SPIRou - un SpectroPolarimètre
Infra-Rouge
a near infrared spectropolarimeter
Science goals
SPIRou plans to essentially concentrate on two main scientific goals:
The first one
is to search for habitable exo-Earths orbiting low-mass & very-low mass stars
(LMSs & vLMSs) using high-accuracy radial velocity (RV) spectroscopic measurements.
This search will expand the initial, exploratory studies being carried out now with
visible instruments (eg HARPS@ESO) and will survey in particular large samples of stars
(eg late-M dwarfs) totally out of reach of existing instruments. In particular, carrying
out a new large-scale survey at nIR wavelengths should enable to boost the sensitivity
to habitable exo-Earths by typically an order of magnitude (with respect to existing
instruments).
The second main goal is to explore the impact of magnetic fields on star &
planet formation, by detecting magnetic fields of various types of young stellar
objects (eg young FUor-like protostellar accretion discs, classical T Tauri stars or
cTTSs) and by characterizing their large-scale topologies. SPIRou will also investigate
the potential presence of giant planets in the inner regions of protostellar accretion
discs & their relation to disc magnetic fields. In particular, this study will strongly
amplify the initial exploration survey presently carried out at optical wavelength within
the framework of the MaPP (Magnetic Protostars and Planets) CFHT Large Program (LP)
in the context of the MagIcS (Magnetic Investigation of various classes of Stars)
international research effort. It will also ideally complement the data that ALMA will
start collecting soon on outer accretion discs & dense prestellar cores.
SPIRou will also be able to tackle many other exciting research topics in stellar
physics (eg dynamo of fully convective stars, weather patterns at the surfaces of brown
dwarfs), in planetary physics (eg winds & chemistry of solar-system planet
atmospheres) galactic physics (eg stellar archeology) as well as in extragalactic
astronomy & cosmology.
Ensuing technical requirements
The following requirements can be derived from the main science
programs on which SPIRou will focus on:
the single-shot spectral domain must include the YJHK photometric bands, ie
cover the whole 0.98-2.4 micrometer range; in particular, the K band very usefully
contributes to most science programs;
the resolving power must reach 75,000, with 2 km/s pixel sampling;
the RV accuracy should be better than 1 m/s; it suggests in particular that the
instrument illumination should be stable and achromatic to within ~0.03" at the
entrance pinhole and that the light should be very efficiently scrambled (eg using
optical fibers) before entering the spectrograph;
the instrument should provide the spectrum of a RV reference, stable to
~0.25 m/s, that can potentially be recorded simultaneously with stellar spectra as a
stability check (even though not needed in principe for normal operation);
the instrument should include an achromatic, dual channel polarimeter, with
swappable channels to achieve accurate and reliable polarimetry; all polarimeter and
pre-polarimeter optics must not generate more than 1% crosstalk between
polarization states;
the peak throughput should be ~15% (telescope & detector included), providing
a peak S/N~110 per 2 km/s pixel in 1 hr at J=12 and K=11;
the instrument thermal background in the K band should not exceed the sky or
telescope thermal emission at 2.4 micrometer, ie should not be brighter than K~13.5;
providing means to estimate the contribution of this thermal emission
simultaneously with stellar spectra may be useful when the sky is bright.
