Main-belt comets in the Palomar Transient Factory survey – I. The search for extendedness

Abstract

Cometary activity in main-belt asteroids probes the ice content of these objects and provides clues to the history of volatiles in the inner solar system. We search the Palomar Transient Factory (PTF) survey to derive upper limits on the population size of active main-belt comets (MBCs). From data collected March 2009 through July 2012, we extracted ∼2 million observations of ∼220 thousand known main-belt objects (40% of the known population, down to ∼1-km diameter) and discovered 626 new objects in multi-night linked detections. We formally quantify the "extendedness" of a small-body observation, account for systematic variation in this metric (e.g., due to on-sky motion) and evaluate this method's robustness in identifying cometary activity using observations of 115 comets, including two known candidate MBCs and six newly-discovered non-main-belt comets (two of which were originally designated as asteroids by other surveys). We demonstrate a 66% detection efficiency with respect to the extendedness distribution of the 115 sampled comets, and a 100% detection efficiency with respect to extendedness levels greater than or equal to those we observed in the known candidate MBCs P/2010 R2 (La Sagra) and P/2006 VW 139 . Using a log-constant prior, we infer 95% confidence upper limits of 33 and 22 active MBCs (per million main-belt asteroids down to ∼1-km diameter), for detection efficiencies of 66% and 100%, respectively. In a follow-up to this morphological search, we will perform a photometric (disk-integrated brightening) search for MBCs.

The Main Belt Comets and ice in the Solar System

“…It reinforces the idea that rotational instability is one of the important fates that comets may suffer, especially for those sub-kilometre sized. In addition to sky surveys such as Pan-STARRS, there have been attempts to search for MBCs with targeted observations of known asteroids in main-belt orbits to search for activity (Hsieh 2009), and in archival data from the Canada-France-Hawaii Telescope Wiegert 2009, 2010;Sonnett et al 2011) and the Palomar Transient Factory (Waszczak et al 2013). Recently, a citizen science project 2 to crowd source the visual identification of MBCs has been employed in the search (Hsieh et al 2016;Schwamb et al 2017).…”

CENTAURS AND SCATTERED DISK OBJECTS IN THE THERMAL INFRARED: ANALYSIS OFWISE/NEOWISEOBSERVATIONS

“…We observed Chiron on 2010 May 22 from the Palomar Transient Factory's 48 inch telescope (PTF; cf. Law et al 2009;Waszczak et al 2013;Ofek et al 2012aOfek et al , 2012b and found an R-band magnitude (m R ) = 17.86 ± 0.03. For a bare nucleus of ∼200 km diameter with ∼15% reflectance (Fernández et al 2002) and an IAU phase curve slope parameter of G = 0.15 (cf.…”

Main-belt comets: sublimation-driven activity in the asteroid belt

“…On the other hand, untargeted searches, such as that conducted by the PANSTARRS1 and Palomar Transient Factory surveys (Hsieh et al 2015a;Waszczak et al 2013), introduce fewer selection biases than targeted surveys, and if conducted as part of larger multi-use surveys, can be much more efficient in terms of required observing effort. The lack of strong selection biases in untargeted surveys also means that more meaningful estimates of the total size of the MBC population can be made than would be possible from a targeted survey.…”

“…Discovery rates of untargeted search will inevitably be lower than those of targeted surveys though, meaning that more objects must be surveyed to achieve each successful new discovery. Reliable automated techniques must also be developed to search large data sets for comet candidates (e.g., Hsieh et al 2012b;Waszczak et al 2013), given that they will generally be too large to analyze manually. Citizen science initiatives, where internet-based tools are used to solicit and gather large numbers of image classifications from members of the general public (e.g., Lintott et al 2008) and which exploit the pattern recognition abilities of the human eye that can be difficult to replicate with automated algorithms, may also prove to be useful for analyses of these large data sets.…”