OSSOS. XII. Variability Studies of 65 Trans-Neptunian Objects Using the Hyper Suprime-Cam

Abstract

We present variability measurements and partial light curves of Trans-Neptunian Objects (TNOs) from a two-night pilot study using Hyper Suprime-Cam (HSC) on the Subaru Telescope (Maunakea, Hawai'i, USA). Subaru's large aperture (8-m) and HSC's large field of view (1.77 deg 2 ) allow us to obtain measurements of multiple objects with a range of magnitudes in each telescope pointing. We observed 65 objects with m r = 22.6-25.5 mag in just six pointings, allowing 20-24 visits of each pointing over the two nights. Our sample, all discovered in the recent Outer Solar System Origins Survey (OSSOS), span absolute magnitudes H r = 6.2-10.8 mag and thus investigates smaller objects than previous light curve projects have typically studied. Our data supports the existence of a correlation between light curve amplitude and absolute magnitude seen in other works, but does not support a correlation between amplitude and orbital inclination. Our sample includes a number of objects from different dynamical populations within the trans-Neptunian region, but we do not find any relationship between variability and dynamical class. We were only able to estimate periods for 12 objects in the sample and found that a longer baseline of observations is required for reliable period analysis. We find that 31 objects (just under half of our sample) have variability ∆ mag greater than 0.4 mag during all of the observations; in smaller 1.25 hr, 1.85 hr and 2.45 hr windows, the median ∆ mag is 0.13, 0.16 and 0.19 mags, respectively. The fact that variability on this scale is common for small TNOs has important implications for discovery surveys (such as OSSOS or the Large Synoptic Survey Telescope) and color measurements.

A statistical review of light curves and the prevalence of contact binaries in the Kuiper Belt

“…The situation is different in Fig. 12c, which shows our analysis of the KBOs in the OSSOS survey (Alexandersen et al, 2019). Here the trend line is strong, with a corrected slope of 0.087 ± 0.037, for 2.3σ significance; this provides 99% confidence (p-value = 0.01) that this trend is real.…”

“…However, when the observations span two or more nights, we include the body in our statistical analysis even though the value of Δm may be somewhat underestimated. Using similar reasoning, Alexandersen et al (2019) argued that most of their two-night observations captured 80-88% of the full light curve amplitude. In the table, the note column indicates our reasoning behind each judgment call.…”

“…The literature contains several reports of observed correlations between absolute magnitude and Δm (Alexandersen et al, 2019;Benecchi and Sheppard, 2013;Thirouin and Sheppard, 2019), which have been interpreted as evidence that smaller KBOs are generally more irregular in shape than large ones. The implied assumption here is that H 0 , the time-averaged value, can be used as a rough proxy for size.…”

“…In this analysis, however, we have opted to plot the measurements from the Outer Solar System Origins Survey (OSSOS) separately from the others (Alexandersen et al, 2019). These show a very different cumulative distribution, with a substantially larger fraction having high amplitudes.…”

“…Part of the explanation for the difference may be that Alexandersen et al (2019) calculated Δm from the difference between the highest and the lowest measurements; this increases the likelihood that noise or one spurious measurement could artificially increase Δm. However, this is not the entire explanation.…”