2014A&A...566A.126M

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. We perform an imaging survey to search for cool (T_eff <1000-1300K) giant planets at separations as close as 5-10AU. 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. We observed a selected sample of 16 stars (age <200Myr, distance <25pc) with the phase-mask coronagraph, SDI, and ADI modes of VLT/NaCo. 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 10AU for 65% and 30AU for 100% of the sample. 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≲800K, which is significantly lower than the methane condensation temperature (∼1300K). 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.