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This page details the sensitivity of GPI in differential-polarimetry mode, with focus on sensitivity to extended objects. As opposed to detection of planets, where there is a simple figure of merit (brightness ratio of the point planet to the point star at a given wavelength), disks are extended objects and sensitivity has to be expressed in a way that recognizes that.
Two disks of the same total brightness, which looked identical to IRAS, may be very different from GPI's point of view if in one case the dust is diffuse and spread over a large area and in another there is a sharp HR4796-like ring. Thus this has been modeled in units of either total intensity (assuming a particular geometry) or surface brightness.
GPI's sensitivity to disk-scattered light relies on differential polarimetry to suppress PSF speckles. We show here predicted polarimetric surface brightness sensitivity per spatial element, assuming polarimetry is the only speckle suppression used (no ADI or other PSF subtraction) and given a residual polarized noise floor of 1% of total intensity (see below).
ADI polarimetry or detection of dust disks is very complicated to model and case-specific, it's clearly very dependent on disk geometry, a face-on circular disk would be subtracted by the ADI just like the seeing halo is, in general it's only useful for edge on systems, but for those it does work quite well.
Plotted in solid lines are PSF radial profiles based on current GPI AO simulations. In dotted lines are the derived 5-σ sensitivity levels per resolution element for 100% linearly polarized light. The left axis shows surface brightness in units of fractional flux per spatial element (0.014" GPI lenslet) while the right axis is scaled to show fractional flux per square arcsecond.
Plot of GPI polarization sensitivity in the J band, with an estimated 1h observation.
Plot of GPI polarization sensitivity in the H band, with an estimated 1h observation.
To assess the detectability of a given circumstellar disk requires an assumed polarization fraction derived from dust models, in much the same way that assessing the detectability of a planet from a contrast plot requires an assumed luminosity for that planet's mass derived from evolutionary models. Surface brightness levels for a few known disks are also indicated for comparison at right. For HD 4796 the polarized surface brightness is taken from Hinkley et al. 2009, for HD 15115 from Kalas et al. 2007, and for AU Mic from Fitzgerald et al. 2007.
The achieved speckle-suppression performance depends both on the science camera calibration and also on our ability to calibrate instrumental polarization for the combined GPI+Gemini South system. We conservatively estimate 1% residual post-suppression speckles based on our modeling of instrumental systematics. This level is comparable to polarimetric performance achieved with prior AO polarimeters at Lick (Perrin et al. 2008) and AEOS (Hinkley et al. 2009).
In the polarization mode you don't need to observe PSF reference stars, because the two orthogonal polarizations essentially serve as each other's PSFs, though one can contemplate using some other form of PSF subtraction if you want to obtain a total intensity image as a followup characterization observation. But polarimetry should be most sensitive for detection.