Near-IR photometry of the small Uranian satellites with Keck at phase angles 0–2°

Keck near-infrared detections of Mab and Perdita

“…This paper makes use of H-band (1.6 µm) and Kp-band (2.1 µm) data taken on 28 October 2019 by the narrow camera of the NIRC2 instrument coupled to the Adaptive Optics (AO) system at Keck Observatory, using Uranus itself for wavefront sensing (Wizinowich et al 2000;van Dam et al 2004). The same data were analyzed by Paradis et al (2023), so we refer the reader to that paper for a detailed description of the observation and data reduction procedures. The standard star was FS2 from the UKIRT Faint Standard Star list (Hawarden et al 2001), 1 and standard photometric calibration procedures resulted in calibration constants C H = 8.87 × 10 −17 and C Kp = 6.46 × 10 −17 to convert from 1 count per second to 1 erg s −1 cm −2 µm −1 .…”

“…The proper motion of Mab is roughly 30 mas (3 pixels) per minute, which means that each frame had to be shifted relative to the previous one in accordance with Mab's expected vector of motion to improve the signal-to-noise ratio (SNR). This so-called "shift-and-stack" technique has been used by other authors to improve the SNR of moving point sources, including to measure photometry of other small moons of Uranus with NIRC2 (Paradis et al 2019(Paradis et al , 2023. Our shift-and-stack procedure differs in several ways from the one implemented by those authors, and we describe it fully in Supplementary Material A.…”

“…Because Mab was detected at low SNR and within a substantial scattered light background, the flux in the wings of the point-spread function (PSF) could not be measured. We therefore determined the flux of Mab using a flux bootstrapping procedure similar to that applied to Keck-observed point sources by other authors (Gibbard et al 2005;de Pater et al 2014;Molter et al 2019;Paradis et al 2023), the specifics of which we summarize in Supplementary Material B. We find the total flux of Mab to be (6.1 ± 0.8 )×10 −16 erg s −1 cm −2 µm −1 in H-band, where the error bars indicate a 1-σ uncertainty, computed by taking the quadrature sum of the ∼13% flux bootstrapping error and the ∼2% photometric calibration error.…”

“…The infrared brightness of Mab is approximately a factor of 2 fainter than observed by Showalter & Lissauer (2006) with HST in the HRC/Clear filter centered at 0.5 µm. 3 We compare the I/F of Mab in the three available filters (HST HRC/clear, H, Kp) with I/F values in the same filters for Puck and Miranda (Karkoschka 2001b;Gibbard et al 2005;Paradis et al 2023) in Figure 3 assuming Mab has either a 6-km or 12-km radius. Ratios between these filters are compiled in Table 1; large ratios likely indicate an icy surface, as explained in the Introduction.…”

“…Those authors proposed two end-member cases: Mab has a ∼6 km radius with a Miranda-like icy surface and an albedo of ∼0.46, or Mab has a ∼12 km radius and an albedo of ∼0.1 like its other nearest neighbor, Puck. Previous near-infrared searches with Keck did not detect the moon; the upper limit of de Pater et al (2006) led to the suggestion that Mab was an icy body like Miranda, with substantial water-ice absorption, and the more sensitive upper limit of Paradis et al (2019) required that Mab's radius be ≲6 km and its surface as dark at 1.6 µm as Puck (albedo ≲0.11; Paradis et al 2023). Strong near-IR absorption from the crystalline water ice band at 1.65 µm is responsible for a factor-of-four darkening in the near-IR spectra of Pluto's outer satellites Nix and Hydra (Cook et al 2018), and this absorption band could explain such a low H-band flux.…”