Properties of slowly rotating asteroids from the Convex Inversion Thermophysical Model

Abstract

Context. Recent results for asteroid rotation periods from the TESS mission showed how strongly previous studies have underestimated the number of slow rotators, revealing the importance of studying those targets. For most slowly rotating asteroids (those with P > 12 h), no spin and shape model is available because of observation selection effects. This hampers determination of their thermal parameters and accurate sizes. Also, it is still unclear whether signatures of different surface material properties can be seen in thermal inertia determined from mid-infrared thermal flux fitting. Aims. We continue our campaign in minimising selection effects among main belt asteroids. Our targets are slow rotators with low light-curve amplitudes. Our goal is to provide their scaled spin and shape models together with thermal inertia, albedo, and surface roughness to complete the statistics. Methods. Rich multi-apparition datasets of dense light curves are supplemented with data from Kepler and TESS spacecrafts. In addition to data in the visible range, we also use thermal data from infrared space observatories (mainly IRAS, Akari and WISE) in a combined optimisation process using the Convex Inversion Thermophysical Model. This novel method has so far been applied to only a few targets, and therefore in this work we further validate the method itself. Results. We present the models of 16 slow rotators, including two updated models. All provide good fits to both thermal and visible data.The obtained sizes are on average accurate at the 5% precision level, with diameters found to be in the range from 25 to 145 km. The rotation periods of our targets range from 11 to 59 h, and the thermal inertia covers a wide range of values, from 2 to <400 J m−2 s−1∕2 K−1, not showing any correlation with the period. Conclusions. With this work we increase the sample of slow rotators with reliable spin and shape models and known thermal inertia by 40%. The thermal inertia values of our sample do not display a previously suggested increasing trend with rotation period, which mightbe due to their small skin depth.

Thermal Properties of 1847 WISE-observed Asteroids

“…We found our derived diameters and albedos to be broadly consistent with the NEATM-derived values with the WISE thermal data set (Mainzer et al 2019). Our derived thermal inertia were broadly consistent with the trends found with diameter (e.g., Delbo' et al 2007) and rotation period (e.g., Marciniak et al 2019;Alí-Lagoa et al 2020;Marciniak et al 2021) in the literature, though in both cases the large uncertainties on thermal inertia limit the conclusions we were able to draw from the data. We applied a linear fit to the diameters and thermal inertia normalized to 1 au of the form log[Γ] = α + β log[D], we find best-fit values of α = 2.667 ± 0.059 and β = −0.467 ± 0.044 for our sample alone and α = 2.509 ± 0.017 and β = −0.352 ± 0.012 when combined with literature estimates.…”

“…Some earlier studies (e.g., Harris & Drube 2016) have suggested that slow rotators should present higher thermal inertia due to thermal observations being able to probe more deeply into an asteroid's surface. Other more recent studies (e.g., Marciniak et al 2019;Alí-Lagoa et al 2020;Marciniak et al 2021) have found no excess of high thermal inertia values among slow rotators (conventionally defined as asteroids with P > 12 hours), nor for low thermal inertia values among fast rotators. In our sample, we likewise find thermal inertia values greater than 100 J m −2 s −0.5 K −1 to be generally present across our entire rotation period range, though we note that our sample is sparsely populated with rotation periods longer than 30 hours (Fig.…”

“…In cases where a publication gives multiple fits for one asteroid, if there are no extenuating circumstances, we reference the values associated with either the smaller χ 2 , or, if the χ 2 are equal, the larger thermal inertia uncertainty. The Marciniak et al (2021) results do not include the 8 asteroids where only an upper limit was found for the thermal inertia.…”

Thermal Properties of 1847 WISE-observed Asteroids

“…It was thus hypothesized that slow rotators (i.e., rotation periods longer than 12 hours) should on average present higher thermal inertia than those of fast rotators. While early studies such as Harris & Drube (2016) and Marciniak et al (2018) appeared to confirm this trend, later studies with larger samples of slow rotators (e.g., Marciniak et al 2019;Alí-Lagoa et al 2020;Marciniak et al 2021) found no such relation.…”

“…The orbital classification is sourced from the JPL Small-Body Database as one of the following: inner or outer MBAs (IMB: a < 2.0 au, MBA: 2.0 < a < 3.2 au, OMB: 3.2 < a < 4.6 au), Jupiter Trojans (TJN), Aten, Apollo, or Amor NEAs (ATE, APO, AMO), Mars crossers (MCA), or trans-Neptunian objects (TNO). The Marciniak et al (2021) results do not include the eight asteroids where only an upper limit was found for the thermal inertia.…”