Centaur and Kuiper Belt Object Phase Curves from the Asteroid Terrestrial-impact Last Alert System Survey

The phase curve of a small Solar System object measures its change in brightness at different phase angles relative to incident sunlight, and is a complex function of the surface properties of that object. Analysing phase curves of small Outer Solar System Objects (OSSOs) not only allows accurate absolute magnitude measurements, but can probe the composition, surface topography, and sizes of regoliths of these objects’ surfaces. This opens up another method of detecting possible correlations with other compositional and dynamical properties across different populations of these objects, which could shed light on possible evolutionary connections between them.

Several observation programs of OSSOs have observed the phase curves of many Kuiper-belt objects (KBOs) and Centaurs (Rabinowitz et al., 2007; Schaefer et al., 2009; Alvarez-Candal et al., 2016; Ayala-Loera et al., 2018; Alvarez-Candal et al., 2019; Verbiscer et al., 2019). However, limited availability of telescope observation time, the large baseline of observations required, coupled with the challenges of ground-based observing (bouts of poor weather, and the limited observability of targets throughout the year) combine to make obtaining photometry of these objects across a wide range of phase angles difficult. Additionally, the large heliocentric distances of these objects typically restricts Earth-based observations to very small phase angles, < 2◦ for KBOs. Many studies make use of photometry obtained from different datasets. This additional data is often obtained with heterogeneous processing and analysis methods, leaving open the possibility of systematic errors in the resultant phase curve. Furthermore, several Centaurs have been known to exhibit brightening due to cometary activity, which would significantly alter an observed phase curve from its true profile, and may be difficult to identify from the light curve if large time-intervals exist between photometric measurements. However, multi-filter broad- band photometry of bright Centaurs and KBOs across a long, continuous observation baseline has been made available by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey (Tonry et al., 2018a,b), and offers an opportunity to study the phase curves of these objects populated with large numbers of observations.

The ATLAS survey has been in continuous operation since its installation in 2015. Using two 0.5-m Schmidt telescopes located at Haleakalā and Mauna Loa, Hawai’i, it regularly observes the sky outside of ∼ 60◦ of the Sun, down to a limiting magnitude of ∼19.7 across declinations +90◦ ≤ δ ≤ −50◦, in two visible-region broadband filters - ’cyan’ (’c’, of wavelength range 420-650 nm) and ’orange’ (’o’, of wavelength range 560-820 nm). Fields are observed with a cadence of 2 nights, with four exposures taken per frame of observation over an interval of 1 hour (Tonry et al., 2018a). Throughout its search to detect dangerous near-Earth asteroids, it has accumulated photometry for a sample of bright KBOs and Centaurs over a long, continuous observation baseline. This allows the analysis of the phase curves of these objects in two colour filters, with a dataset sufficiently large as to preclude the need for including additional data from potentially heterogeneous sources, thus helping to circumvent possible systematic errors.

Our sample of ~20 bright small OSSOs visible to the ATLAS survey telescopes comprises 12 Centaurs and 8 KBOs, spanning semimajor axes of 5.7 au ≤ a ≤ 189.7 au. These objects span approximate absolute magnitudes of −1.1HV13.6, and span apparent magnitudes 14.5c 19.5 and 14.2o19.2, averaged across the observation baseline of ATLAS. The phase curve in a given filter of each object in the sample contains on average 331 data points, exceeding the values of previous studies (Rabinowitz et al., 2007; Schaefer et al., 2009; Alvarez-Candal et al., 2016; Ayala-Loera et al., 2018; Alvarez-Candal et al., 2019), and covers the majority of the maximum range of phase angles observable from Earth during the observation baseline of ATLAS. Example phase curves for the bright Centaur (2060) Chiron using photometry from ATLAS is shown in Figure 1.

 

Figure 1: Phase curve of Centaur (2060) Chiron using ATLAS photometry in cyan (’c’) filter (upper, 281 measurements) and orange (’o’) filter (lower, 997 measurements). Light curve corrections to the phase curves have not been made.

By analysing the phase curves of this sample of Centaurs and KBOs, we can explore their surface properties of these objects based on a large dataset of photometric observations. We will search for correlations between the derived phase curve parameters and other object properties e.g. a dependency of phase coefficients on object colour. We will present our ATLAS sample of phase curves and compare our findings to those from past literature.

2 Acknowledgements

We made use of the ATLAS Forced Photometry Server to obtain our Centaur and KBO photometry. https://fallingstar-data.com/forcedphot/

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (AT- LAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astro- nomical Observatory (SAAO), and the Millennium Institute of Astrophysics (MAS), Chile.

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