Illustration of the performance of SPHERE compared to other instruments for a typical nearby white dwarf (age 2 Gyr, MV = MH = 12). The green regions represent the parameter space for which SPHERE enables significant improvement over the other instruments. Left: sensitivity vs. distance of the white dwarf from the Sun. The quantity ρ represents the angular separation that the instrument is sensitive to. The performance of SPHERE is extrapolated from our observation of GD 50. Because GD 50 is at the faintness limit, the actual performance on a brighter target would be better. The vertical cut-off in the SPHERE curve near 30 pc is artificial and assumes that a star fainter than mV = 14.5 cannot be observed because of the limiting magnitude of the AO. For nearby bright white dwarfs, resolved imaging is more sensitive to low-mass objects than photometric observations. Right: contrast curves for the same WD at a distance of 10 pc. The NACO curve is estimated from archival data (program ID: 079.D-0561(A), PI: Radiszcz) with a similar integration time as SPHERE (3000 s). The HST/NICMOS limit was extrapolated from Debes et al. (2005b) with a typical exposure time of 20 min. The key point is that SPHERE has the unique capability to detect high-mass planets at 2−10 au from the white dwarf, which is not possible with any other instrument.
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