Using Spectroscopic Cornography for Exoplanet Detection

Spectroscopic Coronagraphy for Planetary Radial Velocimetry of Exoplanets

Authors:

Kawahara et al

Abstract:

We propose to apply the coronagraphic techniques to the spectroscopic direct detection of exoplanets via the Doppler shift of planetary molecular lines. Even for unresolved close-in planetary system, we show that a combination of a visible nuller and an extreme adaptive optics system can reduce photon noise of a main star and increases total signal-to-noise ratio of the molecular absorptions of exoplanetary atmosphere: it works as spectroscopic coronagraph. Assuming a 30 m telescope, we demonstrate the benefit of these high-contrast instruments for nearby close-in planets that mimic 55 Cnc b (0.6λ/D of the angular separation in the K-band). We find that the tip-tilt error is the most crucial factor, however, low-order speckles also contribute to the noise. Assuming relatively conservative estimates for future wavefront control technique, the spectroscopic coronagraph can increase the contrast to ∼50−130 times and enable us to obtain a ∼3−6 times larger S/N for warm Jupiters and Neptunes at 10 pc than that without it. If the tip-tilt error can be reduced to ≲0.3 mas (rms), it gains a ∼10−30 times larger S/N and enables to detect warm super Earths with an extremely large telescope. This paper demonstrates the concept of spectroscopic coronagraphy for the future spectroscopic direct detection. Further studies of selection of coronagraphs and tip-tilt sensors will extend the range of application of the spectroscopic direct detection beyond the photon correcting area limit.