Resolving protoplanetary H-alpha emission with the high-resolution spectrograph RISTRETTO

Abstract

Thousands of exoplanets have been discovered to date, but only a handful of them are young planets in the latter stages of their formation. H-alpha emission is a strong tracer of this process, but it is difficult to isolate the planetary H-alpha signal from the stellar signal due to the small angular separation. RISTRETTO is a high-resolution (R~140,000) integral-field unit spectrograph with an attached coronagraph and extreme adaptive system that is currently under construction. Used in tandem with the VLT, its resolving power and seven-spaxel layout is designed to characterise the atmospheres of exoplanets in reflected light. It can also be leveraged to obtain spatially and spectrally resolved observations of planetary H-alpha emission lines. Using a combination of synthetic and real observations, we determined the detection limits for theoretical observations of protoplanets with on-sky separations of 37-600 mas (the equivalent of 4-70 AU at 113 pc). We used theoretical BT-Settl photospheric spectra, observed HARPS spectra of PDS70, and a model for the line emission at the planetary accretion shock to estimate the planetary H-alpha signal. Models with the PyEchelle simulator show that the H-alpha emission from theoretical objects similar to the four known protoplanet candidates (PDS70b, PDS70c, WISPIT2b, and 2MJ1612b) can be spectrally resolved with VLT/RISTRETTO down to a line flux between $F\sim(1.4-2.7)\times10^{-16} erg \;s^{-1} cm^{-2}$ with a 1 hour exposure. We show that the planetary H-alpha line can be spatially resolved from that of its stellar host and that the shape of the line can be used to constrain the pre-shock density (n0) and velocity (v0), thereby enabling new constraints on the accretion rate and flow geometry. This study and the instrument act as a pathfinder for the next generation of high-resolution spectrographs such as ELT/PCS and ELT/ANDES.