Search for cool giant exoplanets around young and nearby stars. VLT/NaCo near-infrared phase-coronagraphic and differential imaging

Context. Spectral differential imaging (SDI) is part of the observing strategy of current and future high-contrast imaging instruments. It aims to reduce the stellar speckles that prevent the detection of cool planets by using in/out methane-band images. It attenuates the signature of off-axis companions to the star, such as angular differential imaging (ADI). However, this attenuation depends on the spectral properties of the low-mass companions we are searching for. The implications of this particularity on estimating the detection limits have been poorly explored so far.
Aims: We perform an imaging survey to search for cool (T_eff< 1000-1300 K) giant planets at separations as close as 5-10 AU. We also aim to assess the sensitivity limits in SDI data taking the photometric bias into account. This will lead to a better view of the SDI performance.
Methods: We observed a selected sample of 16 stars (age <200 Myr, distance <25 pc) with the phase-mask coronagraph, SDI, and ADI modes of VLT/NaCo.
Results: We do not detect any companions. As for the estimation of the sensitivity limits, we argue that the SDI residual noise cannot be converted into mass limits because it represents a differential flux, unlike what is done for single-band images, in which fluxes are measured. This results in degeneracies for the mass limits, which may be removed with the use of single-band constraints. We instead employ a method of directly determining the mass limits and compare the results from a combined processing SDI-ADI (ASDI) and ADI. The SDI flux ratio of a planet is the critical parameter for the ASDI performance at close-in separations (≲1''). The survey is sensitive to cool giant planets beyond 10 AU for 65% and 30 AU for 100% of the sample.
Conclusions: For close-in separations, the optimal regime for SDI corresponds to SDI flux ratios higher than ~2. According to the BT-Settl model, this translates into T_eff ≲ 800 K, which is significantly lower than the methane condensation temperature (~1300 K). The methods described here can be applied to the data interpretation of SPHERE. In particular, we expect better performance with the dual-band imager IRDIS, thanks to more suitable filter characteristics and better image quality.

Based on observations collected at the European Southern Observatory, Chile, ESO programs 085.C-0257A, 086.C-0164A, and 088.C-0893A.