MARSU: Millimag Astrophotometry of Red Spirou/Spip stars as a University space mission

Photometric monitoring is a widely used method for characterizing stars with variable brightness, such as active stars, young accreting stars, and stars with transiting exoplanets. Space telescopes are ideal tools to carry out continuous observations of these variable stars since they avoid the drawbacks of ground-based observations, i.e. day/night alternation and atmospheric variations (stochastic weather, turbulence). We propose to build an infrared CubeSat-type space photometer working in parallel with SPIRou and SPIP, which are ground-based infrared spectropolarimeters & velocimeters operated as new generation instruments for the Canada-France-Hawaii Telescope (CFHT, Maunakea observatory, Hawaii) and Télescope Bernard Lyot (TBL, Pic du Midi, France) respectively. SPIRou is already installed at CFHT whereas the construction of SPIP has just begun, for a commissioning on the telescope in 2021.
The SPIRou / SPIP Legacy Survey is a Large observing program initiated with SPIRou at CFHT, to be extended later-on with SPIP at TBL. It is mostly aimed at detecting & characterizing Earth-like planets in the habitable zone of low-mass stars and at investigating how magnetic fields impact star/planet formation. With a SPIRou / SPIP CubeSat, stars monitored and exoplanets detected with SPIRou / SPIP would benefit from a simultaneous photometric follow-up. As SPIRou and SPIP, the CubeSat will observe at near-infrared wavelengths, where the target stars (nearby M dwarfs and young forming stars) emit most of their light. Combining spectroscopy and photometry enables one to measure both the masses and radii of exoplanets, thus constraining their bulk densities and thereby providing information on their internal structures for distinguishing between, e.g., telluric and gaseous planets. Moreover, Doppler shifts measured by SPIRou and SPIP can be polluted by stellar magnetic activity whose contribution can exceed the velocimetric signatures of exoplanets. Light curves from the SPIRou / SPIP CubeSat will improve the reliability of exoplanet detection and characterization by recording the photometric imprint of stellar activity, leading to a better filtering of this effect. The SPIRou / SPIP CubeSat will also monitor the activity of a sample of young forming stars and catch transits from the close-in giant planets they may host.
The goal of the SPIRou / SPIP CubeSat will be to achieve continuous (duty cycle in excess of 90%) photometric monitoring in the YJH bands (1 – 1.8 μm) for stars up to an H magnitude of ~11, at a precision better than 1 mmag for up to 10 min exposure times, over continuous periods of up to 3 months and simultaneously with SPIRou and SPIP observations. Continuous observations in the antisolar direction impose to use a heliosynchronous orbit. An optional strategy is to alternate between up to three fields of view on a timescale of ~1 hr in order to maximize the probability of detecting planetary transits.
The CubeSat can either host a single or twin camera (each unit occupying 1x1x3U) with improved performances in the second case; this requires the payload to be integrated on a 6U or 12U platform depending on the option. Each telescope unit features an 8.5cm objective with 5 optical lenses and a Short-Wave InfraRed (SWIR) type detector, working in the 0.9 – 1.8 μm wavelength range.