Asteroid Light Curves From the Palomar Transient Factory Survey: Rotation Periods and Phase Functions From Sparse Photometry

Abstract

We fit 54,296 sparsely-sampled asteroid lightcurves in the Palomar Transient Factory (PTF) survey to a combined rotation plus phase-function model. Each lightcurve consists of 20 or more observations acquired in a single opposition. Using 805 asteroids in our sample that have reference periods in the literature, we find the reliability of our fitted periods is a complicated function of the period, amplitude, apparent magnitude and other lightcurve attributes. Using the 805-asteroid ground-truth sample, we train an automated classifier to estimate (along with manual inspection) the validity of the remaining ∼53,000 fitted periods. By this method we find 9,033 of our lightcurves (of ∼8,300 unique asteroids) have 'reliable' periods. Subsequent consideration of asteroids with multiple lightcurve fits indicate a 4% contamination in these 'reliable' periods. For 3,902 lightcurves with sufficient phase-angle coverage and either a reliably-fit period or low amplitude, we examine the distribution of several phase-function parameters, none of which are bimodal though all correlate with the bond albedo and with visible-band colors. Comparing the theoretical maximal spin rate of a fluid body with our amplitude versus spin-rate distribution suggests that, if held together only by self-gravity, most asteroids are in general less dense than ∼2 g/cm 3 , while C types have a lower limit of between 1 and 2 g/cm 3 . These results are in agreement with previous density estimates. For 5-20 km diameters, S types rotate faster and have lower amplitudes than C types. If both populations share the same angular momentum, this may indicate the two types' differing ability to deform under rotational stress. Lastly, we compare our absolute magnitudes (and apparent-magnitude residuals) to those of the Minor Planet Center's nominal (G = 0.15, rotation-neglecting) model; our phase-function plus Fourier-series fitting reduces asteroid photometric RMS scatter by a factor ∼3.

Large Super-Fast Rotator Hunting Using the Intermediate Palomar Transient Factory

“…Furthermore, with sufficiently large samples, the C-type asteroids were for the first time found to show a rotation-period limit that was longer than that of the S-type asteroids Waszczak et al 2015). This is in accordance with the general picture for rubble-pile asteroids that: (a) they cannot rotate exceedingly fast; and (b) those with lower bulk density should have a longer rotation-period limit ( r +D P m 3.3 1 ; ( ) Harris 1996).…”

“…With advances in observational technology, several data sets containing hundreds to thousands of asteroid rotation periods have been acquired through large sky surveys (Masiero et al 2009;Polishook & Brosch 2009;Dermawan et al 2011;Polishook et al 2012;Chang et al 2014aChang et al , 2015. Moreover, numerous asteroid rotation periods, obtained from various time-series-archived data products (see an example of Waszczak et al 2015), and single target observations from the Asteroid Light Curve Database (LCDB; Warner et al 2009), 5 also provide major contributions to this field. Therefore, a more comprehensive understanding of asteroid rotations has emerged and possible applications could be conducted as well (see, e.g., Chang et al 2016).…”

“…While the spin-rate distributions obtained from large skycoverage surveys and archived data show a number decrease with spin rate at frequencies f>5 rev day −1 (Masiero et al 2009;Chang et al 2015;Waszczak et al 2015), a flat distribution was indicated by the single target observations (Pravec et al 2008(Pravec et al , update 2014. However, the tendency of number decrease still remains in the asteroid spin-rate distributions from the large sky surveys after taking into account possible observational bias (Masiero et al 2009;Chang et al 2015).…”

The heart of the swarm: K2 photometry and rotational characteristics of 56 Jovian Trojan asteroids

“…Our period determinations are plotted against data from the literature (Molnar et al 2008;Mottola et al 2011;French et al 2015;Waszczak et al 2015) in Fig. 4.…”

WISE data and sparse photometry used for shape reconstruction of asteroids

“…2. For Lowell data only, the correct period (known from dense lightcurves to be around 6.58 h, Almeida et al 2004;Waszczak et al 2015, and Behrend's web †) is hidden in many local minima. Adding increasingly more WISE data makes the correct period stand out more clearly.…”