from CHARA to the injection in the single mode fibres. 


Design of the CHARA/SPICA spectrograph (not final one)

          The core program of CHARA/SPICA is related to fundamental parameters of stars and planets. The combination of Gaia parallaxes, the high-precision measurement of the transit of exoplanets with the PLATO space mission, and CHARA/SPICA angular diameters will provide the radius of exoplanets with 1% precision. Besides, determining the radii of asterosismic targets (using 'à la carte' approach or scaling relations) will bring constrain on the mass of stars, and these measurements be used to improve stellar evolution models, and, in fine, stellar age. In addition, the imaging capacities of CHARA/SPICA will be used mainly to study binaries (mass determination), limb-darkening and stellar rotation. Besides, constraining surface-brightness color relations associated to several parts of the HR diagram is fundamental in order to determine the angular diameter of any non-active stars, even faint, for PLATO, but also for the distance determination of early-type eclipsing binaries (for e.g. in M31 and M33), which is crucial for the calibration of the Hubble constant (in collaboration with the Araucaria Project).

          One of the main challenge of the CHARA/SPICA instrument (Mourard et al. 2018) is to measure a sample of 1000 stars and to feed the OIDB database of JMMC. Observations are expected to start at the end of 2021. We have defined a first phase of 3 years (2022-2024) aiming at measuring 800 diameters and obtaining 200 images of surface of stars. In the second phase (2025/2026), CHARA/SPICA will cover extensively the PLATO field and additional science programs will be developed (Cepheids, disk of Be stars, interacting binaries, ...; see Stee et al. 2017, white book). After the launch of the PLATO mission (>2027), CHARA/SPICA will continue its science collection and will serve as ground follow-up of interesting PLATO targets. For bright PLATO stars, direct measurements will be done. For faint PLATO stars, we will use surface-brightness color relations (based on the observations of the 800 stars). The CHARA/SPICA group is part of the WP122300 ("Non seismic diagnostics and and model atmosphere" of PLATO) in order to prepare the implementation of the CHARA/SPICA data into the PLATO pipeline. There is also work in collaboration with WP125500 ("Benchmark stars for PLATO").

          By considering 4 stars per sub-spectral type and class reachable by CHARA/SPICA and a time constrain of 3 years 2022-2024 (80 nights per year, 15 stars per night) we obtain these numbers for each WP (800 diameters based on 2 observations per stars, 200 images based on 10 observations per star):   

  • WP3: 92D+8I = 100*,  known asteroseismic giants in the F5-K7 range + Galactic Archeology

  • WP5: 64D+3I = 67*,  stars from K7 to M dwarfs

  • WP7: Analysis of Surface-Brightness Color Relations in all HR diagram.

  • WP11: Study of the limb-darkening of stars


          The standard stars (i.e. the non active ones) will be used to build the SBCRs (WP7) in the different regions of the HR diagram: O, B1, B2, B3  for the calibration of the distance scale (distance of M31/M33 eclipsing binaries), B4-F5 to fill the current gap in the interferometric databases (cf. JMDC), and stars later than F5 for the preparation of the PLATO space mission. These relations will be differentiated with respect to the luminosity class.

More information can be found on the CHARA/SPICA-sg kick-off meeting. All presentation are online  as well as the slides summarizing the discussion in the working groups.

  • WP1 – Exoplanet Host Stars:

R. Ligi, T. Boyajian, A. Chiavassa, A. Gallenne, R. M. Roettenbacher, R. Szabos, M. Wittkowski, T. Guillot, A. Crida, S. Albrecht, Borgniet, S., D. Mourard, N. Nardetto

  • WP2 – Asteroseismology / F5-K7 dwarfs:

O. Creevey, K. Belkacem, T. Boyajian, R. Ligi, T. Morel, R. M. Roettenbacher, R. Szabo, W. J. Chaplin, M. Bazot, , D.B. Palakkatharappil,  D. Mourard, N. Nardetto

  • WP3 – Asteroseismology / F5-K7 Giants:

K. Belkacem, O. Creevey, R. Ligi, R. M. Roettenbacher, R. Szabos, W. J. Chaplin, D. Mourard, T. Morel, Y. Lebreton, S. Deheuvels, M. Bazot, N. Nardetto

  • WP4 – SBCR of B4-F5:

M. Wittkowski, A. Salsi, G. Duvert, A. Chelli, N. Nardetto

  • WP5 – M stars dwarfs:

T. Boyajian, R. Ligi, T. Morel, A. Salsi, D. Graczyk, N. Nardetto

  • WP6 – O, B1, B2, B3:

D. Graczyk, A. Salsi, N. Nardetto

  • WP7 – SBCR Analysis: WP to be connected to WP1, 2, 3, 5 and 6:

N. Nardetto, A. Salsi, T. Boyajian, A. Chiavassa, A. Gallenne, P. Kervella, T. Morel, R. Szabos, M. Wittkowski

  • WP8 – Binaries:

D. Mourard, O. Creevey, A. Gallenne, P. Kervella, Y. Lebreton, F. Millour, N. Nardetto

  • WP9 – rotation:

A. Domiciano, A. Gallenne, P. Kervella, A. Meilland, M. Rieutord, P. Stee, R. M. Roettenbacher, A. Claret, S. Albrecht, M. Borges, N. Nardetto

  • WP10 – YSOs:

S. Kraus, R. Ligi, D. Mourard, A. Meilland, M. Borges

  • WP11 – Limb-darkening:

A. Claret, P. Kervella, D. Mourard, N. Nardetto, A. Domiciano, M. Borges






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